The book discusses the basic methods of measuring electrical and radio engineering quantities on direct current and alternating current in a wide range of frequencies. Measuring circuits, their principles of construction are described and specifications the most widely used measuring instruments. Examples of calculations are given to facilitate the assimilation of the material. The textbook can be used for vocational training of workers in production.

Basic definitions. Features and methods of measurements.
A qualitatively common property of many physical objects ( physical systems, their states, processes occurring in them) are called physical quantities. In electrical and radio engineering, physical quantities are electrical voltage, current, power, energy, and electrical resistance, electrical capacitance, inductance, frequency.

A physical quantity may have different meanings. A certain value is taken as a unit of measurement of a physical quantity. As a rule, this value is one

The measurement of a given physical quantity is the determination of its value experimentally. Quantitative result, i.e. the measurement result is obtained by comparing the found value of a physical quantity with its unit of measurement.

TABLE OF CONTENTS
Introduction
Chapter first. General information about measurements
§1. Basic definitions. Features and measurement methods
§2. Physical quantities and their units of measurement
§3. Measurement errors
§4. Classification and designation system of measuring instruments
Chapter two. Electromechanical measuring instruments
§5. General information
§6. Magnetoelectric system devices
§7. Electromagnetic system devices
§8. Devices of electro-, ferrodynamic and induction systems
§9. Electrostatic system devices
Chapter three. Measurement direct current and voltage
§10. Measuring direct current with a magnetoelectric device
§eleven. Measuring direct current with an electronic microammeter
§12. Measurement DC voltage magnetoelectric device
§13. Measuring DC voltage with electronic devices
Chapter Four. AC Current and Voltage Measurement
§14. General information
§15. Thermoelectric system devices
§16. Rectifier system devices
§17. Ammeters and voltmeters of the rectifier system
§18. Combined instruments
§19. Electronic voltmeters
§20. Digital voltmeters
Chapter five. Measuring parameters of elements of electrical and radio engineering circuits
§21. General information
§22. Direct reading ohmmeters
§23. Voltmeter - ammeter method
§24. Bridge method
§25. Resonance method
Chapter six. Measurement of parameters of diodes, transistors and vacuum tubes
§26. Diode parameters measurement
§27. Parameter measurement bipolar transistors
§28. Parameter measurement field effect transistors
§29. Vacuum tube testing
Chapter seven. Measuring generators
§thirty. General information
§31. Signal generators low frequencies
§32. High Frequency Signal Generators
§33. Microwave Signal Generators
§34. Generators pulse signals
Chapter eight. Electronic oscilloscopes
§35. General information
§36. Cathode-ray tube
§37. Oscilloscope sweeps
§38. Ramp voltage generators
§39. Control channels
§40. Measurement of voltages and time intervals
Chapter Nine. Frequency measurement
§41. General information
§42. Oscillographic frequency comparison method
§43. Comparison of frequencies based on zero beats
§44. Resonant frequency measurement method
§45. Direct-reading analog frequency meters
§46. Direct indicating electronic frequency counters
Chapter ten. Measurement of parameters of modulated oscillations and spectrum
§47. Measuring parameters of modulated oscillations
§48. Spectrum survey
§49. Harmonic Distortion Measurement
Chapter Eleven. Measurements in distributed constant circuits
§50. Measuring lines
§51. Power measurement
Literature.

Ministry of Education and Science of the Russian Federation

Federal State Budgetary Educational Institution

higher professional education

Chuvash State University named after I.N. Ulyanova

Faculty of Radio Engineering and Electronics

Department of PC and C

Laboratory work No. 2, 3

Measurement of electrical and radio parameters

CIRCUITS BY BRIDGE METHOD

Completed by: student of group RTE-11-10

Ivanov A.O.

Checked by: Kazakov V.D.

Cheboksary 2012

Lab 2

MEASUREMENT OF ELECTRICAL AND RADIO ENGINEERING PARAMETERS

CIRCUITS BY BRIDGE METHOD

Goal of the work: introduction to the bridge method of measuring active resistance , inductance L, containers WITH, quality factor of the coil and oscillatory circuits Q and dielectric loss tangent
, studying the principle of operation of devices based on bridge circuits and acquiring skills in operating these devices.

Brief theoretical information

Electrical and radio circuits consist of resistors, inductors, capacitors and connecting wires. To select or test these components, the active resistance must be measured R, inductance , capacity WITH. In addition, losses in capacitors, the quality factor of coils and oscillating circuits are often measured. Losses in capacitors are determined by the dielectric loss tangent
.

Comparison of the measured quantity (resistance, capacitance, inductance) with a reference standard using a bridge during the measurement process can be carried out manually or automatically using direct or alternating current. Bridge circuits have high accuracy, high sensitivity, and a wide range of measured parameter values. On the basis of bridge methods, measuring instruments designed to measure any one value, and universal analog and digital instruments are built.

DC measuring bridge

DC Bridge(Fig. 6) contains four resistors connected in a closed circuit. Resistors ,,,of this contour are called the arms of the bridge, and the connecting points of adjacent arms are called vertices. The chains connecting opposite vertices are called diagonals. Diagonal ab contains a power source and is called power supply diagonal. Diagonal Withd, in which the G indicator (galvanometer) is included, is called measuring diagonal.

Fig.6. Bridge diagram direct current

DC bridges are designed to measure active resistance. The measurement process using bridge circuits is based on the ratio of the resistances of the arms, called equilibrium condition(balance), which looks like:

.

The equilibrium condition of a DC bridge is formulated as follows: for the bridge to be balanced, the products of the resistances of the opposite arms of the bridge must be equal. If the resistance of one of the bridge arms (for example ) is unknown, then, having balanced the bridge by selecting the resistance of the bridge arms ,And , we find it from the equilibrium condition
.

In the equilibrium state of the bridge, the current through the galvanometer is zero and, therefore, fluctuations in the supply voltage and resistance of the galvanometer do not affect the measurement result. Therefore, the main error of a balanced bridge is determined by the sensitivity of the galvanometer and circuit, the error of the arm resistances, as well as the resistance of the wires and contacts.

Radio engineering measurements are also used very widely in various sectors of the national economy. Non-electrical quantities, such as pressure, humidity, temperature, linear elongations, mechanical vibrations, speed and others, can be converted into electrical ones using special sensors and assessed using methods and instruments of electrical and radio engineering measurements.
Radio engineering measurements cover the field of electrical measurements and, in addition, include all types of special radio measurements.
Radio engineering measurements are used and not for assessing electrical quantities. Such quantities as pressure, temperature, humidity, mechanical vibrations, linear elongations when heated, etc. can be converted using special sensors into electrical ones and assessed using instruments and methods of electrical and radio engineering measurements. The purpose of measurements is to obtain the numerical value of the measured quantity.
The subject of radio engineering measurements, in accordance with the program, includes the following sections: basic metrological concepts; brief information about measurement errors, ways to take them into account and reduce their influence on measurement results; measurement of current, voltage and power over a wide frequency range; study of measuring signal generators; electronic oscilloscopes; measurement of phase shift, frequency and time intervals; measurement of modulation parameters, nonlinear distortions; measurements in radio circuits with concentrated and distributed parameters; tension measurements electromagnetic field and radio interference.
Circuit diagram of a lamp voltmeter with a compensation battery. Features of radio engineering measurements of voltages and currents.
In radio engineering measurements, systematic errors that vary over time are often encountered. Thus, highly sensitive devices are characterized by a systematic error caused by regular interference in the form of a pulsed or quasi-harmonic signal induced into the input circuits of the device. To reduce the level of interference, constructive measures are taken: input circuits are shielded, and the grounding point is rationally chosen. A general method for reducing the influence of periodic interference is to average the measurement results over a certain time interval. Averaging is achieved in two ways, often used together: pre-filtering the input signal and performing multiple measurements with subsequent calculation arithmetic mean.
In radio engineering measurements in the audio, low and very low frequency ranges, C-oscillators are mainly used, which at these frequencies have significant advantages over LC-oscillators. This is explained by the fact that the elements of the oscillatory circuits of LC generators for audio frequencies are too bulky (primarily inductors), and their parameters are unstable when temperature changes, which determines the low stability of the frequency of the generated signals. In addition, it is difficult to tune the frequency of LC oscillators in the audio range.
In ordinary radio engineering measurements carried out in laboratory conditions, Tm is assumed to be 292 K (approximately room temperature 19 C), and the ratio Tsh in / 292 is called the noise number.
Appearance voltmeter BB-5624. When performing electrical and radio engineering measurements, it is customary to indicate on instruments the sign of an ungrounded wire in relation to the ground; thus, the opposite rule of signs applies here.
The introduction of radio measurement technology coincided with the beginning of the development of radio communication systems and radio electronics.
The widespread use of radio engineering measurements in various fields of radio engineering entails the emergence of new measurement methods and special measuring instruments. The most specific measurements are over high frequencies, which is explained by the design features of oscillatory systems and energy transmission lines in this range.
The degree of accuracy of radio engineering measurements, as well as electrical ones, is determined by the error, or measurement error.
The basics of radio engineering measurements are outlined. The principles and methods of measuring radio engineering quantities characterizing the parameters of signals, systems and devices of radio communication and radio broadcasting in the entire applicable frequency range are considered. Information is provided on the construction of block diagrams of measuring instruments, errors and methods for taking them into account and reducing their influence. Special attention devoted to digital devices and those made on microcircuits. Brief background information on many measuring instruments is provided.

The team of the radio engineering measurements department (from left to right): first row - engineers Lyudmila Viktorovna Elyagina, Alexey Andreevich Sorokin, Nina Vladimirovna Tokhtarova, Svetlana Georgievna Popova, Aidar Ravievich Gareev, second row - leading engineer Lidiya Nikolaevna Vdovina, engineer Zaniya Shakhbaevna Mur-salimova, chief department Natalya Veniaminovna Solovova, engineer Vladislav Eminovich Elcheev.
Radio engineering measurements are based on both methods used in electrical measurement technology and methods unique to measurements at high frequencies.
Radio engineering measurements of currents and voltages are based on both methods used in electrical measurement technology and methods unique to measurements at high frequencies.
Sometimes in radio engineering measurements, as well as when checking the calibration of some radio measuring instruments, it is necessary to use standard capacitances, inductances and resistances.
Radio engineering measurements are especially important in astronomy, nuclear physics, rocketry and astronautics.
Basic subjects for radio engineering measurements are: electrical engineering and electrical measurements, electronic devices, electronic amplifiers, basics of radio engineering, automation and Computer Engineering. Good knowledge of these subjects ensures a free understanding and solid assimilation of the radio engineering measurements course within the allotted time. curriculum time.
Block diagram of an oscilloscope type C1 - 1. Let's consider some types of radio engineering measurements that can be performed using an oscilloscope of this type.
Some metrologists in the field of radio engineering measurements consider the entropy error to be more accurate and consistent with the modern information approach to characterizing the process of measuring physical quantities. Information approach allows you to analyze measuring devices in both static and dynamic operating modes from a unified position, optimize technical characteristics and evaluate the limiting capabilities of certain measuring instruments.
Since 7997, the department of radio engineering measurements has been headed by Natalya Veniaminovna Solovova.
What are the features of radio engineering measurements?
Radio interference measurements differ from other radio engineering measurements in that they are very large number types of radio interference, as well as the variety of types of radio communications that can be interfered with by this interference.
The All-Union Scientific Research Institute of Physical, Technical and Radio Engineering Measurements (VNIIFTRI) stores the state primary standard of the temperature unit in the range from 13 81 to 273 15 K. The same institute created and stores the state special standard of the temperature unit in the range from 4 2 to 13 81 K based on the temperature scale of a germanium resistance thermometer.
At the All-Union Scientific Research Institute of Physicochemical and Radio Engineering Measurements, work is underway on thermometry and unification of the values ​​of the properties of substances.
Thus, when performing radio engineering measurements, many factors must be taken into account, otherwise it is impossible to obtain sufficiently accurate results. Actually, this is the ability to use measuring instruments and make measurements.

Amplitude-modulated oscillations are required for many radio engineering measurements. Not all generators are equipped with a modulator.
Automation of the processes of radio engineering measurements, testing and maintenance of radio equipment is important.
Turning on instruments for measuring currents.| Turning on a shunt to expand the current measurement limits of the device. Magnetoelectric devices used for radio engineering measurements are usually very sensitive. The current required to completely deflect the needle of such devices is negligible - fractions of a milliampere. In this case, only part of the total current chains.
Adjustment and adjustment operations are based on various electrical and radio engineering measurements. To successfully solve adjustment problems, knowledge of the techniques and sequence of adjustment operations and measurement methods is required. In this regard, the adjustment of equipment is entrusted to the most qualified workers. The regulator must know the basics of electrical and radio engineering, be fluent in circuit and wiring diagrams, and have a good understanding of the operating principle and interrelation of the main elements of the regulated equipment. When using special adjustment stands, the adjuster must perfectly know their structure and operation and be able to correctly use the stand to ensure high accuracy of adjustment.
Measuring instruments used in radio engineering measurements are called radio measuring instruments. Radio measuring instruments are classified according to types of measurements, operating principles, operating conditions and accuracy.
This is extremely important question in radio engineering measurements and, I must say, very complex. After all, a reverse reaction also occurs: not only does the measuring device affect the circuits under study, but they can also change the operating conditions of the measuring device.
Pulse voltage measurement is a common type of radio engineering measurements. Very often, when setting up and adjusting pulse equipment, oscillographic measurement methods are used, which make it possible not only to measure the parameters of pulses, but also to simultaneously observe their shape. The presence of a calibrator in the oscilloscope with smooth adjustment of the output voltage allows the use of the following methods for measuring the amplitude parameters of pulse signals: calibrated scale, comparison and compensation.
Diagram of a resonant wavemeter connected to a circuit for measuring in latest frequency current Let's confirm the latter on following example from the practice of radio engineering measurements.
It should be noted that due to the peculiarities of radio engineering measurements and various requirements for measurement accuracy, the error of radio measuring instruments and measurements varies within significant limits.
In January 2000, L.N. moved to the radio engineering measurements verification department. Vdovina, A.A. Sorokin, S.G. Popov in order to carry out state metrological control in the new division.
Shape of the moving plate of a logarithmic capacitor.| V. a Series equivalent circuit of a capacitor with losses, b vector diagram for it. This property of a logarithmic capacitor turns out to be valuable in radio engineering measurements.

For the correct installation and adjustment of such equipment, a wide variety of radio engineering measurements are required, as a result of which any quantities are quantified. The measured quantity is compared with a unit of measurement using measuring instruments, which in turn are compared with a standard by calibration.
For a student starting to study the principles and methods of basic radio engineering measurements, the knowledge about power sources used in radio measurements that he knows from previously completed courses is quite sufficient.
Characteristic feature technology of adjustment and tuning operations is a wide variety of electrical and radio engineering measurements. In the process of adjusting radio equipment or its components(cascades), as a rule, various faults are detected and eliminated that were not noticed or missed during inspection, for example: incorrect installation, poor quality soldering, lack of current conductivity through the contact connection, as well as defects in the form of defects in the circuit itself.
Reproducing the vibration shape is important task, solved in radio engineering measurements, since many vibration parameters can be immediately assessed from the shape. Oscilloscopes are used to reproduce the waveform.
The equipment under consideration combines devices used both independently for various radio engineering measurements, and as part of kits, installations and systems for specialized time-frequency measurements. Frequency synthesizers and additional devices that expand the capabilities of frequency synthesizers are used to measure the parameters of highly stable frequency signals, monitor the characteristics of quadripoles and narrow-band paths of radio engineering devices, analyze the spectrum of radio signals, and calibrate the frequency scales of receivers and transmitters.
Tutorial is intended for students of secondary specialized institutions in the specialties Radio engineering measurements, Electrical and thermal measurements:, Mechanical measurements, and can also be used by specialists working in the field of measurement technology.
VNIIFTRI-54 was installed in 1954 at the All-Union Scientific Research Institute of Physical, Technical and Radio Engineering Measurements. In the region from 10 7 to 94 9 K, thermodynamic temperatures were plotted on four platinum thermometers. The boiling temperature of oxygen was taken in this scale to be 90 19 K.
A common disadvantage reactive current dividers, limiting their use in radio engineering measurements is a significant voltage drop across the measuring device.
Students of radio engineering faculties of colleges of communication, along with other disciplines, are taught a course in radio engineering measurements. The book presented to the attention of readers was written according to the program of this course.
Errors in resonant circuits and ways to reduce them are discussed in the literature on radio engineering measurements.

Basic parameters of measuring instruments

Any measuring device must have certain parameters that would ensure more accurate measurement results. To the most general parameters measuring instruments include:

Sensitivity is the ratio of the change in the signal at the output of the device to the change in the measured value that caused it.

Sensitivity threshold is the minimum value of the measured value at the input of the device at which it can still be read.

Amplitude range - the minimum and maximum values ​​of the measured quantity, measured with a given accuracy.

Input resistance is the resistance between the terminals of the device to which the measurement object is connected. This parameter is important for voltmeters, oscilloscopes and other devices that, when measuring, create an additional load for the circuit under study. For generators, this parameter is called output impedance.

Measurement accuracy is a parameter that reflects the closeness of the measurement result to the actual value of the measured value.

Performance - the time it takes for the instrument readings to settle.

Type of scale equation - the most convenient is a scale with a linear dependence,

The measurement of any physical quantity consists in determining its value using special technical means by comparison with a certain value of this quantity taken as unity.

All means used directly in measurement are called measuring equipment and are divided according to the nature of participation in the measurement process into three groups: measures, measuring instruments and measuring devices. Measures and measuring instruments are divided into exemplary and working.

Model measures and measuring instruments are used to reproduce and calibrate various measures and measuring instruments. Those exemplary measures and measuring instruments that are designed to implement and store units of measurement of quantities with the highest accuracy achievable in a given state of technology are called standards.

Working measures and measuring instruments serve for practical measurement purposes and are divided into laboratory and technical. Laboratory measures and measuring instruments are higher than technical ones, since when they are used, the measurement accuracy is taken into account using correction tables or formulas.

In his practical activities, a radio mechanic uses electrical and radio engineering measurements to check, adjust, configure and repair household radio and television equipment. When searching for simple faults, they are often limited to measuring voltages, currents and resistances. To find complex faults, as well as configure and adjust radio-television equipment, more complex measurements are used.

Metrological reliability is a parameter that depends on implicit failures of the device associated with parameters leaving the tolerance limits over time.

Units of physical quantities

In our country, on January 1, 1982, GOST 8.417-81 GSI came into force. Units of physical quantities, which provide for the transition to mandatory use of units International system(SI), which provides the basis for the unification of units of physical quantities throughout the world. The basic units of this system are: length (meter), mass (kilogram), time (second), electric current (ampere), thermodynamic temperature (kelvin), amount of substance (mole) and luminous intensity (candela).

Along with the basic SI units, derivatives from them are used, as well as decimal multiples (10, 100, ... times more) and submultiples (10, 100, ... times less) units. Here are the names of some basic and derived units: electricity- ampere (A), electrical voltage - volt (V), electrical power - watt (W), electrical resistance - ohm (Ohm), electrical conductivity - Siemens (Sm), electrical capacitance - farad (F), inductance - Henry ( Hn), frequency - hertz (Hz), time - second (s).

Names and designations of decimal multiples and submultiple units are formed by adding the following prefixes:

Atto (a) 10 -18, femto (f) 10 -15, pico (p) 10 -12, nano (n) 10 -9, micro (mk) 10 -6, milli (m) 10 -3, centi ( s) 10 -2, deci (d) 10 -1, deca (da) 10, hecto (g) 10 2, kilo (k) 10 3, mega (M) 10 6, giga (G) 10 9, tera ( T) 10 12 .

Measurement errors

The purpose of measurement is to obtain the numerical value of the measured value and estimate the permissible error. Error; inevitable even with the most careful measurements. Therefore, it is impossible to obtain the true value of the measured quantity.

To determine measurement errors, instead of the true value, the actual A D value of the measured quantity is used, which is determined by a standard device or as the arithmetic mean A avg of the results of a large number n of measurements:

The absolute measurement error ΔA is the difference between the measurement result A and the actual value of the measured quantity A D: AΔ = A - A D.

The absolute error with the opposite sign, called the correction, is used when working with laboratory instruments.

The use of absolute error to assess measurement accuracy is inconvenient, since it is not the same at different measurement limits. Therefore, the absolute error is compared with one of the obtained values ​​of the measured quantity, i.e., the relative error is determined.

There is a real relative error Y D %, which is defined as the ratio of the absolute error to the actual value of the measured quantity:

Y D = (ΔA/A D) 100, and the reduced relative error Y D %, which is defined as the ratio of the absolute error to the maximum possible value of the measured quantity A pr, i.e. to the upper measurement limit:

Y pr = (ΔA/A pr) ∙ 100

If multi-range instruments are used, then it is necessary to select a measurement limit at which the deviations of the indicator pointer are located closer to the end of the scale. In this case, the actual error is close to the given one. When the pointer is placed at the beginning of the scale, the actual error increases sharply while the reduced error remains unchanged.

The accuracy of measuring instruments is assessed by highest value permissible error, which is indicated on the scale and in the instrument passport in the form of absolute, actual or reduced errors. For electrical measuring instruments, the largest reduced error determines their accuracy class. Nine accuracy classes have been established: 0.02; 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.0.

Radio measuring instruments do not have an accuracy class, since some of them do not have a dial indicator, and where there is one, its readings are affected by electronic circuit with which it is used. To assess the accuracy of radio measuring instruments, absolute and relative errors are used.

The absolute error of the device is indicated as a single value (for example, ±1 Hz - the frequency drift of the generator during network fluctuations) or as a sum of two values, one of which depends and the other does not depend on the measured value (for example, 0.1 F + 4, Hz, is the error in setting the pulse repetition frequency of the generator).

The relative error of the device is indicated as a percentage by one value (for example, ±6%, - the error of a voltmeter when measuring alternating voltage) or as the sum of two values, of which the first determines the error for large measured quantities, and the second for small ones (for example, 1 + 6R ,%, is the error of the universal bridge when measuring resistance).

Depending on the measurement conditions, absolute and relative errors can be basic or additional. The main one is the error of the device, which operates under normal conditions (temperature, humidity, pressure). The main error depends on design features the device, the quality of its manufacture, the accuracy of the scale calibration, etc. Additional is the error of the device operating under conditions other than normal. The value of the additional error is indicated in the form of a summand to the main error or a correction factor to the measurement result.

Depending on the reasons for their occurrence, errors are divided into systematic and random. The former are caused by inaccurate calibration of instrument scales, their malfunction, and the influence of mechanical, thermal or other factors. These errors are repeated in subsequent measurements; they can be detected and eliminated when processing the measurement results. Random errors arise for many reasons that cannot be taken into account (for example, irregular voltage fluctuations in power supplies, random changes external conditions, etc.).

With repeated measurements, random errors turn out to be different both in value and sign. To reduce the influence of random errors on the measurement result, it is necessary to repeat the measurements n times, calculate the arithmetic mean of the measurement results A avg and accept it as the actual value. To assess the influence of a random error, use the mean square error o, which is calculated using the formula

The smaller the mean square error, the more accurate the measurement and the less the influence of random error on the measurement result.

ANALOG ELECTROMECHANICAL INSTRUMENTS

General information

In analog electromechanical measuring instruments for direct assessment, electromagnetic energy supplied to the device directly from the circuit being measured is converted into mechanical energy of the angular movement of the moving part relative to the stationary one.

Electromechanical measuring instruments (EIM) are used to measure current, voltage, power, resistance and other electrical quantities at constant and alternating currents predominantly industrial frequency 50 Hz. These devices are classified as direct action devices. They consist of an electrical transducer (measuring circuit), an electromechanical transducer (measuring mechanism), and a reading device (Fig. 5.1).

Rice. 5.1. Structural scheme analog EIP

Measuring chain. It ensures the transformation of the electrical measured quantity X into some intermediate electrical quantity Y (current or voltage), functionally related to the measured quantity X. The Y quantity directly affects the measuring mechanism (MM).

According to the nature of the transformation, the measuring circuit can be a set of elements (resistors, capacitors, rectifiers, thermocouples, etc.). Various measuring circuits make it possible to use the same MM when measuring heterogeneous quantities, voltage, current, resistance, varying over a wide range.

Measuring mechanism. Being the main part of the design of the device, it converts electromagnetic energy into mechanical energy necessary for the angle of deflection a of its moving part relative to the stationary one, i.e.

α = f(Y) = F(X).

The moving part of the IM is a mechanical system with one degree of freedom relative to the axis of rotation. The moment of momentum is equal to the sum of the moments acting on the moving part.

The differential equation of moments describing the operation of the IM has the form

J( d 2 α/ dt 2) = Σ M, (5.1)

where J is the moment of inertia of the moving part of the IM; α - angle of deflection of the moving part; d 2 α/ dt 2 - angular acceleration.

The moving part of the MI during its movement is affected by:

torque M , determined for all EIP by the rate of change of electromagnetic field energy w e concentrated in the mechanism, according to the deflection angle α of the moving part. Torque is a function of the measured quantity X, and therefore Y (current, voltage, product of currents) and α:

M= (∂w e /∂α) = f(α) Y n , (5.2)

counter moment M α, created mechanically using spiral springs, braces, lead wires and proportional to the deflection angle α of the moving part:

M α = - Wα, (5.3)

Where W- specific counteracting moment per unit angle of twist of the spring (depends on the material of the spring and its geometric dimensions);

moment of calm M usp, i.e. the moment of forces of resistance to movement, always directed towards the movement and proportional to the angular velocity of the deflection:

M successful =- R (dα/ d t), (5.4)

Where R- damping coefficient.

Substituting (5.2) - (5.4) into (5.1), we obtain the differential equation for the deflection of the moving part of the mechanism:

J( d 2 α/ dt 2) = M + M α + M usp, (5.5)

J( d 2 α/ dt 2) + R (dα/ d t) + Wα = M. (5.6)

The steady deflection of the moving part of the MI is determined by the equality of the torque and counteracting moments, i.e. M = Mα , in the case if the first two terms of the left side of the differential equation (5.6) are equal to zero. Substituting into equality M = Mα analytical expressions of the moments, we obtain the equation of the instrument scale, showing the dependence of the deviation angle a of the moving part on the value of the measured quantity and the MI parameters.

Depending on the method of converting electromagnetic energy into mechanical angular movement of the moving part of the IM, electromechanical devices are divided into magnetoelectric, electrodynamic, ferrodynamic, electromagnetic, etc.

Analog EIP reading device. Most often, it consists of a pointer rigidly connected to the moving part of the IM and a fixed scale. There are arrow (mechanical) and light indicators. The scale is a set of marks that are located along a line and depict a series of sequential numbers corresponding to the values ​​of the quantity being measured. Marks take the form of strokes, dashes, dots, etc.

According to the scale There are rectilinear (horizontal or vertical), arc (for an arc up to 180° inclusive) and circular (for an arc of more than 180°).

By the nature of the location of the marks There are scales that are uniform and uneven, one-sided relative to zero, two-sided and non-zero. Scales are graduated either in units of the measured value (named scale) or in divisions (unnamed scale). The numerical value of the measured quantity is equal to the product of the number of divisions read on the scale and the price (constant) of the device. Division value is the value of the measured quantity corresponding to one division of the scale.

Since EIPs are direct action devices, the sensitivity of the device S p is determined by the sensitivity of the circuit S c and the sensitivity of the measuring mechanism S and:

S p = S c S and (5.7)

Analog EIP accuracy classes: 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.0.

Units and parts of measuring instruments. For most EIP, despite the diversity of the IM, it is possible to identify common components and parts - devices for installing the moving part of the IM, to create a counteracting moment, balancing and calming

.

Rice. 5.2. Installation of the moving part of the measuring mechanism

Since any EIP measuring mechanism consists of a moving and a fixed part, to ensure free movement of the moving part, the latter is installed on supports (Fig. 5.2, a), guy wires (Fig. 5.2,6), and a suspension (Fig. 5.2, c). During transportation, the moving part of the MI is fixed motionless using a lock.

Devices for installing the moving part on supports They are a lightweight aluminum tube into which cores (steel pieces) are pressed. The ends of the cores are sharpened and ground to a rounded cone. The cores are supported on agate or corundum bearings. When installing the moving part of the MI on cores, friction occurs between the core and the thrust bearing, which introduces an error in the instrument readings. In devices high class accuracy (laboratory) to reduce friction, the scale is installed horizontally and the axis vertically. In this case, the load is concentrated mainly on the lower support.

Devices for installing the moving part on guy wires They are two thin belts made of a bronze alloy on which the movable part of the IM is suspended.

Rice. 5.3. General details of the moving part of the IM on supports

Their presence ensures the absence of friction in the supports, facilitates the moving system, and increases vibration resistance. Stretch bars are used to supply current to the frame winding and create a counteracting torque.

Devices for installing moving parts on suspensions used in particularly sensitive devices. The moving part of the MI is suspended on a thin metal (sometimes quartz) thread. Current is supplied to the frame of the moving part through a suspension thread and a special torque-free current lead made of gold or silver.

To create a counteracting moment in IM with the installation of the moving part on supports (Fig. 5.3), one or two flat spiral springs 5 ​​and 6, made of tin-zinc bronze, are used. The springs also serve as current leads to the winding of the frame of the moving part. One end of the spring is attached to the axle or axle shaft, and the other - to the driver 4 of the corrector. The corrector, which sets the pointer 3 of a device that is not switched on to zero, consists of a screw 9 with an eccentrically located pin 8 and a fork 7 with a leash. The corrector screw 9 is brought out to the front panel of the device body; when rotating, it moves the fork 7, which causes the spring to twist and, accordingly, the pointer 3 to move. Axis 2 ends with cores resting on thrust bearings 1.

To balance the moving part counterweight weights 10 serve.

Rice. 5.4. Schemes of magnetic induction (a) and air (b) dampers

The measuring mechanism is considered balanced when the center of gravity of the moving part coincides with the axis of rotation. A well-balanced measuring mechanism shows the same value of the measured quantity at different positions.

To create the necessary sedation for MI They are equipped with dampers that develop a torque directed towards the movement (soothing time no more than 4 s). In MI, magnetic induction and air dampers are most often used, less often liquid dampers (when very high damping is required).

Magnetic induction damper (Fig. 5.4, o) consists of a permanent magnet 1 and an aluminum disk 2, rigidly connected to the moving part of the mechanism and freely moving in the field of the permanent magnet. Calming is created due to the interaction of currents induced in the disk when it moves in the magnetic field of a permanent magnet with the flux of the same magnet.

Air damper (Fig. 5.4, b) is a chamber / in which a lightweight aluminum wing (or piston) 2 moves, rigidly connected to the moving part of the IM. When air moves from one part of the chamber to another through the gap (between the chamber and the wing), the movement of the wing is slowed down and the vibrations of the moving part quickly die out. Air dampers are weaker than magnetic induction dampers.

Logometers

Ratiometers are devices of the electromechanical group that measure the ratio of two electrical quantities Y 1 and Y 2:

α = F(Y 1 / Y2) n, (5.41)

where n is a coefficient depending on the MI system.

The peculiarity of ratiometers is that the rotating M and counteracting M α moments in them are created electrically, therefore the ratiometer has two sensing elements, which are affected by the quantities Y 1 and Y 2 that make up the measured ratio. The directions of the quantities Y 1 and Y 2 must be chosen such that the moments M and M α acting on the moving part are directed towards each other; in this case, the moving part will rotate under the influence of a larger moment. To fulfill these conditions, the moments M and M α must depend differently on the angle of deflection of the moving part of the device.

The sources of logometer error are the non-identical design of the two sensing elements, especially in the presence of ferromagnetic materials; the presence in the ratiometer of additional moments M additional (from friction in the supports, momentless connections, imbalance of the moving part). Hence,

M = M α + M add. (5.42)

The presence of an additional moment M additional makes the ratiometer readings dependent on secondary factors (for example, voltage). Therefore, the logometer scale indicates the operating voltage range within which the scale calibration is valid. The upper voltage limit is determined by the maximum power released in the ratiometer circuits, and the lower limit is determined by M add. The needle, which is not connected to the voltage of the ratiometer, occupies an indifferent position due to the absence of a mechanical counteracting moment.

Rice. 5.18. The mechanism of the magnetoelectric logometer

The operation of a magnetoelectric ratiometer is as follows.

The movable part of the IM is placed in the uneven magnetic field of a permanent magnet (Fig. 5.18), containing two frames, rigidly fastened at an angle d = 30°-90° and mounted on a common axis. Currents I 1 and I 2 are supplied to the frames using torqueless current leads. The direction of the currents is such that the current I 1 creates a torque, and I 2 creates a counteracting moment:

M = I 1 (∂Ψ 1 /∂α); M α = I 2 (∂Ψ 2 /∂α), (5.43)

where Ψ 1, Ψ 2 are the fluxes created by the magnet and coupled to the frames.

The moments M and M α change depending on the change in angle α. Maximum values moments will be shifted by an angle d, which makes it possible to obtain a decrease in M ​​and an increase in M ​​α in the working area. At equilibrium, I 1 (∂Ψ 1 /∂α) = I 2 (∂Ψ 2 /∂α), whence

where f 1 (α), f 2 (α) are quantities that determine the rate of change in flux linkage.

From the equality of moments it follows that

α = F(I 1 / I 2) (5.45)

If the ratio of currents is expressed through the desired value X, then

α = F 1 (X). (5.46)

The existence of this functional dependence is possible if the main operating conditions of the ratiometer are met, i.e. at ∂Ψ 1 /∂α ≠ ∂Ψ 2 /∂α, which is ensured by artificially created unevenness of the magnetic field in the air gap of the ratiometer. Magnetoelectric ratiometers are used to measure resistance, frequency and non-electrical quantities,

Electro-radiotechnical measurements