Parametric measuring transducers. Functional transducers: measuring, parametric, generator

The main elements of most of the measuring instruments used are primary measuring transducers, the purpose of which is to convert the measured physical quantity (input quantity) into a measurement information signal (output quantity), as a rule, electrical, convenient for further processing.

Primary converters are divided into parametric and generator. In parametric transducers, the output value is a change in some parameter of the electrical circuit (resistance, inductance, capacitance, etc.), in generator output value - EMF, electric current or charge arising from the energy of the measured value.

There is a large class of measuring transducers whose input values ​​are pressure, force or torque. As a rule, in these transducers, the input value acts on the elastic element and causes its deformation, which is then converted either into a signal perceived by observers (mechanical indicating instruments) or into an electrical signal.

To a large extent, the inertial properties of the transducer are determined by the natural frequency of the elastic element: the higher it is, the less inertial the transducer is. The maximum value of these frequencies when using structural alloys is 50...100 kHz. Crystalline materials (quartz, sapphire, silicon) are used to manufacture elastic elements of high-precision transducers.

Resistive converters are parametric converters, the output value of which is a change in electrical resistance, which can be caused by the influence of quantities of various physical nature - mechanical, thermal, light, magnetic, etc.

The potentiometric transducer is a rheostat, the engine of which is moved under the influence of the measured value (input value). The output quantity is resistance.

Potentiometric transducers are used to measure the position of regulatory bodies (linear and angular), in level gauges, in sensors (for example, pressure) to measure the deformation of an elastic sensitive element. The advantage of potentiometric transducers is a large output signal, stability of metrological characteristics, high accuracy, and insignificant temperature error. The main disadvantage is the narrow frequency range (several tens of hertz).

The work of strain gauges is based on a change in the resistance of conductors and semiconductors during their mechanical deformation (tensor effect). A wire (or foil) strain gauge is a zigzag curved thin wire with a diameter of 0.02 ... 0.05 mm or a foil tape 4 ... Output copper conductors are connected to the ends of the grid. The transducers, being glued to the part, perceive the deformation of its surface layer.

When measuring strains and stresses in parts and structures, as a rule, there is no possibility of calibrating the measuring channels and the measurement error is 2...10%. In the case of using strain gauges in primary measuring transducers, the error can be reduced to 0.5...1% by calibration. The main disadvantage of strain gauges of this type is a small output signal.

To measure small deformations of elastic sensitive elements of measuring transducers, semiconductor strain gauges are used, grown directly on an elastic element made of silicon or sapphire.

When measuring dynamic deformations with a frequency of up to 5 kHz, wire or foil strain gauges with a base of no more than 10 mm should be used, and the maximum deformation for them should not exceed 0.1% (0.02% for semiconductor).

The action of piezoelectric transducers is based on the appearance of electric charges during crystal deformation (direct piezoelectric effect).

Piezoelectric transducers provide the ability to measure rapidly variable quantities (the natural frequency of the transducers reaches 200 kHz), are highly reliable and have small overall dimensions and weight. The main disadvantage is the difficulty in measuring slowly changing quantities and in carrying out static calibration due to electricity leakage from the crystal surface.

An electrostatic converter can be schematically represented as two electrodes (plates) with an area F, located in parallel at a distance d in a medium with a permittivity e.

Typically, these converters are designed in such a way that their output value is a change in capacitance (in this case they are called capacitive), and the input values ​​can be mechanical displacements that change the gap d or area F, or a change in the dielectric constant of the medium e due to a change in its temperature, chemical composition, etc.

In addition to capacitance, EMF is used as the output value of electrostatic converters. generated by the mutual movement of electrodes in an electric field (generator mode). For example, condenser microphones operate in the generator mode, converting the energy of acoustic vibrations into electrical energy.

The advantage of electrostatic converters is the absence of noise and self-heating. However, in order to protect against interference, the connecting lines and the converters themselves must be carefully shielded.

For inductive converters, the output value is a change in inductance, and the input values ​​can be movements of individual parts of the converter, leading to a change in the resistance of the magnetic circuit, mutual inductance between circuits, etc.

The advantages of converters are: linearity of characteristics, low dependence of the output signal on external influences, shocks and vibrations; high sensitivity. Disadvantages - a small output signal and the need for a high frequency supply voltage.

The principle of operation of vibration-frequency converters is based on a change in the natural frequency of a string or a thin bridge when its tension changes.

The input value of the transducer is the mechanical force (or quantities converted into force - pressure, torque, etc.). which is perceived by the elastic element associated with the jumper.

The use of vibration-frequency converters is possible when measuring constant or slowly changing values ​​in time (frequency not more than 100...150 Hz). They are distinguished by high accuracy, and the frequency signal - increased noise immunity.

Optoelectric converters use the patterns of propagation and interaction with matter of electromagnetic waves in the optical range.

The main element of the converters are radiation receivers. The simplest of them - thermal converters - are designed to convert all the radiation energy incident on them into temperature (integral converter).

As radiation receivers, various photoelectric converters are also used, in which the phenomenon of the photoelectric effect is used. Photovoltaic converters are selective, i.e. they are highly sensitive in a relatively narrow wavelength range. For example, the external photoelectric effect (the emission of electrons under the influence of light) is used in vacuum and gas-filled photocells and photomultipliers.

A vacuum photocell is a glass container, on the inner surface of which a layer of photosensitive material is deposited, forming a cathode. The anode is made in the form of a ring or mesh of metal wire. When the cathode is illuminated, a photoemission current arises. The output currents of these elements do not exceed a few microamperes. In gas-filled photocells (inert gases Ne, Ar, Kr, Xe are used for filling), the output current increases by 5...7 times due to gas ionization by photoelectrons.

In photomultipliers, the amplification of the primary photocurrent occurs due to secondary electron emission - "knocking out" electrons from secondary cathodes (emitters) installed between the cathode and anode. The total gain in multistage photomultipliers can reach hundreds of thousands, and the output current can be 1 mA. Photomultipliers and vacuum elements can be used in measurements of rapidly changing quantities, since the phenomenon of photoemission is practically inertialess.

Pressure measurement

To measure the total or static pressure, special receivers with receiving holes are placed in the flow, which are connected by small-diameter tubes (pneumatic lines) to the corresponding primary transducers or measuring instruments.

The simplest total pressure receiver is a cylindrical tube with a perpendicularly cut end, bent at a right angle and oriented towards the flow. To reduce the sensitivity of the receiver to the flow direction (for example, when measuring in flows with a small swirl), special receiver designs are used. For example, total pressure receivers with a flow (Fig. 3.3) are characterized by a measurement error of no more than 1% at bevel angles up to 45 ° at a number M<0,8.

When measuring static pressures near the walls of the channels, receiving holes with a diameter of 0.5 ... 1 mm are made directly in the walls (drainage holes). There should be no unevenness at the drainage site, and the edges of the holes should not have burrs. This type of measurement is very common in the study of flows in pipes and channels in combustion chambers, diffusers and nozzles.

Rice. 3.3. Total pressure receiver diagram:

Rice. 3.4. Scheme of the static pressure receiver:

a - wedge-shaped;

b - disk;

c - L-shaped for measurements at M £ 1.5

To measure static pressures in the flow, wedge-shaped and disk receivers are used, as well as receivers in the form of L-shaped tubes (Fig. 3.4) with receiving holes located on the side surface. These receivers work well at subsonic and low supersonic speeds.

To study the pressure distribution in the cross sections of the channels, combs of total and static pressures containing several receivers, or combined combs having a receiver of both total and static pressures, have become widespread. When measuring in flows with a complex flow structure (combustion chambers, interblade channels of turbomachines), orientable and non-orientable pressure receivers are used to determine the values ​​of the total and static pressures and the direction of the velocity vector. The first of them are intended for measurements in two-dimensional flows, and their design allows, by rotation, to set the receiver in a certain position relative to the local flow velocity vector.

Non-orientable receivers are equipped with several receiving holes (5...7), which are made in the walls of a cylinder or sphere of small diameter (3...10 mm) or located at the ends of tubes cut at certain angles (diameter 0.5...2 mm ), combined into a single structural unit (Fig. 3.5). When flowing around the receiver, a certain pressure distribution is formed around it. Using the pressure values ​​measured using the receiving holes and the results of the preliminary calibration of the receiver in the wind tunnel, it is possible to determine the values ​​of the total and static pressures and the local direction of the flow velocity.

At supersonic flow velocities, shocks occur in front of the pressure receivers, and this must be taken into account when processing the measurement results. For example, from the measured values ​​of the static pressure in the flow p and the total pressure behind the direct shock p * ", it is possible to determine the number M using the Rayleigh formula, and then the value of the total pressure in the flow:

When testing engines and their elements, various devices are used to measure pressure (pointer deformation, liquid, group recording pressure gauges), which allow the operator to control the operating modes of experimental objects. Various primary converters are used in information-measuring systems. As a rule, pressure, or rather the pressure difference (for example, between measured and atmospheric, between full and static, etc.), acts on an elastic sensitive element (membrane), the deformation of which is converted into an electrical signal. Most often, inductive and strain-sensitive transducers are used for measuring constant and slowly changing pressures, and piezocrystalline and inductive transducers when measuring variable pressures.

Rice. 3.5. Scheme of a five-channel pressure receiver:

C x , C y , C z - components of the velocity vector; p i - measured pressure values

As an example, in fig. 3.6 shows the diagram of the Sapphire-22DD converter. Transducers of this type are available in several modifications designed to measure overpressure, differential pressure, vacuum, absolute pressure, overpressure and vacuum in various ranges. The elastic sensitive element is a metal membrane 2, to which a sapphire membrane with sputtered silicon strain gauges is soldered on top. The measured pressure difference acts on a block consisting of two diaphragms 5. When their center is displaced, the force is transmitted to the lever 3 with the help of a rod 4, which leads to deformation of the membrane 2 with strain gauges. The electrical signal from the strain gauges enters the electronic unit 4, where it is converted into a unified signal - direct current 0...5 or 0...20 mA. The electrical power supply of the converter is carried out from a DC source with a voltage of 36 V.

When measuring variable (for example, pulsating) pressures, it is advisable to bring the primary transducer as close as possible to the place of measurement, since the presence of a pneumatic line introduces significant changes in the amplitude-frequency characteristic of the measurement system. The limiting in this sense is the non-drainage method, in which miniature pressure transducers are mounted flush with the surface flowing around the flow (channel wall, compressor blade, etc.). Known transducers having a height of 1.6 mm and a membrane diameter of 5 mm. Systems with pressure receivers and waveguides (l ~ 100 mm) are also used (the method of remote pressure receivers), in which, in order to improve dynamic

characteristics, corrective acoustic and electrical links are used.

With a large number of measurement points in measuring systems, special high-speed pneumatic switches can be used, which provide serial connection to one converter of several tens of measurement points.

To ensure high accuracy, it is necessary to periodically control pressure measuring instruments under working conditions using automatic setters.

Temperature measurement

A variety of measuring instruments are used to measure temperatures. A thermoelectric thermometer (thermocouple) consists of two conductors made of different materials, connected (welded or soldered) to each other by ends (junctions). If the temperatures of the junctions are different, then a current will flow in the circuit under the action of a thermoelectromotive force, the value of which depends on the material of the conductors and on the temperatures of the junctions. During measurements, as a rule, one of the junctions is thermostatted (melting ice is used for this). Then the EMF of the thermocouple will be uniquely related to the temperature of the "hot" junction.

Dissimilar conductors can be included in a thermoelectric circuit. In this case, the resulting EMF will not change if all the junctions are at the same temperature. This property is the basis for the use of so-called extension wires (Fig. 3.7), which are connected to thermoelectrodes of limited length, and such thus saving expensive materials. At the same time, it is necessary to ensure the equality of temperatures at the points of connection of extension wires (T c) and the thermoelectric identity of their main thermocouple in the range of possible temperature changes T c and T 0 (usually not more than 0...200°C). In the practical use of thermocouples, there may be cases when the temperature T 0 is different from 0°C. Then, to take this circumstance into account, the EMF of the thermocouple should be defined as E \u003d E meas + DE (T 0) and find the temperature value from the calibration dependence. Here E meas - the measured value of the EMF; DE(T 0) is the EMF value corresponding to the value of T 0 and determined from the calibration dependence. Calibration dependences for thermocouples are obtained at a temperature of "cold" junctions T 0 equal to 0°C. These dependencies are somewhat different from linear ones. As an example, in fig. 3.8 shows the calibration dependence for a platinum-rhodium-platinum thermocouple.

Some characteristics of the most common thermocouples are given in Table. 3.1.

In practice, thermocouples with an electrode diameter of 0.2 ... 0.5 mm are most common. The electrical insulation of the electrodes is achieved by winding them with asbestos or silica thread, followed by impregnation with heat-resistant varnish, placing the thermoelectrodes in ceramic tubes or stringing pieces of these tubes (“beads”) on them. Cable-type thermocouples, which are two thermoelectrodes placed in a thin-walled shell made of heat-resistant steel, have become widespread. To insulate thermoelectrodes, the inner cavity of the shell is stuffed with MgO or Al 2 O 3 powder. The outer diameter of the shell is 0.5...6 mm.

Table 3.1

To correctly measure the temperature of structural elements, thermocouples must be sealed in such a way that the hot junction and thermoelectrodes near it do not protrude above the surface and that the conditions for heat transfer from the temperature-controlled surface are not disturbed due to the installation of a thermocouple. To reduce the measurement error due to the outflow (or inflow) of heat from the hot junction along the thermoelectrodes due to thermal conductivity, the thermoelectrodes at a certain distance near the junction (7 ... 10 mm) should be laid approximately along isotherms. The scheme of terminating a thermocouple that satisfies the specified requirements is shown in fig. 3.9. The part has a groove with a depth of 0.7 mm, into which the junction and the thermoelectrodes adjacent to it are placed; the junction is welded to the surface by contact welding; the groove is closed with foil 0.2 ... 0.3 mm thick.

The output of thermoelectrodes from the internal cavities of the engine or its components is carried out through a fitting. In this case, it is necessary to ensure that the thermoelectrodes do not disturb the flow structure too much and their insulation is not damaged due to friction against each other and against the sharp edges of the structure.

When measuring the temperatures of rotating elements, thermocouple readings are taken using brush or mercury current collectors. Non-contact current collectors are also being developed.

Diagrams of thermocouples used to measure the temperature of a gas flow are shown in fig. 3.10. Hot junction 1 is a sphere with a diameter d 0 (thermal electrodes can also be butt-welded); thermoelectrodes 2 near the junction are fixed in an insulating two-channel ceramic tube 3, and then removed from the housing 4. In the figure, the housing 4 is shown to be water-cooled (cooling is necessary when measuring temperatures exceeding 1300 ... 1500 K), the supply and removal of cooling water are carried out through the fitting 5 .

At high gas temperatures, methodological errors arise due to heat removal from the junction due to heat conduction through thermoelectrodes to the thermocouple body and radiation to the environment. Heat losses due to thermal conductivity can be almost completely eliminated by providing an extension of the insulating tube equal to 3 ... 5 of its diameters.

To reduce heat removal by radiation, shielding of thermocouples is used (Fig. 3.10, b, c). This also protects the junction from damage, and slowing down the flow inside the shield helps to increase the temperature recovery factor when measuring in high-speed flows.

A method has also been developed for determining the gas temperature from the readings of two thermocouples with thermoelectrodes of different

Rice. 3.9. Thermocouple termination scheme for measuring the temperature of combustion chamber elements

Rice. 3.10. Thermocouple circuits for measuring gas temperature:

a - open junction thermocouple; b, c - shielded thermocouples; g - double-junction thermocouple; 1 - junction: 2 - thermoelectrodes; 3 - ceramic tube; 4 - body; 5 - fittings for water inlet and outlet

diameter (Fig. 3.10, d), which makes it possible to take into account the removal of heat by radiation.

The inertia of thermocouples depends on the design. Thus, the time constant varies from 1...2 s for open junction thermocouples to 3...5 s for shielded thermocouples.

When studying temperature fields (for example, behind a turbine, combustion chamber, etc.), thermocouple combs are used, and in some cases they are installed in rotating turrets, which makes it possible to determine in sufficient detail the temperature distribution over the entire cross section.

The action of the resistance thermometer is based on the change in the resistance of the conductor with a change in temperature. As an electrical resistance, a wire with a diameter of 0.05 ... 0.1 mm is used, made of copper (t \u003d -50 ... + 150 ° C), nickel (t \u003d -50 ... 200 ° C) or platinum ( t=-200...500°С).

The wire is wound on the frame and placed in the case. Resistance thermometers are highly accurate and reliable, but they are characterized by high inertia and are not suitable for measuring local temperatures. Resistance thermometers are used to measure the air temperature at the engine inlet, the temperatures of fuels, oils, etc.

Liquid thermometers use the thermal expansion property of a liquid. Mercury (t=-30...+700°C), alcohol (t=-100...+75°C), etc. are used as working liquids. Liquid thermometers are used to measure the temperature of liquid and gaseous media in laboratory conditions. , as well as when calibrating other instruments.

Optical methods for measuring temperature are based on the laws of thermal radiation of heated bodies. In practice, three types of pyrometers can be implemented: brightness pyrometers, whose operation is based on a change in the thermal radiation of a body with temperature at a certain fixed wavelength; color pyrometers that use the change with temperature of the distribution of energy within a certain part of the radiation spectrum; radiation pyrometers based on the temperature dependence of the total amount of energy emitted by the body.

At present, when testing engines for measuring the temperatures of structural elements, brightness pyrometers created on the basis of photoelectric receivers of radiant energy have found application. As an example, the installation scheme of the pyrometer during temperature measurement of turbine blades on a running engine is shown in fig. 32.11. With the help of lens 2, the "field of view" of the primary transducer is limited to a small (5...6 mm) area. The pyrometer "examines" the edge and part of the back of each blade. Protective glass 1, made of sapphire, protects the lens from contamination and overheating. The signal through the light guide 3 is transmitted to the photodetector. Due to the low inertia, the pyrometer allows you to control the temperature of each blade.

To measure the temperatures of the structural elements of the engine, color temperature indicators (thermal paints or thermal varnishes) can be used - complex substances that, upon reaching a certain temperature (transition temperature), change their color dramatically due to the chemical interaction of the components or phase transitions occurring in them.

Rice. 3.11. Installation diagram of the pyrometer on the engine:

(a) (1 - blowing air supply; 2 - primary converter) and diagram of the primary converter

(b) (1 - protective glass; 2 - lens; 3 - light guide)

Thermal paints and thermal varnishes, when applied to a solid surface, after drying, harden and form a thin film, which is able to change its color at the transition temperature. For example, white thermal paint TP-560 becomes colorless when t=560 °C is reached.

With the help of thermal indicators, you can detect overheating zones in engine elements, including hard-to-reach places. The complexity of the measurements is low. However, their use is limited, since it is not always possible to establish in which mode the maximum temperature was reached. In addition, the color of the thermal indicator depends on the time of exposure to temperature. Therefore, thermal indicators, as a rule, cannot replace other measurement methods (for example, using thermocouples), but they provide additional information about the thermal state of the object under study.

The operation of measuring transducers takes place in difficult conditions, since the object of measurement is, as a rule, a complex, multifaceted process characterized by many parameters, each of which acts on the measuring transducer together with other parameters. We are only interested in one parameter, which is called measured value, and all other process parameters are considered interference. Therefore, each transmitter has its own natural input quantity, which is best perceived by him against the background of interference. In a similar way, one can distinguish natural output value measuring transducer.

From the point of view of the type of signal at its output, non-electrical to electrical converters can be divided into generator ones that produce charge, voltage or current (output value E \u003d F (X) or I \u003d F (X) and internal resistance ZBH \u003d const), and parametric with output resistance, inductance or capacitance, changing in accordance with the change in the input value (EMF E \u003d 0 and the output value in the form of a change in R, L or C in the function X).

The difference between generator and parametric converters is due to their equivalent electrical circuits, which reflect the fundamental differences in the nature of the physical phenomena used in the converters. The generator converter is a source of a directly issued electrical signal, and the measurement of changes in the parameters of a parametric converter is carried out indirectly, by changing the current or voltage as a result of its mandatory inclusion in a circuit with an external power source. An electrical circuit directly connected to the parametric converter generates its signal. Thus, the combination of a parametric converter and an electrical circuit is a source of an electrical signal.

According to the physical phenomenon underlying the work, and the type of input physical quantity, generator and parametric converters are divided into a number of varieties (Figure 2.3):

Generator - on piezoelectric,

thermoelectric, etc.;

Resistive - on contact,

Rheostatic, etc.;

Electromagnetic - to inductive,

Transformer, etc.

According to the type of modulation, all IPs are divided into two large groups: amplitude and frequency, time, phase. The last three varieties have a lot in common and therefore are combined into one group.

Rice. 2.3. Classification of measuring transducers of non-electric quantities into electrical ones.

2. By the nature of the transformation, the input values:

Linear;

Nonlinear.

3. According to the principle of operation of the primary measuring transducer (PMT) are divided into:

Generator;

Parametric.

The output signal of the generator PIP is the EMF, voltage, current and electric charge, functionally related to the measured value, for example, the EMF of a thermocouple.

In parametric PIPs, the measured value causes a proportional change in the parameters of the electrical circuit: R, L, C.

Generators include:

induction;

Piezoelectric;

Some varieties of electrochemical.

Resistive IP - convert the measured value into resistance.

Electromagnetic IP – converted into a change in inductance or mutual inductance.

Capacitive IP – is converted into a change in capacitance.

Piezoelectric power supplies - convert dynamic force into electrical charge.

Galvanomagnetic IP - based on the Hall effect, they convert the acting magnetic field into an EMF.

Thermal IP - the measured temperature is converted into the value of thermal resistance or EMF.

Optoelectronic IP - convert optical signals into electrical signals.

For sensors, the main characteristics are:

Operating temperature range and error in this range;

Generalized input and output resistances;

frequency response.

In industrial applications, the error of sensors used in control processes should be no more than 1–2%. And for control tasks - 2 - 3%.

2.1.3. Schemes for switching on primary measuring transducers

Primary measuring transducers are:

Parametric;

Generator.

Schemes for switching on parametric primary measuring transducers are divided into:

Sequential connection:

Differential switching:

With one primary measuring transducer;

With two primary measuring transducers;

Bridge switching circuits:

Symmetrical unbalanced bridge with one active arm;

Symmetrical unbalanced bridge with two active arms;

Symmetrical unbalanced bridge with four active arms.

Schemes for switching on generator measuring transducers are divided into:

Sequential;

differential;

Compensatory.

Generators do not need an energy source, while parametric ones do. Very often, generators can be represented as a source of EMF, and parametric ones can be represented as an active or reactive resistor, the resistance of which changes with a change in the measured value.

Sequential and differential switching can be applied to both parametric and generator power supplies. Compensation scheme - to the generator. Bridge - to parametric.

2.1.3.1. Schemes for serial connection of parametric measuring transducers

Sequential connection of one parametric measuring transducer (Fig. 2.4):

Rice. 2.4. Sequential connection of one parametric IP.

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- voltage sensitivity;

Power sensitivity;

Rice. 2.5. Output characteristics of a series-connected IP:

a - real; b is ideal.

Sequential connection of two parametric measuring transducers (Fig. 2.6).

Fig.2.6. Sequential connection of two parametric IP.

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Electrical measuring instruments are widely used for measuring non-electric quantities. This became possible thanks to the use of special converters (PR).

The output signals of such converters are transmitted in the form of circuit parameters or EMF (charge) associated with the input signal by a functional dependence. The first are called parametric, the second - generator.

Of the parametric transducers, the most widely used are rheostatic, strain-sensitive, thermosensitive, electrolytic, ionization, inductive and capacitive devices.

Rheostat converters they are an isolated frame on which a conductor and a brush moving along the turns are wound. Their output parameter is the resistance of the circuit.

The measured value Pr can be the movement of the brush in a straight line or in a circle. By improving the perceiving system, Pr can be used to determine the pressure or mass under which the slider will move.

To wind the rheostat, materials are used whose resistance depends little on external factors (temperature, pressure, humidity, etc.). Such materials can be nichrome, fechral, ​​constantan or manganin. By changing the shape and cross section of the core (the length of one turn changes accordingly), it is possible to achieve a nonlinear dependence of the circuit resistance on the movement of the slider.

The advantage of rheostat converters is the simplicity of their design. However, it is impossible to accurately determine the displacement if the output resistance changes within one turn. This is the main disadvantage of such Pr, and characterizes their error.

Strain sensitive transducers (TSChPr). Their work is based on a change in the active resistance of the conductor under the influence of pressure or mechanical deformation. This phenomenon is called the tensor effect.

The input signal for TFPR can be tension, compression or another type of deformation of equipment parts, metal structures, the output signal is a change in the resistance of the transducer.

Strain-sensitive Pr are a thin substrate made of paper or film and a wire of very small cross section glued to it. As a receiving element, a constantan wire is usually used, which has a temperature-independent resistance, with a diameter of 0.02-0.05 mm. Foil TFPR and film strain gauges are also used.

The TF transducer is glued to the measured part in such a way that the axis of the linear expansion of the part coincides with the longitudinal axis of the TST. With the expansion of the measured object, the length of the TFC increases, respectively, its resistance changes.

The advantage of such devices is linearity, simplicity of design and installation. The disadvantages include low sensitivity.

Thermally sensitive transducers (TRPr). As the main elements of such devices, thermistors, thermal diodes, thermotransistors, etc. are used. The thermoelement is included in the electrical circuit in such a way that the circuit current passes through it and the temperature of the measured element is affected.

With their help, temperature, viscosity, thermal conductivity, speed of movement and other parameters of the medium in which the element is located can be measured.

For measurements in the temperature range -260°C to +1100°C, platinum thermistors are used, in the range -200°C to +200°C - copper. In the temperature range -80°C to +150°C, when special accuracy is required, thermal diodes and thermal transistors are used.

TRPR according to the mode of operation is divided into overheating and without preheating. Devices without preheating are used only to measure the temperature of the medium, since the current flowing in them does not affect their heating. The resistance of the element accurately determines the temperature of the medium.

The mode of operation of another type of thermal converters is associated with their preheating to a predetermined value. Then they are placed in the medium to be measured, and the change in its resistance is monitored.

By the rate of change of resistance, one can judge how intensively cooling or heating occurs, which means that it is possible to determine the speed of movement of the measured substance, its viscosity and other parameters.

Semiconductor TPRr are more sensitive than thermistors, so they are used in the field of precise measurements. However, their significant disadvantage is the narrow temperature range and poor reproducibility of the static characteristics of the device.

Electrolytic converters (ELP). It is used to determine the concentration of solutions, since the electrical conductivity of solutions significantly depends on the degree of salt concentration in them.

ELP is a vessel with two electrodes. Voltage is applied to the electrodes, thus, the electrical circuit is closed through the electrolyte layer. Such converters are used on alternating current, since under the influence of direct current, the electrolyte dissociates into positive and negative ions, which introduces an error in the measurements.

Another disadvantage of ELP is the dependence of the electrolyte conductivity on temperature, which makes it necessary to maintain a constant temperature with the help of refrigeration or heating installations.

Inductive and capacitive transducers. As the name implies, the output parameters of such devices are inductance and capacitance. The measured value of simple inductive Pr can be a displacement of 10 to 15 mm, for inductive transformer Pr with an open system, this value can be increased to 100 mm. Capacitive Pr are used to measure displacements of the order of 1 mm.

Inductive Pr are two inductors placed on an open core. The mutual inductance of the coils is influenced by such parameters as: the length of the air gap of the open section, the cross-sectional area of ​​the air gap, the magnetic permeability of the air gap.

Thus, by measuring the mutual inductance of the coils, one can determine how much the above parameters have changed. And they can change when a dielectric plate moves in the air gap. This is the basis of the principle of operation of inductive Pr.

The principle of operation of capacitive Pr is based on a change in the capacitance of the capacitor with a decrease in the active area of ​​the plates, a change in the distance between the plates of the capacitor and a change in the dielectric constant of the interplate space.

Capacitive transducers have a higher sensitivity to changes in input parameters. A capacitive Pr is able to detect a change in capacitance even when moving by thousandths of a millimeter.

Ionization transducers. The principle of operation of the device is based on the phenomenon of ionization of gas and other media under the influence of ionizing radiation, which can be used as ionizing α-, β- and γ-radiation of radioactive substances, or X-rays.

If a gas chamber is exposed to radiation, then an electric current will flow through the electrodes. The magnitude of this current will depend on the composition of the gas, the dimensions of the electrodes, the distance between the electrodes, and the applied voltage.

By measuring the electric current in the circuit, with a known composition of the medium, the distance between the electrodes, the applied voltage, it is fashionable to determine the size of the electrodes, or vice versa, other parameters. They are used to measure the dimensions of parts, or gas compositions, etc.

The main advantage of ionizing Pr is the possibility of non-contact measurement in aggressive environments, under high pressure or temperature. The disadvantage of such Pr is the need for biological protection of personnel from exposure to radiation.

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 spread 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 A) 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. Thus,

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.

Thus,

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 perform measurement 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 analog-to-digital conversion methodology that 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.

5.4.1 D/A and A/D converters

Digital-to-analogue (DAC) and analog-to-digital converters (ADC) are an integral part of automatic control and regulation systems. In addition, since the vast majority of the 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 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 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.

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 circuit 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 in 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.

1. What is the device, working principle and application:

a) photoelectric converters;

Photovoltaic converters are those in which the output signal changes depending on the light flux incident on the converter. Photovoltaic converters or, as we will call them in the following, photocells are divided into three types:

1) photocells with external photoelectric effect

They are vacuum or gas-filled spherical glass containers, on the inner surface of which a layer of photosensitive material is applied, forming a cathode. The anode is made in the form of a ring or grid of nickel wire. In the darkened state, a dark current passes through the photocell as a result of thermionic emission and leakage between the electrodes. When illuminated, the photocathode imitates electrons under the influence of photons of light. If a voltage is applied between the anode and cathode, then these electrons form an electric current. When the illumination of a photocell included in an electrical circuit changes, the photocurrent in this circuit changes accordingly.

2) photocells with internal photoelectric effect

They are a homogeneous semiconductor plate with contacts, for example, made of cadmium selenide, which changes its resistance under the action of a light flux. The internal photoelectric effect consists in the appearance of free electrons knocked out by light quanta from the electronic orbits of atoms remaining free inside the substance. The appearance of free electrons in a material, such as a semiconductor, is equivalent to a decrease in electrical resistance. Photoresistors have high sensitivity and a linear current-voltage characteristic (CVC), i.e. their resistance does not depend on the applied voltage.

3) photovoltaic converters.

These converters are active light-sensitive semiconductors that, when light is absorbed, due to photoelectric effects in the barrier layer, create free electrons and EMF.

A photodiode (PD) can operate in two modes - photodiode and generator (valve). Phototransistor - a semiconductor receiver of radiant energy with two or more p - "-junctions, in which a photodiode and a photocurrent amplifier are combined.

Phototransistors, like photodiodes, are used to convert light signals into electrical signals.

b) capacitive transducers;

A capacitive transducer is a capacitor whose capacitance changes under the action of a measured non-electric quantity. As a capacitive converter, a flat capacitor is widely used, the capacitance of which can be expressed by the formula C \u003d e0eS / 5, where e0 is the dielectric constant of air (e0 \u003d 8.85 10 "12F / m; e is the relative permittivity of the medium between the capacitor plates; S- facing area; 5-distance between facings)

Since the measured non-electrical quantity can be functionally related to any of these parameters, the design of capacitive transducers can be very different depending on the application. To measure the levels of liquid and granular bodies, cylindrical or flat capacitors are used; for measuring small displacements, rapidly changing forces and pressures - differential capacitive transducers with a variable gap between the plates. Consider the principle of using capacitive transducers to measure various non-electrical quantities.

c) thermal converters;

A thermal converter is a current-carrying conductor or semiconductor with a high temperature coefficient, which is in heat exchange with the environment. There are several ways of heat exchange: convection; thermal conductivity of the medium; thermal conductivity of the conductor itself; radiation.

The intensity of heat exchange between the conductor and the environment depends on the following factors: the speed of the gas or liquid medium; physical properties of the medium (density, thermal conductivity, viscosity); environment temperature; geometric dimensions of the conductor. This dependence of the temperature of the conductor, and consequently, its resistance on the listed factors can be

be used to measure various non-electric quantities characterizing a gas or liquid medium: temperature, velocity, concentration, density (vacuum).

d) ionization converters;

Ionization transducers are such transducers in which the measured non-electric quantity is functionally related to the current of the electronic and ionic conductivity of the gaseous medium. The flow of electrons and ions is obtained in ionization converters either by ionization of the gas medium under the influence of one or another ionizing agent, or by thermionic emission, or by bombarding the molecules of the gas medium with electrons, etc.

Mandatory elements of any ionization transducer are a source and a receiver of radiation.

e) rheostat converters;

A rheostat transducer is a rheostat, the engine of which moves under the action of a measured non-electric quantity. A wire is wound with a uniform pitch on a frame made of insulating material. The insulation of the wire at the upper border of the frame is stripped, and the brush slides over the metal. The additional brush slides over the slip ring. Both brushes are isolated from the drive roller. Rheostatic transducers are made both with a wire wound on a frame and a rheochord type. Nichrome, manganin, constantan, etc. are used as wire material. In critical cases, when the requirements for wear resistance of contact surfaces are very high or when contact pressures are very low, platinum alloys with iridium, palladium, etc. are used. The rheostat wire must be covered with either enamel or a layer of oxides to isolate adjacent turns from each other. Engines are made of two or three wires (platinum with iridium) with a contact pressure of 0.003 ... 0.005 N or lamellar (silver, phosphor bronze) with a force of 0.05 ... 0.1 N. The contact surface of the wound wire is polished; the width of the contact surface is equal to two or three wire diameters. The frame of the rheostatic transducer is made of textolite, plastic or aluminum coated with insulating varnish or oxide film. Frame shapes are varied. The reactance of rheostat transducers is very small and can usually be neglected at frequencies in the audio range.

Rheostatic transducers can be used to measure vibration accelerations and vibration displacements with a limited frequency range.

f) strain gauge transducers;

A strain gauge transducer (strain gauge) is a conductor that changes its resistance during tensile or compression deformation. The length of the conductor / and the cross-sectional area S change with its deformation. These deformations of the crystal lattice lead to a change in the resistivity of the conductor p and, consequently, to a change in the total resistance

Application: to measure deformations and mechanical stresses, as well as other static and dynamic mechanical quantities that are proportional to the deformation of an auxiliary elastic element (spring), such as path, acceleration, force, bending or torque, gas or liquid pressure, etc. These measured quantities can be used to determine derived quantities, such as mass (weight), tank filling level, etc. Paper-based wire strain gauges, as well as foil and film strain gauges, are used to measure relative strains from 0.005 ... 0.02 to 1.5 ... 2%. Free wire strain gauges can be used to measure strains up to 6...10%. Strain gauges are practically inertialess and are used in the frequency range of 0...100 kHz.

g) inductive transducers;

Inductive measuring transducers are designed to convert position (displacement) into an electrical signal. They are the most compact, noise-resistant, reliable and economical measuring transducers for solving the problems of automating the measurement of linear dimensions in mechanical engineering and instrumentation.

The inductive transducer consists of a housing in which a spindle is placed on the rolling guides, at the front end of which there is a measuring tip, and at the rear - an armature. The guide is protected from external influences by a rubber cuff. The armature connected to the spindle is located inside the coil fixed in the body. In turn, the coil windings are electrically connected to the cable fixed in the housing and protected from kinks by a conical spring. At the free end of the cable there is a connector used to connect the converter to the secondary device. The body and spindle are made of hardened stainless steel. The adapter connecting the armature to the spindle is made of titanium alloy. The spring that creates the measuring force is centered, which eliminates friction when the spindle moves. This design of the transducer provides a reduction in random error and variation in readings to a level of less than 0.1 µm.

Inductive transducers are widely used mainly for measuring linear and angular displacements.

h) magnetoelastic transducers;

Magnetoelastic transducers are a type of electromagnetic transducers. They are based on the phenomenon of changes in the magnetic permeability μ of ferromagnetic bodies depending on the mechanical stresses σ arising in them, associated with the action of mechanical forces P on the ferromagnetic bodies (tensile, compressive, bending, twisting). A change in the magnetic permeability of the ferromagnetic core causes a change in the magnetic resistance of the core RM. A change in RM leads to a change in the inductance of the coil Llocated on the core. Thus, in the magnetoelastic transducer we have the following chain of transformations:

P -> σ -> μ -> Rm -> L.

Magnetoelastic transducers can have two windings (transformer type). Under the action of a force due to a change in magnetic permeability, the mutual inductance M between the windings and the induced EMF of the secondary winding E change. The conversion circuit in this case has the form

P -> σ -> μ -> Rm -> M -> E.

The effect of changing the magnetic properties of ferromagnetic materials under the influence of mechanical deformations is called the magnetoelastic effect.

Magnetoelastic transducers are used:

For measuring high pressures (greater than 10 N/mm2, or 100 kg/cm2), since they directly perceive pressure and do not need additional transducers;

To measure strength. In this case, the measurement limit of the device is determined by the area of ​​the magnetoelastic transducer. These transducers deform very little under the action of force. Yes, at l= 50 mm, △ l < 10 мкм они имеют высо­кую жесткость и собственную частоту до 20... 50 кГц. Допустимые напряжения в материале магнитоупругого преобразователя не дол­жны превышать 40 Н/мм2 .

i) electrolytic resistance converters;

Electrolytic converters are a type of electrochemical converters. In the general case, an electrochemical converter is an electrolytic cell filled with a solution with electrodes placed in it, which serve to turn the converter into a measuring circuit. As an element of an electrical circuit, an electrolytic cell can be characterized by the EMF it develops, the voltage drop from the passing current, resistance, capacitance and inductance. By highlighting the relationship between these electrical parameters and the measured non-electrical quantity, as well as suppressing the action of other factors, it is possible to create transducers for measuring the composition and concentration of liquid and gaseous media, pressures, displacements, speeds, accelerations, and other quantities. The electrical parameters of the cell depend on the composition of the solution and electrodes, chemical transformations in the cell, temperature, the rate of movement of the solution, etc. The relationship between the electrical parameters of electrochemical converters and non-electrical quantities is determined by the laws of electrochemistry.

The principle of operation of electrolytic converters is based on the dependence of the resistance of an electrolytic cell on the composition and concentration of the electrolyte, as well as on the geometric dimensions of the cell. Liquid column resistance of the electrolytic converter:

R = ρh/S = k/૪

where ૪= 1/ρ is the specific conductivity of the electrolyte; k - converter constant, depending on the ratio of its geometric dimensions, usually determined experimentally.