Communications – electrical: acoustic wave systems and devices – Echo systems – Distance or direction finding
Patent
1989-03-15
1990-06-12
Tarcza, Thomas H.
Communications, electrical: acoustic wave systems and devices
Echo systems
Distance or direction finding
367127, 367908, G01S 1500
Patent
active
049339150
DESCRIPTION:
BRIEF SUMMARY
The present invention refers to a method of indicating the time of an acoustic pulse at the transmission or reception of the same, the acoustic pulse comprising a wave package, the amplitude envelope of which having an ascending and a descending flank. The invention also refers to a device for carrying out such a time indication of an acoustic pulse being transmitted or received in an acoustic system which comprises means for the generation of an electric signal that corresponds to the pulse.
Advantageously, the method and the device according to the invention can be used for the measurement of transit time in an echo type acoustic system, i.e. a system in which the transit time of an acoustic pulse is measured from the time of transmission of the pulse and to the time it is again received after having been reflected.
Such acoustic measurement systems exist in several variants and may be used in many different areas, for example measuring level or volume in tanks, containers or similar in measuring distance, for example auto-focusing in camera lenses, in measuring of flow, in medical diagnostics, such as ultrasound examination, in position determination etc.
Due to the fact that, in particular, the method and the device according to the invention solve the problems of today in the measurements of transit time in connection with echo type acoustic liquid level measurement in tanks and containers, a brief description will follow of the design and function of a known acoustic liquid level measurement sytem and then the problems existing in such a system will be elucidated.
In acoustic liquid level measurement of the so-called echo type an acoustic transducer is provided at the highest point in a container which contains the liquid, the level or volume of which is to be measured. The acoustic transducer is fed from a transmitter with a first electric signal, such as a wave package of a relatively short duration. In response to this first signal the transducer generates a corresponding acoustic pulse which is transmitted downwards towards the surface of the liquid. After reflection against the surface the pulse is again picked up by the transducer which in response thereof generates a second electric signal which is fed to a receiver. The time interval between the first and the second electric signal, i.e. the transit time of the acoustic pulse, is determined and the distance from the transducer to the surface of the liquid can be calculated with a knowledge of the propagation velocity of the acoustic pulse in the medium in question.
Obviously, in connection with such a transit time measurement it is important to be able to make an accurate determination of the time of reception of the reflected pulse or echo. In this connection there are a number of factors that can cause incorrect measurement results in the indication of time of the transmitted and the received acoustic pulse, respectively.
One known way of indication of time of an acoustic pulse is simply to detect the presence or absence of the pulse. In this case the accuracy in the indication of time of the pulse or the electric signal is essentially dependent on the width of the amplitude envelope of the wave package.
In order to achieve a higher accuracy in the indication of time of acoustic pulses different types of threshold level detectors have been proposed. In these cases the indication of time is performed by a detector that detects the amplitude of the pulse and approves the pulse if its amplitude exceeds a preset threshold level. The indication of time of the pulse occurs at the moment the amplitude of the pulse exceeds the threshold level.
In many cases threshold level detection on the positive or ascending flank does not give a sufficient accurate indication of time of the pulse and an attempt to increase the accuracy by increasing the slope of the ascending flank involves an increased bandwidth which is unfavourable and in turn causes a worse signal-to-noise ratio.
Further, the threshold level technique has the disadvantage to be sensitive to
REFERENCES:
patent: 4022058 (1977-05-01), Brown
patent: 4334431 (1982-06-01), Kohno et al.
patent: 4480485 (1984-11-01), Bradshaw et al.
Pihulic Daniel T.
Tarcza Thomas H.
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