Measuring transducers of non-electric quantities are divided into parametric and generator. In parametric transducers, the output value is the increment of the electrical circuit parameter ( R, L, M, S), so an additional power supply is required when using them.

In generator converters, the output quantity is the EMF, the current or charge of which is functionally related to the measured non-electric quantity.

When creating measuring transducers of non-electric quantities, they strive to obtain a linear conversion function. The difference between the actual calibration characteristic and the nominal linear conversion function causes the nonlinearity error, which is one of the main components of the resulting error in the measurement of non-electrical quantities. One of the ways to reduce the nonlinearity error is to select such values as the input and output values of the converter, the relationship of which is closer to a linear function. So, for example, when measuring linear displacements using a capacitive transducer, either the gap between the plates or the area of their overlap can change. In this case, the transformation functions turn out to be different. When the gap changes, the dependence of the capacitance on the displacement of the movable plate is essentially non-linear; it is described by a hyperbolic function. However, if as the output value of the converter we use not its capacitance, but resistance at a certain frequency, then the measured displacement and the specified capacitance turn out to be related linear dependence.

Another effective way to reduce the nonlinearity error of parametric transducers is their differential construction. Any differential measuring transducer is in fact two similar measuring transducers, the output values of which are subtracted, and the input value acts on these transducers in the opposite way.

Structural scheme instrument with a differential measuring transducer is shown in Figure 16.1.

Measured value X acts on two similar measuring transducers IP1 And IP2, and the corresponding increments of the values of the output quantities 1 And at 2 have opposite signs. In addition, there is some constant initial value x0 quantities

at the inputs of these converters, which is usually determined by the design parameters of the converters. Output quantities 1 And at 2 are subtracted, and their difference 3 measured by electrical measuring device EIU (analogue or digital).

Let's assume that the converters IP1 And IP2 are identical, and their transformation functions are quite accurately described by a second-order algebraic polynomial. In this case, the values 1 And at 2 at the outputs of the converters can be written as (16.1) /14/

After subtraction we get (16.2) /14/

Figure 16.1 - Structural diagram of the differential

educator

This shows that the resulting transformation function y 3 \u003d f (x) turned out to be linear. Because 3 does not depend on a 0, then the systematic additive errors of the measuring transducers are compensated. In addition, compared with a single transducer, the sensitivity is almost doubled. All this determines the widespread use of differential measuring transducers in practice.

Let us briefly consider the main types of used parametric converters of non-electric quantities.

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 change electrical resistance 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.

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

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, under the influence of temperature, the p-n resistance junction 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 permittivity 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 formatting the report and Control 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, among which the majority are non-electric 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 the mandatory presence of a 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 value Y 0 (zero level), corresponding to zero or another initial value of the input value 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, the 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, A). 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).

A– 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, A.

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, the scale of the EIP indicator device is graduated 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 nodes feedback 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, V).

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)

Where	 active component of magnetic resistance;
l i , S i ,  i	 respectively, the length, cross-sectional area and relative magnetic permeability of the i-th section of the magnetic circuit;
	 magnetic constant;
	 length and cross-sectional area of the air section of the magnetic circuit;

	 reactive component of magnetic resistance;
	 power losses in the magnetic circuit due to eddy currents and hysteresis;
	- angular frequency;
	- magnetic flux in the magnetic circuit.

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. A change in 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 A).

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, V). 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, A 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, V 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. The measuring transducer or is part of any measuring instrument(measuring installation, 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 input signal, absence of movable current-collecting contacts, high speed, the possibility of obtaining the necessary transformation law by choosing the appropriate design parameters, no influence of the input circuit on the measuring circuit.

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 displacement rate, 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 generate electrical charges on a surface when a mechanical load is applied.

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

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 form 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.

The most important metrological characteristics of converters are: nominal static conversion characteristic, sensitivity, basic error, additional errors, or influence functions, output signal variation, output impedance, dynamic characteristics, etc.

The most important non-metrological characteristics include dimensions, weight, ease of installation and maintenance, explosion safety, resistance to mechanical, thermal, electrical and other overloads, reliability, cost of manufacture and operation, etc.

Depending on the type of output signal, all measuring transducers are divided into parametric And generator. They are also classified according to the principle of action. Only the transmitters that have received the most use are discussed below.

13.1 Parametric transducers

General information. In parametric transducers, 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. On fig. 11-5 schematically shows some designs of rheostat converters for angular (Fig. 11-5, A) and linear (Fig. 11-5, b and c) movements. The converter consists of a winding applied to the frame and a brush. For the manufacture of frames, dielectrics and metals are used. The winding wire is made of alloys (an alloy of platinum with iridium, constantan, nichrome and fechral). For winding, insulated wire is usually used. After the winding is made, the wire insulation is cleaned off at the points of contact with the brush. The converter brush is made either from wires or from flat springy strips, and both pure metals (platinum, silver) and alloys (platinum with iridium, phosphor bronze, etc.) are used.

Rice. 11-5. Rheostat transducers for angular (a), linear (b) displacements and for the functional transformation of linear displacements (c)

The dimensions of the converter are determined by the value of the measured displacement, the resistance of the winding and the power released in the winding.

To obtain a nonlinear transformation function, functional rheostat converters are used. The desired character of the transformation is often achieved by profiling the frame of the converter (Fig. 11-5, V).

In the rheostatic converters under consideration, the static conversion characteristic has a stepped character, since the resistance changes in jumps equal to the resistance of one turn, which causes an error. Sometimes rheochord transducers are used, in which the brush slides along the axis of the wire. These transducers do not have the specified error. Rheostatic transducers are included in measuring circuits in the form of balanced and non-equilibrium bridges, voltage dividers, etc.

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

Strain sensitive 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. 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.

Strain-sensitive transducers, widely used at present (Fig. 11-6), are a thin zigzag laid and glued to a strip of paper (substrate /) wire 2 (wire grate). The converter is connected to the circuit using welded or soldered leads 3. The transducer is glued to the surface of the part under study so that the direction of the expected deformation coincides with the longitudinal axis of the wire grating.

For the manufacture of transducers, mainly constantan wire with a diameter of 0.02-0.05 mm is used. (S== 1.9 - 2.1). Constantan has a low temperature coefficient of electrical resistance, which is very important, since the change in the resistance of transducers during deformations, for example, of steel parts is commensurate with the change in the resistance of the transducer with a change in temperature. As a substrate, thin (0.03-0.05 mm) paper is used, as well as a film of varnish or glue, and at high temperatures, a layer of cement.

Foil transducers are also used, in which foil and film strain gauges are used instead of wire, obtained by sublimation of a strain-sensitive material with its subsequent deposition on a substrate.

Adhesives are used to glue the wire to the substrate and the entire transducer to the part (celluloid solution in acetone, BF-2, BF-4 glue, bakelite, etc.). For high temperatures (above 200 °C), heat-resistant cements, silicone varnishes and adhesives, etc. are used.

Converters are available in different sizes depending on the purpose. Most often, transducers with a grating length (base) from 5 to 50 mm are used, having a resistance of 30-500 ohms.

A change in temperature causes a change in the transformation characteristics of strain gauges, which is explained by the temperature dependence of the resistance of the transducer and the difference in the temperature coefficients of linear expansion of the material of the strain gauge and the part under study. The effect of temperature is usually eliminated by applying appropriate temperature compensation methods.

A pasted strain gauge transducer cannot be removed from one part and pasted onto another. Therefore, to determine the characteristics of the transformation (coefficient S), one resorts to selective calibration of the converters, which gives the value of the coefficient S with an error of ±1%. Methods for determining the characteristics of strain gauges are regulated by the standard. The advantages of these converters are the linearity of the static conversion characteristic, small dimensions and weight, and simplicity of design. Their disadvantage is their low sensitivity.

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

Heat exchange takes place between the thermistor and the investigated medium during the measurement process. Since the thermistor is included in the electrical circuit, with the help of which its resistance is measured, a current flows through it, releasing heat in it. The heat exchange of the thermistor with the medium occurs due to the thermal conductivity of the medium and convection in it, the thermal conductivity of the thermistor itself and the fittings to which it is attached, and, finally, due to radiation. The intensity of heat transfer, and hence the temperature of the thermistor, depends on its geometric dimensions and shape, on the design of protective fittings, on the composition, density, thermal conductivity, viscosity and other physical properties of the gas or liquid medium surrounding the thermistor, as well as on the temperature and speed of movement of the medium .

Rice. 11-7. Device(s) and appearance fittings (b) platinum thermistor

Thus, the dependence of temperature, and hence the resistance of the thermistor on the factors listed above, can be used to measure various non-electric quantities characterizing a gas or liquid medium. When designing the transducer, the aim is to ensure that the heat exchange of the thermistor with the medium is mainly determined by the measured non-electric quantity.

According to the mode of operation, thermistors are overheated and without deliberate overheating. In converters without overheating, the current passing through the thermistor practically does not cause overheating, and the temperature of the latter is determined by the temperature of the medium; these transducers are used to measure temperature. In overheating converters, electric current causes overheating, depending on the properties of the medium. Overheating transducers are used to measure speed, density, composition of the medium, etc. Since overheating thermistors are affected by the temperature of the medium, circuit methods are usually used to compensate for this effect.

To measure temperature, the most common thermistors are made of platinum or copper wire.

Standard platinum thermistors are used to measure temperature in the range from -260 to + 1100 ° C, copper - in the range from - 200 to + 200 ° C (GOST 6651-78). Low-temperature platinum thermistors (GOST 12877-76) are used to measure temperatures in the range from -261 to -183 °C.

On fig. 11-7, A The device of a platinum thermistor is shown. In the channels of the ceramic tube 2 there are two (or four) sections of the helix 3 made of platinum wire connected in series. Solder the leads to the ends of the spiral 4, used to include a thermistor in the measuring circuit. The fastening of the leads and the sealing of the ceramic tube are made with glaze /. The channels of the tube are covered with anhydrous aluminum oxide powder, which acts as an insulator and a retainer for the spiral. Anhydrous alumina powder, having high thermal conductivity and low heat capacity, provides good heat transfer and low inertia of the thermistor. To protect the thermistor from mechanical and chemical influences of the external environment, it is placed in protective fittings (Fig. 11-7, b) made of stainless steel.

The initial resistances (at 0 ° C) of platinum standard thermistors are 1, 5, 10, 46, 50, 100 and 500 Ohms, copper - 10, 50, 53 and 100 Ohms.

The permissible value of the current flowing through the thermistor when it is included in the measuring circuit must be such that the change in the resistance of the thermistor during heating does not exceed 0.1% of the initial resistance.

Static conversion characteristics in the form of tables (calibration) and permissible deviations of these characteristics for standard thermistors are given in GOST 6651-78.

In addition to platinum and copper, sometimes nickel is used to make thermistors.

To measure temperature, semiconductor thermistors (thermistors) of various types are also used, which are characterized by greater sensitivity (TCS thermistor-

the resistance is negative and at 20 °C 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 characteristic:

Where rt And Ro- thermistor resistance at temperatures T And That; That- initial temperature of the operating range; IN- coefficient.

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

To measure temperatures from -80 to -f-150 ° C, thermal diodes and thermotransistors are used, in which resistance changes under the influence of temperature R- i-junction and voltage drop at this junction. The voltage sensitivity of the thermotransistor is 1.5-2.0 mV/K, which significantly exceeds the sensitivity of standard thermocouples (see Table 11-1). These converters are usually included in bridge circuits and circuits in the form of voltage dividers.

The thermal inertia of standard thermistors according to GOST 6651-78 is characterized by an indicator of thermal inertia v^, defined as the time required for the temperature difference between the medium and any point of the converter introduced into it to become equal to 0.37 of that value when the converter is introduced into an environment with a constant temperature , which she had at the time of the onset of a regular thermal regime. The thermal inertia index is determined from that part of the transient thermal process curve of the converter, which corresponds to the regular mode, i.e., has an exponential character (in a semi-logarithmic scale - a straight line). The value of e^ for various types of standard transducers ranges from several tens of seconds to several minutes.

When fast-resistance thermistors are needed, very thin wire (microwire) is used for their manufacture, or small volume thermistors (bead) or thermotransistors are used.

Rice. 11-8. Gas analyzer converter based on the principle of thermal conductivity measurement

Rs. 11-9. Dependence of gas thermal conductivity on pressure

Thermistors are used in instruments for the analysis of gas mixtures. Many gas mixtures differ from each other and from air in thermal conductivity.

In devices for gas analysis - gas analyzers - an overheating platinum thermistor (Fig. 11-8) placed in a chamber is used to measure thermal conductivity 2 with the analyzed gas. The design of the thermistor, fittings and chamber, as well as the value of the heating current, are chosen such that heat exchange with the medium is carried out mainly due to the thermal conductivity of the gaseous medium.

To eliminate the influence of external temperature, in addition to the operating temperature, a compensation chamber with a thermistor filled with a gas of constant composition is used. Both chambers are made in the form of a single block, which provides the chambers with the same temperature conditions. During measurements, the working and compensation thermistors are included in the adjacent arms of the bridge, which leads to compensation for the effect of temperature.

Thermistors are used in devices for measuring the degree of rarefaction. On fig. 11-9 shows the dependence of the thermal conductivity of the gas located between the bodies A And B, from his pressure.

Thus, the thermal conductivity of a gas becomes dependent on the number of molecules per unit volume, i.e., on pressure (degree of rarefaction). The dependence of the thermal conductivity of a gas on pressure is used in vacuum gauges - devices for measuring the degree of rarefaction.

To measure thermal conductivity in vacuum gauges, metal (platinum) and semiconductor thermistors are used, placed in a glass or metal container, which is connected to a controlled environment.

Thermistors are used in devices for measuring the speed of a gas flow - hot-wire anemometers. The steady state temperature of an overheating thermistor placed in the path of the gas flow depends on the flow rate. In this case, convection (forced) will be the main way of heat exchange between the thermistor and the medium. The change in the resistance of the thermistor due to the removal of heat from its surface by a moving medium is functionally related to the velocity of the medium.

The design and type of the thermistor, fittings and heating thermistor current are chosen such that all heat transfer paths are reduced or excluded, except for convective.

The advantages of hot-wire anemometers are high sensitivity and speed. These devices make it possible to measure speeds from 1 to 100-200 m/s using a measuring circuit, with the help of which the temperature of the thermistor is automatically maintained almost unchanged.

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.

On fig. 11-10, for example, graphs of the dependences of the electrical conductivity of some electrolyte solutions on concentration are shown With solute. It follows from this figure that, in a certain concentration range, the dependence of electrical conductivity on concentration is unambiguous and can be used to determine With.

Rice. 11-10. Dependence of the electrical conductivity of electrolyte solutions on the concentration of the dissolved substance

Rice. 11-11. Laboratory electrolytic converter

The transducer used in the laboratory to measure the concentration is a vessel with two electrodes (electrolytic cell) (Fig. 11-11). For industrial continuous measurements, the transducers are flow-through, and constructions are often used in which the walls of the vessel (metal) play the role of the second electrode.

The electrical conductivity of solutions depends on temperature. Thus, when using electrolytic converters, it is necessary to eliminate the effect of temperature. This problem is solved by stabilizing the temperature of the solution using a refrigerator (heater) or by using temperature compensation circuits with copper thermistors, since the temperature coefficients of conductivity of copper and electrolyte solutions have opposite signs.

When passing direct current electrolysis of the solution occurs through the converter, which leads to a distortion of the measurement results. Therefore, solution resistance measurements are usually carried out on alternating current (700-1000 Hz), most often using bridge circuits.

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.

Rice. 11-12. Magnetic circuit with gaps and two windings

The inductance and mutual inductance can be changed by acting on the length b, the cross section of the air section of the magnetic circuit s, on the power losses in the magnetic circuit, and in other ways. This can be achieved, for example, by moving the movable core (armature) / (Fig. 11-12) relative to the fixed 2, the introduction of a non-magnetic metal plate 3 into the air gap, etc.

On fig. 11-13 schematically show Various types inductive converters. An inductive transducer (Fig. 11-13, a) with a variable length of the air gap b is characterized by a non-linear dependence L=f(b). Such a converter is usually used when the armature moves by 0.01-5 mm. Significantly lower sensitivity, but linear dependence L=f(s) variable air gap transducers differ (Fig. 11-13, b). These converters are used for movements up to 10-15 mm.

Rice. 11-13. Inductive transducers with variable gap length (a), with variable gap section (b), differential (V), differential transformer (d), differential transformer with open magnetic circuit (e) magnetoelastic (e)

An armature in an inductive transducer experiences an (undesirable) force of attraction from an electromagnet

Where Wm- energy magnetic field; L- converter inductance; / - current passing through the converter winding.

Widespread inductive differential converters (Fig. 11-13, V), in which, under the influence of the measured value, two gaps of electromagnets change simultaneously and, moreover, with different signs. Differential transducers in combination with an appropriate measuring circuit (usually a bridge) have a higher sensitivity, less non-linearity of the conversion characteristic, are less influenced by external factors and a reduced resultant force on the armature from the electromagnet than non-differential transducers.

On fig. 11-13, G shows the switching circuit of a differential inductive converter, whose output values are mutual inductances. Such converters are called mutually inductive or transformer. When the primary winding is powered by alternating current and with a symmetrical position of the armature relative to the electromagnets, the EMF at the output terminals is zero. When the armature is moved, an emf appears at the output terminals.

To convert relatively large displacements (up to 50-100 mm), transformer converters with an open magnetic circuit are used (Fig. 11-13, O).

Apply transformer converters of the angle of rotation, consisting of a fixed stator and a movable rotor with windings. The stator winding is fed with alternating current. Rotation of the rotor causes a change in the value and phase of the EMF induced in its winding. Such transducers are used for measuring large angular displacements.

Inductosins are used to measure small angular displacements (Fig. 11-14). Rotor / and stator 2 inductosyn is supplied with printed windings 3, having the form of a radial raster. The principle of action of inductosin is similar to that described above. By applying the windings in a printed way, it is possible to obtain a large number of pole winding pitches, which ensures high sensitivity of the converter to a change in the angle of rotation.

Rice. 11-14. Device (a) and type of printed winding (b) inductosyn

If the ferromagnetic core of the converter is subjected to mechanical stress F, then due to a change in the magnetic permeability of the core material, the magnetic resistance of the circuit will change, which will entail a change in the inductance L and mutual inductance M windings. Magnetoelastic transducers are based on this principle (Fig. 11-13, e).

The transducer design is determined by the range of measured displacement. Converter dimensions are selected based on the required output signal power.

To measure the output parameter of inductive converters, bridge (equilibrium and non-equilibrium) circuits, as well as a compensation (in automatic devices) circuit for differential transformer converters, are most widely used.

Inductive transducers are used to convert displacement and other non-electrical quantities that can be converted into displacement (force, pressure, moment, etc.).

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 the frequency characteristics of the device.

Rice. 11-15. Capacitive transducers with a variable distance between the plates (a), differential (b), differential with a variable active area of the plates (c) and with a changing permittivity of the medium between the plates (d)

Capacitive converters. 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 permittivity of the medium between them.

On fig. 11-15 schematically shows the arrangement of various capacitive transducers. The converter in fig. 11-15, A is a capacitor, one plate of which moves under the action of the measured value X relative to the fixed plate. The static characteristic of the transformation C(b) is non-linear. The sensitivity of the transducer increases with decreasing distance 6. Such transducers are used to measure small displacements (less than 1 mm).

In capacitive transducers, there is an (undesirable) force of attraction between the plates

Where W 3- energy of the electric field; U and C are the voltage and capacitance between the plates, respectively.

Differential transducers are also used (Fig. 11-15, b), which have one movable and two fixed plates. When exposed to the measured value X these converters simultaneously change capacitances. On fig. 11-15, V shows a differential capacitive transducer with a variable active area of the plates. Such a transducer is used to measure relatively large linear (more than 1 mm) and angular displacements. In these transducers, it is easy to obtain the required conversion characteristic by profiling the plates.

Transducers (e) are used to measure the level of liquids, the humidity of substances, the thickness of dielectric products, etc. For example (Fig. 11-15, G) the device of the capacitive level gauge converter is given. The capacitance between the electrodes lowered into the vessel depends on the level of the liquid, since a change in the level leads to a change in the average permittivity of the medium between the electrodes. By changing the configuration of the plates, one can obtain the desired character of the dependence of the instrument readings on the volume (mass) of the liquid.

To measure the output parameter of capacitive transducers, bridge circuits and circuits using resonant circuits are used. The latter make it possible to create devices with high sensitivity, capable of responding to displacements of the order of 10~7 mm. Circuits with capacitive converters are usually fed with high frequency current (up to tens of megahertz), which is caused by the desire to increase the signal entering the measuring device, and the need to reduce the shunting effect of the insulation resistance.

ionization converters. 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 p-rays, then a current will flow between the electrodes included in the electrical circuit (Fig. 11-16). 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, the properties and intensity of ionizing radiation, etc. These dependencies are used to measure various non-electrical quantities: the density and composition of the gaseous medium, the geometric dimensions of parts and etc.

Rice. 11-16. Scheme of the ionization converter

Rice. 11-17. Volt-ampere characteristic of the ionization transducer

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

To measure the degree of ionization, converters are used - ionization chambers and ionization counters, the action of which corresponds to different areas volt-ampere characteristic gas gap between two electrodes. On fig. 11-17 shows the dependence of the current in the chamber (Fig. 11-16) with a constant gas composition on the applied voltage U and radiation intensity. Location on A characteristics, the current increases in direct proportion to the voltage, then its growth slows down and in the area B reaches saturation. This indicates that all the ions generated in the chamber reach the electrodes. Location on IN the ionization current begins to grow again, which is caused by secondary ionization when primary electrons and ions collide with neutral molecules. With a further increase in voltage (section G) ionization current ceases to depend on the initial ionization and comes

continuous discharge (section D) which is no longer dependent on exposure to radioactive radiation.

Plots A and B current-voltage characteristics describe the action of ionization chambers, and sections IN And G - ionization counters. In addition to ionization chambers and counters, scintillation (luminescent) counters are used as ionization converters. The principle of operation of these counters is based on the occurrence in certain substances - phosphors (silver-activated zinc sulfide, cadmium sulfide, etc.) - under the influence of radioactive radiation of light flashes (scintillations), which are recorded in the counters by photomultipliers. The brightness of these flashes, and hence the current of the photomultiplier, are determined by radioactive radiation.

The choice of the type of ionization transducer depends largely on the ionizing radiation.

Alpha rays (the nuclei of the helium atom) have a high ionizing power, but have a low penetrating power. In solids, a-rays are absorbed in very thin layers (a few to tens of micrometers). Therefore, when using a-beams, the a-emitter is placed inside the transducer.

Beta rays are a stream of electrons (positrons); they have a much lower ionizing power than a-rays, but they have a higher penetrating power. The path length in solids reaches several millimeters. Therefore, the emitter can be located both inside and outside the converter.

A change in the distance between the electrodes, the area of overlap of the electrodes, or the position of the source of radioactive radiation relative to the ionization chambers or counters affects the value of the ionization current. Therefore, these dependencies are used to measure various mechanical and geometric quantities.

On fig. 11-18 as an example, an ionization membrane manometer is shown, where / is an emitter; 2 - membrane; 3 - fixed electrode isolated from the membrane. Between electrodes 2 a 3 a potential difference sufficient to achieve saturation current is applied. When the pressure changes R the membrane flexes, changing the distance between the electrodes and the value of the ionization current.

Rice. 11-18. Ionization diaphragm pressure gauge

Rice. 11-19. Gas discharge counter

Gamma rays - electromagnetic oscillations very small wavelength (10 ~ 8 -10 ~ "cm), arising from radioactive transformations. Gamma rays have a high penetrating power.

The designs of ionization chambers and counters are varied and depend on the type of radiation.

To register individual particles, as well as to measure small y-radiations, so-called gas-discharge counters are widely used, the action of which is described by sections IN and G current-voltage characteristics. The device of the gas-discharge counter is shown in fig. 11-19. The counter consists of a metal cylinder /, inside of which a thin tungsten wire is stretched 2. Both of these electrodes are placed in a glass cylinder. 3 s inert gas. When the gas is ionized, current pulses appear in the counter circuit, the number of which is counted.

As sources of a-, p- and y-radiation, radioactive isotopes are usually used. Radiation sources used in measuring technology must have a significant half-life and sufficient radiation energy (cobalt-60, strontium-90, plutonium-239, etc.).

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 drawback of these devices is the need to use biological protection at high activity of the radiation source.

13.2 Generator transducers

General information. In generator converters, the output quantity is the EMF or charge, functionally related to the measured non-electric quantity.

Thermoelectric converters. These converters are based on the thermoelectric effect that occurs in a thermocouple circuit.

With a temperature difference of points / and 2, the connection of two dissimilar conductors A and B(Fig. 11-20, a), forming a thermocouple, thermo-EMF arises in the thermocouple circuit.

To measure thermo-EMF, an electrical measuring device (millivoltmeter, compensator) is included in the thermocouple circuit (Fig. 11-20, b). The connection point of the conductors (electrodes) is called the working end of the thermocouple, the points 2 And 2" - free ends.

In order for the thermo-EMF in the thermocouple circuit to be unambiguously determined by the temperature of the working end, it is necessary to maintain the temperature of the free ends of the thermocouple the same and unchanged.

Rice. 11-20. Thermocouple (a) and the method of including the device in the thermocouple circuit (b)

The calibration of thermoelectric thermometers - devices that use thermocouples to measure temperature, is usually carried out at a temperature of the free ends of 0 ° C. Calibration tables for standard thermocouples are also compiled under the condition that the temperature of the free ends is equal to 0 °C. At practical application thermoelectric thermometers, the temperature of the free ends of the thermocouple is usually not equal to 0 ° C and therefore a correction must be introduced.

For the manufacture of thermocouples currently used for temperature measurement, mainly special alloys are used.

To measure high temperatures, thermocouples of the TPP, TPR and TVR types are used. Thermocouples made of noble metals (TPP and TPR) are used in measurements with increased accuracy. In other cases, non-precious metal thermocouples (TXA, TXK) are used.

To protect against external influences (pressure, aggressive gases, etc.), thermocouple electrodes are placed in protective fittings, structurally similar to thermistor fittings (Fig. 11-7, b).

For the convenience of stabilizing the temperature of the free ends, sometimes the thermocouple is extended using the so-called extension wires made either from the corresponding thermoelectrode materials or from specially selected materials that are cheaper than the electrode ones and satisfy the condition of thermoelectric identity with the main thermocouple in the range of possible temperatures of the free ends ( usually from 0 to 100 °C). In other words, the extension wires must have the same temperature dependence of thermo-EMF in the specified temperature range as that of the main thermocouple.

The inertia of thermocouples is characterized by an indicator of thermal inertia. Designs of fast-response thermocouples are known, in which the thermal inertia index is 5-20 s. Thermocouples in conventional fittings have a thermal inertia of several minutes.

Induction transducers are used to measure the speed of linear and angular displacements. The output signal of these converters can be integrated or differentiated in time using electrical integrating or differentiating devices. After these transformations, the informative signal parameter becomes proportional to displacement or acceleration, respectively. Therefore, induction transducers are also used to measure linear and angular displacements and accelerations.

Induction transducers are most widely used in instruments for measuring angular velocity (tachometers) and in instruments for measuring vibration parameters.

Induction converters for tachometers are small (1-100 W) generators of constant or alternating current usually independently excited by a permanent magnet, the rotor of which is mechanically connected to the shaft under test. When using a direct current generator, the angular velocity is judged by the EMF of the generator, and in the case of an alternating current generator, the angular velocity can be determined from the value of the EMF or its frequency.

On fig. 11-21 shows an inductive transducer for measuring amplitude, speed, and acceleration of reciprocating motion. The converter is a cylindrical coil /, moving in the annular gap of the magnetic circuit 2. Cylindrical permanent magnet 3 creates a constant radial magnetic field in the annular gap. The coil, when moving, crosses the lines of force of the magnetic field, and an emf appears in it, proportional to the speed of movement.

Rice. 11-21. Induction transducer

The errors of induction transducers are determined mainly by the change in the magnetic field over time and with temperature changes, as well as temperature changes in the resistance of the winding.

The main advantages of induction transducers are the relative simplicity of design, reliability and high sensitivity. The disadvantage is the limited frequency range of the measured values.

Piezoelectric transducers. Such transducers are based on the use of the direct piezoelectric effect, which consists in the appearance electric charges on the surface of some crystals (quartz, tourmaline, Rochelle salt, etc.) under the influence of mechanical stresses.

A plate is cut out of a quartz crystal, the edges of which must be perpendicular to the optical axis Oz, mechanical axis OU and electrical axis Oh crystal (Fig. 11-22, a and b).

Fx along the electric axis on the faces X charges appear Q x = kF x , Where k- piezoelectric coefficient (module).

When exposed to the force plate Fy along the mechanical axis on the same faces X charges arise Q y = kF y a/b, Where A And b- dimensions of the plate faces.

Mechanical action on the plate along the optical axis does not cause the appearance of charges.

The device of a piezoelectric transducer for measuring variable gas pressure is shown in fig. 11-23. Pressure R through a metal membrane / transmitted to sandwiched between metal gaskets 2 quartz plates 3.

Rice. 11-22. Quartz crystal (a) and plate (b), carved from it

Ball 4 contributes to a uniform distribution of pressure over the surface of the quartz plates. The middle spacer is connected to pin 5 passing through a bushing of good insulating material. When subjected to pressure R a potential difference occurs between pin 5 and the converter housing .

In piezoelectric transducers, quartz is mainly used, in which the piezoelectric properties are combined with high mechanical strength and high insulating qualities, as well as with the independence of the piezoelectric characteristic from temperature over a wide range. Polarized ceramics of barium titanate, titanate and lead zirconate are also used.

Rice. 11-23. Piezoelectric pressure transducer

The dimensions of the plates and their number are chosen based on design considerations and the required charge value.

The charge that occurs in the piezoelectric transducer "flows" along the insulation and the input circuit of the measuring device. Therefore, devices that measure the potential difference on piezoelectric transducers must have a high input resistance (10 12 -10 15 Ohm), which is practically ensured by the use of electronic amplifiers with high input resistance.

Due to the "drain" of the charge, these converters are used to measure only rapidly changing quantities (variable forces, pressures, vibration parameters, accelerations, etc.).

Piezoelectric transducers are used - piezoresonators, which use both direct and reverse piezoelectric effects. The latter is that if an alternating voltage is applied to the electrodes of the transducer, then mechanical oscillations will occur in the piezo-sensitive plate, the frequency of which (resonant frequency) depends on the thickness h plate, modulus of elasticity E and density p of its material. When such a converter is included in the resonant circuit of the generator, the frequency of the generated electrical oscillations is determined by the frequency f p . When changing values h, E or p under the influence of mechanical or thermal influences, the frequency /p will change and, accordingly, the frequency of the generated oscillations will change. This principle is used to convert pressure, force, temperature and other quantities into frequency.

Galvanic converters. The converters are based on the dependence of the electromotive force of a galvanic circuit on the chemical activity of electrolyte ions, i.e. on the concentration of ions and redox processes in the electrolyte. These converters are used to determine the reaction of a solution (acidic, neutral, alkaline), which depends on the activity of the hydrogen ions of the solution.

Distilled water has a weak but well-defined electrical conductivity, which is explained by the ionization of water. The chemical activity a is equal to the product of the equivalent concentration and the activity coefficient (tending to unity with infinite dilution of the solution).

If an acid is dissolved in water, which forms H + ions during dissociation, then the concentration of H + ions in the solution will become greater than in pure water, and the concentration of OH ~ ions will be lower due to the recombination of part of the H + ions with OH ions.

Thus, the chemical activity of the hydrogen ions of a solution is a characteristic of the reaction of the solution. The solution reaction is numerically characterized by the negative logarithm of the activity of hydrogen ions - the pH value. For distilled water, the pH value is 7 pH units.

The range of changes in the pH of aqueous solutions at t = 22 °С is 0-14 pH units.

To measure pH, a method based on measuring the electrode (boundary) potential is used.

If a metal electrode is immersed in a solution containing its ions of the same name, then the electrode acquires a potential. The hydrogen electrode behaves similarly.

To obtain the electrode potential between hydrogen and solution, it is necessary to have a so-called hydrogen electrode. A hydrogen electrode can be created by taking advantage of the adsorption property of hydrogen on the surface of platinum, iridium, and palladium. Typically, the hydrogen electrode is a platinum black coated platinum electrode to which hydrogen gas is continuously supplied. The potential of such an electrode depends on the concentration of hydrogen ions in the solution.

In practice, it is impossible to measure the absolute value of the boundary potential. Therefore, a galvanic converter always consists of two half-cells electrically connected to each other: a working (measuring) half-cell, which is a test solution with an electrode, and a comparative (auxiliary) half-cell with a constant boundary potential, consisting of an electrode and a solution with a constant concentration. A hydrogen electrode with a normal constant concentration of hydrogen ions is used as a reference half cell. For industrial measurements, a more convenient reference calomel electrode is used.

Rice. 11-24. Galvanic Converter

On fig. 11-24 shows a transducer for measuring the concentration of hydrogen ions. A calomel electrode serves as a comparative half-element. It is a glass vessel 4, on the bottom of which a small amount of mercury is placed, and on top of it is a paste of calomel (Hg2Cb). A solution of potassium chloride (KC1) is poured on top of the paste. The potential arises at the calomel-mercury interface. For contact with mercury, a platinum electrode 5 is soldered into the bottom of the vessel. The potential of the calomel electrode depends on the concentration of mercury in the calomel, and the concentration of mercury ions, in turn, depends on the concentration of chloride ions in the potassium chloride solution.

A hydrogen electrode is immersed in the test solution. Both half-cells are connected by an electrolytic key, which is a tube 2, usually filled with a saturated solution of KC1 and closed with semi-permeable plugs 3. The EMF of such a transducer is a function of pH.

In industrial-type devices, instead of working hydrogen electrodes, more convenient antimony or quinhydrone electrodes are used. The so-called glass electrodes are also widely used.

To measure the EMF of galvanic converters, compensation devices are mainly used. For glass electrodes, the measuring circuit must have a high input resistance, since the internal resistance of glass electrodes reaches 100-200 MΩ. When measuring pH with galvanic transducers, corrections must be made for temperature effects.