CA1109144A - Distance amplitude compensation system - Google Patents

Abstract

ABSTRACT

A distance amplitude compensation system is disclosed herein for comparing received signals with a reference signal having a time varying amplitude. The preferred embodiment of the invention disclosed herein is particularly adapted for use in an ultrasonic nondestructive testing system to vary the reference signal of the system as a function of time to compensate for variations of the echo signal produced by changes in the amplitude of the ultrasonic energy as it propagates through the workpiece as a result of attenu-ation or other causes. A pulser/receiver transmits ultrasonic signals into the workpiece, receives echo signals returned therefrom and produces received signals corresponding to the echo signals. A

Description

BACKGROUND

The present invention relates to a distance amplitude compensation system and more particularly to a ~system for providing a reference signal that has an amplitude that varies as a function of tLme.
The use of ultrasonic pulse echo techniques to test workpieces has been long known. Such techniques typicallY provide for the periodic generation of high frequency electric pulses and applying them to a piezoelectric element which transforms the electric vibrations into mechanical vibrations which are then lOtransmitted into the object under test. The transmitted pulses are reflected from the rear boundary of the workpiece as well as from any internal discontinuities in the workpiece such as cracks~
inclusions and flaws. The presence of such defects can therefore be determined by detecting the reception of pulses reflected by the piezoelectric element which transforms the detected mechanical pulse vibrations into electric voltage signals. The electric signals may be processed to be viewed on a cathode ray tube or other similar utilization device to determine flaws in the work-piece.
~0 The magnitude of signals detected which indicate flaws would be ordinarily a function of the size of the defects. How-ever, the signal is also a function of the distance of the defect in the workpiece below the entering surface of the object. It is therefore necessary in order to obtain visual and automatic inter-pretation of flaw size to recognize the continuous change in echo signal from a given size flaw as a ~unction of its distance from the entering surface of the workpiece under test. This change in echo signal is non-linear and usually bidirectional. Thus, the response signal increases for a short distance in the woxkPiece 3Q (near field zone), reaches a peak (near field limit), and then continues to decrease throughout the remainder of the test piece, (far field).

~~ 3~-205 This problem was recognized by ~eg~aæ~ as documented in his U.S. patent 3,033,029. This prior art patent describes a gain control system with complex wave shapes for the control voltage and discusses the result of the near field and far field effects of the beam geometry as well as the exponential attenua-tion with depth in the material. As shown in Figure 1 of the ~eg*~rt patent, not only does the quantity of returned energy B vary with distance within the test piece, but it varies non-linearly as a complex function of distance (or time). The lower Lo amplitude response for short distances is due to the near field ~ t effect. As illustrated in the ~Tieg~art patent, the signals first increase with distance below the surface and then decrease with distance below the surface.
Various prior art devices have sought to compensate for this effect by the use of distance amplitude compensation techniques, such as shown in Wicghart, which change the gain of the receiver.
Such devices have typically been referred to as distance amplitude compensation systems.
These conventional devices have sought to provide distance ~0 amplitude compensationiby changins the gain of the receiver amplifier rapidly with each sweep trace, e.g., several dB in a few microseconds. Such devices are disclosed in adar Handbook, by Skolnik, Ed., McGraw-~ill, New York, N.Y. There are several limitations and disadvantages to conventional distance a~plitude compensation systems using the time-varied gain method. Very fast gain changes cannot be made easily because transients are intro-duced in the amplifier which produce false signals. The distance amplitude compensation function cannot be abruptly terminated at the start of the back reflection, which is ordinarily very large 30 compared to flaw signals. This aggravates the problem of displaying the desired back echo on-screen when applying "back-echo gain" control. The linearity and/or dynamic range of the amplifier may be adversely affected by the distance amplitude _~ 34-205 compensation control using this time-varied-gain method.
Another problem with typical prior art systems relates to the set-up of such devices. Ordinarily, the operator must be provided with a distance amplitude response function curve which was previously obtained experimentally. Alternatively, the operator may be provided with a set of distance amplitude test blocks and be required to establish his own curve. If the effect of gain control function is not displayed on the CRT, the operator must use trial and error techniques to establish the echo signals 1~ from various depth blocks, an almost impossible task. In general, o~ly one echo at a time from each block can be displayed. Even i,f the distance amplitu~e compensation control voltage can be displayed on the CRT, various trial and error adjustments of the several distance amplitude compensation controls must be made to compensate for the near field slope, far field slope~ amplitude, and delay so that the desired gain effect may be obtained.
It can be shown that for the usual distance ampli~ude compensation method of receiver gain control, the voltage needed at the amplifier is a non-linear function of the distance amplitude ~ response curve. If curve matching is used to set up the distance amplitude compensation system, a rather elaborate and very precise e~lectronic method must be employed to present the inverse of the distance amplitude compensation control signal in both shape and absolute level referenced against the echo amplitude signals to be corrected. For curve matching, the distance amplitude compensa-tion waveshape must be displayed on an alternate sweep trace in order not to be superimposed on the regular video trace. Display svstems in which this is not done are especiallv difficult to use because there is inadequate CRT screen height to show large ~ ~ ~ e_ s~ <~
30 signals added to the distance amplitude compensation wavcshpe curve.
Various prior art devices use a flaw gate which produces an output or alarm when the echo amplitude in the gate portion of the time sweep rises above a selected level, as shown, for example, in U.S. Patent No. 2,883,860 to Henry. Such conventional devices use a rectangular gating function having a length corresponding to the material depth to be examined and a constant amplitude corres~onding to the alarm level. The gating function of such devices has essentially constant sensitivity along its length and, to achieve automatic distance amplitude compensation, the receiver amplifier g~in in such devices is controlled b~ distance amplitude compensation technigues as descri~ed above with the inherent pitfalls as set forth above.

su~r~P~Y

The present invention provides a distance amplitude compensation system for providing a comparison of a received signal ith a reference signal have a time varying amplitude. To obtain this, the present invention provides a comparator circuit whic~
compares received signals with the output waveshape of a distance amplitude compensation generator. If the amplitude of any of the received signals exceeds the time varying amplitude of the distance amplitude co~pensation generator, an alarm circuit is activated.
The comparison between the received signals and the reference signal from the distance amplitude compensation generator may 20 also be viewed on the screen of a CRT.
The present invention is particularly adapted to be used in an ultrasonic non-destructive testing system to provide an A
scan presentation on a CRT screen to indicate the depth in a workpiece at which a flaw is located as well as the amplitude of the flaw signal. The invention may alternatively be used to provide a B scan presentation on a CRT screen to indicate the ~law depth in an object as well as the flaw distribution in cross sectional view. This application of the invention is particularly significant in ~edical testing of organs as well as ~on-destructive testing of 30 inani~ate workpieces. The present invention may also be used to provide C scan presentation on a cathode ray screen to indicate _ -- 34-205 fla~, distribution in a ~70rkpiece in plan view. A gated alarm system also provides an electrical signal for an alarm to indicate a flaw in the workpiece.
A pulser provides an initial signal through a suitable transducer to a workpiece or other object to be tested and a receiver receives echo signals and amplifies them. The gain of the receiver is constant and the output signals from the receiver remain a function of attenuation within the ~70rkpiece.
~ he attenuated received signals are transmitted to a 10 comparator system. A distance amplitude compensation generator transmits a reference signal having a time varying amplitude to the comparator system and the received signals are compared with the reference signal by the comparator system. The comparator system includes alarm ~.eans for providing an alarm signal when any of the received signals exceeds the reference signal generated by the distance amplitude compensation generator.
The received signals and the reference signal are also transmitted during alternate portions of a clock pulse cycle to the screen of a cathode ray tu~e to display ~oth the received 20 signals and the reference signal. Although the received signals are displayed only during one half of the clock pulse cycle and the reference signAl have a time varying amplitude, is displayed during the other half of the cloc~ pulse cvcle, the time period between the displaying of the received signals an~ the reference signals is so short that the eye of an observer discerns a composite di`splay of both signals. The CRT thus produces a visual comparison of~ the received signals with the reference signal to determine whether the received signals exceed the reference signal thereby signaling a flaw or other discontinuity in the workpiece under test.

In the application of the present in~ention for non-destructive testing, the signal applied .o the ~orkpiece is an ultrasonic signal and the distance amplitude compensation generator is constructed using a suitable network of capacitors and resistors to provide an exponential rise corresponding to the near field and ~i 34-205 ~ &~'~

an exponential decay corresponding to the far field attenuation to provide a distance amplitude compensation reference signal.
The reference signal is applied to the comparator which includes a threshold comparator system for transmitting TTL (transistor transistor logic) pulses to a gate which in 'urn enables an alarm latch if the received signals, which correspond to the echo signals from the ultrasonic test pulse, exceed the reference signal. The amplitude of the TTL pulses are equal, and the ~idth of each pulse is determined by the width of the corresponding eeho signal~at the threshold level deter~ined by the amplitude of the distance amplitude compensation reference signal.
The received signals as well as the reference signal are also applied to a vertical analog switch which is operative to transmit the received signals as well as the reference signal, during alternate portions of a cloc~ pulse cycle, to the vertical amplifier of a suitable CRT. Unblanki~g circuit means is coupled to the CRT to enable it to display the receive signals corresponding to the echo signals from the workpiece, as ~7ell as the reference signal during alternate portions of the clock pulse cycle to 20 provide a comparison between the received signals and the reference signal to enable an observer to determine whether the workpiece has any flaw or discontinuity, which produces an ec~o signal which exceeds the threshold determined by the reference signal.
It may be seen that although the received signal does continue to be a function of the attenuation of the pulse signal in the workpiece, the received signal is compared to an alarm level in the comparator and gate circuit which varies with time as a function of the attenuation. As a result, the actuation of the alarm can be made a direct function of the size of the discontinuity in the workpiece and independent of the attenuation of the pulse signal in the workpiece.

According to a first broad aspect of the present invention, there is provided a distance amplitude compensation system for comparing signals transmitted through a workpiece with a reference signal having a time varying amplitude, said system including: receiver means for receiving said trans-mitted signals and converting them to video signals and having an output circuit, distance amplitude compensation signal generator means for generating a reference signal having a time varying amplitude for compensating for the variation of said transmitted signals in the workpiece, and having an output circuit, and comparator means having an input circuit coupled to the output circuit of said receiver means and an input circuit coupled to the output circuit of said distance amplitude compensation generator for comparing said video signals with said reference signal.
According to a second broad aspect of the present invention, there is provided a system for comparing received signals corresponding to energy received from an object with a reference signal having a time varying ampli-tude comprising: means for transmitting ultrasonic energy into the object, receiver means for receiving said ultrasonic energy after it has propagated ~hrough the object~ said receiver means being effective to produce a received signal corresponding to said energy received from the object, signal gener-~0 ator means adapted to produce a reference signal having a time varying ampli-tude corresponding to the variations in the ultrasonic energy as it propa-gates through the object, and signal comparator means coupled to said receiver means and said signal generator means for comparing the receiving and refer-ence signals with each other.
According to a third broad aspect of the present invention, there is provided a method for comparing received signals corresponding to ultra-sonic energy received from a workpiece being tested with a reference signal including the steps of: transmitting ultrasonic energy into the workpiece under test; receiving the ultrasonic energy after it has propagated through the wo~kpiece; cQnverting the received ultrasonic energy after it has prop-agated through the workpiece into an electrical signal; generating a reference -8a-signal having a time va~ying amplitude corresponding to variations in the ultrasonic energy as it propagates through the workpiece; and comparing the received electrical signal with the reference signal.

-8b-~ ..

~ 34-205 - DRAI~INGS
FIGURE 1, is a basic block diagram of the present invention.
FIGURE 2, is a detailed block diagram of the present invention.
FIGURES 3A through 3N are timing diagrams of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1 there is shown the distance amplitude compensation system of the present invention having a pulser means 11 for transmitting an electrical wavetrain to a 10 transducer 12 which generates ultrasonic signals which are transmitted to a workpiece 13. The echo signals are received by the transducer 12 and transmitted to a rèceiver means shown as receiver 14 which converts the signals to video signals.
The video signals are transmitted to a comparator and gate circuit 16 and a distance amplitude compensation signal generator means 17 transmits a reference signal having a time varying amplitude to the comparator and gate circuit 16. If the received signals, which correspond to echo signals from the work-piece 13, exceed the time varying amplitude of the reference signal 20 generated by the distance amplitude compensation generator 17, an alarm signal is generated by the alarm output of tne comparator and gate circuit 16. The output of the distance amplitude compen-sation generator 17 and the output of the receiver 14 are also coupled to vertical analog switch means 63 which is clocked to transmit the reference signal and the received signals during different portions of the clock pulse cycle to a suitable CRT 18 for dis~laying the received signals as well as the reference signals on the screen of the CRT.

~4-;~U~
!''~

Although the present invention may be utllized in a wide variety of applications, it is particularly adapted to be embodied in an ultrasonic nondestructive testing system for inspecting the internal structures of various types of workpieces 13. The pulses from the pulser 1~ shown in Figure 3B, are acousti-cally coupled to the workpiece 13 by a suitable transducer 12 which preferably includes a piezoelectric crystal which produces acous-tical signals which are transmitted through the front surface 19 of the workpiece and are propagated through the workpiece. Echoes 10 Of the ultrasonic pulses are reflected from any discontinuities in the workpiece 13 such as holes, openings, cracks, inclusions, fissures and flaws. In addition, if any of the energy reaches the rear surface 21 of the workpiece, it is reflected. At least a portion of the propagated signals are returned as echo signals to the transducer 12 and are amplified by the receiver 14 which includes a constant gain a~plifier.
The receiver 14 has a constant gain and provides an output of video signals as shown in Figure 3~ which correspond to the echo signals received by the transducer 12. As shown in Figure 20 3E the echo signals normally include a pulse 30 corresponding to the front surface 19 of the workpiece 13. This is followed at some later point of time by another pulse 40 which corresponds to the back surface 21 of the work~iece 13. The time delay between these two pulses 30 and 40 corresponds to the time required for the ultra-sonic energy generated by the transducer 12 to propagate from the -~\
front surface 19 through the workpiece 13 and to the rear surface and return to the front surface 19.
If there are one or more discontinuities inside the workpiece 13, there would be a corresponding number of pulses 32, 3~, 36 and 38 as shown in Figure 3E between the front pulse 30 and the rear pulse 40. The time delay for the individual pulses is the function of the distance to the reflecting discontinuity.

The magnitudes of each of the echo pulses and, correspond-ingly each video pulse 30 through 40, is a function of the appare~t si'ze of the associated reflecting discontinuities. However, the magnitude of the received pulse is also a function of whether the dïscontinuity is in the near field zone or the far field zone.
In the near field zone, the ultrasonic energy tends to converge toward a focal point. In the far field zone beyond the focal point, the energy tends to diverge. As a result, if a given size discontinuity is situated in the near field zone, as it recedes ~0 from the transducer 12 toward the focal point, the energy tends to concentrate and therefore the pulse tends to increase. As the same discontinuity recedes beyond the focal point, the energy diverges and the pulse decreases. In this zone, the pulse tends to decrease as an exponential function of time so that the pulse is a function of the distance of the discontinuity in the workpiece from the transducer.
As shown in Figure 2, the entire distance amplitude compensation system is synchronized by clock pulses generated by a clock pulse generator 50 which produces clock pulses as shown in -20 Figure 3A. By way of example, the frequency of the clock pulse generator may vary between 100 hertz to 10 kilohertz. However, for most applications, the clock pulse generator S0 will operate at a frequency in the order of 1 kilohertz.
The clock pulse is transmitted to the pulser 11 which generates a pulse shown in Figure 3B which is transmitted to the transducer 12 to convert the pulse to ultrasonic energy which is propagated into the workpiece 13 as described above. The echo pulses received by the transducer 12 are fed to the receiver 14 which includes a constant gain amplifier circuit and is operative 30 to convert the received echo pulses to video signals as shown in Figure 3E. The clock pulse A is also transmitted to the distance amplitude compensation waveform generator 17 as shown in Figure 2.

' 34~205 '~LP'fi~

The distance amplitude compensation generator provides a waveform as shown in Figure 3F which is shaped to accommodate the change in amplitude of the echo signal in the workpiece due to the near field zone effect and the far field zone effect. The wave shape shown in Figure 3F may be generated by charging and discharging capacitors through adjustable networks to provide the exponential rise and decay. This signal may be provided by circuits such as that shown in U.S. Patent No. 3,033,029 to Weighart.
The clock pulse generator 50 also couples a clock pulse to 10 unblanking generator means shown as a sweep and unblanking generator 51 as shown in Figure 2. The sweep and unblanking generator 51 is a standard sweep generator and provides a saw-tooth wave as shown in Figure 3C to a horizontal amplifier 52 which is coupled to the horizontal control plates of a standard CRT 18. A second output of the sweep and unblanking generator 51 provides a rectangular pùlse signal as shown in Figure 3D which is fed to unblanking ; switch means shown as an unblanking analog switch 53. The unblan~ing analog switch 53 is in turn coupled to an unblanking amplifier 54 which controls the intensity modulation of CRT 18 to enable it when 20 the unblanking amplifier is activated.
Thus, the sweep and unblanking generator 51 is operati~e to generate a sweep signal as shown in Figure 3C to control the horizontal trace of the CRT 18 for the time duration of the sweep signal. The rectangular pulse generated by the sweep and unblanking generator 51 as shown in Figure 3D effectively turns on the CRT to enable the trace to appear for the duration of the unblanking pulse shown in Figure 3D.
The output of the distance amplitude compensation generator 17 is fed to an alar~ level control 56 which is a level shifter 30 for shifting the DC level of the waveform shown in Figure 3F.

~' ; 34-205 The shifted waveform is applied to one input of a threshold comparator means 57. The video signals sho-~n in Figure 3E
and received from the receiver 14 are fed to a second input of the threshold comparator 57. The threshold comparator 57 is a compara-tor circuit which generates pulses which have a fixed amplitude but a width and position which correspond to the video signals 3E
that exceed the threshold determined by the output waveform 3F that is-fed to the alarm threshold comparator 57.
The ou~put pulses from the threshold comparator 57, 1~ shown in Figure 3K are referred to as TTL (transistor transistor logic) video pulses and are fed to one input of an AND gate 58.
The other input is from gate pulse generating means 61 for generating a gate pulse commencing at a predetermined point of time and having a predetermined time duration. Such devices are well known in the art and may be formed of suitable gate having delays for initiating a gate pulse at a predetermined point of time after the beginning of the clock pulse and terminating the gate pulse at a subsequent predetermined point of time. The gate pulse generating means 61 receives the clock pulse from the clock pulse generator 50 and 20 d~elays the initiation of a gate pulse for a selectable predetermined period of time. The output of the gate pulse generating means 61 provides an unblanking pulse shown in Figure 3G for a predetermined period of time. The gate pulse generating circuit 61 includes suitable delay circuitry and the duration of the delay may be controlled by any standard control mechanism.
The output rectangular pulse from gate pulse generating means 61, shown in Figure 3G is also fed to the second input of AND
gate 58 to thereby enable the AND gate 58 to pass the TTL pulses, shown in Figure 3Y~ during the duration of the pulse shown in Figure 3G.
30 The output of AND gate 58 is therefore a train of TTL pulses shown in Figure 3L which is fed through an alarm latch 62 which may be . 34-205 coupled to any suitable alarm signaling device to provide an alarm output signal. The alarm output signal may be in the ~orm of an audible signal for a predetermined period of time which audibly informs the operator that a flaw or discontinuity has been detected. Alternatively, the alarm output signal may be coupled to any mechanical or electronic utilization device to record the detection of the flaw or discontinuity.
~ he output of the alarm level control 5~ also feeds the output waveform from the distance amplitude compensation generator, ~ s~own in Figure 3F to vertical switch means shown as a vertical analog switch 63. The output ~f the receiver 14 is also fed to a second input of the vertical analog switch 63 and the output of the vertical analog switch 63 produces video signals during the main half of the clock pulse cycle and a distance amplitude compen-sation waveform during the alternative half of the clock pulse cycle as shown in Figure 3H. The vertical analog switch 63 as well as the unblanking analog switch 53 preserve the amplitude of the input signal and permit the transmission thereof when activated.
The analog switches are known in the art and may be formed, for ~0 example, of a suitable configuration of field effect transistors CFET ' s ) .
The output of the vertical analog switch 63 shown in Figure 3H is fed to a vertical amplifier 64 which controls the vertical deflection of the trace on the CRT 18. As indicated above, the horizontal deflection is obtained by the horizontal amplifier 52 ..
which in turn, is controlled by the swee? and unblanking generator 51 to produce the signal shown in Figure 3C. The C~T 18 is activated by the unblanking amplifier 54 which, in turn, is controlled by ; the unblanking analog switch 53 for a period of time as shown in ~0 the waveform of Figure 3I. The CRT 18 ,herefore displays both the video signals as well as the distance amplitude compensation waveform to provide a visual comparison between the echo signals received by the receiver 14 with the distance amplitude compensation reference signal.
The width of the distance amplitude compensation waveform displayed on the screen of the CRT 18 is therefore controlled by the width of the pulse generated by the gate pulse generator 61 during the alternate half of the clock pulse generator as shown in Figure 3G. The width of the pulse shown in Figure 3G during the alternate half of the clock pulse cycle corresponds to the locations of the gated area with respect to the distance between the front and rear surfaces of the workpiece. Thus the width of the distance amplitude compensation waveform also corre-sponds to the depth of the gated portion of the workpiece.
The video signals received by the receiver 14, which correspond to the echo pulses received by the transducer 12, are displayed on the screen of the CRT 18 during the main portion of the clock pulse cycle shown in Figure 3A. The distance amplitude compensation waveform, shown in ~igure 3F and generated by the distance amplitude compensation genera~or 17 is displayed on the screen of the CRT 18 during the alternate half of the clock pulse cycle shown in Figure 3A. This alternate display on the video signals during the first, or main, half of the clock pulse cycle and the distance amplitude compensation waveform displayed during the second, or alternate half of the clock pulse cycle occurs so rapidly that the eye of a human viewer discerns a composite picture ; of the video signals and the distance amplitude compensation wave-form as shown in Figure 3N. When the video signal shown in Figure 3N exceeds the distance amplitude compensation waveform, the viewer ` is visually informed that a flaw or discontinuity has been detected in the object, such as workpiece 13, under test.

_ 34-205 I

- The clock pulse generator provides clock pulses having main and alternate portions each having opposite states as shown in Figure 3A. During the main half of the clock pulse cycle shown in Figure 3A, the pùlser 11 generates a pulse which, in the preferred embodiment, is in the order of 1 microsecond in duration, and the sweep and unblanking generator 51 generates a sweep signal shown in Figure 3C and a main unblanking rectangular wave, 3D, during the alternate half of the clock pulse cycle in the same manner that these pulses are generated during the main half of the clock pulse cycle.
10 Since the pulse signal shown in Figure 3B is generated at the beginnnng of both the main and alternate halves of the clock pulse cy¢le, the echo signals are received and converted to video signals shown in Figure 3E during both halves of the clock pulse cycle.
The distance amplitude compensation waveform shown in Figure 3F and generated, as described above, by the distance amplitude compensation generator 17 is also produced at the beginning of both the main and alternate halves of the clock pulse cycle.
The vertical analog switch 63 transmits the video pulses ~ during the main half of the clock pulse c~cle and the distance amplitude compensation waveform during the alternate half of the clock pulse cycle as shown in Figure 3H. This is achieved by the transmission of the signal on the video input shown in Figure 3E
during the main half of the clock pulse cycle and the blocking off of the channel from the distance amplitude compensation generator to prevent the waveform shown in Figure 3F to be transmitted during the main half of the clock pulse cycle. Correspondingly, during the alternate half of the clock pulse cycle, the video input, Figure 3E, is not transmitted and the distance amplitude compensa-30 tion generator waveform, Figure 3F, is transmitted.

The clock pulse signal, Figure 3A, is also supplied to the unblanking analog switch 53 and during the main half of the clock pulse cycle, output of the main unblanking input terminal is transmitted to provide an output signal of the unblanking analog switch 53 shown in Figure 3I having a time duration determined by the time duration of the main unblanking pulse shown in Figure 3D
during the main half of the clock pulse cycle. The input signal to the unblanking analog switch 53 from the gate pulse generator 61 - is not transmitted during this main half of the clock pulse cycle.
10 Correspondingly, during the alternate half of the clock pulse cycle, the signal from the sweep and unblanking generator 51 having a waveform shown in Figure 3D, is not transmitted, and the input from the gate pulse generator 61 having a waveform shown in Figure 3G
i$ transmitted. Thus, the waveform shown in Figure 3I is produced.
The alternate transmission of the two inputs of the unblanking analog switch 53 and the vertical analog switch 63 is achieved by means well known in the art. It may be achieved, for example, by coupling two FET's each to one of the circuit inputs and coupling their outputs together. The clock pulse generator is 20 coupled to each of the two FET's in such a manner so as to transmit the input to one during the main half of the clock pulse cycle and transmit the signal to the second FET during the alternate half of the clock pulse cycle.
Since, as indicated above, the CRT 18 is enabled for the duration of the pulses shown in Figure 3I, during the first half of the clock pulse cycle, the CRT displays the video signals received during the time duration of the main unblanking pulse of Figure 3D, shown during the main half of the clock pulse cycle of Figure 3I.

During the alternate half of the clock pulse cycle, the CRT displays the distance amplitude compensation waveform generated during the alternate half of the clock pulse, shown in Figure 3G, for the time duration of the unblanking pulse generated by the gate pulse generating means 61 as shown in Figure 3I. As a result, the CRT
screen 18 produces the signals shown in Figure 3J with the video signals displayed during the main half of the clock pulse cycle and the distance amplitude compensation waveform displayed during the alternate half of the clock pulse cycle. The width of the distance amplitude compensation waveform displayed on the screen of the CRT 18 is ~herefore controlled by the width of the gate pulse generator so that the width of the display waveform corresponds to the location of the gated area with respect to the top and bottom surfaces 18 of the workpiece. As indicated above, since the time duration between the main and the alter-nate half of the clock pulses are relatively small, the human eye discerns a camposite waveform of video signals and a dis-tance amplitude compensation waveform such as that shown in Figure 3N.
Thus, the present invention provides circuit means having an alarm threshold which can be varied in time in a selec-table manner for providing an alarm signal when the echo signals exceed the alarm threshold. The AND gate 58 enables the opera-tions of the circuit means for predetermined time durations shown in Figure 3G. It is seen that although the AND gate 58 is shown coupled between the threshold comparator 57 and the alarm latch 62 in the preferred embodiment, the AND gate may be alternatively coupled between the receiver 14 and the thresh-old comparator 57.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings.
It is therefore to be understood that within the scope of the appended claims, the invention can be practiced otherwise than as specifically dèscribed.

.... ..

Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A distance amplitude compensation system for comparing signals transmitted through a workpiece with a reference signal having a time varying amplitude, said system including: receiver means for receiving said transmitted signals and converting them to video signals and having an output circuit, distance amplitude compensation signal generator means for generating a reference signal having a time varying amplitude for compensat-ing for the variation of said transmitted signals in the workpiece, and having an output circuit, and comparator means having an input circuit coupled to the output circuit of said receiver means and an input circuit coupled to the output circuit of said distance amplitude compensation generator for comparing said video signals with said reference signal.

2, The system as defined in claim 1 and wherein said comparator means includes alarm threshold comparator means for producing a train of pulses each of which corresponds to any of said video signals which has an amplitude which exceeds the threshold of said reference signal, each pulse of said train of pulses being equal in amplitude and having a width which varies in proportion to the width of the associated video signal at the threshold amplitude level of said reference signal, said alarm threshold comparator means having an output circuit.

3. The system defined in claim 2 and further including alarm output signalling means having an input circuit coupled to the output circuit of said alarm threshold comparator means for providing an alarm output signal in response to said train of pulses.

4. The system as defined in claim 2 and further including: gate pulse generating means for generating a gate pulse commencing at a predetermined point of time and having a predetermined time duration, said gate pulse generating means having an output circuit, AND gate means having an output circuit, a first input circuit coupled to the output circuit of said gate pulse generator means and a second input circuit coupled to the output circuit of said comparator means, and alarm latch means having an input circuit coupled to the output circuit of said AND gate means for providing an alarm output signal, whereby the transmission of said train of pulses to said AND
gate during the application of said gate pulse energizes said alarm latch means to provide the alarm output signal.

5. The system as defined in claim 1 and further including a utilization device having an input circuit coupled to the output circuits of said receiver means and said distance amplitude compensation signal generator means for providing a visual display of the comparison of said video signals with said signal having a time varying amplitude.

6. The system as defined in claim 5 and wherein said utilization de-vice including a cathode ray tube having a screen and a control circuit, and vertical switch means having a first input circuit coupled to the output cir-cuit of said distance amplitude compensation generator means and a second input circuit coupled to the output circuit of said receiver means, and having an output circuit coupled to the control circuit of said cathode ray tube to control the vertical trace on the screen to transmit said video signals and said reference signal to said cathode ray tube to be displayed on the screen.

7. The system as defined in claim 6 and further including unblanking switch means coupled to said cathode ray tube for unblanking said cathode ray tube for predetermined time periods to display said video signals and said reference signal.

8. The system as defined in claim 7 and further including gate pulse generating means coupled to the input circuit of said unblanking switch means for generating a gate pulse commencing at a predetermined point of time and having a predetermined period of time for controlling said unblanking switch means for unblanking said cathode ray tube and displaying said reference signal for said predetermined period of time.

9. The system as defined in claim 6 and further including; clock pulse generator means coupled to said vertical switch means and said unblank-ing switch means, said clock pulse generator means being operative to gener-ate a train of clock pulses, each of which has a main portion having one polarity and an alternate portion having a second polarity, whereby said vertical switch is energized to transmit said video signals to said cathode ray tube during said main portion of said clock pulse and to transmit said reference signal having a time varying amplitude to said cathode ray tube during said alternate portion of said clock pulse.

10. A system for comparing received signals corresponding to energy received from an object with a reference signal having a time varying ampli-tude comprising: means for transmitting ultrasonic energy into the object, receiver means for receiving said ultrasonic energy after it has propagated through the object, said receiver means being effective to produce a received signal corresponding to said energy received from the object, signal gener-ator means adapted to produce a reference signal having a time varying ampli-tude corresponding to the variations in the ultrasonic energy as it propagates through the object, and signal comparator means coupled to said receiver means and said signal generator means for comparing the receiving and refer-ence signals with each other.

11. An ultrasonic non-destructive testing system for inspecting an object, said system including the combination of clock pulse generator means for producing a series of timing pulses, transmitting means coupled to said clock pulse generator means for transmitting pulses of ultrasonic energy into the object synchronously with said timing pulses, said ultrasonic energy being effective to propagate through said object whereby echoes thereof are reflect-ed from discontinuities, with the time and the magnitude of each echo being a function of the distance to the discontinuity and the size of the discontinu-ity, receiver means for receiving said echoes and producing a signal having a time and magnitude corresponding to the echoes, signal generator means coupled to said clock pulse means for producing a reference signal having a time varying amplitude which is synchronized with said timing pulses, com-parator means coupled to said receiver means and said signal generator means, said comparator means being effective to compare the received signal and the reference signal, and a utilization device coupled to said signal comparator and effective to produce a signal when the received signal exceeds the refer-ence signal.

12. An ultrasonic non-destructive testing system for inspecting a work-piece, said system including the combination of: pulser means for repeatedly transmitting pulses of ultrasonic energy to a workpiece, said pulses of energy being effective to propagate through said workpiece whereby echoes of said pulses are reflected from discontinuities therein, receiver means for receiving the echoes returned from the workpiece and producing a received signal corresponding to the received energy, whereby the magnitude of the signal is a function of the size of the discontinuity and its depth in the material, signal generator means adapted to produce a reference signal that varies as a function of time corresponding to the variation of said ultra-sonic energy as it propagates through the workpiece, a cathode ray tube for producing a visual display of a signal, means for alternately coupling said receiver means and said signal generator means to said cathode ray tube for producing superimposed displays of the received signal and the reference signal, and a signal comparator coupled to said receiver means and said signal generator, said comparator means being effective to compare said received signal with the reference signal and produce an alarm signal when the received signal exceeds the reference signal.

13. The ultrasonic nondestructive testing system of claim 12 further including: clock pulse generator means coupled to said signal generator means and said pulser means to produce clock pulses to synchronize the operation of said signal generator means with the operation of said pulser means.

14. The ultrasonic nondestructive testing system of claim 12 and further including: clock pulse generator means coupled to said means for alternately coupling said receiver means and said signal generator means to said cathode ray tube for producing superimposed displays of the received signal and the reference signal on said cathode ray tube.

15. A method for comparing received signals corresponding to ultrasonic energy received from a workpiece being tested with a reference signal includ-ing the steps of: transmitting ultrasonic energy into the workpiece under test; receiving the ultrasonic energy after it has propagated through the workpiece; converting the received ultrasonic energy after it has propagated through the workpiece into an electrical signal; generating a reference signal having a time varying amplitude corresponding to variations in the ultrasonic energy as it propagates through the workpiece; and comparing the received electrical signal with the reference signal.

16. The method as defined in claim 15 and further including the step of producing an alarm signal when the received electrical signal exceeds the reference signal.

17. The method as defined in claim 16 and further including the step of displaying the reference signal and the received electrical signal on the screen of a cathode ray tube.