Communications – electrical: acoustic wave systems and devices – Echo systems – Speed determination
Patent
1991-11-21
1993-06-29
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Speed determination
367 90, G01S 1500
Patent
active
052240754
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an apparatus for measuring the velocity of a moving body such as a ship and a water body, and more particularly, relates to a system for detecting the speed of a ship or the velocity of a water current flow by utilizing the Doppler effect generated with ultrasonic signals propagating in the water.
TECHNICAL BACKGROUND OF THE INVENTION
Echo signals reflected by things in water such as planktons and small dust and the like and by the seabed and resulting from ultrasonic signals emitted from a transmitting-and-receiving transducers mounted on the bottom of a ship have Doppler shifted frequencies due to the Doppler effect and allow the detection of the speed of the own ship or the velocity of a water current by measuring the Doppler shifted frequencies.
For example, when the speed V of the ship is desired to be detected, an ultrasonic signal (having a carrier frequency "f.sub.o ") is emitted downwardly in an oblique direction (at an angle .theta. with respect to the horizontal) by means of a transmitting-and-receiving transducer as shown in FIG. 4. A Doppler frequency shift "f.sub.d " included in echo signals reflected by the seabed will be as follows: of a ship can be obtained from the following equation which is a modification of the equation (1). following three methods are conceivable to obtain it.
1 Frequency Tracking Method
A variable frequency oscillator is provided in a measuring device and is controlled so that a difference between the frequency of output signals from the oscillator and the frequency of echo signals becomes zero. The frequency of the echo signals will be obtained from the varied frequencies.
With this method, however, the oscillator is a complex circuit in which a PLL system and the like is used. There are limitations in the frequency tracking speed and in the range thereof. It is difficult to detect the Doppler shift frequency with high precision based on echo signals resulting from one transmission and reception of ultrasonic wave signals. Moreover, due to the limitation of the tracking speed, a tracking circuit is required for a beam formed by each of transducers and for each liquid layer to be measured, so that a large number of parts are needed for an apparatus such as a current flow metering system using a plurality of beams and receiving echo signals from a plurality of water layers to be investigated.
2 Pulse Counting Method
As shown in FIG. 5, this is a method to set a certain duration of time from "t0" to "t1" and to count the number of pulses of the echo signal detected within the time duration. Since frequency is expressed by the number of waves (number of pulses) in one second, the Doppler shifted frequency can be obtained in accordance with n/.DELTA.t, where the time duration for measurement is .DELTA.t and the number of pulses counted in the time duration is "n".
However, in order to detect the frequency with high precision, a time duration for measurement must be set to be long. This increases the thickness of a layer to be investigated that corresponds to the time from a time instant "t0" to another time instant "t1" (as shown in FIG. 4). As a result, resolution in the depth direction is degraded, and thus, it becomes difficult to measure a Doppler shift frequency of an echo signal produced at a desired depth.
3 Period Detecting Method
This is a method to obtain a time .DELTA.t required for counting a certain number of pulses "n" included in an echo signal, as opposed to the method 2, and then to calculate an average period T (=.DELTA.t
) per one pulse included in the echo signal and to invert the period (1/T) to obtain a Doppler shifted frequency. This method measures a time duration corresponding to n pulses included in the echo signal with count clock pulses having an extremely high repetition frequency as compared to the frequency of the echo signal so that it is possible to obtain a Doppler shifted frequency with higher precision as compared to the method 2.
However, echo signals reflected by a
REFERENCES:
patent: 3953823 (1976-04-01), Katakura
patent: 4104912 (1978-08-01), Clavelloux et al.
Aoyama Shigeru
Iino Hiroshi
Ishihara Shinji
Furuno Electric Co. Ltd.
Pihulic Daniel T.
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