Communications – electrical: acoustic wave systems and devices – Echo systems – Distance or direction finding
Patent
1995-12-18
1997-09-23
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Distance or direction finding
G01S 1508
Patent
active
056711905
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and a device for distance measurement which are suitable for ultrasonic distance sensors which work on the basis of single-transducer arrangements in pulse-echo operation. The invention is advantageously usable for the purposes of contactless distance measurement, position determination of objects and filling-level measurement, with a wide dynamic measurement range.
2. Description of the Prior Art
Ultrasonic distance sensors which work according to the pulse-echo method, transmit a sonic signal and receive in the form of an echo the signal reflected from the object. The propagation time of the acoustic signal transmitted by the sonic transmitter until arrival at the sonic receiver gives a measure of the absolute path distance, by which is meant the path distance from the sonic transmitter to the object and back. A single sonic transducer is frequently used for alternate transmission and reception. However, the reflected sonic signal cannot be received until the transmission process has decayed sufficiently. For this reason, ultrasonic sensors which operate with a single transducer always have an ambiguity zone, the extent of which depends both on the properties of the transducer and on the excitation function. This intrinsic ambiguity zone is disadvantageous for many practical applications since, on the one hand, the measurement range is restricted to the extent that echo signals whose propagation time is less than the time corresponding to the ambiguity zone can no longer be detected and, on the other hand, in the case of detected echoes, it is no longer possible to establish whether the first echo actually detected is the primary echo or an echo produced by multiple reflection, a so-called multiple echo. This can lead to large measurement errors if a predetermined minimum distance between the ultrasonic distance sensor and the object of the measurement is not reached.
Methods and circuit arrangements by which it is intended to minimize the ambiguity zone of the transducer are known. As described in German OS 33 39 984 and in U.S. Pat. No. 3,683,324, the solution to the problem consists in using a controllable amplifier to reduce the amplitude of the decaying crosstalk signal after the transmission process to such an extent that a particular threshold value is not exceeded. Short-range echoes which are above this threshold value can, however, be detected.
Another solution consists in shortening the reverberation time by exciting the transducer after the transmission process with phase-inverted pulses, and thereby retarding the transducer. Time-optimized short echo signals are obtained by exciting the sonic transducer with the temporal signal corresponding to the inverse transducer transfer function, for example according to Magori, V.: Signal processing for smart ultrasonic sensors. Proc. 3rd Annual European Computer Conference, Hamburg, May 1989, pp 3-21 to 3-26. Because of the comparatively long temporal extent of the inverse excitation signal, the ambiguity zone of the sensor is not, however, substantially reduced in this case.
For measurement tasks dealing primarily with small distances, very wide-band ultrasonic transducers are preferably used, which are inherently distinguished by very short reverberation times.
The known solutions for shortening the ambiguity zone remain adversely affected by the disadvantage that echoes within these ambiguity zones are not detectable, and spurious measurements can therefore be triggered.
Ultrasonic distance sensors with a single transducer are known, in which the transducer is arranged in the sensor head in such a way that the ambiguity zone of the transducer is compensated for by an adapted sound delay path within the sensor head, as described in DE 83 32 005. In this way, spurious measurements as a result of ambiguous echoes are prevented, since the object of the measurement cannot be closer than a minimum distance from the transducer. The delay path may in this case be stra
REFERENCES:
patent: 3683324 (1972-08-01), Hoxsie
patent: 4580251 (1986-04-01), Koukovinis
patent: 4675854 (1987-06-01), Lau
"Signal Processing For Smart Ultrasonic Sensors," Magori, Proc. 3rd Annual European Computer Conf., Hamburg, Germany, May 1989, pp. 3-21 to 3-26.
Kroemer Nils
Vossiek Martin
Pihulic Daniel T.
Siemens Aktiengesellschaft
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