Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-03-23
2001-04-10
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06215226
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a liquid gauging system, in general, and more particularly to an improved method and apparatus for determining ultrasonically the quantity of liquid in a tank.
Ultrasonic liquid gauging systems, like a fuel gauging system for an aircraft, for example, generally include one or more ultrasonic transducers at each fuel tank of the aircraft, generally disposed at the bottom thereof, and one or more target reflectors disposed in the tank at predetermined distances from the ultrasonic transducer. In operation, an incipient ultrasonic burst signal is transmitted from the transducer, conducted through the liquid, reflected from the height of the liquid, i.e. the liquid/air interface, and returned to the transducer where it is received. A round trip time period from inception to reception of the ultrasonic burst signal is measured to determine the height of the liquid in the tank. In order to determine liquid height the velocity of sound of the liquid is needed. One technique for determining velocity of sound of the liquid is to utilize the time measurements for the ultrasonic burst reflections from the one or more target reflectors in the tank. Since the distance between a target reflector and the transducer is known the velocity of sound may be determined from said distance and the time measurement for the target reflector.
But this presumes that the velocity of sound of the liquid is substantially constant over a large liquid height profile around the target reflector. Unfortunately, this may not always be the case, especially if the liquid in the tank is thermally stratified. Accordingly, having the velocity of sound at one height of the liquid may not be sufficient across the over all height profile of the tank liquid, especially if accuracy of liquid quantity measurement is of paramount importance. Thus, it would be an important improvement to be capable of determining the velocity of sound cumulatively at the height of the liquid in the tank under thermally stratified conditions.
In addition, stratification may also occur due to a separation of different liquids in the tank. For example, reflections which may occur from the stratified liquid levels, may compromise the time measurements of the reflections from the target reflectors. Therefore, a liquid gauging system may also be improved by distinguishing between the different reflections in order to obtain accurate time measurements from the reflections of the target reflectors.
Also, current ultrasonic transducers like that illustrated in cross sectional view in
FIG. 3A
, for example, include a bottom layer of piezoresonator material which is of a different acoustic impedance than that of the liquid in the tank about the operational frequency passband of the ultrasonic burst or pulse transmitted and received therefrom. Generally a second or top layer of material is disposed between the piezoresonator material and the tank liquid for matching the acoustic impedances of the piezoresonator material and the tank liquid to render an efficient energy transfer. However, this acoustic impedance matching has not always been accurate due primarily to the available material for use as the second layer. Accordingly, an improvement in efficiency of energy transfer can occur if the acoustic impedance matching is made for accurate than currently implemented.
The embodiment of the invention which will be described in a succeeding section ameliorates the aforementioned drawbacks, thus providing a more accurate and improved ultrasonic liquid gauging system.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, an ultrasonic transducer comprises a first or bottom layer of piezoresonator material, a second or middle layer of material having a thickness of approximately one-quarter wavelength, which is based on the frequency of an ultrasonic pulse and the velocity of sound through the second layer of material, and a third or top layer of material having a thickness of approximately one-quarter wavelength, which is based on the frequency of the ultrasonic pulse and the velocity of sound through the third layer of material. An ultrasonic pulse is transmitted from the first layer and conducted through the second and third layers into a tank of liquid. Echos from the transmitted pulse are conducted through the second and third layers and received at the first layer. The materials of the second and third layers have corresponding acoustic impedances which together are chosen to match the acoustic impedance of the piezoresonator material to the acoustic impedance of the tank liquid about the operational frequency passband of the ultrasonic pulse. In one embodiment, the acoustic impedances of the second and third layer materials are determined from a substantially flat responding transfer function of the acoustic impedances of the first layer material and the tank liquid. In the same embodiment, the material of the second layer also includes the characteristics of a low density and medium Youngs modulus. Boron nitride was found to have the aforementioned characteristics and suitable for the material of the second layer. The boron nitride layer may be grown by pyrolytic chemical vapor deposition.
Another aspect of the present invention involves a circuit and method of determining the phases of the ultrasonic burst echo signals received by the ultrasonic transducer. According to this aspect, an ultrasonic burst echo signal is received and positive and negative envelope signals are generated therefrom. The phase of the echo signal is then determined based on the corresponding positive and negative envelope signals.
Another aspect of the present invention involves a system and method for discriminating between echo sources of an ultrasonic burst echo signal resulting from an incipient ultrasonic burst signal transmitted from an ultrasonic transducer wherein the incipient signal has an initial phase. In accordance with this aspect, the echo signal is received and the phase thereof determined and compared with the initial phase of the incipient signal to discriminate between echo sources thereof.
A further aspect of the present invention involves a method of determining ultrasonically the height of a thermally stratified liquid in a tank using at least one ultrasonic transducer disposed at the bottom of the tank for transmitting an ultrasonic signal towards the height surface of a liquid and for receiving ultrasonic reflections. The method includes the steps of measuring the temperature of the liquid at at least two different heights, determining the velocity of sound in the liquid at at least two different predetermined heights, establishing an approximation of a velocity of sound versus temperature profile for the liquid, determining an approximation of a velocity of sound versus height profile for each of at least two height regions based on the temperature measurements, the velocity of sound determinations, and the established approximation of velocity of sound versus temperature profile for the liquid, measuring the time of an ultrasonic reflection from the height surface of the liquid, determining a velocity of sound for the ultrasonic reflection from the height surface based on the target ultrasonic reflection times and the velocity of sound versus height profile approximations, and determining the height of the liquid from the time measurement of the ultrasonic reflection from the height surface and the determined velocity of sound therefor.
A still further aspect of the present invention involves a circuit for exciting an ultrasonic transducer disposed at a tank of liquid remote from the circuit wherein the circuit includes means for generating an electrical excitation signal for the transducer, and step-up transformer means including a transformer having a primary side coupled to the generating means and a secondary side coupled differentially to the remotely disposed transducer for conveying the excitation signal to the transducer. The circuit affords a DC i
Calfee, Halter & Griswol LLP
Dougherty Thomas M.
Simmonds Precision Products Inc.
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