Suppression of second-time-around echoes in high firing rate ultrasonic transducers

F~UTTERWQRTH ~E I N IE M A N N

0963-8695(94)00011-5

NDT&E International, Vol. 28, No. 2, pp. 89-93, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0963-8695/95 $10.00 + 0.00

Suppression of second-timearound echoes in high firing rate ultrasonic transducers P. W e b b * and C. Wykes t * Department of Mechanical and Manufacturing Engineering, De Montfort University, The Gateway, Leicester LE1 9BH, UK t Department of Manufacturing Engineering and Operations Management, The University of Nottingham, University Park, Nottingham NG7 2RD, UK

Received 15 July 1994 With the development of faster and more sophisticated computer hardware, sensor data can be processed at increasing rates. The increased demand on sensors which results can lead to problems with the validity of sensor information. One such effect that can occur with ultrasonic sensors is the reception of second-time-around echoes. These are most likely to be present when observing a target in front of a large reflector. A method used in radar processing to eliminate such interference is described and its application to an ultrasonics phased array object location system is presented.

Keywords: ultrasonics, signal processing, range determination

common commercially available ultrasonic range measurement system, the system manufactured by the Polaroid Corporation 181 primarily as a camera range finder.

Ultrasonics has been applied widely to the problem of robot guidance and collision avoidance systems for both fixed and mobile robots 11-71. In their simplest form these systems detect the presence of a target within the beam pattern of the ultrasound transducer and provide a range measurement of the target position. In such systems range is usually measured by the 'time-of-flight' principle in which the range is found by measuring the time taken by energy from a transmitting source to travel to a target and be reflected back to the transducer. If the velocity of the energy in the propagating medium is known then the range of the target can be calculated from: R-

An ultrasonic phased array system developed at Nottingham provides both range and bearing information about multiple targets from a single transmitted signal. The system is based upon a capacitive transducer manufactured within the Department of Manufacturing and Operations Management at the University of Nottinghamt9-~ 11. One of the problems that can arise in such a system is that of second-time-round echoes where the ultrasound can be multiply reflected and will be detected by the system as a target which does not, in fact, exist.

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Variable pulse repetition frequency (PRF) is a technique widely used in radar 112'131, particularly in short range systems operating with a high PRF 1141, to deal with this problem. It has been applied by Borensteint1~] to reduce the interference between multiple single transducers used in a robot collision avoidance system. Its application to the suppression of second-time-around echoes in an ultrasonic array system is described here.

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The most common method of measuring the time of flight is the use of the pulse echo method. A pulse or series of pulses is transmitted, the transducer then assumes a listening mode and the time of arrival of any reflected pulses is recorded. This method is employed in the most

89

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received signals are shown in Figure 2 for the target arrangement shown in Figure 1. The single PRF returns are as shown in the figure. If the time between firings is varied, then the STA echo will appear to move in time relative to the transmit pulse. If the received signals from two firings are compared and only targets which remain static in relation to each other are considered then all STA targets will be removed. This is again shown in Figure 2. This use of a variable PRF also has the benefit of removing noise, as noise spikes seldom correlate between transmitter firings.

Second-time-around echoes occur when an echo is received from a target beyond the normal range of the system (Figure 1). This happens when the transmitter is fired whilst energy from a previous firing is still being returned by reflection. Since the second-time-around (STA) return is received shortly after the transmitter has fired again it appears as a target at a much reduced range. The problem can be avoided by reducing the firing rate and 'listening' only in the time where true targets occur. This does, however, reduce the data rate that can be obtained from the sensor.

Practical implementation

The principle of the variable PRF system is that the time separation between transmitter firings is varied and only those target returns which occur in more than one returned signal are considered to be true targets. Idealized

The system was tested with a four-element array and ultrasonic data acquisition and processing system, operating at 100 kHz, used to guide a robot in a pick and place operation t2].

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A purpose-built data acquisition system was used to store the received signals for processing by a digital signal processor (DSP) [5]. The raw signals from the array are amplified and bandpass filtered before being mixed with 100 kHz in-phase and quadrature signals. The resulting

in-phase and quadrature signals are then sampled at a frequency of 200 kHz and digitized. This data is then processed using a Texas Instruments TMS320C30 DSP mounted on a Loughborough Sound Images PC AT compatible plug in board. This is mounted in an IBM

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compatible 25 MHz 386 personal computer. Communication with a robot is provided via a standard RS232 serial line.

effects. In the system a sample-by-sample comparison is performed. A sample is accepted as valid if two samples with an amplitude difference within ___25% are adjacent. The effectiveness of this technique is shown in Figures 3 and 4. It can be seen that the STA returns have disappeared and the background noise level is significantly reduced with the variable PRF system active. The data was obtained with the target layout shown in Figure 5. The steel plate was inserted to simulate the presence of objects in front of a solid vertical surface. This is a situation that is likely to occur regularly due to the

The variable PRF system, for the removal of STA echoes, has been implemented in software on the DSP. The PRF is changed by altering the time delay between transmitter firings. Two sets of data are captured by the DSP and then compared. The process is effectively a logical 'and' but the comparison must be made in such a way as to compensate for the difference in amplitudes that may occur between the two data sets, due to environmental

92

Suppression of second-time-around echoes

References

widespread occurrence of vertical surfaces in a real environment. This technique is therefore particularly suited to use in robot anti-collision systems.

1 Brady, M., Durrant-Whyte, H., Probert, P. and Hu, H. 'A sensing autonomous vehicle for advanced manufacturing' Proceedings 2

3

Conclusions The use of a variable PRF has produced a significant reduction in sensor ambiguities caused by STA reflections. The system has proved particularly effective in enabling objects in front of large, flat reflectors to be detected unambiguously.

4 5 6

One disadvantage of the system is that each new target has to be 'looked at' twice, resulting in a decrease in response speed. However, if only short range coverage is required the system will still be faster than waiting for all the energy from a previous transmission to die away.

7 8 9

10 11

Acknowledgements

12

The authors would like to thank the Department of Manufacturing Engineering and Operations Management at the University of Nottingham for providing the facilities and the Science and Engineering Research Council for financial support. They would also like to thank Nick Cope for help in the construction of the ultrasonic arrays used in this work.

13 14 15

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IEEE International Conference on Roboti¢s and Automation, Cincinnati, (13-18 May 1990) pp 140-145 Webb, P., Gibson, !. and Wykes, C. 'Robotic guidance using ultrasonic arrays' J Robot Syst to be published Peremans, H, Audeneart, K. and Van Campenhout, J.M. 'A high resolution sensor based on tri-aural perception' IEEE Trans Robot Autom RA-9 (1990) pp 1533-1660 Borenstein, J. and Koren, Y. 'Obstacle avoidance with ultrasonic sensors' IEEE J Robot Autom RA-4 (1988) pp 213-218 Webb, P.F. 'An ultrasonics based system for the extraction of range and bearing data for multiple targets' PhD Thesis University of Nottingham (1994) Estochen, E.L 'Applications of acoustic sensors to robotic seam tracking' Proc IEEE Ultrasonics Symp Denver (October 1987) Crowley, J . L 'World modelling and position estimation for a mobile robot using ultrasonic ranging' Proc 1EEE Ultrasonics Symp (1989) pp 674-680 Ultrasonic Range Finders, Polaroid Corporation (1982) Carr, H. 'Electrostatic transducers for airborne ultrasound' PhD Thesis, University of Nottingham (1989) Munro, W.S.H. 'Ultrasonic phased arrays for use in imaging and automatic vehicle guidance' PhD Thesis, University of Nottingham (1990) Munro, W.S.H. and Wykes, C. 'Arrays for 100kHz airborne ultrasound' Ultrasonics 32 1 (1994) pp 57-63 Gerlaeh, K. 'Second time around radar return suppression using PRI modulation' IEEE Trans Aerosp Electr Syst 25 6 (1989) pp 854-862 Hovanessian, S.A. 'An algorithm for calculation of range in a multiple PRF radar' 1EEE Trans Aerosp Electr Syst 12 2 (1976) pp 287-290 Hovanessian, S.A. 'Medium PRF performance analysis' IEEE Trans Aerosp Electr Syst 18 3 (1992) pp 286-295 Borenstein, J. and Koren, Y. 'Noise rejection for ultrasonic sensors in mobile robot applications' Proc IEEE International Cot!f on Robotics and Automation, Nice, France (1992) pp 1727-1732