SU1219992A1 - Method of contactless measurement of conducting body magnetic permeability - Google Patents

Abstract

The invention can be used in determining the magnetic parameters of conductive samples using the eddy current method. The purpose of the invention is to improve the measurement accuracy. The phase angle is measured between the voltage of the measuring winding and the reference voltage in two modes: without biasing and biasing until saturation of the material of the body. Frequencies of an alternating magnetic field are obtained at which the phase angle between the voltage of the measuring winding and the reference voltage has the same value. The value of the magnetic permeability of the body is determined as a fraction of the value of the frequency found in the saturation mode by the value of the frequency found without bias. The value of magnetic permeability is not affected by changes in such parameters as the geometric dimensions of the body, inductance and resistance of the feed-through coil, the supply voltage, etc. The description provides a mathematical expression for determining the magnitude of the phase shift between the voltage of the measuring winding and the reference voltage. In addition, a block diagram of the device that implements this method. 1 il. (Ls

Description

one

The invention relates to magnetic measurements and can be used in determining the magnetic parameters of conductive samples by the eddy current method.

The aim of the invention is to improve the measurement accuracy.

This goal is achieved by two-stroke measurement of the phase angle by the voltage of the measuring winding and the reference voltage in the saturation mode of the material of the test body without biasing and obtaining such values of the frequencies of the alternating magnetic field at which the angle of the shift, the phases between the measuring winding voltage and the reference voltage will be equal to the same value, so that the values of magnetic permeability are not affected by changes in parameters such as the geometric dimensions of the body, inductance and active with the resistance of the feed coil, the supply voltage

Nin and so on

The drawing shows a block diagram of a device implementing the proposed method for measuring magnetic permeability.

The device includes the Start button 1, one-shot 2, counter 3, clock pulse generator (GTI) 4 circuits AND 5 and 6, digital-to-analog converter (DAC) 7, switch 8 analog signals, controlled sinusoidal oscillator 9, amplifiers 10 and 11 power, eddy current sensor 12, consisting of exciting 13, measuring 14 windings and bias winding 15, phase meter 16, circuit 17 - and 18 matches, circuit OR 19 and 20, control circuit 21, register 22, shift register 23, divider 24, Schrogrammable read-only memory (Г Memory) 25, klyuch.26, resistors 27 and 28, the capacitor 29.

The start button 1 is connected to the input of the one-shot 2. The output of the one-shot 2 is connected to the set input of the initial state of the control circuit 21. The direct output of the GTI 4 is connected to the inputs of two-input circuits AND 5 and 6. The inverse output of the GTI 4 is connected to the trigger input of the phase meter 16, the output of the conversion of which is connected to the gate input of the register record 23. The output of the circuit 5 is connected to the summing input of the counter199922

ka 3, and the output circuit And 6 - with the subtracting input of the counter 3. The output code of the counter 3 is fed to the input of the DAC 7 and to the input of the EPROM 25. Output 5 of the EPROM 25 is connected to the input of the register 22 and the input of the divider 24, the second input of which is connected to the output register 22. A digital code entering the input of the CDL 7, is converted into an analogue voltage signal 10, coming through the switch 8 to the input of the controlled harmonic oscillator 9 and resistor 28. The frequency of the oscillator 9 can vary smoothly 5 input voltage values. The sinusoidal signal from the generator 9 is amplified by the power amplifier 10 and is fed through a resistor 27 to the exciter winding 2Q 13 of the sensor 12. Phase meter. 16 measures the phase angle between the voltages at the output of the amplifier 10 and the output of the measuring winding 14 of the sensor 12, and also converts the difference of 25 phases of the input voltages into a corresponding digital code. The code value is fed to the inputs of the circuits I7 and 18 coincidence and the input of register 23. The coincidence circuit 17 is adjusted to a specific value calculated according to the proposed recommendations. The coincidence circuit 18 works when the previous code value written in register 23 is equal to the current code value, coming from the phase meter 16. The outputs of the matching circuits 17 and 18 are connected to the inputs of the OR circuit 19, the output of which is connected to the input of the control circuit 21. The key 26 shunts the capacitor 29 when a signal is detected at the output 1 of the control circuit 21. The outputs I and P of the control circuit 21 are connected to the inputs of the OR circuit 20, the output of which is connected to the second output of the circuit 5. The output P of the control circuit 21 is also connected to the input gate of the register entry 22 and the control input of the switch 8. The output 1II of the control circuit 21 is connected with the clock input of the divider 24. The output of the amplifiers 11 is connected to the bias winding 15 of the sensor 12.

Resistor 27 provides constant current in the supply circuit of the excitation winding. The resistor 28 sets the time 55 charge of the capacitor 29.

The device for measuring magnetic permeability works as follows.

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45

3

Using the Start button 1, the single-vibration 2 is started. The signal from the output of the single-vibrator 2 zeroes the counter 3 and sets the control circuit 21 to the initial state corresponding to the output code 1000. The signal from the output 1 of the control circuit 21 goes through the OR circuit 20 5, allowing the supply of pulses from the GTI 4 to the summing input of the counter 3.

The value of the code in the counter 3 increases, which leads to an increase in the frequency of the harmonic oscillations of the generator 9 and, consequently, the frequency of the supply voltage in the winding 13 of the pulse.

The pulses from the inverse output of the GTI 4 control the operation of the phase meter 16 to start it with a new pulse each time. The frequency of the GTI 4 is chosen so that the duration of the transient process in the controlled generator 9 and the windings of the sensor 12 is less than half the period of the pulses of the GTI 4. Under such conditions, at the moment the phase meter 16 starts, its inputs will have steady signals. Depending on the frequency of the supply voltage, as well as on the magnitude of the magnetic permeability of the product placed in the sensor 12, the phase angle between the voltages on the excitation winding 13 and on the measurement winding 14 varies. For some value of the generator frequency 9, the value of the offset angle phases calculated by this method and for which the I7 coincidence circuit is set up and the value of the current phase angle coincide.

In this case, the circuit 17 will work together; according to a signal from which at the output M of the circuit, 21 controls will receive a signal. By this signal, the register 22 will record the value of the frequency code from the output of the EPROM 25, designed to take into account the nonlinearity existing between the values of the frequency of the controlled generator 9 and the values of the input control current determined by the code stored in the counter 3. At the same time, the switch 8 switches. The output voltage of the CPU 7, amplified by the amplifier 11, will go to the bias winding 15. Counter 3 continues to count incoming pulses, hence the bias current will increase and measure 99924

be the phase angle between the voltages measured by the phase meter 16. The previous value of the phase shift angle code, coming from the output of the shift register 23, information on the output of which is delayed by one cycle compared to the input code, and the current code value coming from the phase / J output 16, will be compared by coincidence circuit 18 which will work with their equality. The absence of a change in the phase angle with increasing bias current will be observed

moan about the onset of the regime

saturation of the material of the investigated parts. The signal at the output of the control unit circuit 21 is obtained. The switch 8 switches over on this signal, the E1AP 7 output is connected to the input of the controlled oscillator 9, the pulses c are stopped. GTI 4 to the summing input of counter 3 and permission to feed pulses from the output of GTI 4 through the circuit AND 6 to the subtractive input of the meter 3 will be received. The bias current in the winding 15 of the sensor 12 will not change, since the input voltage of the switch is remembered by a capacitor 29 Counter 3 will start decreasing its code. This will cause a decrease in the oscillation frequency of the oscillator 9. When the matching circuit 17 operates, the output of the control circuit 21 is followed by a signal

which will fix the code value in the counter 3, and the divisor 24 will divide the code entering the EPROM 25 into the code stored in register 22. At the output we will get a code proportional to

sample magnetic permeability value.

The proposed method is based on the fact that when the voltage frequency that feeds the eddy current is changed

the transducer changes the phase difference between this voltage and the voltage from the measuring winding of the feed coil; in addition, the magnetic permeability of the bulk sample is equal to unity.

The EMF of the measuring winding of the vortex transient converter can be found by the following formula:

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b-j.,. w, H.CR; -K-.,

(eleven

de magnetic permeability

vacuum;

- - cyclic frequency of the alternating magnetic field Wy - number of turns of the measuring

windings;

HO is the intensity of the magnetic field in the air;

R is the radius of the measuring winding;

R is the radius of the investigated part; 1 - relative magnetic permeability of the material; Ig, 1 are the modified cylindrical functions of the first kind, of the zero and first orders, respectively;

K -JuJ {6; 6 - specific electrical conductivity

material studied details. From the expression (1) it can be seen that

arg Э f (R) (2) Let (n) N, then by formula (1) you can find arg Э (И), and at the time of the first equality of the EMF argument of the measuring winding given

(3) (4)

(5) (6)

the value can be written

 N, from where

where uJ is the frequency at which the first stage of measurement ends. At the moment of the second equality of the EMF argument of the measuring winding, the specified value can be written

-jR uJ, 6 | fo N, A 1 from where l And

Substituting expression (6) into expression (4), we get

| U (7

The value of N is advisable to choose from the condition of the maximum sensitivity of the voltage of the measuring winding of the eddy current transducer to a change in the Generalized parameter N -JRcJf i t however, the condition should be met: V «l MHz (uJ 211J)

Usually, INi 2, ", 5,. Herewith, the value of a given value is calculated by formula (1) at kH Y.

The proposed method of measuring magnetic permeability increases the measurement measurement by decreasing

the number of parameters affecting the original result, as a result of which the measurement error, depending on the error of each parameter, decreases.

The influence on the measurement accuracy of the radius of products, on which the value of magnetic permeability depends, calculated by the compared method, is excluded. Eliminates the effect on the final result and the values of inductance and active resistance of the winding

five

to calculate the value of the magnetic

permeability known way. In this method, the value of these parameters may affect the amplitude and phase, but it is excluded by stabilizing the current in the exciting coil.

Thus, the result of the measurement does not depend on the electrophysical parameters or on the geometric dimensions of the investigated part.

Moreover, as a result of the two-stroke measurement process, the influence on the result obtained by such hardly readable factors, such as temperature, changes in supply voltage, etc., is reduced.

Claims (1)

Invention Formula A method for contactless measurement of the magnetic permeability of a conducting body, including the excitation of eddy currents in it by an alternating magnetic field created by the pass-through coil, and the determination of the parameters of its electrical circuit, so that, with the aim of increasing No accuracy, measurements are carried out in two modes, without biasing and biasing to saturate the material of the body, and in both modes, by varying the frequency of the alternating magnetic field, set the phase angle between the measuring winding voltage and op molecular voltage equal to a predetermined value calculated by the expression -.y-ig.-s „н.к.« .1 5: m |:}, where is the magnetic permeability of the vacuum; UL) is the cyclic frequency of the alternating magnetic field; Wi, is the number of measuring turns 0 five winding through coil; - magnetic field strength in the air; - radius of the measuring winding of the feed coil; - radius of the investigated part; - relative magnetic permeability of the material; - modified cylindrical functions of the first kind respectively zero and first time; N-is selected from the series 2 ... 5, fixes the obtained frequency values, and the magnetic permeability value of the body is determined as dividing the frequency value found in the saturation mode by the frequency value found without magnetization.