Abstract:
This invention relates to immersion ultrasonic pulse-echo testing of tubes or bars for internal defects wherein means are provided to continuously monitor the operation of the test channel. As ultrasonic energy enters the workpiece, the round workpiece surface causes acoustic energy scatter responsive echo signals which are sensed by the transmitter probe, are amplified and applied to a threshold circuit. If the amplitude of the scatter responsive signal falls below a predetermined level, an output signal indicative of &#34;loss of operation&#34; is produced. Means are provided to inhibit such output signal in the event of only a momentary decrease of the scatter responsive signal below a predetermined value.

Description:
FIELD OF INVENTION 
     This invention concerns a method and apparatus for monitoring the operation of one or of a plurality of transmit transducer probes and/or of amplifiers when testing tubes and bars by the ultrasonic immersion pulse-echo test method. The invention particularly concerns the monitoring of the electro-acoustic apparatus when testing workpieces for cracks in proximity to the surface. 
     BRIEF SUMMARY OF THE INVENTION 
     Ultrasonic immersion test arrangements are known for the purpose stated above wherein one or two test probes are disposed in a plane normal to the tube or rod axis and wherein the sound beams are incident via a water path upon the workpiece surface at an angle from about 14° to 27° from normal; the water serving as a coupling path. When a single test probe is used, such probe operates as a transmitter as well as a receiver for the ultrasonic energy. When two test probes are used, one probe operates as the transmitter and the other as receiver of the ultrasonic energy. Preferably in the latter case, the position of the receiver test probe is advanced relative to the rotation of the tube axis by 90° or 180°. The two-test probe arrangement is particularly advantageous when rough workpiece surfaces are present since in this case substantially none or only a small portion of the sound beam is reflected back by reflection at the workpiece surface to the receiver test probe. In this manner, it is possible to evaluate test signals (defect echoes) without interference from the surface induced signals. 
     However, these test systems cannot be monitored continuously for proper operation since, in accordance with their operational principle, they lack a reference signal, for example a rear wall echo signal. 
     For checking the test apparatus it is necessary to test at regular intervals for comparison a workpiece having a known defect. Continuous monitoring of the operation during the progress of the test process is not possible with the known arrangements. This fact is particularly disturbing since these test methods are used largely in fully automatic ultrasonic test systems. Moreover, for increasing the test speed frequently several test systems are operated in parallel, performing the same test function and, hence, an urgent need exists to monitor on a continuing basis the faultless operation of the several test channels operating in parallel. As understood herein a test channel is defined as the operating chain comprising the electronic pulse generator, transmit probe, receiver probe and amplifier. 
     This invention deals with the problem of providing a method for continuously monitoring the operation of a test system as noted above. 
     The invention is based on the fact that when an ultrasonic energy angle beam is transmitted into a curved workpiece surface, scattered reflections are always produced, such reflections being more or less prominent, but always present during the relative helical scan motion of workpieces. 
     These scattered signals are produced in a known manner resulting from surface roughness and are present on account of the beam width of the entering sound beam and because of the curvature of the tube surface during a certain portion of the transit time. 
     The novelty of the present invention resides in the feature that the absence of the scatter responsive signals, which regularly are visible on the cathode ray tube screen as a burst of spurious signals, is evaluated as a disorder in the respective test channel. 
     For practicing the invention, it is significant therefore, to extract the burst of scatter responsive echo signals and to amplify and monitor those signals. Switch means are used to preclude a momentary absence of these signals, e.g., two or three times, to be registered as an operational disorder. 
     In accordance with the present invention, means are provided for receiving at test probes the sound scatter responsive signals arising by reflection at the tube or bar surfaces and to amplify and pass the signals through a gate, and moreover, to signal as an operation disturbance the condition when the scatter responsive signals decrease below a predetermined threshold value. 
     The instant invention will be more clearly apparent by reference to the following description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 3 depict known immersion test arrangements; 
     FIGS. 3 through 6 depict the echo signal indications (for instance inside lengthwise defect) pertaining to FIGS. 1 through 3; 
     FIGS. 7 and 8 disclose arrangements for monitoring the operation utilizing scatter indication; 
     FIGS. 9 and 10 disclose arrangements for monitoring the operation utilizing scatter indication and the inclusion of defect signal amplifiers in the monitor circuit. 
     FIGS. 11 and 12 disclose arrangements for monitoring the operation utilizing scatter indication and inclusion in the circuit of defect signal amplifiers in a single monitor system. 
     FIG. 13 shows a safety circuit to inhibit faulty signals, and 
     FIG. 14 is a schematic diagram for multi-channel test apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 6 depict the known immersion test arrangements with their respective echo representation, in the present example an inside axial defect. An ultrasonic beam 2 transmitted from the transmit-receive test probe 1 enters the surface 3 of a workpiece 4, e.g., a tube, and after reflection at a defect 5 is reflected back toward surface 3 and is manifest as an amplitude peak 6 adjacent to a scatter signal 7 on a cathode ray tube screen as shown in FIGS. 4 and 6. The first peak Si denotes the transmit pulse responsive signal. 
     One embodiment of the invention is shown in FIG. 7 which discloses aside from the amplifier 8 used for defect signal evaluation also a second amplifier 9 coupled in parallel, the latter providing a relatively greater amplification. Therefore, the amplifier 9 is capable of providing continuously a scatter responsive echo signal of sufficient output signal amplitude. The circuit depicts, in addition, a set of monitors 10 and 11 for the defect responsive signal and the scatter responsive signal respectively. An ultrasonic pulse generator 12 cyclically produces the transmit pulse signal as is well understood in the art of ultrasonic testing. 
     In a second and third embodiment, see FIGS. 2, 3 and 8, the same result is achieved if the transmit probe 1b is coupled also to an amplifier 9 which amplifies the surface induced scatter responsive echo signals that are reflected back to the transmit probe 1b, whereas the amplifier 8 coupled in circuit with the receive probe 1a amplifies the defect responsive signals. The gain of the amplifiers 8 and 9 can be adjusted independently from each other. 
     It has proven advantageous to include the defect signal amplifiers in the monitoring circuit. 
     To this end, the arrangements shown in FIGS. 7 and 8 can be improved. Specifically, the heretofore shown arrangements do not include the defect responsive signal amplifiers in the monitoring circuit. This disadvantage can be remedied by providing after the defect signal amplifier 8 a further amplifier stage 13 for amplifying the scatter responsive signals, see FIG. 9. In accordance with another embodiment, see FIG. 11, a single amplifier 14 having a logarithmic gain characteristic is used for providing both of the monitoring functions. The output signal from the amplifier 14 is coupled to a monitor system 15 which is adjusted for two different threshold values. For instance, signal 15a is the &#34;defect signal&#34; and is generated when a defect responsive signal exceeds a first threshold value, and signal 15b is the &#34;loss of operation&#34; signal and is produced when the scatter responsive signal is less than a predetermined second threshold value. Since an amplifier with a logarithmic gain characteristic has such a large dynamic range, it is possible to evaluate both signal indications, i.e., defect and scatter indications, at the output of the amplifier with two differently adjusted threshold values, especially in view of the fact that both signals frequently exhibit a considerable difference in signal amplitude. 
     In conjunction with the embodiments per FIGS. 2 and 3 in addition to the amplifier 13 for the scatter responsive signals a further monitor cycle is required in order to test the operation of the defect responsive signal amplifier 8. During the first time cycle (I), see FIGS. 10 and 12, the transmit probe 1b operates as transmitter as is normally the case. The other probe 1a operates as receiver for defect responsive echo signals 6 and the transmit probe 1b operates also for receiving the scatter responsive signals 7. During the following cycle (II), which serves merely for monitoring the operation of the defect signal amplifier 8, the receive transducer probe 1b operates as transmitter 1a and the ensuing scatter responsive signals 7 are amplified by the defect signal amplifier 8 and then further amplified in an amplifier 13 for causing the scatter responsive signals to be rendered capable of being monitored and evaluated. Instead of the last-mentioned combination, once again a logarithmic gain amplifier 14 can be used as stated previously in conjunction with the embodiment per FIG. 11. In this event, see FIG. 12, during the test cycle I, the defect responsive output signal 15a from the threshold device 15 is used for defect evaluation and the scatter absent signal 15&#39;b indicating a malfunction would be apparent at the output of monitor 11. During the ensuing monitor cycle II when the previous receive probe 1a becomes a transmitting probe, the scatter responsive signal received by the transmitting probe is fed via the logarithmic amplifier 14 to the threshold device 15 for causing in the event of a malfunction a scatter absent signal 15b. Hence, the condition of signal 15b is used for monitoring the operation of the defect echo signal channel. 
     With reference to FIG. 13, in order to prevent the condition that a single, very brief, lack of the scatter indication 7 is registered as an operational fault, it is possible to include in all of the arrangements a suitable inhibit circuit. A counting circuit 17 coupled after the monitor circuit 9 achieves this result by providing that only after an adjustable, predeterminable number of sequential failures, i.e., failure of the scatter signals to reach the threshold level, a signal 18 is produced, the presence of which serves to indicate lack of proper operation, see FIG. 3 in conjunction with the embodiment per FIG. 10. As is apparent to those skilled in the art, the counting circuit must cyclically be reset. 
     If, as shown in FIG. 14, several independently operating test channels are present, the monitoring function can be achieved by multiplication of the previously described circuits. However, particularly in connection with the arrangement shown per FIG. 2, multi-channel systems are known which do not operate independently of one another. For the arrangement, shown for instance in German patent publication OS No. 21 38 458.7, there are provided four test probes 1a and 1b which are disposed equidistantly about the circumference of the workpiece and which operate in accordance with the principle shown in FIG. 2. In accordance with the present invention an improvement, see FIG. 14, is provided to the effect that during cyclic testing at all times one pulse generator 12 is coupled in circuit with an associated test probe 1a, and the associated amplifier 8 coupled in series with a respective receive probe 1b is rendered operative for signal reception. Each of the four test probes, which are cyclically rendered operative by conventional means, not shown, operate therefore once as a transmitter 1b and once as a receiver 1a. The solution in accordance with the present invention of the problem to monitor the proper function of all elements comprising the test apparatus is achieved by utilizing the arrangement previously described in conjunction with FIG. 9. The cycle signals designated I, II, III and IV must be observed which are coupled to the individual test probe or probe pairs respectively, for periodically conditioning the probes for the monitoring and the testing cycle. 
     The fault signals can be coupled to an intermediate amplifier 19, to a monitor 11 and to a preset signal sequence counter 17 which is scanned during cycles I, II, III and IV (FIG. 14 bottom) to provide, depending on the circumstances , a fault alarm signal for each of the first, second, third or fourth test probe. An analogous circuit, of course, can be made also with amplifiers 14 having a logarithmic gain characteristic. The operation of such latter circuit is then similar to that described in conjunction with FIG. 12. 
     It will be apparent to those skilled in the art that the defect responsive monitors and the scatter responsive monitors used heretofore are controlled by time gates in the known manner in order to render the monitors operative during predetermined periods of the test cycle.