In parametric converters, the output value is the parameter of the electrical circuit (R, L, M, C). When using parametric transducers, an additional power source is required, the energy of which is used to form the output signal of the transducer.

Rheostat converters. Rheostatic transducers are based on the change in the electrical resistance of the conductor under the influence of the input value - displacement. A rheostat transducer is a rheostat whose brush (moving contact) moves under the influence of a measured non-electric quantity.

The advantages of converters include the possibility of obtaining high conversion accuracy, significant output signals, and relative simplicity of design. Disadvantages - the presence of a sliding contact, the need for relatively large movements, and sometimes significant effort to move.

Rheostatic transducers are used to convert relatively large displacements and other non-electric quantities (forces, pressures, etc.) that can be converted into displacement.

Strain gauge transducers(sensors). The operation of the converters is based on the tensor effect, which consists in changing the active resistance of the conductor (semiconductor) under the action of the mechanical stress and deformation caused in it.

Rice. 11-6. Strain gauge wire transducer

If the wire is subjected to mechanical stress, such as stretching, then its resistance will change. Relative change in wire resistance , where S is the coefficient of strain sensitivity; is the relative deformation of the wire.

The change in the resistance of the wire under mechanical action on it is explained by a change in the geometric dimensions (length, diameter) and the resistivity of the material.

In those cases where high sensitivity is required, strain-sensitive transducers made in the form of strips of semiconductor material are used. The coefficient S for such converters reaches several hundred. However, the reproducibility of the characteristics of semiconductor converters is poor. At present, integrated semiconductor strain gauges are mass-produced, forming a bridge or half-bridge with thermal compensation elements.

Equilibrium and non-equilibrium bridges are used as measuring circuits for strain gauges. Strain gauges are used to measure deformations and other non-electrical quantities: forces, pressures, moments.

Temperature sensitive transducers(thermistors). The principle of operation of converters is based on the dependence of the electrical resistance of conductors or semiconductors on temperature.

To measure temperature, the most common thermistors are made of platinum or copper wire. Standard platinum thermistors are used to measure temperatures in the range from -260 to +1100 ° C, copper - in the range from -200 to +200 "C.

Semiconductor thermistors (thermistors) are also used to measure temperature. various types, which are characterized by greater sensitivity (TCS of thermistors is negative and at 20 "C it is 10-15 times higher than the TCR of copper and platinum) and have higher resistances (up to 1 MΩ) at very small sizes. The disadvantage of thermistors is poor reproducibility and non-linearity of the conversion characteristics:

where R T and Ro are the resistances of the thermistor at temperatures T and To, To is the initial temperature of the operating range; B - coefficient.

Thermistors are used in the temperature range from -60 to +120°C.

To measure temperatures from -80 to +150 ° C, thermal diodes and thermotransistors are used, in which resistance changes under the influence of temperature p-n transition and the voltage drop across that junction. These converters are usually included in bridge circuits and circuits in the form of voltage dividers.

The advantages of thermal diodes and thermal transistors are high sensitivity, small size and low inertia, high reliability and low cost; disadvantages - a narrow temperature range and poor reproducibility of the static conversion characteristics.

Electrolytic converters. Electrolytic converters are based on the dependence of the electrical resistance of an electrolyte solution on its concentration. They are mainly used to measure the concentration of solutions.

Inductive transducers. The principle of operation of the converters is based on the dependence of the inductance or mutual inductance of the windings on the magnetic circuit on the position, geometric dimensions and magnetic state of the elements of their magnetic circuit.

Figure 11-12 Magnetic circuit with gaps and two windings

The inductance of the winding located on the magnetic circuit, where Zm is the magnetic resistance of the magnetic circuit; is the number of turns of the winding.

Mutual inductance of two windings located on the same magnetic circuit, , where and - the number of turns of the first and second windings. The magnetic resistance is given by

where - the active component of the magnetic resistance (we neglect the scattering of the magnetic flux); - respectively, the length, cross-sectional area and relative magnetic permeability of the i-th section of the magnetic circuit; mo - magnetic constant; d is the length of the air gap; s - cross-sectional area of ​​the air section of the magnetic circuit, - reactive component of magnetic resistance; P - power losses in the magnetic circuit due to eddy currents and hysteresis; w - angular frequency; Ф - magnetic flux in the magnetic circuit.

The above relations show that the inductance and mutual inductance can be changed by affecting the length d, the cross section of the air section of the magnetic circuit s, power losses in the magnetic circuit, and in other ways.

Compared to other displacement transducers, inductive transducers are distinguished by high power output signals, simplicity and reliability in operation.

Their disadvantage is the reverse effect of the transducer on the object under study (the effect of an electromagnet on the armature) and the effect of the armature inertia on frequency characteristics device.

Capacitive transducers. Capacitive transducers are based on the dependence of the electrical capacitance of the capacitor on the dimensions, the relative position of its plates and on the dielectric constant of the medium between them.

For a two-plate flat capacitor, electric capacitance , where is the electric constant; - relative permittivity of the medium between the plates; s is the active area of ​​the plates; d is the distance between the plates. The sensitivity of the transducer increases with decreasing distance d. Such transducers are used to measure small displacements (less than 1 mm).

A small working movement of the plates leads to an error from changing the distance between the plates with temperature fluctuations. By choosing the dimensions of the transducer parts and materials, this error is reduced.

The transducers are used to measure the level of liquids, the humidity of substances, the thickness of products made of dielectrics.

Rice. 11-16. Scheme of the ionization converter

Ionization transducers. The converters are based on the phenomenon of gas ionization or the luminescence of certain substances under the action of ionizing radiation.

If a chamber containing a gas is irradiated, for example, with b-rays, then between the electrodes included in electrical circuit(Fig. 11-16), current will flow. This current depends on the voltage applied to the electrodes, on the density and composition of the gaseous medium, the size of the chamber and electrodes, and the properties and intensity of ionizing radiation. These dependencies are used to measure various non-electric quantities: the density and composition of the gaseous medium, the geometric dimensions of parts.

As ionizing agents, a-, b- and g-rays of radioactive substances are used, much less often - x-rays and neutron radiation.

The main advantage of devices using ionizing radiation is the possibility of non-contact measurements, which is of great importance, for example, when measuring in aggressive or explosive environments, as well as in environments under high pressure or high temperatures. The main disadvantage of these devices is the need to use biological protection at high activity of the radiation source.

Ministry of Education of the Republic of Belarus

educational institution

"Belarusian State University

informatics and radio electronics"

Department of Metrology and Standardization

Parametric measuring transducers

Guidelines for laboratory work E.5B

for students of the specialty 54 01 01 ‑ 02

"Metrology, standardization and certification"

all forms of education

UDC 621.317.7 + 006.91 (075.8)

BBC 30.10ya73

Compiled by V.T. Revin, L.E. Bataille

The guidelines contain the purpose of the work, brief information from the theory, a description of the laboratory setup, a laboratory task and the procedure for performing the work, as well as instructions for preparing a report and test questions to test students' knowledge. The main types of parametric measuring transducers (rheostatic, inductive and capacitive), their main characteristics and schemes of inclusion in the measuring circuit are considered. Performing laboratory work involves determining the main metrological characteristics (conversion function, sensitivity, basic error, sensitivity determination error) of the considered measuring transducers, as well as mastering the technique of measuring non-electrical quantities using measuring transducers and finding errors in determining the values ​​of non-electrical quantities.

UDC 621.317.7 + 006.91 (075.8)

BBC 30.10 am 73

1 Purpose of work

1.1 Studying the principle of operation, design and main characteristics of rheostatic, capacitive and inductive measuring transducers of non-electric quantities into electrical ones.

1.2 Study of methods for measuring non-electric quantities using rheostatic, capacitive and inductive measuring transducers.

1.3 Practical definition of the main characteristics of measuring transducers and measurement of linear and angular displacements with their help.

2 Brief information from the theory

A feature of modern measurements is the need to determine the values ​​of many physical quantities, most of which are non-electrical quantities. To measure non-electric quantities, electrical measuring instruments are widely used, due to a number of their significant advantages. These include high measurement accuracy, high sensitivity and speed of measuring instruments, the possibility of remote measurements, automatic conversion of measurement information, automatic control of the measurement process, etc. A feature of electrical measuring instruments designed to measure non-electric quantities is obligatory presence primary measuring converter of a non-electric quantity into an electrical one.

The primary measuring transducer establishes an unambiguous functional relationship between the output electrical quantity Y and the input non-electrical quantity X: Y= f(X).

Depending on the type of output signal, primary measuring transducers are divided into parametric and generator.

IN parametric In measuring transducers, the output quantity is an electrical circuit parameter: resistance R, inductance L, mutual inductance M or capacitance C. When using parametric transducers, an additional power source is always required, the energy of which is used to generate the output signal of the transducer.

IN generating Measuring transducers output quantities are EMF, current, voltage, or charge. When using generator transducers, auxiliary power supplies are used only to amplify the received signal.

According to the principle of operation, parametric measuring transducers are divided into rheostatic, strain-sensitive (strain resistors), thermally sensitive (thermistors, thermistors), capacitive, inductive, ionization.

The dependence of the output value of the measuring transducer Y on the input value X, described by the expression Y = f (X), called conversion function. Often the output value of the converter Y depends not only on the input measured value X, but also on some external factor Z. Therefore, in general terms, the transformation function can be represented by a functional dependence: Y = f (X, Z).

When developing measuring transducers of non-electrical quantities, they strive to obtain a linear conversion function. To describe a linear transformation function, it suffices to specify two parameters: the initial value of the output quantity Y 0 (zero level), corresponding to zero or another initial value of the input quantity X, and the parameter S, which characterizes the slope of the transformation function.

In this case, the transformation function can be represented as follows:

The parameter S, which characterizes the slope of the transformation function, is called the sensitivity of the converter. Transducer sensitivity is the ratio of the change in the output value of the measuring transducer ΔY to the change in the input value ΔX that caused it:

. (2)

The sensitivity of the transducer is a quantity that has a dimension, and the dimension depends on the nature of the input and output quantities. For a rheostat transducer, for example, sensitivity has the dimension of Ohm/mm, for a thermoelectric transducer - mV/K, for a photocell - µA/lm, for an engine - rev/(sV) or Hz/V, for a galvanometer - mm/µA and etc.

The most important problem in the design and use of a measuring transducer is to ensure the constancy of its sensitivity. The sensitivity should depend as little as possible on the values ​​of the input variable X (in this case, the transformation function is linear), the rate of change of X, the operating time of the converter, as well as the impact of other physical quantities that characterize not the object itself, but its environment (such quantities are called influencing). With a non-linear transformation function, the sensitivity depends on the values ​​of the input variable: S = S(X) .

The range of values ​​of non-electric quantities converted using a measuring transducer is limited, on the one hand, by the conversion limit, and, on the other hand, by the sensitivity threshold.

Conversion limit converter is the maximum value of the input quantity that can be accepted by the converter without damaging it or distorting the conversion function.

Sensitivity threshold- this is the minimum change in the value of the input variable that can cause a noticeable change in the output value of the converter.

Ratio Y = f(X) expresses in a general theoretical form the physical laws underlying the work of converters. In practice, the conversion function is determined experimentally in numerical form as a result of the calibration of the converter. In this case, for a series of exactly known values ​​of X, the corresponding values ​​of Y are measured. , which allows you to build a calibration curve (Figure 1, but). Using the constructed calibration curve, according to the values ​​of the electrical quantity Y obtained as a result of the measurement, it is possible to find the corresponding values ​​of the desired non-electric quantity X (Figure 1, b).

but– construction of a calibration curve according to the measured values ​​of X and Y;

b use of a calibration curve to determine the input value X

Figure 1 - Calibration characteristic of the measuring transducer

The most important characteristic of any measuring transducer is its basic error, which is due to the principle of operation, imperfection of the design of the converter or its manufacturing technology and manifests itself at normal values ​​of the influencing quantities or when they are within the range of normal values.

The basic error of the measuring transducer can have several components, due to:

The inaccuracy of exemplary measuring instruments, with the help of which the transformation function was determined;

The difference between the real calibration characteristic and the nominal conversion function; approximate (tabular, graphical, analytical) expression of the transformation function;

Incomplete coincidence of the conversion function with increasing and decreasing measured non-electric quantities (hysteresis of the conversion function);

Incomplete reproducibility of the characteristics of the measuring transducer (most often sensitivity).

When calibrating a series of converters of the same type, it turns out that their characteristics are somewhat different from each other, occupying a certain band. Therefore, in the passport of the measuring transducer, some average characteristic is given, called nominal. Differences between the nominal (passport) and real characteristics of the converter are considered as its errors.

The calibration of the measuring transducer (determination of the actual conversion function) is carried out using measuring instruments for non-electrical and electrical quantities. As an example, Figure 2 shows a block diagram of a setup for calibrating a rheostat transducer. A ruler is used as a means of measuring linear displacement (non-electrical quantity), and a digital meter L, C, R E7-8 is used as a means of measuring electrical quantity - active resistance.

Figure 2 - Structural diagram of the installation for calibrating a rheostat converter

The calibration process of the transducer is as follows. With the help of the movement mechanism, the movable contact (engine) of the rheostatic converter is sequentially set to the digitized marks of the scale of the ruler, and at each mark the active resistance of the converter is measured using the E7-8 device. The measured values ​​of linear displacement and active resistance are entered in the calibration table 1.

Table 1

In this case, we obtain the conversion function of the measuring transducer, given in tabular form. To obtain a graphical representation of the transformation function, you must use the recommendations shown in Figure 1, but.

However, it should be borne in mind that the measurement of linear displacement and active resistance was carried out with an error due to the instrumental errors of the measuring instruments used. In this regard, the definition of the transformation function was also made with some error (Figure 3).

Figure 3 - Errors in determining the transformation function

Since the sensitivity of the transducer S, given by the slope of the conversion function, is determined by formula (2), then the calculation of the error in determining the sensitivity of the converter Δ S should be carried out on the basis of the algorithm for calculating the error of the result of indirect measurement. In general, the calculation formula for Δ S as follows:

where
,

Δ y 1 And Δ y 2 – errors in determining the output values ​​y 1 and y 2 ,

Δ x 1 And Δ x 2 – errors in determining the input values ​​x 1 and x 2 .

Additional errors of the measuring transducer, due to its principle of operation, imperfection of the design and manufacturing technology, appear when the influencing quantities deviate from normal values.

In addition to the characteristics discussed above, measuring converters of non-electrical quantities into electrical ones are characterized by: output signal variation, output impedance, dynamic characteristics. The most important technical characteristics also include: dimensions, weight, resistance to mechanical, thermal, electrical and other overloads, reliability, ease of installation and maintenance, explosion safety, manufacturing cost, etc. .

Measuring transducers vary according to the principle of signal conversion.

When analog direct conversion(Figure 4) the measured non-electrical quantity X is fed to the input of the primary measuring transducer (PMT). The output electrical value Y of the transducer is measured by an electrical measuring device (EIM), which includes a measuring transducer and an indicator device.

Figure 4 - Block diagram of the device with analog direct conversion of the measured non-electrical quantity

Depending on the type of output quantity and the requirements for the device, an electrical measuring device can be of varying degrees of complexity. In one case, this is a magnetoelectric millivoltmeter, and in the other, a digital measuring device. Usually scale indicator device EIP is calibrated in units of the measured non-electric quantity. The measured non-electric quantity can be repeatedly converted to match the limits of its measurement with the limits of the PIP conversion and to obtain a more convenient type of input action for the PIP. To perform such transformations, enter into the device preliminarilybody converters of non-electrical values ​​into non-electrical ones.

With a large number of intermediate converters in direct conversion devices, the total error increases significantly. To reduce the error, use differential outmeasuring transducers, which have lower additive error, less non-linear conversion function and higher sensitivity compared to direct conversion devices.

Figure 5 shows a block diagram of a device with a differential measuring transducer (DIP). The converter includes a DZ differential link with two outputs, two conversion channels (P1 and P2) and a VU subtractor. When the input measured value x changes from the initial value x 0 to the value (x 0 + Δx), the output values ​​x 1 and x 2 at the output of the remote sensing receive increments with different signs. After their conversion to P1 and P2, the values ​​at the output of the converters y 1 and y 2 are subtracted. As a result, the output value of the DIP (y = y 1 -y 2) supplied to the measuring mechanism of the MI is proportional only to the increment Δx of the measured non-electrical quantity.

Figure 5 - Block diagram of the device with differential conversion of the measured non-electrical quantity

In appliances with transformation based on the principle of compensation (balancing) in the device for comparing the US of the converter, a comparison takes place measurable magnitude and homogeneous to it changeable the value created by the UOS feedback node (Figure 6) The values ​​are compared until they are completely balanced. As feedback nodes, reverse converters are used that convert an electrical quantity into a non-electric one (for example, incandescent lamps, electromechanical converters, etc.).

Figure 6 - Block diagram of the device with a compensation measuring transducer

Compared to direct conversion devices, compensatory comparison devices provide higher accuracy, faster response, and consume less energy from the object of study.

Electrical instruments for measuring non-electrical quantities can be either analog or digital.

Rheostat converters

Rheostatic transducers are based on a change in the electrical resistance of a conductor under the influence of an input value - linear or angular displacement. A rheostat transducer is a rheostat (a frame with a wire winding applied to it), the movable contact of which performs linear or angular movement under the influence of a measured non-electric quantity. Schematic representations of some designs of rheostatic transducers are shown in Figure 6, a-c. The dimensions of the transducer are determined by the limiting values ​​of the measured displacement, the resistance of the winding and the electrical power dissipated in the winding. To obtain a nonlinear transformation function, functional rheostat converters are used. The desired form of the transformation function is achieved by profiling the frame of the converter (Figure 6, in).

In rheostatic converters, the static conversion characteristic has a stepped character, since the resistance changes in jumps equal to the resistance of one turn. This causes the appearance of the corresponding error, the maximum value of which can be represented as:

, (4)

where R is the maximum resistance of one turn;

R is the impedance of the transducer.

IN rheochord converters in which the moving contact slides along the axis of the wire, this error can be avoided.

Rheostatic transducers are included in measuring circuits in the form of balanced and non-equilibrium bridges, voltage dividers, etc.

Figure 7 - Rheostatic measuring transducers

The main disadvantages of rheostatic transducers are the presence of a sliding contact, the need for relatively large movements, and sometimes significant effort to move. The advantages include simplicity of design and the ability to obtain significant levels of output signals.

Rheostatic transducers are used to measure relatively large linear and angular displacements, as well as other non-electric quantities that can be converted into displacement (force, pressure, etc.).

Inductive transducers

The principle of operation of inductive converters is based on the dependence of the intrinsic or mutual inductance of the windings on the magnetic circuit on the relative position, geometric dimensions and magnetic resistance of the elements of the magnetic circuit. From electrical engineering it is known that the inductance L winding located on the magnetic core (magnetic circuit) is determined by the expression:

, (5)

where Z M  magnetic resistance of the magnetic circuit;

w- the number of turns of the winding.

Mutual inductance M two windings located on the same magnetic circuit with magnetic resistance Z M, is defined as

, (6)

where w 1 And w 2  number of turns of the first and second windings.

The magnetic resistance is given by:

, ` (7)

The above relations show that the inductance and mutual inductance can be changed by changing the length δ or the cross section S of the air section of the magnetic circuit, the power loss P in the magnetic circuit, etc.

Figure 8 schematically shows different types of inductive transducers. Changing the mutual inductance can be achieved, for example, by moving the movable core (armature) 1 relative to the fixed core 2, by introducing a non-magnetic metal plate 3 into the air gap (Figure 8 but).

Figure 8 - Inductive measuring transducers

Inductive transducer with variable air gap length  (Figure 8, b) is characterized by a nonlinear dependence L = f ( ). Such a converter has a high sensitivity and is usually used when moving the armature of the magnetic circuit in the range from 0.01 to 5 mm.

Significantly lower sensitivity, but linear dependence of the transformation function L = f(S) converters with a variable air gap cross section differ (Figure 8, in). Such transducers are used to measure displacements up to 10-15 mm.

Inductive differential converters are widely used (Figure 8, G), in which the movable armature is placed between two fixed cores with windings. When the armature is moved under the influence of the measured value, the lengths change simultaneously and with different signs δ 1 And δ 2 air gaps of the converter, while the inductance of one winding will increase, and the other will decrease. Differential converters are used in combination with bridge measuring circuits. Compared to non-differential converters, they have a higher sensitivity, less non-linearity of the conversion function, and are less influenced by external factors.

To convert relatively large displacements (up to 50 - 100 mm), transformer converters with an open magnetic circuit are used (Figure 8, d).

If the ferromagnetic core of the converter is subjected to mechanical action by force F, then due to a change in the magnetic permeability of the core material, the magnetic resistance of the circuit will change, which will also entail a change in the inductance L and mutual inductance M of the windings. The principle of operation of magnetoelastic transducers is based on this dependence (Figure 8, e).

Inductive transducers are used to measure linear and angular displacements, as well as other non-electrical quantities that can be converted into displacement (force, pressure, torque, etc.). The transducer design is determined by the range of measured displacements. Converter dimensions are selected based on the required output signal power.

To measure the output parameter of inductive converters, bridge (equilibrium and non-equilibrium) and generator measuring circuits, as well as circuits with using resonant circuits, which have the greatest sensitivity due to the large steepness of the conversion function.

Compared to other displacement transducers, inductive transducers are distinguished by high power output signals, simplicity and reliability in operation.

Their main disadvantages are: the reverse effect on the object under study (the effect of an electromagnet on the armature) and the influence of the armature inertia on the frequency characteristics of the device.

Capacitive transducers

The principle of operation of capacitive measuring transducers is based on the dependence of the electric capacitance of the capacitor on the dimensions, the relative position of its plates and the permittivity of the medium between them.

The electric capacitance of a flat capacitor with two plates is described by the expression:

, (8)

It can be seen from this expression that a capacitive converter can be built based on the use of dependencies C =f( ), C =f(S) or C = f( ).

Figure 9 schematically shows the design of various capacitive transducers.

Figure 9 - Capacitive measuring transducers

The converter in figure 9, but is a capacitor, one plate of which moves under the action of a measured non-electric quantity X relative to a fixed plate. Static characteristic of the converter using dependence C =f( ) is non-linear. The sensitivity of the transducer increases with decreasing distance between the plates . Such transducers are used to measure small displacements (less than 1 mm).

Differential capacitive transducers are also used (Figure 9, b), which have one movable and two fixed plates. Under the influence of the measured value X, these converters simultaneously change the capacitances C1 and C2.

Figure 9, in shows a differential capacitive converter with a variable active area of ​​​​the plates, which uses the dependence C =f(S) . Transducers with this design are used to measure relatively large displacements. In these transducers, the required conversion characteristic can easily be obtained by profiling the plates.

Dependency Transformers C =f( ) used to measure the level of liquids, the humidity of substances, the thickness of products made of dielectrics, etc. As an example in Figure 9, G the device of the converter of the capacitive level gauge is given. The capacitance between the electrodes lowered into the vessel depends on the level of the liquid.

To measure the output parameter of capacitive measuring transducers, bridge, generator measuring values ​​and circuits using resonant circuits are used. The latter make it possible to create devices with high sensitivity that are capable of responding to linear displacements of the order of 10 µm. Circuits with capacitive converters are usually fed with high frequency current (up to tens of MHz).

car body test reliability

Measuring transducer -- technical means with normalized metrological characteristics, which serves to convert the measured value into another value or measuring signal, convenient for processing, storage, further transformations, indication and transmission, but not directly perceived by the operator. A measuring transducer is either part of a measuring device (measuring setup, measuring system) or used together with any measuring instrument.

By the nature of the transformation, the following converters are distinguished:

An analog measuring transducer is a measuring transducer that converts one analog value (analog measuring signal) to another analog value (measuring signal);

An analog-to-digital measuring transducer is a measuring transducer designed to convert an analog measuring signal into a numerical code;

A digital-to-analog measuring transducer is a measuring transducer designed to convert a numerical code into an analog value.

According to the place in the measuring circuit, the following converters are distinguished:

The primary measuring transducer is a measuring transducer, which is directly affected by the measured physical quantity. The primary measuring transducer is the first transducer in the measuring circuit of the measuring instrument;

The sensor is a structurally isolated primary measuring transducer;

The detector is a sensor in the field of measurements of ionizing radiation;

Intermediate measuring transducer -- a measuring transducer that occupies a place in the measuring circuit after the primary transducer.

The transmitting measuring transducer is a measuring transducer designed for remote transmission of a signal of measuring information;

Scale measuring transducer -- a measuring transducer designed to change the size of a quantity or measuring signal by a given number of times.

According to the principle of operation, the converters are divided into generator and parametric.

Generator - these are converters that, under the influence of the input value, themselves generate electrical energy (with an output value - voltage, or current). Generator measuring transducers can be included in the measuring circuit, where there is no energy source. Examples of generator measuring transducers are thermoelectric and photoelectric measuring transducers.

Parametric - these are transducers that, under the influence of the measured value, change the value of the output value depending on the principle of operation (with an output value in the form of a change in resistance, capacitance, and depending on the value of the input value), these include thermistive, capacitive measuring transducers.

According to the physical regularity on which the operation of the transducer is based, all measuring transducers can be divided into the following groups:

resistive;

Thermal;

electromagnetic;

Electrostatic;

Electrochemical;

Piezoelectric;

photovoltaic;

Electronic;

Quantum.

Let's consider some groups of measuring transducers in more detail.

Resistive transducers are currently the most common. The principle of operation is based on the change in their electrical resistance when the input value changes.

Figure 1. - Diagram of a resistive measuring transducer

When constructing a resistive measuring transducer, one strives to ensure that the change in resistance R occurs under the action of one input value (less often two).

The advantages of this converter include: simplicity of design, small size and weight, high sensitivity, high resolution at a low level of the input signal, the absence of movable current-collecting contacts, high speed, the possibility of obtaining the necessary transformation law by choosing the appropriate design parameters, the absence of input circuit influence to the measuring one.

Electromagnetic measuring transducers - such transducers make up a large group of transducers for measuring various physical quantities and, depending on the principle of operation, are parametric and generator.

Parametric converters include those in which the output mechanical action is converted into a change in the parameters of the magnetic circuit - magnetic permeability, magnetic resistance RM, winding inductance L.

To generator - induction-type converters that use the law of electromagnetic induction to obtain an output signal. They can be made on the basis of transformers and electrical machines. The last group is tachogenerators, selsyns, rotary transformers.

The values ​​of L and M can be changed by decreasing or increasing the gap, changing the position of the armature, changing the cross section S of the magnetic flux, turning the armature relative to the stationary part of the magnetic circuit, introducing a plate of ferromagnetic material into the air gap, respectively reducing 0 and the magnetic resistance of the gap.

Measuring transducers that convert the natural input value in the form of displacement into a change in inductance are called inductive.

Converters that convert movement into a change in mutual inductance M are commonly called transformer.

Figure 2 - Scheme of a measuring transducer based on a change in magnetic resistance

In transformer converters, a change in the mutual inductance M can be obtained not only by changing the magnetic resistance, but also by moving one of the windings along or across the magnetic circuit.

If compressive, tensile or twisting forces are applied to the closed magnetic circuit of the converter, then under their influence the magnetic permeability 0 of the core will change, which will lead to a change in the magnetic resistance of the core and, accordingly, to a change in L or M.

Converters based on a change in magnetic resistance due to a change in the magnetic permeability of a ferromagnetic core under the influence of mechanical deformation are called magnetoelastic. They are widely used to measure forces, pressures, moments.

If in the gap of a permanent magnet, or an electromagnet, through the winding of which a direct current is passed, the winding is moved, then, according to the law of electromagnetic induction, an EMF appears in the winding equal to

where is the rate of change of the magnetic flux interlocking with the turns of the winding W.

Since the rate of change of the magnetic flux is determined by the speed of the winding in the air gap, the converter has a natural input value in the form of a linear or angular velocity, and an output value in the form of an induced EMF. Such converters are called inductive.

Piezoelectric transducers - the principle of operation of such sensors is based on the use of direct and inverse piezoelectric effect.

The direct effect is the ability of some materials to form electric charges on the surface under the application of mechanical load.

The opposite effect - a change in mechanical stress or geometric dimensions forms a material under the influence of an electric field.

As piezoelectric materials, natural materials are used - quartz, tourmaline, as well as artificially polarized ceramics based on barium titanite, lead titanite and lead zirconate.

Quantitatively, the piezoelectric effect is estimated by the piezoelectric modulus Kd, which establishes the relationship between the emerging charge Q and the applied force F, which can be expressed by the formula:

Let's consider another type of measuring transducer - thermal transducers.

Their principle of operation is based on the use of thermal processes (heating, cooling, heat exchange) and the input value of such sensors is temperature.

However, they are used as transducers not only of temperature, but also of such quantities as heat flow, gas flow rate, humidity, liquid level.

When building thermal converters, such phenomena as the occurrence of thermo-EMF, the dependence of the resistance of a substance on temperature are most often used.

A thermocouple is a sensing element consisting of two different conductors or semiconductors connected electrically and converting the controlled temperature into EMF.

The principle of operation of a thermoelectric converter is based on the use of a thermoelectromotive force that arises in a circuit of two dissimilar conductors, the junctions (junctions) of which are heated to different temperatures.

The sign and value of thermo-EMF in the circuit depend on the type of material and the temperature difference at the junctions.

With a small temperature difference between the junctions, the thermo-EMF can be considered proportional to the temperature difference:

A thermocouple can be used to measure temperature.

Various precious metals (platinum, gold, iridium, rhodium and their alloys), as well as base metals (steel, nickel, chromium, nichrome alloys) are used as materials for thermocouples.

Silicon and selenium thermocouples (semiconductors) are relatively rarely used, they have low mechanical strength, have high internal resistance, although they provide a large thermo-EMF compared to metals.

Thermo-EMF occurs only in junctions of dissimilar materials. When comparing different materials, the thermo-EMF of platinum is taken as the base, in relation to which the thermo-EMF of other materials is determined.

To increase the output EMF, a series connection of thermocouples is used, forming a thermopile.

Advantages of thermocouples - the possibility of measurements in a wide range of temperatures; simplicity of the device; operational reliability.

Disadvantages - not high sensitivity, large inertia, the need to maintain a constant temperature of free junctions.

Thermistor converters work on the basis of the property of a conductor or semiconductor to change its electrical resistance with a change in temperature.

For such sensors, materials are used that have high stability, high reproducibility of electrical resistance at a given temperature, significant resistivity, stability of chemical and physical properties when heated, and inertness to the influence of the medium under study.

These materials primarily include platinum, copper, nickel, and tungsten. The most common are platinum and copper thermistors.

Platinum thermistors are used in the range from 0 to 6500 C; from 0 to - 2000 C. Their disadvantage is that they lose their stability of characteristics, and the brittleness of the material increases at high temperatures.

Copper thermistors are used in the temperature range from 50 to 1800C, they are quite resistant to corrosion, cheap.

Their disadvantages: high oxidizability when heated, as a result of which they are used in a relatively narrow temperature range in environments with low humidity and in the absence of aggressive gases.

Semiconductor thermistors differ from metal ones in their smaller size and inertia. The disadvantage is the non-linear dependence of resistance on temperature.

Thermistors are commonly used to measure temperature. In this case, the load current passing through them should be small. If this current is large, then the overheating of the thermistor in relation to the environment can become significant. The set value of overheating and, accordingly, the resistance in this case will be determined by the conditions of heat transfer from the surface of the thermistor.

Figure 3 - General view of thermoelectric converter

If a heated thermistor is placed in a medium with variable thermophysical characteristics, then it becomes possible to measure a number of physical quantities: the flow rate of liquid and gases, the density of gases.

The sensitivity of copper wire thermistors is constant, while the sensitivity of platinum ones changes with temperature. With the same values ​​of R 0, the sensitivity of copper thermistors is higher.

The range of measured temperatures using thermistors with platinum and copper sensitive elements is from - 200 to + 1100 0 С.

When measuring high temperatures, non-contact measuring instruments are used - pyrometers, which measure temperature by thermal radiation. Pyrometers are serially produced, providing temperature measurement in the range from 20 to 6000 0 С.

The non-contact method of temperature measurement is based on the temperature dependence of black body radiation, i.e. a body capable of completely absorbing radiation of any wavelength incident on it.

Resistance thermometers.

Resistance thermometers, like thermocouples, are designed to measure the temperature of gaseous, solid and liquid bodies, as well as surface temperatures. The principle of operation of thermometers is based on the use of the property of metals and semiconductors to change their electrical resistance with temperature. For conductors made of pure metals, this dependence in the temperature range from -200°C to 0°C has the form:

R t \u003d R 0,

and in the temperature range from 0 °С to 630 °С

R t =R 0 [ 1+At+Bt 2 ],

where R t , R 0– conductor resistance at temperature t and 0 °С; A, BС – coefficients; t– temperature, °C.

In the temperature range from 0°C to 180°C, the dependence of the conductor resistance on temperature is described by the approximate formula

R t =R 0 [ 1+αt],

where α - temperature coefficient of resistance of the conductor material (TCS).

For bare metal conductors α ≈ 6-10 -3 ... 4-10 -3 deg -1.

Measuring temperature with a resistance thermometer is reduced to measuring its resistance R t, with subsequent transition to temperature according to formulas or calibration tables.

Distinguish wire and semiconductor resistance thermometers. A wire resistance thermometer is a thin wire made of pure metal, fixed on a frame made of temperature-resistant material (sensing element), placed in protective fittings (Figure 6.4).

Figure 6.4 - Sensitive element of the resistance thermometer

The leads from the sensing element are connected to the head of the thermometer. The choice for the manufacture of resistance thermometers of wires from pure metals, rather than alloys, is due to the fact that the TCR of pure metals is greater than the TCR of alloys and, therefore, thermometers based on pure metals are more sensitive.

The industry produces platinum, nickel and copper resistance thermometers. To ensure interchangeability and uniform calibration of thermometers, their resistance values ​​are standardized R0 and TKS.

Semiconductor resistance thermometers (thermistors) are beads, disks or rods made of semiconductor material with leads for connection to the measuring circuit.

The industry commercially produces many types of thermistors in various designs.

Thermistor sizes are usually small - about a few millimeters, and some types are tenths of a millimeter. For protection from mechanical damage and environmental influences, thermistors are protected by glass or enamel coatings, as well as metal covers.

Thermistors usually have a resistance of units to hundreds of kilo-ohms; their TCS in the operating temperature range is an order of magnitude greater than that of wire thermometers. As materials for the working body of thermistors, mixtures of oxides of nickel, manganese, copper, cobalt are used, which are mixed with a binder, give it the desired shape and sinter at a high temperature. Thermistors are used to measure temperatures in the range from -100 to 300°C. The inertia of thermistors is relatively small. Their disadvantages include the non-linearity of the temperature dependence of the resistance, the lack of interchangeability due to the large variation in the nominal resistance and TCR, as well as the irreversible change in resistance over time.

For measurements in the temperature range close to absolute zero, germanium semiconductor thermometers are used.

Measurement of the electrical resistance of thermometers is carried out using bridges of constant and alternating current or compensators. A feature of thermometric measurements is the limitation of the measuring current in order to exclude the heating of the working body of the thermometer. For wire resistance thermometers, it is recommended to select such a measuring current that the power dissipated by the thermometer does not exceed 20...50mW. The allowable power dissipation in thermistors is much less and it is recommended to determine it experimentally for each thermistor.

Strain sensitive transducers (sensors).

In design practice, it is often necessary to measure mechanical stresses and strains in structural elements. The most common converters of these quantities in electrical signal are strain gauges. The operation of strain gauges is based on the property of metals and semiconductors to change their electrical resistance under the action of forces applied to them. The simplest strain gauge can be a piece of wire rigidly attached to the surface of a deformable part. The stretching or compression of the part causes a proportional stretching or compression of the wire, as a result of which its electrical resistance changes. Within the limits of elastic deformations, the relative change in the resistance of the wire is related to its relative elongation by the ratio:

∆R/R = K T ∆l/l,

where l, R are the initial length and resistance of the wire; Δl, ∆R– increment of length and resistance; K T– coefficient of strain sensitivity.

The value of the strain gauge coefficient depends on the properties of the material from which the strain gauge is made, as well as on the method of fastening the strain gauge to the product. For metal wires of various metals K T = 1... 3,5.

Distinguish between wire and semiconductor strain gauges. For the manufacture of wire strain gauges, materials are used that have a sufficiently high strain sensitivity coefficient and a low temperature coefficient of resistance. The most commonly used material for the manufacture of wire strain gauges is constantan wire with a diameter of 20...30 microns.

Structurally, wire strain gauges are a lattice consisting of several loops of wire glued to a thin paper (or other) substrate (Figure 6.5). Depending on the substrate material, strain gauges can operate at temperatures from -40 to +400°C.

Figure 6.5 - Tensiometer

There are designs of strain gauges attached to the surface of parts with the help of cements, capable of operating at temperatures up to 800°C.

The main characteristics of strain gauges are the nominal resistance R, base l and gauge factor K T The industry produces a wide range of strain gauges with a base size from 5 to 30 mm, nominal resistances from 50 to 2000 Ohm, with a strain gauge factor of 2 ± 0.2.

A further development of wire strain gauges are foil and film strain gauges, the sensitive element of which is a lattice of foil strips or the thinnest metal film deposited on lacquer-based substrates.

Strain gauges are made on the basis of semiconductor materials. The tensor effect is most pronounced in germanium, silicon, etc. The main difference between semiconductor strain gauges and wire strain gauges is a large (up to 50%) change in resistance during deformation due to the large value of the strain gauge coefficient.

Inductive transducers.

Inductive transducers are used to measure displacements, dimensions, shape deviations and surface arrangement. The converter consists of a fixed inductor with a magnetic core and an armature, which is also part of the magnetic core, moving relative to the inductor. To obtain the greatest possible inductance, the magnetic circuit of the coil and the armature are made of ferromagnetic materials. When the armature (associated, for example, with the probe of the measuring device) is moved, the inductance of the coil changes and, consequently, the current flowing in the winding changes. Figure 6.6 shows diagrams of inductive transducers with a variable air gap δ (Figure 6.6 but) used to measure displacement within 0.01 ... 10 mm; with variable air gap area S0(Figure 6.6 b), used in the range of 5...20mm.

Figure 6.6 - Inductive displacement transducers

6.2. Operational amplifiers

Operational amplifier(op amp) is a differential amplifier direct current with a very high gain. For a voltage amplifier, the transfer function (gain) is given by

To simplify design calculations, it is assumed that the ideal op-amp has the following characteristics:

1 Open-loop gain equals infinity.

2 Input impedance Rd equals infinity.

3 output impedance R o = 0.

4 The bandwidth is infinity.

5 V o=0 at V 1 \u003d V 2(no zero offset voltage). The last characteristic is very important. Because V1-V2 = Vo/A, then if Vo has a finite value, and the coefficient A is infinitely large (typical value is 100000), we will have

V 1 - V 2= 0 and V1 = V2.

Because the input impedance for a differential signal is ( V 1 - V 2) is also very large, then we can neglect the current through Rd.These two assumptions greatly simplify the development of circuits on the OS.

rule 1. When the op-amp operates in the linear region, the same voltages act on its two inputs.

rule 2. The input currents for both op amp inputs are zero.

Consider the basic circuit blocks on the op-amp. In most of these circuits, the op amp is used in a closed loop configuration.

6.2.1. Unity Gain Amplifier (Voltage Follower)

If in a non-inverting amplifier we put Ri equal to infinity, a RF equal to zero, then we will come to the circuit shown in Figure 6.7.

Figure 6.7 - Voltage follower

According to rule 1, the input voltage also acts on the inverting input of the op-amp Vi, which is directly transferred to the output of the circuit. Consequently, V o = V i, And output voltage monitors (repeats) the input voltage. For many analog-to-digital converters, the input impedance depends on the value of the analogous input signal. With the help of a voltage follower, the constant input resistance is ensured.

6.2.2. Adders

An inverting amplifier can sum multiple input voltages. Each input of the adder is connected to the inverting input of the op amp through a weighting resistor. The inverting input is called the summing node because all input currents and the feedback current are summed here. The basic circuit diagram of the summing amplifier is shown in Figure 6.8.

As in a conventional inverting amplifier, the voltage at the inverting input must be zero, therefore, the current flowing into the op-amp is also zero. In this way,

Figure 6.8 - Basic circuit diagram of summing amplifier

Since zero voltage acts on the inverting input, after the appropriate substitutions, we get:

Resistor Rf determines the overall gain of the circuit. resistance R 1 , R 2 ,...R n set the values ​​of the weight coefficients and input impedances of the respective channels.

6.2.3. Integrators

An integrator is an electronic circuit that produces an output signal that is proportional to the integral (over time) of the input signal.

Figure 6.9 - Schematic diagram of the analog integrator

Figure 6.9 shows the circuit diagram of a simple analog integrator. One output of the integrator is connected to the summing node, and the other - to the output of the integrator. Therefore, the voltage across the capacitor is also the output voltage. The output signal of the integrator cannot be described by a simple algebraic relationship, since at a fixed input voltage, the output voltage changes at a rate determined by the parameters Vi, R And FROM. Thus, in order to find the output voltage, you need to know the duration of the input signal. Voltage across the initially discharged capacitor:

where i f through the capacitor and t i- integration time. For positive Vi we have i f = V i /R. Insofar as i f = i i then, taking into account the inversion of the signal, we obtain:

From this relation it follows that V o is determined by the integral (with the opposite sign) of the input voltage in the range from 0 to t i multiplied by the scale factor 1/ RC. Voltage Vic is the voltage across the capacitor at the initial moment of time ( t = 0).

6.2.4. Differentiators

The differentiator produces an output signal proportional to the rate of change of the input signal over time. Figure 6.10 shows the circuit diagram of a simple differentiator.

Figure 6.10 - Schematic diagram of the differentiator

The current through the capacitor is:

If the derivative dV i /dt positive, current i i flows in such a direction that a negative output voltage is generated V o. In this way,

This method of signal differentiation seems simple, but in its practical implementation there are problems with ensuring the stability of the circuit at high frequencies. Not every op amp is suitable for use in a differentiator. The selection criterion is the speed of the op-amp: you need to choose an op-amp with a high maximum slew rate and a high gain-bandwidth product. High-speed field-effect transistor op-amps work well in differentiators.

6.2.5. Comparators

A comparator is an electronic circuit that compares two input voltages and produces an output signal that depends on the state of the inputs. The basic circuit diagram of the comparator is shown in Figure 6.11.

Figure 6.11 - Schematic diagram of the comparator

As you can see, here the op-amp works with an open feedback loop. A reference voltage is applied to one of its inputs, and an unknown (comparable) voltage is applied to the other. The output signal of the comparator indicates whether the level of the unknown input signal is above or below the reference voltage level. In the circuit in Figure 6.11, the reference voltage V r is applied to the non-inverting input, and an unknown signal is applied to the inverting input Vi.

At Vi > V r voltage is set at the output of the comparator V0=-Vr(negative saturation voltage). Otherwise, we get V0 = +V r. You can swap the inputs - this will invert the output signal.

6.3. Switching of measuring signals

In information and measurement technology, when implementing analog measuring transformations, it is often necessary to make electrical connections between two or more points. measuring circuit in order to cause the necessary transient process, dissipate the energy stored by the reactive element (for example, discharge the capacitor), connect the power supply of the measuring circuit, turn on the analog memory cell, take a sample of the continuous process during sampling, etc. In addition, many measuring instruments carry out measuring transformations sequentially over a large number of electrical quantities distributed in space. To implement the above, measuring switches and measuring keys are used.

Measuring switch A device is called a device that converts spatially separated analog signals into signals separated in time, and vice versa.

Measuring switches for analog signals are characterized by the following parameters:

- dynamic range switched values; transmission coefficient error;

Speed ​​(switching frequency and or the time required to perform one switching operation); the number of switched signals;

Limit number of switchings (for switches with contact measuring keys) .

Depending on the type of measuring keys used in the switch, the contact And contactless switches. The measuring key is a two-terminal network with a pronounced non-linearity volt-ampere characteristic. The transition of the key from one state (closed) to another (open) is performed using a control element.

6.4. Analog to digital conversion

Analog-to-digital conversion is an integral part of the measurement procedure. In indicating devices, this operation corresponds to the reading of the numerical result by the experimenter. In digital and processor measuring instruments, analog-to-digital conversion is performed automatically, and the result either goes directly to the display, or is entered into the processor to perform subsequent measurement conversions in numerical form.

The methods of analog-to-digital conversion in measurements are developed deeply and thoroughly and are reduced to the representation of instantaneous values ​​of the input action at fixed points in time by the corresponding code combination (number). The physical basis of analog-to-digital conversion is gating and comparison with fixed reference levels. The most widespread are ADCs of bitwise coding, sequential counting, tracking balancing, and some others. The issues of the methodology of analog-to-digital conversion, which are related to the development trends of ADCs and digital measurements in the coming years, include, in particular:

Eliminate read ambiguity in the fastest matching ADCs, which are becoming more common with the development of integrated technology;

Achieving fault tolerance and improving the metrological characteristics of ADCs based on the redundant Fibonacci number system;

Application for analog-to-digital conversion of the statistical test method.

6.4.1 Digital, analog and analog-to-digital converters

Digital-to-analog (DAC) and analog-to-digital converters (ADC) are an integral part of automatic control and regulation systems. In addition, since the vast majority of measured physical quantities are analog, and their processing, indication and registration, as a rule, are carried out digital methods, DAC and ADC are widely used in automatic means measurements. So, DAC and ADC are part of digital measuring instruments(voltmeters, oscilloscopes, spectrum analyzers, correlators, etc.), programmable power supplies, cathode ray tube displays, plotters, radar systems installations for the control of elements and microcircuits are important components various converters and generators, computer information input-output devices. Broad prospects for the use of DACs and ADCs are opening up in telemetry and television. Serial production of small-sized and relatively cheap DACs and ADCs will enable even wider use of discrete-continuous conversion methods in science and technology.

Exists three Varieties of constructive and technological execution of DAC and ADC: modular, hybrid And integral.

At the same time, the share of production of integrated circuits (ICs) of DACs and ADCs in the total volume of their production is constantly increasing, which is largely facilitated by the widespread use of microprocessors and digital data processing methods.

DAC- a device that creates an output analog signal (voltage or current) proportional to the input digital signal. In this case, the value of the output signal depends on the value of the reference voltage U on, which determines the full scale of the output signal. If any analog signal is used as the reference voltage, then the output signal of the DAC will be proportional to the product of the input digital and analog signals. In an ADC, the digital code at the output is determined by the ratio of the converted analog input signal to the full scale reference signal. This relationship is also satisfied if the reference signal changes according to some law. An ADC can be thought of as a ratio meter or voltage divider with a digital output.

6.4.2. Principles of operation, basic elements and block diagrams of the ADC

Currently developed a large number of types of ADCs that meet a variety of requirements. In some cases, the predominant requirement is high accuracy, in others - speed of conversion.

According to the principle of operation, all existing types of ADCs can be divided into two groups:

ü ADC with comparison of the input converted signal with discrete levels stresses;

ü ADC of integrating type.

In the ADC with a comparison of the input signal to be converted with discrete voltage levels, a conversion process is used, the essence of which is to form a voltage with levels equivalent to the corresponding digital codes, and comparing these voltage levels with the input voltage to determine the digital equivalent of the input signal. In this case, voltage levels can be formed simultaneously, sequentially or in a combined way.

A step-sawtooth sequential counting ADC is one of the simplest converters (Figure 6.12).

Figure 6.12 - Structural diagram of the sequential counting ADC

SS - comparison scheme; Sch - pulse counter; RP - memory register; DAC - digital-to-analog converter.

By the "Start" signal, the counter is set to the zero state, after which, as clock pulses arrive at its input with a frequency f T the output voltage of the DAC increases linearly in steps. When the voltage reaches U output values U input comparison circuit stops counting pulses in the counter FROM h, and the code from the outputs of the latter is entered into the memory register. The capacity and resolution of such ADCs is determined by the capacity and resolution of the DAC used in its composition. The conversion time depends on the level of the input voltage to be converted. For an input voltage corresponding to the full scale value, FROM h must be filled and at the same time it must generate a full scale code at the DAC input. This requires an 11-bit DAC conversion time to (2 n-1) times the clock period. For fast analog-to-digital conversion, the use of such ADCs is impractical.

In the tracking ADC (Figure 6.13), the summing FROM h replaced by up/down counter RS h to monitor the changing input voltage. The output signal KN determines the direction of counting, depending on whether the input voltage of the ADC exceeds or not the output voltage of the DAC.

Figure 6.13 - Structural diagram of ADC tracking type

Before starting measurements RS h is set to the state corresponding to the middle of the scale (01...1). The first conversion cycle of the tracking ADC is similar to the conversion cycle in the sequential counting ADC. In the future, conversion cycles are significantly reduced, since this ADC has time to track small deviations of the input signal over several clock periods, increasing or decreasing the number of pulses recorded in RS h, depending on the sign of the mismatch of the current value of the converted voltage U input and output voltage of the DAC.

ADC successive approximation(bit-by-bit balancing) have found the widest distribution due to their rather simple implementation while simultaneously ensuring high resolution, accuracy and speed, they have a slightly lower speed, but a significantly higher resolution in comparison with ADCs that implement the parallel conversion method (Figure 6.14).

To increase the speed, a pulse distributor RI and a successive approximation register are used as a control device. Comparison of the input voltage with the reference voltage (DAC feedback voltage) is carried out starting from the value corresponding to the most significant bit of the generated binary code.

When starting the ADC with the help of the RI, it is set to the initial state RPP: 1000...0. At the same time, a voltage corresponding to half of the conversion range is generated at the DAC output, which is ensured by switching on its most significant bit.

Figure 6.14 - Structural diagram of the bitwise balancing ADC

SS - comparison circuit: T - flip-flop, RPP - successive approximation register; RI - pulse distributor.

If the input signal is less than the signal from the DAC, the code 0100...0 is generated at the digital inputs of the DAC in the next cycle using the RPP, which corresponds to the inclusion of the 2nd most senior digit. As a result, the output signal of the DAC is halved.

If the input signal exceeds the signal from the DAC, in the next cycle the code 0110...0 is generated at the digital inputs of the DAC and the additional 3rd bit is turned on. In this case, the output voltage of the DAC, which has increased by one and a half times, is again compared with the input voltage, and so on. The described procedure is repeated n times (where n is the number of bits of the ADC).

As a result, the output of the DAC will generate a voltage that differs from the input by no more than one LSB of the DAC. The result of the conversion is taken from the RPP output.

The advantage of this scheme is the possibility of constructing multi-bit (up to 12 bits and more) converters of relatively high speed (with a conversion time of the order of several hundred nanoseconds).

In a direct-read (parallel type) ADC (Figure 6.15), the input signal is simultaneously applied to the inputs of all VFs, the number T which is determined by the capacity of the ADC and is equal to m = 2n-1, where n-number of ADC bits. In each KN, the signal is compared with a reference voltage corresponding to the weight of a certain discharge and taken from the nodes of a resistor divider powered by an ION.

The CV output signals are processed by a logic decoder that generates a parallel code, which is the digital equivalent of the input voltage. Such ADCs have the highest performance. The disadvantage of such ADCs is that with increasing bit depth, the number of required elements almost doubles, which makes it difficult to build multi-bit ADCs of this type. The conversion accuracy is limited by the accuracy and stability of the KN and the resistor divider. To increase the bit depth at high speed, two-stage ADCs are implemented, while the lower-order bits of the output code are removed from the outputs of the second stage Dsh, and the high-order bits are removed from the outputs of the first stage Dsh.

Figure 6.15 - Structural diagram of a parallel ADC

ADC with pulse width modulation(single-cycle integrating)

The ADC is characterized by the fact that the level of the input analog signal U input is converted into a pulse, the duration of which t pulse is a function of the value of the input signal and is digitized by counting the number of periods of the reference frequency that fit between the start and end of the pulse. The output voltage of the integrator under the action of the connected to its input U on changes from zero level with the speed:

At the moment when the output voltage of the integrator becomes equal to the input U in, KN is triggered, as a result of which the formation of the pulse duration ends, during which the ADC counters count the number of periods of the reference frequency.

The pulse duration is determined by the time during which the voltage U input changes from zero to U in:

The advantage of this converter lies in its simplicity, and the disadvantages are in the relatively low speed and low accuracy.

Figure 6.15 - Structural diagram of a single-cycle integrating ADC

Questions to control the assimilation of knowledge:

1 What physical principles are used in primary converters?

2 How are IP classified according to the type of measured value?

3 The main criteria for matching primary converters with the object of measurement.

4 IP structure, operating principles, transformation function and application features.

5 Explain the basic circuit blocks on operational amplifiers (inverting and non-inverting amplifiers, voltage followers, etc.).

6 What are the metrological characteristics of analog calculators (adders, integrators, differentiators)?

7 Measuring switches, their characteristics, equivalent circuits, designations on circuit diagrams.

8 Implementation of analog-to-digital conversion in ADC of serial counting.

9 Operating principles. Basic elements, block diagrams and characteristics of ADC and DAC.

Resistance thermometers. Resistance thermometers, like thermocouples, are designed to measure the temperature of gaseous, solid and liquid bodies, as well as surface temperatures. The principle of operation of thermometers is based on the use of the property of metals and semiconductors to change their electrical resistance with temperature. For conductors made of pure metals, this dependence in the temperature range from –200 °C to 0 °C has the form:

R t \u003d R 0,

and in the temperature range from 0 °С to 630 °С

R t \u003d R 0,

where R t , R 0 - conductor resistance at temperature t and 0 °С; A, B, C - coefficients; t- temperature, °С.

In the temperature range from 0 °C to 180 °C, the dependence of the conductor resistance on temperature is described by the approximate formula

R t \u003d R 0,

where α - temperature coefficient of resistance of the conductor material (TCS).

For bare metal conductors α≈ 6-10 -3 ...4-10 -3 deg -1 .

Measuring temperature with a resistance thermometer is reduced to measuring its resistance R t , s subsequent transition to temperature according to formulas or calibration tables.

Distinguish wire and semiconductor resistance thermometers. A wire resistance thermometer is a thin wire made of pure metal, fixed on a frame made of a temperature-resistant material (sensing element), placed in protective fittings (Fig. 5.4).

Rice. 5.4. Resistance thermometer sensing element

The leads from the sensing element are connected to the head of the thermometer. The choice for the manufacture of resistance thermometers of wires from pure metals, rather than alloys, is due to the fact that the TCR of pure metals is greater than the TCR of alloys and, therefore, thermometers based on pure metals are more sensitive.

The industry produces platinum, nickel and copper resistance thermometers. To ensure interchangeability and uniform calibration of thermometers, their resistance values ​​are standardized R0 and TKS.

Semiconductor resistance thermometers (thermistors) are beads, disks or rods made of semiconductor material with leads for connection to the measuring circuit.

The industry commercially produces many types of thermistors in various designs.

Thermistor sizes are usually small - about a few millimeters, and some types are tenths of a millimeter. To protect against mechanical damage and exposure to the environment, thermistors are protected by glass or enamel coatings, as well as metal cases.

Thermistors usually have a resistance of units to hundreds of kilo-ohms; their TCS in the operating temperature range is an order of magnitude greater than that of wire thermometers. As materials for the working body of thermistors, mixtures of oxides of nickel, manganese, copper, cobalt are used, which are mixed with a binder, give it the desired shape and sinter at a high temperature. Thermistors are used to measure temperatures in the range from -100 to 300°C. The inertia of thermistors is relatively small. Their disadvantages include the non-linearity of the temperature dependence of the resistance, the lack of interchangeability due to the large variation in the nominal resistance and TCR, as well as the irreversible change in resistance over time.

For measurements in the temperature range close to absolute zero, germanium semiconductor thermometers are used.

Measurement of the electrical resistance of thermometers is carried out using DC and AC bridges or compensators. A feature of thermometric measurements is the limitation of the measuring current in order to exclude the heating of the working body of the thermometer. For wire resistance thermometers, it is recommended to select a measuring current such that the power dissipated by the thermometer does not exceed 20 ... 50 mW. The permissible power dissipation in thermistors is much less and it is recommended to determine it experimentally for each thermistor.

Strain sensitive transducers (sensors). In design practice, it is often necessary to measure mechanical stresses and strains in structural elements. The most common converters of these quantities into an electrical signal are strain gauges. The operation of strain gauges is based on the property of metals and semiconductors to change their electrical resistance under the action of forces applied to them. The simplest strain gauge can be a piece of wire rigidly attached to the surface of a deformable part. The stretching or compression of the part causes a proportional stretching or compression of the wire, as a result of which its electrical resistance changes. Within the limits of elastic deformations, the relative change in the resistance of the wire is related to its relative elongation by the relation

ΔR/R=K Τ Δl/l,

where l, R- initial length and resistance of the wire; Δl, ∆R- increment of length and resistance; K Τ - strain gauge factor.

The value of the strain gauge coefficient depends on the properties of the material from which the strain gauge is made, as well as on the method of fastening the strain gauge to the product. For metal wires of various metals K Τ= 1... 3,5.

Distinguish between wire and semiconductor strain gauges. For the manufacture of wire strain gauges, materials are used that have a sufficiently high strain sensitivity coefficient and a low temperature coefficient of resistance. The most commonly used material for the manufacture of wire strain gauges is constantan wire with a diameter of 20 ... 30 microns.

Structurally, wire strain gauges are a lattice consisting of several loops of wire glued to a thin paper (or other) substrate (Fig. 5.5). Depending on the substrate material, strain gauges can operate at temperatures from -40 to +400 °C.

Rice. 5.5. Tensiometer

There are designs of strain gauges attached to the surface of parts with the help of cements, capable of operating at temperatures up to 800 °C.

The main characteristics of strain gauges are the nominal resistance R, base l and gauge factor K Τ . The industry produces a wide range of strain gauges with a base size from 5 to 30 mm , nominal resistances from 50 to 2000 Ohm, with a strain gauge factor of 2 ± 0.2.

A further development of wire strain gauges are foil and film strain gauges, the sensitive element of which is a lattice of foil strips or the thinnest metal film deposited on lacquer-based substrates.

Strain gauges are made on the basis of semiconductor materials. The strain effect is most pronounced in germanium, silicon, etc. The main difference between semiconductor strain gauges and wire strain gauges is a large (up to 50%) change in resistance during deformation due to the large value of the strain gauge coefficient.

Inductive transducers. Inductive transducers are used to measure displacements, dimensions, shape deviations and surface arrangement. The converter consists of a fixed inductor with a magnetic core and an armature, which is also part of the magnetic core, moving relative to the inductor. To obtain the greatest possible inductance, the magnetic circuit of the coil and the armature are made of ferromagnetic materials. When the armature (associated, for example, with the probe of the measuring device) is moved, the inductance of the coil changes and, consequently, the current flowing in the winding changes. On fig. 5.6 shows diagrams of inductive transducers with a variable air gap d (Fig. 5.6 but) used to measure displacement within 0.01 ... 10 mm; with a variable air gap area S δ (Fig. 5.6 b) used in the range of 5 ... 20 mm.

Rice. 5.6. Inductive displacement transducers

5.2. Operational amplifiers

An operational amplifier (op-amp) is a DC differential amplifier with a very high gain. For a voltage amplifier, the transfer function (gain) is given by

To simplify design calculations, it is assumed that the ideal op amp has the following characteristics.

1. The open-loop gain is infinity.

2. The input resistance R d is equal to infinity.

3. Output resistance R 0 = 0.

4. The bandwidth is infinity.

5. V 0 \u003d 0 at V 1 \u003d V 2 (there is no zero bias voltage).

The last characteristic is very important. Since V 1 -V 2 \u003d V 0 / A, then if V 0 has a finite value, and the coefficient A is infinitely large (typical value 100000) we will have

V 1 - V 2 \u003d 0 and V 1 \u003d V 2.

Since the input impedance for a differential signal (V 1 - V 2)

is also very large, then the current through R d can be neglected. These two assumptions greatly simplify the development of circuits on the op-amp.

Rule 1. When the op-amp operates in the linear region, the same voltages act on its two inputs.

Rule 2. The input currents for both op amp inputs are zero.

Consider the basic circuit blocks on the op-amp. In most of these circuits, the op amp is used in a closed loop configuration.

5.2.1. Unity Gain Amplifier

(voltage follower)

If in a non-inverting amplifier we set R i equal to infinity, and R f equal to zero, then we will come to the circuit shown in Fig. 5.7.

According to rule 1, the input voltage V i also acts on the inverting input of the op-amp, which is directly transmitted to the output of the circuit. Therefore, V 0 = V i , and the output voltage follows (replicates) the input voltage. For many analog-to-digital converters, the input impedance depends on the value of the analogous input signal. With the help of a voltage follower, the constant input resistance is ensured.

5.2.2. Adders

An inverting amplifier can sum multiple input voltages. Each input of the adder is connected to the inverting input of the op amp through a weighting resistor. The inverting input is called the summing node because all input currents and the feedback current are summed here. The basic circuit diagram of the summing amplifier is shown in fig. 5.8.

As in a conventional inverting amplifier, the voltage at the inverting input must be zero, therefore, the current flowing into the op-amp is also zero. In this way,

i f = i 1 + i 2 + . . . + i n

Since zero voltage acts on the inverting input, after appropriate substitutions, we obtain

V 0 \u003d -R f ( +. . . + ).

Resistor R f determines the overall gain of the circuit. Resistance R 1, R 2, . . . R n set the values ​​of the weight coefficients and input impedances of the respective channels.

5.2.3. Integrators

An integrator is an electronic circuit that produces an output signal that is proportional to the integral (over time) of the input signal.

On fig. Figure 5.9 shows a schematic diagram of a simple analog integrator. One output of the integrator is connected to the summing node, and the other to the output of the integrator. Therefore, the voltage across the capacitor is also the output voltage. The output signal of the integrator cannot be described by a simple algebraic relationship, since with a fixed input voltage, the output voltage changes at a rate determined by the parameters V i , R and C. Thus, in order to find the output voltage, you need to know the duration of the input signal. Voltage across the initially discharged capacitor

where i f is through the capacitor and t i is the integration time. For positive

Vi we have i i = V i /R. Since i f = i i , then, taking into account the inversion of the signal, we obtain

From this relationship it follows that V 0 is determined by the integral (with the opposite sign) of the input voltage in the range from 0 to t 1 multiplied by the scale factor 1/RC. The voltage V ic is the voltage across the capacitor at the initial time (t = 0).

5.2.4. Differentiators

The differentiator produces an output signal proportional to the rate of change of the input signal over time. On fig. 5.10 shows a circuit diagram of a simple differentiator.

current through the capacitor.

If the derivative is positive, current i i flows in such a direction that a negative output voltage V 0 is generated.

In this way,

This method of signal differentiation seems simple, but in its practical implementation there are problems with ensuring the stability of the circuit at high frequencies. Not every op amp is suitable for use in a differentiator. The selection criterion is the speed of the op-amp: you need to choose an op-amp with a high maximum slew rate and a high gain-bandwidth product. High-speed field-effect transistor op-amps work well in differentiators.

5.2.5. Comparators

A comparator is an electronic circuit that compares two input voltages and produces an output signal that depends on the state of the inputs. The basic circuit diagram of the comparator is shown in fig. 5.11.

As you can see, here the op-amp works with an open feedback loop. A reference voltage is applied to one of its inputs, and an unknown (comparable) voltage is applied to the other. The output signal of the comparator indicates whether the level of the unknown input signal is above or below the reference voltage level. In the circuit in Fig. 5.11, the reference voltage V r is applied to the non-inverting input, and the unknown signal V i is fed to the inverting input.

When V i > V r, the output of the comparator is set to voltage V 0 = - V r (negative saturation voltage). Otherwise, we get V 0 = +V r. You can swap the inputs - this will lead to the inversion of the output signal.

5.3. Switching of measuring signals

In information and measuring technology, when implementing analog measuring transformations, it is often necessary to make electrical connections between two or more points of the measuring circuit in order to cause the necessary transient process, dissipate the energy stored by the reactive element (for example, discharge a capacitor), connect the power supply of the measuring circuit, turn on the analog cell memory, take a sample of a continuous process during discretization, etc. In addition, many measuring instruments carry out measuring transformations sequentially over a large number of electrical quantities distributed in space. To implement the above, measuring switches and measuring keys are used.

A measuring switch is a device that converts spatially separated analog signals into signals separated in time, and vice versa.

Measuring switches for analog signals are characterized by the following parameters:

dynamic range of switched values;

transmission coefficient error;

speed (switching frequency or the time required to perform one switching operation);

the number of switched signals;

the limiting number of switchings (for switches with contact measuring keys).

Depending on the type of measuring keys used in the switch, the contact and contactless switches.

The measuring key is a two-terminal circuit with a pronounced nonlinearity of the current-voltage characteristic. The transition of the key from one state (closed) to another (open) is performed using a control element.

5.4. Analog to digital conversion

Analog-to-digital conversion is an integral part of the measurement procedure. In indicating devices, this operation corresponds to the reading of the numerical result by the experimenter. In digital and processor measuring instruments, analog-to-digital conversion is performed automatically, and the result either goes directly to the display, or is entered into the processor to perform subsequent measurement conversions in numerical form.

The methods of analog-to-digital conversion in measurements are developed deeply and thoroughly and are reduced to the representation of instantaneous values ​​of the input action at fixed points in time by the corresponding code combination (number). The physical basis of analog-to-digital conversion is gating and comparison with fixed reference levels. The most widespread are ADCs of bitwise coding, sequential counting, tracking balancing, and some others. The issues of the methodology of analog-to-digital conversion, which are related to the development trends of ADCs and digital measurements in the coming years, include, in particular:

Disambiguation of readings in the fastest matching ADCs, which are becoming more common with the development of integrated technology;

Achieving fault tolerance and improving the metrological characteristics of ADCs based on the redundant Fibonacci number system;

Application for analog-to-digital conversion of the statistical test method.

5.4.1 D/A and A/D converters

Digital-to-analog (DAC) and analog-to-digital converters (ADC) are an integral part of automatic control and regulation systems. In addition, since the vast majority of measured physical quantities are analog, and their processing, indication and registration, as a rule, are carried out by digital methods, DACs and ADCs have found wide application in automatic measuring instruments. Thus, DAC and ADC are part of digital measuring instruments (voltmeters, oscilloscopes, spectrum analyzers, correlators, etc.), programmable power supplies, cathode ray tube displays, graph plotters, radar systems of installations for monitoring elements and microcircuits, are important components various converters and generators, computer information input/output devices. Broad prospects for the use of DACs and ADCs are opening up in telemetry and television. Serial production of small-sized and relatively cheap DACs and ADCs will enable even wider use of discrete-continuous conversion methods in science and technology.

There are three types of DAC and ADC design and technology: modular, hybrid and integrated. At the same time, the share of the production of integrated circuits (ICs) of DACs and ADCs in the total volume of their production is constantly increasing, which is largely facilitated by the widespread use of microprocessors and digital data processing methods. A DAC is a device that produces an output analog signal (voltage or current) that is proportional to the input digital signal. In this case, the value of the output signal depends on the value of the reference voltage U op, which determines the full scale of the output signal. If any analog signal is used as a reference voltage, then the output signal of the DAC will be proportional to the product of the input digital and analog signals. In the ADC, the digital code at the output is determined by the ratio of the converted input analog signal to the reference signal corresponding to the full scale. This relationship is also satisfied if the reference signal changes according to some law. An ADC can be thought of as a ratio meter or voltage divider with a digital output.

5.4.2. Principles of operation, basic elements and block diagrams of the ADC

Currently, a large number of types of ADCs have been developed to meet a variety of requirements. In some cases, the predominant requirement is high accuracy, in others - speed of conversion.

According to the principle of operation, all existing types of ADCs can be divided into two groups: ADCs with a comparison of the input converted signal with discrete voltage levels and ADCs of the integrating type.

An ADC with a comparison of the input converted signal with discrete voltage levels uses a conversion process, the essence of which is to generate a voltage with levels equivalent to the corresponding digital codes, and compare these voltage levels with the input voltage in order to determine the digital equivalent of the input signal. In this case, voltage levels can be formed simultaneously, sequentially or in a combined way.

Serial counting ADC with a stepped sawtooth voltage is one of the simplest converters (Fig. 5.12).

By the "Start" signal, the counter is set to the zero state, after which, as clock pulses arrive at its input with a frequency f t the output voltage of the DAC increases linearly in steps.

When the voltage U out reaches the value U in, the comparison circuit stops counting pulses in the counter SC, and the code from the outputs of the latter is entered into the memory register. The capacity and resolution of such ADCs is determined by the capacity and resolution of the DAC used in its composition. The conversion time depends on the level of the input voltage to be converted. For an input voltage corresponding to the full scale value, the MF must be filled and at the same time it must generate a full scale code at the DAC input. This requires an n-bit DAC conversion time of (2 n - 1) times the clock period. For fast analog-to-digital conversion, the use of such ADCs is impractical.

IN tracking ADC(Fig. 5.13) the summing Cch has been replaced with a reversible counter Rch to keep track of the changing input voltage. The CV output signal determines the direction of counting depending on whether or not the ADC input voltage exceeds the DAC output voltage.

Before starting measurements, the RF is set to the state corresponding to the middle of the scale (01 ... 1). The first conversion cycle of the tracking ADC is similar to the conversion cycle in the sequential counting ADC. In the future, conversion cycles are significantly reduced, since this ADC has time to track small deviations of the input signal over several clock periods, increasing or decreasing the number of pulses recorded in the RFC, depending on the sign of the mismatch between the current value of the converted voltage Uin and the output voltage of the DAC.

SAR ADC (Bitwise Balanced) have found the widest distribution due to their rather simple implementation while simultaneously providing high resolution, accuracy and speed, they have a slightly lower speed, but a significantly higher resolution in comparison with ADCs that implement the parallel conversion method.

To increase the speed, a pulse distributor RI and a successive approximation register are used as a control device. Comparison of the input voltage with the reference voltage (DAC feedback voltage) is carried out starting from the value corresponding to the most significant bit of the generated binary code.

When starting the ADC with the help of RI, the RPP is set to its initial state:

1000 . . .0. At the same time, a voltage corresponding to half of the conversion range is generated at the DAC output, which is ensured by switching on its most significant bit. If the input signal is less than the signal from the DAC, the code 0100 is generated at the digital inputs of the DAC in the next cycle using the DAC. . 0, which corresponds to the inclusion of the 2nd most senior category. As a result, the output signal of the DAC is halved.

If the input signal exceeds the signal from the DAC, in the next cycle, the code 0110 ... 0 is generated at the digital inputs of the DAC and the additional 3rd bit is turned on. In this case, the output voltage of the DAC, which has increased by one and a half times, is again compared with the input voltage, etc. The described procedure is repeated. n times (where n is the number of bits of the ADC).

As a result, the output of the DAC will generate a voltage that differs from the input by no more than one LSB of the DAC. The result of the conversion is taken from the RPP output.

The advantage of this scheme is the possibility of constructing multi-bit (up to 12 bits and more) converters of relatively high speed (with a conversion time of the order of several hundred nanoseconds).

In ADC direct reading(parallel type)(Fig. 5.15) the input signal is simultaneously applied to the inputs of all VFs, the number m which is determined by the capacity of the ADC and is equal to m = 2 n - 1, where n is the number of ADC bits. In each KN, the signal is compared with a reference voltage corresponding to the weight of a certain discharge and taken from the nodes of a resistor divider powered by an ION.

The CV output signals are processed by a logic decoder that generates a parallel code, which is the digital equivalent of the input voltage. Such ADCs have the highest performance. The disadvantage of such ADCs is that with increasing bit depth, the number of required elements almost doubles, which makes it difficult to build multi-bit ADCs of this type. The conversion accuracy is limited by the accuracy and stability of the KN and the resistor divider. To increase the bit depth at high speed, two-stage ADCs are implemented, while the lower-order bits of the output code are removed from the outputs of the second stage of the LN, and the higher bits are removed from the outputs of the LN of the first stage.

ADC with pulse width modulation (single-ended integrating)

The ADC is characterized by the fact that the level of the input analog signal Uin is converted into a pulse, the duration of which t imp is a function of the value of the input signal and is digitized by counting the number of periods of the reference frequency that fit between the beginning and end of the pulse. The output voltage of the integrator under the action of the connection

valued to its input U op changes from the zero level with a speed

At the moment when the output voltage of the integrator becomes equal to the input voltage U in, the CV is triggered, as a result of which the formation of the pulse duration ends, during which the number of periods of the reference frequency is counted in the ADC counters. The pulse duration is determined by the time during which the voltage U out changes from zero to U in:

The advantage of this converter lies in its simplicity, and the disadvantages are in the relatively low speed and low accuracy.