Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1996-10-25
1998-06-30
Budd, Mark O.
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
310337, 73290V, H01L 4104, G01F 2328
Patent
active
057739137
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a piezoelectric sensor probe for sensing a characteristic of a medium surrounding the probe. The invention has particular application to sensing the level of oil in an oil storage tank of the type used in domestic central heating. For a domestic user, it is essential to know when the oil in such a tank is running low, and for the tanker driver it is essential to know when the tank is full.
Apart from manual inspection and the use of a dipstick, present methods of sensing the level of oil in a storage tank make use of level gauges, such as the Atkinson type, which rely on a visible sighting of fluid in a transparent tube. However, such tubes frequently become dirty and/or blocked, and there is a substantial risk of fluid leakage.
GB-A-2097964 shows a pair of piezoelectric sensors spaced one above the other to detect the level of liquid around a submersible pump. Each sensor forms part of a respective oscillator circuit which oscillates only if the medium surrounding the element has the appropriate acoustic impedance, and each sensor is received in a recess in the outer casing of the pump.
EP-A-0515254 discloses a fluid level indicator having vertically spaced piezoelectric emitters carried on an insulating support which also carries the conductors for conducting electrical signals to the respective emitters. The emitters are excited sequentially, and the emitted signals are received by a common receiver carried on a separate vertical support spaced from the support carrying the emitters.
U.S. Pat. No. 3,220,258 discloses a liquid level detector in which spaced apart electro-acoustic transducers are supported on a rigid wall, plate or post. The transducers may be piezoelectric devices and may form the resonant element in a crystal oscillator. Changes in impedance or frequency of oscillation of the transducer are detected to determine the level of the liquid.
EP-A-0384373 has a sound emitter at the bottom of a fuel tank for emitting pulses of ultrasound into the liquid, the pulses being reflected at the interface between the light and heavy oils. A receiver at the bottom of the tank receives the reflected pulses, and can be combined with the emitter to form an electro-acoustic crystal. A probe stick extending into the fuel tank carries additional electro-acous zic receivers one above the other which receive the emitted soundwaves and pass electrical signals along wires in a tube forming part of the probe stick.
According to the present invention there is provided a piezoelectric sensor probe comprising an elongate insulative support member supporting a plurality of piezoelectric sensing elements spaced along the length of the member, the support member having respective conductors extending from one end of the member to each of the sensing elements for conducting electrical signals to and from each sensing element, characterised in that each sensing element is mounted on the support member in spaced relationship therewith such that the sensing element is free to vibrate substantially independently of the support member when an electrical signal is applied thereto.
Preferably, an electrical connector is provided at the end of the support member for connecting the respective conductors from each sensing element to a monitoring circuit for monitoring an electrical characteristic of the sensing elements dependent on the medium surrounding the sensing element. For example, in a fluid level detector, the monitor circuit monitors a characteristic dependent on the presence or absence of a fluid at the level of the respective elements. The electrical characteristic may comprise, for example, the resonant frequency of the sensing element.
The support member preferably comprises a strip of printed circuit board, and the conductors then comprise conductive tracks on the board.
The piezoelectric sensing element preferably comprises a piezo ceramic disc bonded to a metal resonating plate, the plate being secured to the printed circuit board in a spaced relationship therewith and being electrically
REFERENCES:
patent: 3220258 (1965-11-01), Rod
patent: 4446454 (1984-05-01), Pyle
patent: 4679160 (1987-07-01), Whitener
patent: 5349994 (1994-09-01), Koeninger
Budd Mark O.
Greigg Edwin E.
Greigg Ronald E.
Sensor Systems (Jersey) Limited
Williams Timothy A.
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