US3043132A - Sonic tester - Google Patents

Description

`Iuly 10, 1962 N. W. SCHUBRING SONIC TESTER Filed dot. le, 195s #l -M s k\b-\b A gk/ The present invention relates to non-destructive testing and, more particularly, to means for sonically inspecting a workpiece for hidden structural defects.

It has been found that similar workpieces have similar natural resonant frequencies at which they will vibrate. Variations in the Workpieces produce corresponding variations in the resonant frequencies thereof. If the workpieces are structurally sound, the variations in the resonant frequencies will be within certain prescribed limits. However, if there are `defects such as blow holes, etc. present in the workpiece, the variations -in the natural resonant frequencies will be outside of the limits. Accordingly, the structural acceptability of a workpiece can be readily determined by measuring the resonant frequency or frequencies thereof.

Normally, workpieces having simple shapes will have only one fundamental resonant frequency. However, if the workpiece has a complex shape, such as an engine crankshaft, it may have several separate and independent natural resonant frequencies at which it will vibrate as a fundamental mode. ln the event a defect is present in the workpiece, one or more` of these resonant frequencies will be outside certain predetermined limits. One means of determining the natural resonant frequencies is to excite the workpiece by means of a variable sweep frequency oscillator and observe the amplitude of the resultant forced vibrations in the workpiece in the manner disclosed and claimed in copending application Serial No. 739,894, :tiled June 4, 1958 and assigned to the common assignee.

The foregoing system measures each and every resonant frequency of the workpiece and provides lan accurate determination of the structural soundness of the` workpiece. However, it has been found that workpieces produced by a common process will be subjected to certain types of defects at certain locations. These defects will be accompanied by changes in only certain of the natural resonant frequencies. Accordingly, it is now proposed to provide a system in which the workpiece will be forced to vibrate across an entire frequency spectrum but only certain critical resonant frequencies will be observed. In the event these frequencies yare within acceptable limits, the workpiece will pass inspection and in `the event one or more of the resonantfrequencies are outside these limits, the piece will be .rejected as defective. More particularly, this is accomplished by providing a separate channel for each of the resonant frequencies under consideration. Each channel includes bandpass filtering means that will suppress all signals having frequencies outside of the acceptable limits and passing only signals having frequencies within the acceptable limits. lf a resonant frequency occurs within the contines of the band, the signals passed by the channel will have an amplitude greater than some predetermined amount. If all of the channels pass `a resonant signal, the work-piece will be indicated as acceptable. However, if one or more channels do not pass a signal resulting from a resonant condition, the workpiece will be rejected.

rln actual practice it has been found that the limits on the cut-olf frequencies at the ends of the band are very critical, i.e., the variations in the resonant frequencies of defective and acceptable workpieces are very small. 'ln order to obtain `a sharp cut-off at both ends of the band, a pair of trigger circuits are provided that are responsive Cil 3,043,l32 Patented July 10, 1962 to the frequency of the oscillator or the frequency of the induced vibrations. These trigger circuits are effective to control the conductivity of a gating circuit. When the sweep' frequency passes through the opening critical limit, the gating circuit will become conductive and will pass a signal indicative of the induced vibrations. When the sweep frequency passes through the closing critical limit, the gating circuit will become non-conductive. Thus, in the interval between the `actuation of the two -trigger circuits, i.e the limits of the band, the gate circuit will be opened to pass a signal indicative of the induced vibrations and will thus become conductive and non-conductive across a very, narrow frequency band and will have very sharp cut-olf frequencies.

The FIGURE is a block diagram of a testing apparatus embodying the present invention.

Referring to the drawings' in more detail, the invention is embodied in a testing apparatus 10 particularly adapted for sonically inspecting a workpiece 12 for hidden structural defects. The present apparatus 10 will detect excessive variations in either one or lboth of two resonant frequencies resulting from defects in theworkpiece 12. However, if desired, the complexity of the apparatus 16 may be easily expanded -to detect corresponding variations in any number of resonant frequencies.

In `the present instance the workpiece 12 comprises a q Icasting having Van intricate shape such as a crankshaft for an internal combustion engine so that it will have several independent resonant frequencies at which it will vibrate as a fundamenta-l mode. 11n order to induce forced vibrations in the workpiece 12, -a driver transducer 14 is placed in intimate engagement with a surface of the workpiece 12. Thus, any mechanical vibrations in the driver 14 will excite the workpiece 12 'and force it to vibrate at a corresponding frequency.

The driver transducer 14 is operatively interconnected with the output of a variable frequency sweep oscillator 16. The oscillator 16 may be of any suitable variety wherein the frequency of the oscillations will progressively change across some frequency spectrum. In the present instance, the frequency spectrum is substantially identical to the audio range, i.e., up to approximately 2O kc. and the oscillator 16 will start oscillating at the lower end of the spectrum and gradually increase without any disi continuity to the upper limit or start at the upper limit of the spectrum and gradually decrease without any discontinuity to the lower limit. t may thus be seen that the output of this oscillator 16 will comprise a continuous wave, the frequency of which will progressively increase or decrease across the audio spectrum and the transducer driver 14 will excite the workpiece 12 in a similar fashion.

ln `a sound casting the tolerances for variations in the resonant frequencies will be -a very narrow band Iand the amplitudes of the induced vibrations at resonance will be much higher than during non-resonant conditions. In

other words the Q of the mechanical structure is very high. However, increasing the sweep rate will tend to reduce the effective Q and thereby reduce the maximum amplitudes and broaden the band width. This change becomes .very marked when the frequency passes through the resonant band in such a short period of time that the. casting does not have an adequate period to `absorb enough energy to support the larger vibrations. In addition, at the higher sweep rates, the period of time required to dissipate the larger quantities of energy absorbed during resonance will be long compared to the period of time required for the sweep frequency to pass beyond the resonant band.

Thus, the extended periods required to absorb and dissipate the energy required to support resonant vibrations will have the apparent effect of decreasing the effective Q and increasing lthe band width. However, these effects 3 i will be of a very minor magnitude until the sweep rate becomes quite high. After this point any further increases inthe sweeprrate will produce very marked eects on VIthe apparent Qandband width.

Accordingly, in choosing the rate at which the frequency of the oscillatorchanges, ie., the sweep rate, the rateV should be high enough to reduce the irispection'time toA will be effective to produce an-electrical signal having an instantaneous amplitude corresponding to vthe instantaneous amplitude of the vibrations occurring inthe workpiece `12. The output of the pickup 18 is interconnected with the input to an amplifier which is adapted to increase the strength of the pickup signal to a more useful level. The output ofthe .amplifier 20 is interconnected with a plurality of parallel channels A and'B.` The number kof these channels A and B corresponds to the number of resonant frequencies which must be observed to determine the acceptability of the workpiece 12. In the present'instance only two channels are Vshown since only two resonant frequencies are Vto be investigated. However, by duplication any desired number of channels may be employed.

All of the channels A and B are substantially identical and each include bandpass lter means and a ilip-op circuit'ZSA and 28B. The iiip-ilopY circuit 28A and 28B is responsive to the amplitude of the signal from the pickup I18 and will normally have no output. However, in the event the amplitude of the'signal at the input reaches a level corresponding to a resonant vibration, it will trip the flip-flop circuit 28A and/ or 28B land cause the channel A and/ or B to have an output signal. The'bandpass filter means in each channel is tuned to pass frequencies Vthat are Within the tolerance limits for the resonant freonantrfrequencies of an acceptable casting and a non-acceptable one are very small. Accordingly, the tolerances as to permissible variations in the resonant frequencies afi/raise Y circuits 38 or 40 to no longer produce an output signal. The signals from the nip-flop circuits 38 and 40 will be effective to cause the gate circuits 42 or 44 to become conductive only during the period when the sweep frequency is between the upper and lower limits of the band of acceptability. As a result, the nip-flop circuits 3S or 4o will respond only to -signals in that frequency range.

In the event a resonant condition occurs within the desired frequency rang a signal of increased amplitude will beV fed to the flip-op circuit 28A or 28B and cause Van output signal to be fed into a coincidence gate 4o. If

all of ,the channels A and B supply such a signal to the coincidence gate 46, it will energize Irelay 4S. This will actuate suitable means to causefthe workpiece to be accepted. If a signalis not received from each channel A and B, the relay 48 will cause the workpiece to be rejected.

In order to employ the-present apparatusr 10 for testing a workpiece 12 for structural defects, the workpiece 12 placed 'on suitable-supports for permitting vibrationsv i therein. The driver transducer V174 and pickuptransducer 18 are then connected tothe workpiece 12. The switch 50 is then manually closed for, a short period to return the frequency of the oscillator 16'to its lower limit and to cause the flip-flop circuits 28A and ZSB to be shut off so Ino signals will be fed to the coincident gate circuit 46. Flip-flop circuits 3S and 40 will also be returned to neutral. The switch 52 is then closed to start the oscillator v16 sweeping the spectrum.

As the sweep commences the pickup 18 will produce a signal that will be Ifed to. Ithe inputs of gates 42 and 44 in Y each channel A and B. However, Vthe gates 42 and 44 will not permit the signal to pass therethrough. When the sweep frequency reaches frequency f1, the trigger circuit 30'will trip the rip-op circuit 38 and thus open the gater 42, thereby permitting the ip-ilop circuit 28A to receive a signal from the pickup 1-8. When the sweep frequency reaches f2, the triggerl circuit 32 again trips the flip-op circuit 38.V This'will close the gate 42 and prevent a signal reaching the .nip-flop circuit 28A.V In the event the workpiece 12 is of acceptable quality, a res- Yonant vibration will have occurred when the gate 42 was are very small and very critical. In order to have adee quate resolution to distinguish between an acceptable andz non-acceptable resonant condition, the bandpass iilter means in each channel must have very sharp cut-off limits and be capable of adjustment downto a very narrow bandpass.V Y f Y f A Accordingly, each of the bandpass :[ilter means includes a pair of trigger circuits 30, 3:2V or 34, 36, a lflip-flop circuit 38 or 40 and a gating circuit 42 or 44 respectively. The trigger circuits 30, 32,l 34 and 36 are responsive to V-the frequency of the'oscillator'l. The circuits 30 and 34 include resonant circuits tuned to the frequencies of the opening limits of the bands and the circuits 32 and 36 include resonant circuits tuned tothe-frequencies of theclosing limits of the bands. The circuits 30, Y32, 34, and 36 each may includerectifying and smoothing means in the output thereof Vto provide' a pulse output adapted to `drive one oftheflip-op circuits 38 or 4i). Thus, for

Y the case of an increasing frequency sweep,for example,

whenever the frequency increases S upwardly through a lowerrlimit, the trigger circuit 30 or-34 will cause the ipop circuit 38 or y40 to produce an output signal and whenever the frequency increases upwardly through Van upper limit, the trigger circuits 32 or 36 will cause the `flip-flop Vfurther signals.

. reach open andthe ip-flop circuit 2SA will have received a signal of adequate amplitude to cause it to pass a signal to Y the coincident gate 46.y In the event the workpiece 12 is unacceptable, a resonant vibrationmay or may not have occurred within the tolerance limits depending'on the nature of the defect. Accordingly, the flip-flop circuit may .or may not have produced Yan output signal.

As the sweep equency continues toV change, it will reach frequency f3 and will cause the trigger circuit 34 to trip the ip-iiop circuit 4l) and thereby open the gate 44. Thus the signal from the pickup 18 will reach the flip-flop circuit 28B. As the frequency continues its sweep, it will frequency f4. When this occurs the trigger circuit 36 will trip the iip-ilop circuit 40 and close the gate 44, thereby preventing the lijp-flop circuitrZSB receiving any If n the workpiece v12 is of acceptable 'K quality, a second resonant condition kwill have occurred between frequencies f3 and fr and the flip-liep circuit 28B will have supplied a second signal to the coincident gate 46. As a result, the coincident gate y46 will have energized the relay 48 and caused the acceptance of the workpiece 12.- If the piece is defective, a resonant condition may or may not have occurred between frequencies Y f3' and f4 depending on the nature of the defect.

However, if there is a defect, one orrbothof the ilipflop circuits 2SA or v23B wouldnot have received a larger signalduring Vthe bandpass frequencies.A Since both circuitsV 28A and 28B are not feeding signals to the coincidence gate 46, the relay 48 will cause the workpiece to be rejected as being defective. Y

' It is to be understood that, although the invention has been described with specic reference to a particular ern- Ybodirnent thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as deiined by the appended claims.

What is claimed is:

1. In a ydevice for detecting flaws in a workpiece, the combination of a variable frequency oscillator, a driver operatively interconnected with said oscillator and adapted to induce variable frequency mechanical Vibrations in said workpiece, a vibration pickup responsive to the amplitude of the vibrations induced in said workpiece, a plurality of channels operatively interconnected with said vibration pickup and including bandpass lter means, each of said channels being eifective to produce an output signal whenever the amplitudes of said vibrations are in eX- cess of some predetermined amount and the frequencg. thereof is within the limits of the band for said channel, a coincidence circuit interconnected-with said channels and effective to produce an output signal only when each of said channels produces an output signal.

2. In `a device for detecting aws in a workpiece, the combination of a variable frequency oscillator, a driver operatively interconnected with said oscillator and adapted to induce variable frequency mechanical vibrations in said workpiece, a vibration pickup responsive to the amplitude of the vibrations induced in said workpiece, a plurality of separate channels interconnected with said vibration pickup, each of said channels including means responsive to the frequency of said Vibrations and adapted to produce an output signal if the amplitudes of the induced vibrations are greater than some predetermined -amount when the frequency of said vibration is between some predetermined upper and lower limits, a coincident circuit interconnected Iwith all of said channels and eective to produce an output signal only when each of said channels produces an output signal.

3. In a device for detecting flaws in a workpiece, the combination of a sweep frequency oscillator, a driver operatively interconnected with said oscillator and adapted to induce variable frequency mechanical vibrations in said workpiece, a vibration pickup responsive to the amplitude of the vibrations induced in said workpiece, gating means interconnected with said vibration pickup, trigger circuit means interconnected with said gating means and responsive to the frequency of said oscillations, said trigger circuit means being effective to cause said gating means to become conductive whenever the frequencies of said Vibrations are within certain limits.

4. In a device for detecting iaws in a workpiece, the

combination of a sweep frequency oscillator, a driver operatively interconnected with said oscillator and adapted to induce variable frequency vibrations in said workpiece, a vibration pickup responsive to the amplitude of the vibrations induced in said workpiece, a plurality of channels operatively interconnected with said pickup, each or said channels including a gating circuit and a pair of trigger circuits, each of said trigger circuits being responsive to the frequency orr said Vibrations whereby each pair of trigger circuits will be effective to cause their associated gating circuits to become conductive whenever the frequencies of said vibrations are within certain limits, each of said gating circuits being effective to cause its channel to produce an output signal whenever the gating circuit is conductive and the amplitudes of said vibrations are greater than some predetermined amount.

5. ln a device for detecting flaws in a workpiece, the combination of a sweep frequency oscillator, a driver operatively interconnected with said oscillator and adapted to induce variable frequency vibrations in said workpiece, a vibration pickup responsive to the amplitude of the vibrations induced in said workpiece, a plurality of channels operatively interconnected with said pickup, each of said channels including a gating circuit and a pair of trigger circuits, each of said trigger circuits being responsive to the frequency of said vibrations whereby each pair of triger circuits will be effective to cause their associated gating circuits to become conductive whenever the frequencies of said vibrations are within certain limits, the gating circuits in each of said channels being effective to cause its channel to produce an output signal whenever the amplitudes of said vibrations are greater than a predetermined amount when said gating circuit is conducting, a coincidence circuit operatively interconnected with each of said channels and effective to produce an output signal whenever all of said channels produce an output signal.

References Cited in the tile of this patent UNITED STATES PATENTS 2,393,225 Andalikiewicz Ian. 22, 1946 2,468,648 Abbott et al Apr. 26, 1949 2,635,746 Gordon Apr. 21, 1953 2,640,926 Wu June 2, 1953 2,699,499 Jordan Ian. 11, 1955 2,800,789 Henry July 30, 1957 2,876,638 Diamond Mar. 10, 1959

Images