SU1245988A1 - Eddy-current device for non-destructive testing - Google Patents

Abstract

The invention relates to instrumentation technology and can be used for non-destructive testing of product properties by an electromagnetic method. The purpose of the invention is to increase the sensitivity in remote monitoring. The eddy current device for non-destructive testing contains a generator, a transformer-type working and compensating converter, shielded connecting cables and a unit for recording the error signal from the converters. Due to the parallel connection of the excitation windings of the converters, good agreement with the low-impedance output resistance of the generator is achieved. By connecting the measuring windings of the transducers to the RC chains, their resistances are well matched to the low-impedance input resistance of the error signal recorder. The execution of shielded connecting cables ensures good sensitivity of the device during remote control 1 sludge. (L to 4ib JV CO 00 00

Description

"

The invention relates to instrumentation and measurement techniques and can be used for not deactivating the control of product properties by an electromagnetic meter. 5

The purpose of the invention is to increase the sensitivity of remote control by eliminating the effect of shunt-connected cable capacitances on the current distribution in the windings of the transformer 10 device bodies.

The drawing shows the block diagram of the device.

The device contains a generator 1, a compensation 2 connected to it and a working 3 eddy current transducer. Compensation transducer 2 contains winding 4 forwarded, measuring winding 5, connection 20

25

One end to ground, and the other end to parallel-connected resistor 6 and capacitor 7, made adjustable. Model 8 is placed in the hole of the compensating converter.

The working converter 3 contains the excitation winding 9, the measuring winding 10, which is also connected at one end to ground, and the second to the resistor 11 and 30 connected in parallel to the capacitor 12. The controlled product 13 is placed in the opening of the converter and the excitation windings 4 and 9 are connected in parallel and through the shielded cable 14 are connected to the code of the generator 1, made with a low output impedance. The measuring windings 5 and 10 of both transducers are connected to ground at one end, and the second 40 via RC chains 6, 7 and 11, 12. and shielded cable 15 to the input of the error signal registration unit 16, made with a low input impedance.45

The output impedance of the generator 1 and the input impedance of the error signal detection unit 16 are chosen to be negligible in comparison with the capacitance resistance of the cable 14 and 15 and the complex impedance of the RC chains.

The eddy current device operates as follows.

From generator 1, alternating voltage is supplied to windings 4 and 9 of excitation of working 3 and non-compensating 2 converters. In compensation12459882

A sample product 8 is placed in the transducer 2, and a controlled product 13 is placed in the working transducer 3. The magnetic field created by the currents flowing through the excitation windings creates eddy currents in the controlled and exemplary products, which induce EMF in the measuring windings. The value of EMF depends on the conductivity of the product, and hence on the presence of defects in it. In the absence of defects in a controlled product, its electrical properties are the same as the properties of a model product, the voltages across the measuring windings of both transducers are equal in amplitude. These voltages are transformed by RC circuits 6, 7, and 11, 12 into oppositely directed currents, times 15

which are transmitted via the communication line to the input of the error signal recording unit 16. In the absence of defects, the differential current in the communication line is absent.

The presence of a defect in a controlled product leads to a change in its conductivity. In this case, the voltages at the outputs of the measuring windings differ in amplitude. A differential current appears on the communication line, which is fed to the input of the error signal registration unit 16. This unit converts the current into a voltage, which, after amplification, is applied to an indicator (not shown). The indicator can be used to judge the presence and magnitude of the defect in the controlled object.

In the eddy current device, parasitic capacitances of cables shunt the output of the alternating voltage generator 1 and the input of the error signal registration unit 16. Due to the fact that the input impedance of the error signal detection unit 16 and the output impedance of generator 1 can be made sufficiently low, the shunting effect of the parasitic capacitances is insignificant. Moreover, this influence, even if it exists, does not lead to an additional mismatch, since it equally affects the signals of both transducers. In particular, in the absence of a defect, in a monitored product, the parasitic capacitances of cables cannot affect the zero signal on

five

0 5 0 5

about

five

which are transmitted via the communication line to the input of the error signal recording unit 16. In the absence of defects, the differential current in the communication line is absent.

The presence of a defect in a controlled product leads to a change in its conductivity. In this case, the voltages at the outputs of the measuring windings differ in amplitude. A differential current appears on the communication line, which is fed to the input of the error signal registration unit 16. This unit converts the current into a voltage, which, after amplification, is applied to an indicator (not shown). The indicator can be used to judge the presence and magnitude of the defect in the controlled object.

In the eddy current device, parasitic capacitances of cables shunt the output of the alternating voltage generator 1 and the input of the error signal registration unit 16. Due to the fact that the input impedance of the error signal detection unit 16 and the output impedance of generator 1 can be made sufficiently low, the shunting effect of the parasitic capacitances is insignificant. Moreover, this influence, even if it exists, does not lead to an additional mismatch, since it equally affects the signals of both transducers. In particular, in the absence of a defect, in a monitored product, the parasitic capacitances of cables cannot affect the zero signal on

input of the error signal recorder, in the presence of a defect, the instability of the capacitance of the connecting cables can only slightly affect the signal size, which can be easily compensated by adjusting the gain during calibration of the instrument. In this connection, the length of the connecting cables can be considerable.

The eddy current device provides the possibility of eliminating the spurious mismatch signal caused by the nonidentity of the transducers (technological variation of parameters). The elements of the RC-chain of the compensation converter, which are made with variables, are used for this pin. By adjusting the variable resistor 6 and the variable capacitor 7, the indicator reading results in zero when the defect-free sections of the products are installed in both openings of the transducer. The operation is performed before the start of control, which also eliminates the parasitic signal from the temporal instability of the RC chains and transducers. Thereafter, the sensitivity of the device can be changed within the desired limits by adjusting the gain of the error detection unit.

A mismatch signal, carrying information about the presence of a defect in the monitored product, is formed at the output of the converter, therefore the influence of the cables is eliminated. Effect between

Editor N. Yatsola

Compiled by V.Filinov. -.

Tehred I.Popovich Proofreader I.Musk

3992/36

Circulation 778 Subscription

VNIIPI USSR State Committee

for inventions and discoveries 113035, Moscow, Zh-35, Raushsk yab., d. 4/5

Production and printing company, Uzhgorod, st. Project, 4

of coil capacitances is not significant, since the redistribution of currents in the excitation windings of the converters is compensated for by adjusting the elements of the 5 JS chain of the compensation converter.

Claims (1)

Invention Formula 0 Eddy current device for non-destructive testing, containing a generator, a mismatch signal recording unit, two connecting shielded cables, a working and compensation eddy current transducers, each of which) Гх consists of a parallel CS circuit, field winding and measuring winding, one ends of all windings are grounded , 0 and the other ends of the field windings. The first cable connected to the generator, characterized in that, in order to increase the sensitivity during remote control, 5 excitation windings of eddy current transducers are connected in parallel, the measuring windings of transducers are connected via RC cable with a second cable to the recording unit 0, the error signal, the output impedance of the generator and the input impedance of the registration unit for the error signal were chosen negligibly as small as compared to the capacitive impedance of the cables and the complex impedance of the RC chains, one of which is adjustable.