Ryo Toh

Target Ranging Using Ultrasonic Sensitivity-Compensated Signal and Pulse Compression

A new approach of frequency-modulated pulse compression for target ranging using ultrasonic pulse-echo is discussed. To acquire a receiving signal with a broader bandwidth, a flatter spectrum, and a higher signal-to-noise ratio, the use of a sensitivity-compensated transmitting signal is proposed. To compensate for the uneven and narrow bandwidth of the receiving signal brought forth by the sensitivities of ultrasonic transducers, the sensitivity-compensated signal is calculated by inversing the spectrum of the response function majorly composed of the sensitivities of transmitters and receivers. Moreover, instead of the transmitting signal, a reference receiving signal with an expanded flat spectrum measured priorly is employed for cross-correlation calculation with the receiving signal. The efficiency of the proposed method, compared with both the inverse and matched filtering methods using a chirp wave as the transmitting signal, is studied by a target ranging experiment in air. The results show that the spectrum of the receiving signal is compensated for and expanded using the sensitivity-compensated signal, and that unevenness of less than -20 dB in the spectrum of the receiving signal of the chirp wave is compensated for efficiently. Furthermore, the results of pulse compression show that, using the proposed method, the signal-to-noise ratio of the compressed pulse can be expected to be improved by more than that derived by the inverse filtering method, while the pulse width is shortened and the resolution is improved up to about 1/3 of that acquired by the matched filtering method with a chirp wave.

An experimental study on speed measurement using sensitivity compensated signal and linear prediction processing

Ultrasonic pulse-echo method is widely employed for acoustical measurement in ocean or remote sensing of automobiles and robots. For target ranging, the time-of-flight (TOF), while for speed measurement, the Doppler frequency shift, are usually employed, respectively. However, owing to the resolution of the frequency, lower speed is difficult to be measured. In order to acquire TOF with higher accuracy, pulse compression method using frequency-modulated (FM) signal is usually employed. However, owing to the sensitivities of the ultrasonic transducers, the spectrum of the received signal will be uneven and narrow-banded, that lessens the effectiveness of the pulse compression. In order to acquire the received echo signal with broader and flatter spectrum, we have proposed sensitivity compensated (SC) transmitting signal and linear prediction (LP) processing for expanding effective spectrum. In this paper, the efficiency of these methods for measuring lower speed using TOF of dual-pulses is studied experimentally. As comparison to the traditional linear FM transmitting signal (Chirp wave), two SC signals are discussed. The amplitude modulated SC (AMSC) signal is calculated from the quotient of spectra of the Chirp wave and its reference received signal by inverse filtered processing. Because the amplitude characteristic of the inversed spectrum reflects directly to the time domain, the AMSC signal has a same waveform as an AM Chirp wave. On the other hand, the FMSC signal is calculated by equalizing the energy of the SC spectrum to the duration time of the corresponding frequency part in the time domain, and it becomes a non-linear FM signal with identical amplitude. A matched filtered processing is employed for pulse compression. Furthermore, in order to acquire shorter pulse width, an LP processing expanding the effective spectrum of the pre-compressed pulse is introduced. Therefore, five kinds of measurement result, a) using Chirp wave, b) using AMSC signal, c) using FMSC signal, d) using AMSC signal with LP, and e) using FMSC signal with LP, are compared, in total. The results show that the pulse width of a single compressed pulse derived by d) is shortened to be about 1/5 of that by b), but that derived by e) shows no obvious improvement. While the results of speed measurement of 1.0 m/s ~2.0 m/s, the accuracy of b) ~d) are similar (with error less than 10%) with clear improvement from that of a), but that of e) is the worst. The tendency of these results can be described by the corresponding spectra, that FMSC does not compensate the unevenness of spectrum brought forth by the sensitivities sufficiently.

Lateral Detecting Limit of Underground Imaging Owing To Directivity of Sound Source

Considering the directivity of an electromagnetic-induction-type (EMI) sound source and the nonlinearity of the amplitude correlation synthesis processing (ACSP) method, the lateral detecting ability of the three-dimensional ACSP method for imaging objects buried underground is studied by both numerical simulation and experimental field testing. In both the numerical simulation and the experiment, three objects with identical dimensions buried at an identical depth are measured under six geometrical conditions with different spread angles from the EMI sound source. In addition, two underground objects with different lateral intervals are also imaged by the numerical simulation. The results of both the numerical simulation and the experiment indicate that the lateral detecting ability is mainly determined by the directivity of the EMI sound source and that the lateral detection of the imaging method is efficient for spread angles from the EMI sound source of less than about 35°.

Driving Energy of Reinforcing Steel Bar for Discriminating Background Medium of Concrete

A method for discriminating the background media of concrete using the quality factor of elastic waves multireflected in the concrete was proposed. A reinforcing steel bar buried inside the concrete is employed as a sound source driven by the induction of impact electromagnetic field radiated from a spiral coil placed on the surface of the concrete. In this paper, the appropriate energy for driving the reinforcing steel bar, to obtain stable quality factors with clear differences between different background media, is studied experimentally. With various driving energies, multireflected elastic waves corresponding to three types of background medium, air, sand, and water, are measured. The quality factors are calculated by the linear predictive coefficient analysis method. The results show that the quality factors tend to increase when the driving energy is at its lower region, and they remain comparatively stable when the driving energy is higher than a certain value. For reinforcing steel bars with different diameters, the curves of quality factors versus driving energies agree well by introducing a newly defined volume-normalized driving energy. For the reinforcing steel bar of 13 mm diameter, the appropriate driving energy for background media discrimination is approximately 0.5 J.

A study on the effect of sound velocity estimation for underground imaging

An efficient technique for detecting objects buried underground is expected for archaeological exploration and civil engineering. We have proposed a three dimensional underground imaging method by using an amplitude correlation synthesis processing and an electromagnetic induction type sound source. Because the sound velocity of sand and soil depends on a lot of physical parameters that varies with the area, it is needed to be measured before imaging processing. Up to now, the sound velocity employed for signal processing is measured by receivers buried underground near the area to be detected. In order to improve the efficiency of field detection, a method of estimating the underground sound velocity by the sound velocity measured from the ground surface is studied. In this study, the comparison experimental results of the velocity of underground transverse wave, the velocity of underground longitudinal wave, and the sound velocity measured from the ground surface are introduced. Furthermore, the effect ...

Terahertz Pulse Reflection Imaging Using the Time-Domain Correlating Synthesis Method

The synthetic aperture method is an attractive technique for terahertz nondestructive imaging without strict restraints on the beam form or the Rayleigh length of focus. However, the limitation of the aperture of the receiver array results in distortion and restricted lateral resolution of the reconstructed image. This paper introduces a nonlinear imaging processing principle, the correlating synthesis method, which has been applied in ultrasonic measurements. Using a pulse reflection mode terahertz time-domain spectrometry system and pulse compression preprocessing, we reconstructed images of metal cylinders. The results of comparisons with a synthetic aperture method verify the efficiency of the correlating synthesis method for improving the lateral resolution and eliminating fake images.

Relation of resonant frequency and appropriate linear predictive order for evaluating concrete strength by quality factor

Concrete is a popular construction material used broadly in many kinds of structures. The compressive strength is the most important performance of the concrete responsible to the safety of such structures. Although method of sampling a specimen is commonly employed, it makes damage to the structures. While Schmidt Hammer measures the hardness of the surface, but it relates to the compressive strength with poor accuracy owing to the influence of the conditions of the surface. Considering that the ultrasound propagating inside the concrete with higher strength will decrease less, the quality factor of the ultrasound signals should be higher, we have proposed a new method that evaluates the concrete strength non-destructively using the quality factor of resonant peak of multi-reflected ultrasound propagated inside the concrete. The quality factor is calculated by a linear predictive coefficient (LPC) processing on the ultrasonic signals, where the order of the LPC is an important parameter, by which the number of the poles of the LPC model is determined. Neither too many poles, which include too much details of the original signal with noise, nor too few poles, which lack of the proprieties at the resonant peaks, may express the propriety of ultrasonic propagation clearly. In this paper, as one of the approach of optimizing the parameters employed in measurement and signal processing, the relation between the resonant frequency and the appropriate order of LPC is discussed by numerical simulation and experiment. In the experiment, three types of concrete specimen with identical dimensions but different strengths (48.4 N/mm2, 37.1 N/mm2 and 27.9 N/mm2 respectively) and four kinds of sound sources and receivers with different resonant frequency (28 kHz, 46 kHz, 68 kHz and 105 kHz, respectively) are employed. In the numerical simulation, a low-pass IIR filter is introduced to simulate the absorption of ultrasonic propagation in the concrete. Though the absorption is very complicated and hard to be reconstructed by numerical simulation, the absorption differences corresponding to different strengths are simulated by changing the coefficients of the IIR filter as a simplified model to study the relation between resonant frequencies and appropriate orders of LPC for strength evaluation. The results of both the experiment and the numerical simulation show a similar tendency that with a higher resonant frequency, the appropriate order of LPC turns higher and its range turns narrower. Therefore, in the view of stability of the quality factor employed to evaluate the concrete strength, measurement with lower resonant frequency is expected.