GB2041535A - A measuring and/or testing device - Google Patents

Abstract

A device for testing and/or measuring electrically conductive material comprises means 1 for generating at least two frequencies or frequency components omega 1- omega n which via a transmitter 2 induce a magnetic field in the material, and a control servo 5 for compensation of signals of the frequencies or frequency components derived from the transmitter 2. The compensation is such that an error signal resulting from a varying parameter such as varying lift-off, temperature or dimension is generally suppressed. This can be achieved by suppression of relatively slow signal variations but not the more rapid signal variations arising from the changes or deviations to be measured. <IMAGE>

Description

SPECIFICATION A measuring and/or testing device The present invention refers to a device for testing and/or measuring electrically conducting materials, for example bodies, powders, liquids etc. with respect to changes/deviations such as, for example, defects, changes in materials, changes of the electrical properties etc., said device including at least one generator for generating at least one frequency which is adapted, via at least one transmitter, for example coil, to create magnetic fields in the test body, and at least one control circuit for automatic compensation (for example balancing) of at least one signal and/or single component recieved, directly or indirectly, from/via transmitters. The invention also includes external influence which causes changes/deviation.

There exist prior devices of a similar type for testing, where, for example, eddy currents are induced in the test object, changes/deviations being indicated in the form of changes of the width of the eddy currents which can be detected with certain limitations. The limitations are often caused by the fact that undesired signals disturb (mask) the signal desired to be detected. An example of such a disturbing signal is the signal which is caused directly or indirectly by the varying position of the transmitter in relation to the electrically conducting material (for example so-called liftoff).

In literature there are mentioned inductive type devices, but these differ in principle from the present invention.

In these prior devices active filters are often used to suppress undesired signals. Owing to limitations in the Q factor of these filters it is, however, difficult to suppress these undesired signals efficiently when they appear with a great amplitude. Also, great position signals, for example, require great dynamics (a great operating range).

If by force one has to work with too large a proportion of undesired signals in the signal processing that will also influence the result of measurements in an unfavourable direction.

The invention aims at a solution of these and other problems associated therewith, and the device according to the invention is characterized by the fact that the control rate of at least one control circuit, for example control servo, is adapted so that relatively slow signals/signal variations are suppressed, wholly or partly, while more rapid signals/signal variations arising from changes/deviations do not have time to be suppressed to the same extent, whereby the change/deviation can be more easily/better detected/ indicated and/or measured.

The device includes at least one control circuit for automatic compensation, for exam ple balancing, of at least one signal and/or signal component received, directly or indirectly, from/via transmitters.

A simple variant of the device according to the present invention, useful in practice as shown by tests, consists, inter alia, of a surface transmitter which moves along the surface of a plate slab, the distance between the transmitter and the slab surface varying owing to irregularities in the siab surface. This variation in position (distance) gives rise to great disturbing (undesired) signals. These signals are often intrinsically of relatively low frequency. On the other hand, when the surface transmitter passes a crack (defect) the crack gives rise to a signal of relatively high frequency compared to the distance signal. This is true provided that the dimension of the transmittter as seen in relation to the defect is relatively small/limited, which most often is the case.By compensating the transmitter signal at a relatively early stage of the following signal processing via an automatic process having a relatively long so-called time constant (for example control servo) a suppression of the undesired signal or signals is obtained while the signal or signals arising from the defect/crack is/are relatively uninfluenced and thus may be detected/indicated.

As several frequencies often are used in this type of device as is stated for example in the Swedish patent No. 7507857-6 it is often advantageous automatically to compensate the undesired signals selectively for the respective frequency. Then this requires that the control circuit of the respective frequency has the same time constant (control speed), since otherwise a so-called dynamic fault may arise owing to bad follow-ability. Matched control speed also refers to frequency tuned control circuits, i.e. that the control circuits, for example, emphasize or attenuate certain frequencies.

An embodiment of the invention will be described below in connection with the drawings wherein Fig. 1 is a block diagram of the deive in accordance with the present invention, Fig. 2 illustrates the relative positions of a transmitter and a surface to be tested as well as voltage diagrams showing the uncompensated (D) and compensated (E) transmitter signal respectively and Fig. 3 is a block diagram of the control circuit 5 shown in Fig. 1.

In practice it is often advantageous to carry out the compensation before the signal arising from the transmitter has been detected (for example rectified), that is as seen from Fig. 1 where the invention is exemplified more in detail. As seen from the text the invention appears best to advantage when relative movement occurs between the transmitter and the material to be tested and/or measured.

This relative movement is often a reality in the practical case but can also be generated artificially if required for example by letting the transmitter move forwards and backwards at a certain speed.

The invention is exemplified in Fig. 1 where a surface transmitter (2) is supplied with current of certain frequency contents from the transmitter feeder (1). The secondary voltage from the transmitter is supplied via the summation point (3) to the following amplifier (4) inclusive of possible filters etc. The output signal from amplifier 4 then forms the input signal to the control circuit (5) the output signal of which tends, via the summation point (3), to compensate away the secondary voltage/current from the secondary winding of the transmitter. The transmitter signal thus compensated and amplified via the relatively slow control process including the influence by the material to be tested is then detected via, for example, phase controlled detectors (6).

In principle it may be argued that in this way a selective circuit has been obtained nearest to be compared with a complicated filter but with special properties in respect of dynamics etc. Another tangible advantage is that phase discrimination, for example as described in the Swedish patent application No.

7613708-2, can be more easily carried out when so-called lift-off vectors are suppressed before the discrimination which considerably improves the result of measurement. An additional advantage is that slow temperature drifts etc. often can be entirely suppressed/ eliminated since these intrinsically are of very low frequency.

The invention may advantageously be used in those cases where several frequencies are used, for example when it is desired to evaluate crack depths etc. Here, for example, one may mix several frquencies simultaneously and then at the signal processing via filters separate the respective frequency and thereupon compensate each frequency individually, alternatively use a multiplex process if that is considered advantageous. Computers inclusive of suitable adapters may of course replace a conventional analogous circuit design when it comes to realizing devices according to the present invention.

By phase controlling the control circuit, for example, one may selectively compensate a certain variable, for example lift-off vector, which is advantageous if it is desired to disturb other signals/vectors very little.

In practice it may also be suitable on certain conditions to compensate, for example when a predetermined limit has been exceeded in respect of, for example, the degree of modulation. Typical examples of application where the invention may be used are in the case of testing of blanks or specimens in respect of cracks and other defects, testing of bars, tubes etc. In these examples of application both through-transmitters and surface transmitters may be used depending upon the test material or test specimen.

The control circuit included in the device many times is intrinsically complex and then includes, for example, phase controlled rectifiers, integrators, variable resistors etc.

The control circuit often generates one or more vectors, for example directed opposite to the transmitter signal, whereby compensation is obtained via the compensation or sum may tion point 3 in Fig. 1.

When the expression transmitter signal is used in this specification it is of course understood that therein is included influencing the test material via the electrical coupling between transmitter and test material.

Fig. 2 shows a surface transmitter (A) which moves across a surface (B) in which there is a crack (C) which it is desired to indicate and the non-compensated transmitter signal (D) as well as the compensated transmitter signal in respect of the lift-off dependence. The lift-off signal is, as will be seen, of considerably lower frequency than the fault signal. In a conventional device the lift-off signal would probably have over-modulated the input amplifiers which does not occur in the device here described owing to the compensating/ balancing process. The compensated transmitter signal is preferably applied to an active filter at the signal processing, tuned to the actual fault signal frequency so as to improve the result of mseasurement still further.

To sum up it may be said that the present invention supplements the Swedish patent No. 7507857-6 and the Swedish patent application 7613708-2 in those cases where relative movement occurs between the transmitter and the material to be tested and/or measured. Naturally, the invention may be varied in many ways within the scope of the following claims.

Since it is desired in an ever increasing number of cases to carry out some form of signal interpretation it is becoming more and more common in connection with non-destructive material testing to operate with more than one frequency, for example to estimate the depth of cracks etc. This is the reason why this invention in the first place has been directed to the use of several frequencies, i.e.

the control circuit includes compensating/balancing more than one frequency simultaneously, which often involves several control functions independent of each other. Then, again, it is important to point out that followability of the operation of the control functions is often required as well as often synchronism, so that dynamic faults may not arise owing, for example, to some control function lagging behind for example in the case of rapid prn esses. There exist prior devices in which compensation, for example balancing, of a bridge circuit is carried out for one frequency via slow mechanical control arrangements. Here, however, the purpose is not to suppress unde sired signals as for example lift-off signals but only to occupy a static position necessary for the measurement.The problem becomes more complex in the present invention owing to the fact that an optimum control rate (in relation to the relative movement of the transmitter) is required and that several frequencies can be compensated simultaneously by the control circuits possessing follow-ability (in respect of rate of operation, for example) between themselves.

In the prior devices the compensation most often takes place so that the transmitter is connected in some form of bridge circuit and that the control circuit actuates some bridge arm whereby balance is obtained. A more elegant way is, of course, to generate a desired compensation vector via at least two separate phases (e.g. 0 degrees and 90 degrees, respectively) as is seen from Fig. 1. If the signal processing is made frequency selective it is then easy to multiply this vector generation so that each (carrier) frequency will gets its own compensation vector.

The compensated signal (E) in Fig. 2 clearly shows the signal arising from the crack. Normally, this signal would have the shape of a half-cycle but owing to the fact that the control circuit also tries to attenuate this pulse the pulse shape is transformed to become a full cycle or something similar. This is advantage, since the following filter may then be given a higher Q factor whereby the signal arising from the crack can be emphasized still better.

The formulation "at least one signal obtained from/via a transmitter, directly or indirectly" also includes signal combinations, i.e.

composite signals as appear for example in the case of Fourier transformation where accordingly the whole signal combination is compensated. This compensation most often takes place further back in the signal processing and often after detection of the signals taking place. Examples of transformation appear from the Swedish patent application No.

76.1 3708-2.

In those cases where vector transformation is used (for example Libby's method or as appears from the Swedish patent application No. 76.13708-2) for suppressing undesired signals the present invention represents a good complement, for example by the feature that the lift-off vector via the compensation is made a minimum from the very beginning, i.e. there are made less demands on the transformation, whereby the result is improved.

For example, one may combine vectors, or functions thereof, resulting from at least two frequencies after phase-selective detection for obtaining maximum sensitivity to defects, for example, and then via the control function compensate the whole function complex in respect of the lift-off dependence.

In the term vector transformation all principles are included which utilize information resulting from at least two frequencies/frequency components for testing and/or measurement.

With vector transformation a method or a device for error detection is contemplated wherein more than one frequency (or frequency component) is used and wherein phase sensitive detection (phase discrimination) of any kind, e.g. via phase sensitive rectifiers, is performed. Examples of this are to be found in H. L. Libby, "Multiparameter Eddy Current Concepts" and in Swedish patent 7507857-6 and Swedish patent Application 7613708-2. In said Libby reference there is a description of a multiparameter eddy current tester which is a typical example of what in this case is meant by vector transformation. This known method ip based upon two fixed working points in the impedance diagram of the transmitter.If combined with an automatic compensation in accordance with the present invention movable or dynamic working points will be achieved which is more advantageous in comparison with the static or fixed compensation procedure described in said reference.

In this specification the expression followability relates to control sequencies or control characteristics which for each individual frequency do not bring about any substantial dynamical measurement errors, for example in connection with compensation of a varying lift-off. With follow-ability also the follow-capability achieved due to quick control actions is contemplated.

The control circuit 5 of Fig. 1 may be designed as indicated in Fig. 3 wherein the circuit for sake of simplicity has been shown to work with a number of frequencies, namely only two frequencies , and so2 In frequency separators 7 and 8 the frequencies , and w, respectively are extracted. At voltage dividers 9 and 10 respectively the loop gain is set.

Blocks 11-14 each comprise a phase sensitive or phase controlled rectifier the control signals F, and F2 and which are taken from the transmitter feeder 1 of Fig. 1. Reference designations 15-18 refer to integrators the time constants of which are , and T2 respectively. Reference designations 19-22 refer to variable resistors (AGC), to which a constant signal of frequency 1 and o, respectively is supplied. The output signals from said AGC resistors 19-22 are added in a summation amplifier 23 the output signal of which will compensate the transmitter signal via summation point 3.

By selecting e.g. F1, F2 and r, and 2 and the characteristic of the control servo the requested follow up ability during compensation is attained.

Fig. 1 and Fig. 3 are only examples of several possible modifications of the invention. Several of the functions performed by the circuits shown therein may be realized with a correspondingly programmed computer.

Claims (11)

CLAIMS.

1. A device for testing and/or measuring electrically conducting material, for example bodies, powders, liquids, blanks, wires, tubes etc. with respect to changes or deviations, for example defects in or on the material, said device comprising means including at least one generator, for generating at least two frequencies or frequency components, which via at least one transmitter, for example a coil, is arranged to create a magnetic field in the material, and at least one control circuit, for example a control servo, for the compensation, for example automatic zero balancing, of at least two signals of different frequencies or frequency contents, said signals being derived directly or indirectly from the transmitter, characterised in that said compensation is carried out in such a way that any error, for example dynamic faults resulting from a varying parameter, for example varying lift-off, temperature, dimension etc. is generally suppressed, whereby the changes and/or deviations can be better tested or measured.

2. A device according to the preceding claim, characterized by the fact that the control rates of at least two control functions are adapted so that relatively slow signal variations are suppressed, wholly or partly, while more rapid variations arising from changes/ deviations have no time to be suppressed to the same extent, whereby the change/deviation can be better detected and/or measured.

3. A device according to either of the preceding claims, characterized by the fact that at least one transmitter moves in relation to the test material for example at a certain speed.

4. A device according to any of the preceding claims, characterized by the fact that the signal variations desired to be suppressed wholly or partly arise from a varying position, for example distance, of transmitter-test material.

5. A device according to any of the preceding claims, characterized by the fact that at least one control circuit includes the use of programmable electronics, for example computer.

6. A device according to any of the preceding claims, characterized by the fact that compensation takes place selectively, for example in respect of a certain variable such as, for example, lift-off.

7. A device according to any of the preceding claims, characterized by the fact that compensation takes place intermittently, for example in the presence of certain conditions.

8. A device according to any of the preceding claims, characterized by the fact that the device includes the use of so-called vector transformation.

9. A device according to any of the preceding claims, characterized by the fact that vector transformation and compensation are combined.

10. A device according to any of the preceding caims, characterized by the fact that at least one undesired signal or signal combination is not suppressed to a desired degree by the control/compensation function and that the remaining signal or signal combination is used directly or indirectly for further suppression of the undesired signal or signal combination, via vector transformation.

11. A device according to any of the preceding claims, characterized by the fact that automatic compensation of at least one signal and/or signal component received, directly or indirectly, from transmitter is carried out before detection, for example phase controlled rectification and/or vector transformation.