Abstract:
A method and apparatus for producing a color strip display of eddy current test data from a signal produced by an eddy current detector and which varies in amplitude and in phase relative to a reference signal as the detector is displaced relative to a test body. In order to produce the display, a signal representative of the relative displacement of the detector, a signal representative of the amplitude of the detector signal, and a signal representative of the relative phase of the detector signal are produced and the representative signals are supplied to a color display device for producing a strip display which extends along a path determined by the signal representative of the relative displacement of the detector, has an amplitude perpendicular to the path which is a function of the signal representative of the amplitude of the detector signal, and has a color the hue of which is a function of the signal representative of the relative phase of the detector signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and an apparatus for producing displays of several related but independently variable parameters of an occurrence. 
     The invention is particularly applicable to the evaluation of test data produced by an eddy current detector and which contains information relative to the position of the detector relative to the body of the detector, the amplitude of the detector output signal, and phase shifts between that output signal and a phase reference. Such testing is performed, for example, to detect flaws, such as cracks, in steam generator tubes. 
     Known eddy current detectors include two coils mounted in adjacent arms of a bridge. An alternating current is passed through the bridge to generate an electromagnetic field. This field will be influenced by an adjacent metal body to vary the impedance of one or both coils, thereby creating an unbalance voltage in the bridge, which voltage is the difference between the voltages across the two coils. Thus, the waveform of the unbalance voltage is constituted by a carrier wave at the alternating current frequency modulated by the difference between the impedance variations in the two coils. 
     FIG. 1 illustrates the voltages induced across the two coils of an eddy current detector when moving past a flaw in the body being tested. The respective voltage curves 25 and 26 are offset in time because they are spaced apart in the direction of movement of the eddy current detector. The curves shown represent the modulation of the carrier wave, or waves, which are not represented in FIG. 1. Therefore, the phase variations ocurring in the carrier wave, or waves, are not represented. 
     The duration of each curve 25, 26 depends on the length of the flaw in the direction of detector movement while the peak amplitude of each curve depends on the extent of the flaw. The carrier wave shift in the coil voltage is a function of the nature, and particularly the depth, of the faw. 
     FIG. 2 shows curve 28 representing the resulting bridge unbalance voltage which is the difference between curves 25 and 26 of FIG. 1. 
     Heretofore, evaluation of eddy current test data has involved generation of a strip chart constituted by the waveform of the bridge unbalance voltage amplitude, as the ordinate, with respect to displacement of the eddy current detector along a surface of the metal body being tested, as the abscissa. 
     Such evaluation further includes analysis of the phase displacement between the applied alternating current and the unbalance bridge voltage for a section of the strip chart in which the waveform has an unusual configuration. This can be achieved by obtaining representations of the unbalance voltage components which are in phase with and in quadrature, or 90° out of phase, with the alternating current applied to the bridge. 
     Then a Lissajous figure representing a plot of the in-phase amplitude vs. the quadrature phase amplitude can be generated to permit more detailed analysis of the defect which produced the associated waveform segment. For example, a Lissajous figure permits determination of the depth of a crack or of the type of defect. However, a Lissajoua figure does not contain any information relating to detector position along the test body surface. This general technique is described, for example, in U.S. Pat. No. 3,895,290. 
     Thus, the above-described procedure requires two independent displays, neither of which contains all available information relating to detector position, unbalance voltage amplitude and unbalance voltage phase. 
     It is also known to generate two strip charts each depicting the amplitude of a respective unbalance voltage component relative to detector position. In this case, phase information is still not present in a readily observable form. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to produce displays which simultaneously present information representing three related but independently variable parameters. 
     A more specific object of the invention is to generate a display which simultaneously depicts in a readily discernable manner three parameters of an eddy current detector signal. 
     The above and other objects are achieved, according to the invention, by a method for producing a color strip display of eddy current test data from a signal produced by an eddy current detector and which varies in amplitude and in phase relative to a reference signal as the detector is displaced relative to a test body, comprising: producing a signal representative of the relative displacement of the detector, a signal representative of the amplitude of the detector signal, and a signal representative of the relative phase of the detector signal; and supplying the representative signals to a color display device for producing a strip display which extends along a path determined by the signal representative of the relative displacement of the detector, has an amplitude perpendicular to the path which is a function of the signal representative of the amplitude of the detector signal, and has a color the hue of which is a function of the signal representative of the relative phase of the detector signal. 
     More generally, the objects of the invention are achieved by a method for producing a color strip display containing information relating to three related but independently variable parameters, comprising: producing signals representing successive values of each parameter with respect to a common time scale; applying the signals associated with a first one of the parameters to a color display device for producing a strip display which extends along a path corresponding to the first parameter with successive points along the path corresponding to successive values of the first parameter; applying the signals associated with a second one of the parameters to the color display device for causing the amplitude of the strip display perpendicular to the path to be a function of successive values of the second parameter at successive points along the path; and applying the signals associated with the third one of the parameters to the color display device for causing the hue of the strip display to be a function of successive values of the third parameter at successive points along the path. 
     Objects of the invention are further achieved by the provision of apparatus for producing a color strip display of eddy current test data from a signal produced by an eddy current detector and which varies in amplitude and in phase relative to a reference signal as the detector is displaced relative to a test body, comprising: means for producing a signal representative of the relative displacement of the detector, a signal representative of the amplitude of the detector signal, and a signal representative of the relative phase of the detector signal; and means for supplying the representative signals to a color display device for producing a strip display which extends along a path determined by the signal representative of the relative displacement of the detector, has an amplitude perpendicular to the path which is a function of the signal representative of the amplitude of the detector signal, and has a color the hue of which is a function of the signal representative of the relative phase of the detector signal. 
     Objects of the invention are additionally achieved by the provision of apparatus for producing a color strip display containing information relating to three related but independently variable parameters, comprising: means for producing signals representing successive values of each parameter with respect to a common time scale; means for applying the signals associated with a first one of the parameters to a color display device for producing a strip display which extends along a path corresponding to the first parameter with successive points along the path corresponding to successive values of the first parameter; means for applying the signals associated with a second one of the parameters to the color display device for causing the amplitude of the strip display perpendicular to the path to be a function of successive values of the second parameter at successive points along the path; and means for applying the signals associated with the third one of the parameters to the color display device for causing the hue of the strip display to be a function of successive values of the third parameter at successive points along the path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 and 2 are waveform diagrams which have already been described. 
     FIG. 3 is a waveform diagram illustrating a novel principle of displays according to the invention. 
     FIG. 4 is a block circuit diagram of apparatus for implementing the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When performing eddy current testing, the alternating current applied to the bridge of an eddy current detector may be a sinusoidal current at a single frequency or may contain several frequencies. By way of example, the alternating current may be composed of sinusoidal waves having frequencies, f, of 400 kHz, 100 kHz and 10 kHz so that the bridge unbalance voltage will be composed of three signals each a modulated version of the carrier wave at a respective one of these frequencies. Each frequency is useful for examining flaws at a different depth in the body being tested. Therefore, a procedure utilizing several different frequencies offers certain advantages. 
     For each carrier frequency, f, the corresponding unbalance voltage signal will be divided into a component Ex f  which is in phase with the associated carrier frequency, f, and a component Ey f  which is in quadrature with the associated carrier frequency, f. 
     In addition, to eliminate unwanted signals, it can be useful to derive inphase and quadrature components based on a mixture of the components associated with several carrier frequencies. For example, the following component signals have been found to be useful for observing certain types of flaws: 
     
         Ex.sub.mix =a·Ex.sub.400 +b·Ex.sub.100 +c·Ey.sub.400 +d·Ey.sub.100 ; 
    
     
         Ey.sub.mix =e·Ey.sub.400 +f·Ey.sub.100 +g·Ex.sub.400 +h·Ex.sub.100 ; 
    
     where Ex 400  and Ex 100  are the in-phase components of the 400 kHz and 100 kHz unbalance voltage signals, Ey 400  and Ey 100  are the quadrature components of the 400 kHz and 100 kHz unbalance voltage signals, etc. The coefficients a-h are derived statistically, according to the principles known in the art, under consideration of the particular body, the types of flaws to be observed, and the signals to be eliminated. 
     A display according to the invention can be in the form of a waveform which extends along an axis and which contains information identifying three related parameters. If the waveform axis corresponds to the length of a steam generator tube being tested, each point along the axis of the waveform corresponds to a location along the length of the tube. Thus, the location of a waveform point along the axis constitutes a first parameter. 
     The amplitude of the waveform at each point along the axis corresponds to the amplitude of the eddy current detector unbalance voltage signal associated with the corresponding point along the length of the tube and constitutes the second signal parameter. 
     The color of the waveform at each point along the axis corresponds to the phase shift experienced by the unbalance voltage signal when the detector is at the corresponding point along the tube length and constitutes the third signal parameter. 
     A display according to the invention will be derived from the value pairs: Ex 400 , Ey 400  ; or Ex 100 , Ey 100  ; or Ex 10 , Ey 10  ; or Ex mix , Ey mix . Four displays can be produced simultaneously each based on a respective value pair. When each display extends along a linear axis representing the reference for the first parameter, the four displays can be produced adjacent one another with their first parameters in mutual registry. 
     However, in contrast to the prior art in which display information is derived from the amplitudes of the unbalance voltage components, the signal values for a display according to the invention are based on the rate of change of the unbalance voltage components. 
     FIG. 3 shows a curve 30 which represents the absolute value of the rate of change, or first time derivative, of curve 28 of FIG. 2. Curve 30 is an approximation of, and corresponds conceptually to, the envelope of a display according to the invention with respect to a flaw which produced unbalance voltage curve 28. In a display according to the invention, the region 32 between the peak portion of curve 30 and the waveform base line, or axis, will be filled with a color field the hue of which corresponds to the depth of the associated flaw. 
     Utilization of rate-of change values offers the advantage of eliminating data falsifications which could result from voltage fluctuations. For example, even if curve 28 of FIG. 2 should shift relative to its voltage reference level, this would not affect the corresponding rate-of change values. 
     One embodiment of a system for implementing the invention is shown in FIG. 4. An eddy current detector 1, which can be constructed according to conventional practice and which includes two coils connected in a bridge circuit, is mounted to be introduced into a tube 2 to be tested, and to be displaced along the tube axis. The supply of operating power to detector 1 is not shown. 
     Signals are conducted from detector 1 to a signal processing circuit 3 via conductors 13 and 14. One of these conductors carries the bridge unbalance voltage signal and the other conductor carries a signal corresponding to the alternating current and constituting the phase reference for deriving in-phase and quadrature components of the bridge unbalance voltage signal associated with each alternating current frequency. 
     In circuit 3, which can be constructed according to known principles, the unbalance voltage signals are separated according to carrier frequency and each signal is then divided, for example, by correlation or mixing, into its E x  and E y  components, as shown at the outputs of circuit 3. 
     The components associated with the frequencies of 400 and 100 kHz are conducted to a combining circuit 5, also constructed according to known principles, in which the components Ex mix  and Ey mix  are formed. 
     All Ex and Ey components are conducted to a sampling circuit 7 which samples each component at a sufficiently high rate to preserve the information contained therein and then supplies the successive sample values of each component to a difference circuit 9 where the difference between successive sample values of each component is generated. The broken lines in block 7 and 9 indicate that each component remains in a separate signal processing channel. Circuits 7 and 9 can be constructed according to known principles. 
     The successive difference values, ΔEx and ΔEy, derived for each component pair are supplied to a respective phase function circuit 11 and amplitude function circuit 12. Only the function circuits associated with ΔEx mix  and ΔEy mix  are shown, for ease of understanding. In the complete system, similar function circuits will be provided for each component pair. 
     Circuit 11 generates successive signal values which are a function of φ=arc tan (ΔEy mix  /ΔEx mix ) for each pair of difference values appearing simultaneously at the outputs of circuit 9. These signal values are supplied to a color control input of a color graphics display unit 15 having a screen 17 to control the hue of an associated trace 18 constituting the waveform associated with Ey mix  and Ex mix . 
     The signal produced by circuit 11 is selected to vary the color content of the display in the region of each peak of the associated trace over a range extending, for example, from yellow-green through yellow and orange to red, the yellow-green end of the range corresponding to a minimum value of phase and, for example, a crack depth of less than 20% of the tube wall thickness, the red end corresponding to a maximum value of φ and, for example, a crack extending completely through the tube wall. In general, φ will vary as a function of the depth of a flaw, such as a crack. 
     Circuit 12 generates signal values which are a function of the amplitude of the mixture signal. Specifically, the signal values produced by circuit 12 are proportional to |√(αEx mix ) 2  +(ΔEy mix )Hu 2|. These signal values are supplied to a deflection input of unit 15 to control the horizontal deflection of the trace in a single direction from its axis, or base line. In general, the amplitude value produced by circuit 12 will be representative of the extent of a flaw, such as a crack. 
     In the illustrated arrangement, the axis, or base line, of each trace 18 is vertical and each point along the axis corresponds to a respective position of detector 1 along the axis of tube 2. In order to control the vertical deflection of each trace 18, i.e., its displacement along the vertical axis, unit 15 is provided with a suitable vertical deflection signal via a conductor 20 connected to an input contact 21. This signal is correlated with the displacement of detector 1. 
     Circuits 11 and 12 perform relatively simple functions and could be constructed according to known principles. 
     Unit 15 could also be of the type having only one color control input and one horizontal deflection input. In this case, as is well known, each circuit 11 and each circuit 12, there being four of each in the complete system, could be connected to a multiplexer which supplies a signal value from each circuit in turn to the associated terminal input. An incremental horizontal deflection voltage would be added to each successive amplitude signal to associate the resulting deflection signal with its corresponding trace. 
     In the resulting display, each trace 18 will have, at each point along its axis, an amplitude representative of the rate of change of the associated unbalance voltage signal, or the corresponding rate of change of the mixture signal, and each peak region of the trace will have a color whose hue is representative of the ratio of the incremental change in the quadrature component to the incremental change in the in-phase component of the unbalance voltage signal. 
     Since the trace extends along an axis correlated with the position of detector 1 along the axis of tube 2, trace 18 will additionally provide information as to the location, length and type of flaw being detected. 
     Normally, the test data values produced by circuits 11 and 12 will be recorded for study at a later time. For this purpose, the successive values of f(φ) and f(Δ) can be recorded in digital form. One system which has been used to produce a display according to the invention from such recorded signals was composed of a Data General MV Series Computer, a Data General Dasher D 200 terminal including an operating keyboard and monitor and a Raster Technologies Model 120 or 125 color graphics display system. A suitable program for producing a display according to the invention, consisting of four parallel traces, appears at the end of the specification. This program is written in Fortran 77. 
     The display is generated so that the waveform produced by the amplitude signal forms an envelope which is filled in with the appropriate color, i.e., a color field extends from the amplitude curve to the base line. This facilitates observation of the color information. 
     The display according to the invention can also be generated in the form of a color print-out. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims. ##SPC1##