VIBRATION PARAMETER ESTIMATION TECHNIQUE FOR ULTRASONIC MEASUREMENT SYSTEMS

Abstract

A moving window discrete Fourier transform based synchronization technique is proposed in this research work for a non-contact ultrasonic vibration measurement system. When an ultrasonic wave is transmitted towards an moving object, it experiences a Doppler shift at the receiving end depending on the vibrating frequency and distance of the moving object. Hence, the received ultrasonic signal is phase modulated. The Doppler shift contains all the information of moving object such as vibration amplitude, frequency, and phase. Firstly, vibration signal estimation techniques employed in ultrasonic measurement sys- tems are reviewed. The literature review is focused on the Doppler signal extraction methods including the theory and analysis pertaining to the background of ultrasonic vi- bration measurement techniques. The phase modulated ultrasonic received signal in the vibration measurement system involves constant phase shift due to ultrasonic path length, phase shift introduced by Doppler e ect due to vibrating object, and the parametric phase shift caused by interaction of high frequency ultrasonic wave and low pressure developed by vibrating object. Among the three phase shifts, the Doppler phase shift contains the vibrating signal information and the extraction of Doppler signal helps in knowing the velocity of the vibrating object, amplitude and phase of the vibration. Various tech- niques of Doppler phase shift retrieval are categorized based on the modulation index estimation procedures from the received ultrasonic signal. An analysis is made based on the information retrieval methods for estimation of vibration signal parameters, range of the vibration amplitude, frequency, and the preferable carrier frequency for transmission. Further, the analysis also presents various types of ultrasonic vibration measurement ap- plications. Apart from the Doppler e ect, the theoretical background of parametric e ect in vibration measurement has been analyzed. Secondly, a synchronization technique is presented for extracting the Doppler signal for extracting the Doppler signal in ultrasonic vibration measurement system using moving- window discrete Fourier transform (MWDFT) as a lter. A pair of ultrasonic transducers is used in which one transducer emits the continuous ultrasonic wave of 40 kHz toward the vibrating disc and another transducer receives the phase modulated ultrasonic signal. In this technique, the received ultrasonic signal is tracked for variation in carrier. The quadrature detection is performed with the help of MWDFT. The FM-to-AM conver- sion is done by tuned MWDFT lter. A simple multiplier followed by another MWDFT suppresses the carrier from the received signal and hence the Doppler signal is retrieved. The retrieved signal is further processed to extract the phase modulating signal by sam- pling period adjustment of the MWDFT. This signal contains the vibration amplitude and frequency information. These parameters had been obtained for an electrodynamic vibration system by implementing the synchronization technique in eld programmable gate array (FPGA). Thirdly, when the ultrasonic carrier deviation is large because of the vibration frequency, the received signal appears as AM-FM signal instead of phase or frequency modulated sig- nal. However, the vibration information is present only in frequency modulating signal. To separate the AM signal from FM signal, the MWDFT-PLL is modi ed in such a way that the sampling pulse generator produces pulses suitable for MWDFT bin-1 and MWDFT bin-0 for the retrieval of frequency and amplitude modulation signals, respectively. Hence, this method presents how the frequency and amplitude variation of mono-component AM- FM could be retrieved simultaneously. The proposed technique involves the feed-forward and feedback demodulation techniques. In case of feed-forward demodulation, the FM signal is converted into AM using a lter and the feedback demodulation involves phase locking schemes. The proposed scheme is capable of extracting Instantaneous Frequency (IF) and Instantaneous Amplitude (IA) for the signals comprising of large variation in frequency and amplitude. The wide operating range of the PLL scheme facilitates in achieving the large variation in AM-FM signal demodulation. In addition, the MWDFT incorporated in the PLL would extract the message signals in presence of noise. The proposed scheme is implemented in FPGA to validate the performance of MWDFT-PLL in decomposition of the mono-component AM-FM signal. Fourthly, to improve the performance of the proposed scheme for (i) wider range and (ii) good SNR of demodulated frequency, a closed-loop MWDFT based frequency locked-loop (FLL) is proposed. With this scheme, message signal is retrieved from sinusoidal frequency ii modulated signals. A MWDFT acts as a tuned lter, when the sampling frequency is integral multiple of the center frequency. The spectra of MWDFT exhibits almost at amplitude and phase responses when it is placed in a closed-loop with xed sampling frequency. These at characteristics for variation in input frequency is utilized for tracking frequency modulated carrier. The small leakage observed in terms of magnitude and phase errors of MWDFT is further corrected by adaptive sampling frequency control. The proposed MWDFT-FLL o ers frequency demodulation/estimation under following conditions (i) biased input, (ii) large frequency deviation, (iii) frequency modulation (FM) of decaying sinusoid, and (iv) noise and harmonics. Finally, the velocity measurement of moving object is carried out by applying the MWDFT- PLL and FLL algorithms. In PLL algorithm, a constant velocity of the moving object is measured in d.c. shift at the out signal. But for FLL algorithm, when a sinusoidal signal of carrier frequency 40 kHz is transmitted toward the moving object having con- stant velocity, the linear change in carrier phase shift is experienced because of Doppler e ect in the re ected wave. The linear change in phase shift is re ected as step change in estimated frequency. Thus, the constant velocity of the moving object is measured in terms of step change in carrier frequency.