The random noise ultrasonic system represents the state-of-the-art in terms of sensitivity and resolution for ultrasonic NDE systems. The principle impediment to its use in the field is the lack of real-time inspection capability. If a pair of pseudorandom binary noise sources are substituted for the white noise source in the random noise system, a real-time capability is achieved. An instrument using such sources will be demonstrated and its capabilities will be discussed

Elias, C M

Moran, T J

English

The random noise ultrasonic system represents the state-of-the-art in terms of sensitivity and resolution for ultrasonic NDE systems. The principle impediment to its use in the field is the lack of real-time inspection capability. If a pair of pseudorandom binary noise sources are substituted for the white noise source in the random noise system, a real-time capability is achieved. An instrument using such sources will be demonstrated and its capabilities will be discussed.</p

Elias, C

Moran, T

Digital Repository @ Iowa State University (ISU)

A PSEUDORANDOM BINARY NOISE ULTRASONIC SYSTEM C. M. Elias SRL, Inc. Dayton, Ohio 45440 and T. J. Moran AFML/LLP Wright-Patterson AFB, Ohio 45433 ABSTRACT .The random nois~ ultrasonic system represents the state-of-the-art in terms of sensitivity and res-olutlon for ultrason1c NDE systems. The principle impediment to its use in the field is the lack of real-time inspection capability. If a pair of pseudorandom binary noise sources are substituted for the white noise source in the random noise system, a real-time capability is achieved. An instrument using such sources will be demonstraied and its capabilities will be discussed. INTRODUCTION The random signal ultrasonic system (1) repre-sents the state-of-the-art for NDE ultrasonic sys-tems. Correlation techniques are utilized to ach-ieve very large signal to noise ratios and at the same time achieve ranqe resolution limited only by the bandwidth of the ultrasonic transducers. A white noise source is used in either a cw or long burst mode to excite the transducer which insoni-fies the specimen, allowing high average power and deep penetration into highly attenuating materials to be obtained. Due to the uniqueness of a given noise burst, the reference signal for the correla-tion must be a suitably delayed version of the sig-nal used to excite the transducer insonifying the specimen. This delayed reference signal is usually obtained by splitting the signal from the noise source into two channels and exciting a second transducer in an acoustic reference delay path. The path length is mechanically varied to obtain the required variable delay. Complete correlation occurs when the reference delay is equal to the signal delay. The entire specimen is scanned by sweeping the reference time delay. It is this mechanical scan which has been the principle imped-iment to use of this system outside of the labora-tory. Data acquisition times are simply too long for field use. This paper describes one possible method for speeding up data acquisition while retaining most of the signal to noise enhancement of the random signal system. A pseudorandom binary noise {PBN) source (2) is used in place of the white noise source. Since such a source is reproducible, a se-cond identical source is used as the reference and the delay is obtained digitally. Elimination of the second acoustic path increases the effective bandwidth of the system as compared to the random signal system, improving range resolution. Resol-ution with the PBN system should be comparable to that obtained with the best conventional systems. An instrument was designed and built incor-porating a pair of identical PBN sources consist-ing of 16 stage shift registers with 4 taps and producing a 65,535 bit maximum length sequence {3). It is the purpose of this paper to present the re-sults of an evaluation of the performance of this first generation instrument. A detailed discus-sion of the design will be given elsewhere (4). 74 SUMMARY Present Instrument Capabilities: 1 65,535 bit pseudorandom maximum sequence. • 25 MHz maximum master clock rate, 12.5 MHz PBN rate. 1 100 repetitions maximum per point. 1 999 clock period max. burst length. • 9,999 clock period maximum acoustic cycle period. 1 Operates easily with 2V peak to peak trans-ducer drive. Possible Applications: • Greater penetration into highly attenuative materia 1 s. • Measure attenuation at high frequencies in thin layers using phase modulated CW carrier. • Apply quantitative NDE techniques which re-quire good signal to noise for accuracy. REFERENCES 1. Newhouse, V. L., Furgason, E. S., Bilgutay, N. M. and Cooper, G. R., "Random Signal Flaw Detection," Proc. of 1974 IEEE Ultrasonics Symp., IEEE Cat. No. 74-CH0-896-1-SU, p. 711-715 {1974). 2. Golomb, S. W., Shift Register Sequences, Holden-Day Inc. (1967) Chapter 3. 3. Motorola MeMOS Handbook, Motorola Inc. (1974), Chapter 11, Section D. 4. Elias, C. M., "An Ultrasonic Pseudorandom Sig-nal Correlation System." To be published. SIGNAL CHANNEL REFERENCE CHANNEL 1---+IPNG 2 SYSTEM BLOCK D lA GRAM Fig. 1 Pseudorandum binary noise generators (PNG) 1 and 2 produce identical 65,535 bit se-quences. The control logic incorporates a stable oscillator to control both the bit length and the number of clock times PNG 2 is delayed relative to PNG 1. REF. CHANNE T(N)- Nth TRANSMITTED BURST R(N)- Nth RECEIVED BURST T(1) R(1) LOW CORRELATION MAXIMUM CORRELATION LOW CORRELATION TIME SIMPLIFIED SYSTEM OPERATION F1g. ' PNG 1 and PNG 2 are initially set to the same state. The first noise burst from each ~ener­ator consists of a selected number of bits (PJ of the sequence. Each succeeding burst consists of the next P bits of the sequence. The burst from PNG 2 is initially delayed a selected number of clock periods relative to PNG 1. This delay is held fixed for a set number of repetitions before being advanced one clock period to the next point where it is again repeated as before. This process is repeated until the maximum delay is reached, where the system either halts or repeats the scan as desired. PSEUDORANDOM BINARY M-SEQUENCE: FIRST 100 BITS Fig. 3 75 CURHELATO' OUTPUT • 'l - i2.5 /'SEC BURST LENGTH ~ '" '"" "'"' "' . '"' ~r Fig. 4 Linear and log plots of the cor-relator output using a short drive burst and a small number of repetitions at each reference delay point. 5 MHz transducers and through transmission in water. CUHRELATOR OUTPUT - ~u p SEC BURST LENGTH ~~ REPETIT!UIIi PER POINT a >--~~l+---~,_J rv--~J "' "' "' u~ --------.---~··----.---·-.,.- ·-- ·---, nNE. SECd~Os .to-~ ,.. a.• Fig. 5 Linear and log plots of the cor-relator output using the longest burst and largest number of repetitions t~e cu~rent instrument is capable of produc1ng w1th a 20 MHz master oscillator. Note the im-provement in signal to sidelobe ratio as compared to Figure 4. m :.s w -30 m 0 _J w 0 Vi X ®-CALCULATED I • ~-50 X-EXPERIMENTAL 10 100 REPETITIONS ,10 00 Fig. 6 Graph illustrating the effect of varying the number of repetitions on the sidelobe size. Calculated points were obtained from a computer simulation of the instrument. The solid line is a linear fit to the calculated data. The break in slope was found to occur when the product of the number of bits in the burst and the number of rep-etitions exceeds the sequence length. A 375 bit burst length was used. 6 ' 10 2'o ' 30 ' 4'0 FREQUENCY <IlHz) A. FREQUENCY SPECTRUM OF PSEUDORANDOM BINARY NOISE SEQUENCE OPERATING AT A 10 MHz CLOCK FREQUENCY. FREQUENCY <MHz) -0 ;;; -203 u.J § -40~ ~ B. FREQUENCY SPECTRUM OF 20 MHz CARRI ER SIGNAL PHASE f'oODULATED USING PBN SEQUENCE HAVING SAME SPECTRUM AS SHOWN IN A. Fig. 7 Spectra of the PBN sequence used in this instrument and a CW carrier phase modulated by the · same sequence. The signal shown in B should be useful for driving high frequency transducers since the center frequency may be matched to that of the transducer. 76 A. CORRELATOR OUTPUT WITH ACOUSTI C DELAY IN REFERENCE CHANNEL B. CORRELATOR OUTPUT WITH COMPLETELY DIGITALLY DELAYED REFERENCE CHANNEL Fig. 8 Illustration of the pulse width degradation which occurs when the band-width of the reference channel is reduced by the addition of an acoustic delay path as in the random signal system . The pulse width shown in B is comparable to that ob-tained with conventional systems. A. CONVENTIONAL PULSER-RECEIVER OUTPUT B. PSEUDORAHDOII BINARY NOISE SYSTER OUTPUT Fig. 9 Illustration of the signal to noise enha~cement which may be obtained with the present instrument. 

A Pseudorandom Binary Noise Ultrasonic System

Elias, C.

Moran, T.

https://dr.lib.iastate.edu/handle/20.500.12876/58309

A Pseudorandom Binary Noise Ultrasonic System

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Digital Repository @ Iowa State University (ISU)

Digital Repository @ Iowa State University (ISU)