GB2134270A - Piezoelectric sensors - Google Patents

Abstract

The vibration and shock sensitivity of a piezoelectric sensor is reduced by connecting the electrodes of a primary sensor 11 in voltage opposition to the electrodes of a compensating secondary sensor 12 having a substantially similar piezoelectric element mounted on a common substrate with the primary current sensor. The piezoelectric element of the secondary sensor is loaded with a distributed mass approximating the distributed mass effectively loading the piezoelectric element of the primary sensor. The primary sensor 11 senses current I in conductors 14a, 14b in the arrangement shown. <IMAGE>

Description

SPECIFICATION Piezoelectric sensors The present invention relates to piezoelectric sensors and, more particularly, to a piezoelectric sensor having reduced vibration and shock sensitivity.

Piezoelectric devices, such as the piezoelectric current sensor described and claimed in our co-pending U.S. Patent application Serial No. 432,207, filed October 1, 1982, often possess unwanted sensitivity to shock and vibration, due to the piezoelectric element acting as an accelerometer. Thus, the sensor cannot differentiate between force applied due to a desired phenomenon, e.g. a compressive force responsive to a current to be measured, and that due to an undesired phenomenon, e.g. a compressive force due to shock and/or vibration. Reducing the shock and vibration accelerometer sensitivity of a piezoelectric sensor will allow the sensor to be used in industrial and other harsh environments without resorting to cumbersome mechanical dampening structures.

Accordingly, it is an object of the present invention to provide a piezoelectric sensor having reduced sensitivity to shock and vibration.

In accordance with the invention, the shock and vibration sensitivity of a sensor having a primary piezoelectric device, for measuring a physical quantity, is reduced by series connection of the electrodes of the device in polarity- opposition with the electrodes of a secondary piezoelectric device. Both the primary and secondary devices are so arranged as to be subjected to the same mechanical phenomena, while only the primary device is subjected to the physical quantity to be sensed thereby. The substantially equal output voltages of the devices, caused by the shock and vibration phenomena, are thus subtracted from one another, to reduce the shock and vibration sensitivity of the composite sensing device.

In a preferred embodiment, a piezoelectric current sensor, of the type described and claimed in our aforementioned co-pending U.S. application, is fabricated upon a common substrate with a compensating secondary sensor. The active element of the secondary sensor is loaded with a distributed mass approximating the distributed mass loading the active element of the primary sensor, whereby the differential potential between the primary and secondary sensors is substantially reduced responsive to shock and vibration influences on the common system.

In the accompanying drawings: The sole Figure is a perspective sectional view illustrating one example of a piezoelectric sensor embodying the present invention.

Referring to the sole Figure, a piezoelectric sensor 10 utilizes a common substrate 10' upon one surface 10'a of which is fabricated a primary piezoelectric sensor 11 and a secondary piezoelectric sensor 1 2 for shock and vibration compensation of primary sensor 11.

For example, the illustrated sensor 10 is to provide an output voltage VOUT, between a positive output terminal 1 Oa and a negative output terminal 1 Ob, with magnitude proportional to the square of a current I to be sensed by primary sensor 11.

Primary sensor 11 may be the piezoelectric current sensor described and claimed in the aforementioned co-pending application. Thus, primary sensor 11 has: a first current-carrying conductor 14a fabricated upon substrate surface 10'a; a first insulative layer 1 6a fabricated upon first conductor 14a; a first electrode 1 8a and a second electrode 1 7b fabricated upon opposite surfaces of a piezoelectric element 20, parallel to substrate surface 10'a; a second insulative layer 1 6b fabricated upon the upper electrode 1 8b of the piezoelectric element; and a second conductor 1 4b fabricated upon that surface of second insulative layer 1 6b furthest from the substrate.As is more fully explained in the aforementioned copending application, a flow of the same current I through each of first and second conductors 14a and 14b produces an output voltage V, at electrode 18a, with respect to electrode 18b, proportional to the square of sensed current I. Primary sensor output voltage V1 also has a component due to any extraneous force F1 having a component through the thickness of piezoelectric element 20, i.e. in the direction of arrows F1. This extraneous output voltage is often sufficient, depending upon the magnitude of the shock and vibration force F1, to reduce the sensitivity of primary sensor 11, and is often sufficiently large to completely mask the desired sensor output.

The secondary compensating sensor 1 2 is fabricated upon the same substrate surface 10'a as the primary sensor 11, with a predetermined, preferably small, separation therebetween. Secondary sensor 1 2 is fabricated by depositing a first electrode 22a upon one surface of a piezoelectric element 24 of substantially the same size, shape and properties as piezoelectric element 20. A second electrode 22b is fabricated upon the opposite face of piezoelectric element 24 from the face upon which first electrode 22a is fabricated.

The piezoelectric element 24, bearing electrodes 22a and 22b, is attached ta common substrate surface 10'a with the electrodes 22 substantially parallel to electrode 1 8 of the primary sensor. Thereafter, a member 26 is fabricated upon second electrode 22b. Member 26 is fabricated of any suitable material, such as a plastic and the like, and has a mass selected so as to distributively load sensing element 24 substantially similar to the distri buted mass loading upon primary sensor element 20 (as primarily provided by conductors 14 and insulative layers 16). The secondary sensor provides an output potential V2, from first electrode 22a to second electrode 22b, responsive only to the instantaneous magnitude of a force F2 operating upon element 24 in the direction of arrows F2.Secondary sensor 1 2 is specifically located and configured so as to not be acted upon by the physical phenomena to be sensed by sensor 10. Thus, as shown, only primary sensor 11 has conductors 1 4 for carrying a current to be sensed. Similarly, if sensor 10 is to sense a physical phenomenon such as pressure, secondary sensor 1 2 is placed within a pressuretight enclosure, such that only primarly sensor 11 is subjected to the pressure to be sensed.

Substrate 10' is of a material selected such that any shock and vibration force F, to which the entire sensor assembly 10 is subjected, will be so transmitted as to appear at the primary and secondary sensors as substantially equal shock and vibration forces F1 and F2. The sensor electrodes 1 8 and 22 are series connected, as by conductive leads 18, 30 and 32, to be in voltage opposition. Thus, the sensor output voltage VOUT = V1 - V2.

Since the primary sensor output voltage V1 contains components due to the desired stimulus, i.e. a voltage Vd, and a component due to shock and vibration, i.e. a voltage Vsv, and the secondary sensor contains a component due only to shock and vibration, i.e. a voltage V2 = V'sv, the sensor output voltage is given by the expression VOUT = V1 -V2 = Vd + VSV - SV As VsV and V'sv are respectively responsive to respective forces F1 and F2, and these forces are made substantially equal by means of the properties of substrate 10', the instantaneous magnitude of these voltages are made substantially equal by use of substantially similar sensing elements 20 and 24, subjected to substantially similar distributed mass loading.

Therefore, the sensor output voltage becomes VOUT = Vd + aVsv, where a is a constant (indicative of the match and tracking of VsV to VsV) and is substantially less than 1; the shock and vibration "noise" voltage is substantially reduced at the output and the signal-to-"noise" ratio of sensor 10 is increased.

Many modifications and variations to the above-described example of the invention will be apparent to those skilled in the art. In particular, it must be again emphasized that the shock and vibration compensation principle of the sensor can be applied to any piezoelectric sensing system, for sensing many other physical parameters besides the already-mentioned electrical current and pressure parameters.

Claims (11)

1. A sensor for sensing a physical quantity, comprising: first and second sensor output terminals; a primary piezoelectric sensing element having an output signal responsive both to the physical quantity to be sensed and to shock and vibration forces acting upon the primary piezoelectric sensing element; a secondary piezoelectric sensing element having an output signal substantially responsibe only to shock and vibrational forces acting upon the secondary piezoelectric sensing element; a common substrate supporting said primary and secondary sensing elements in a manner such that both sensing elements are subjected to substantially identical portions of the shock and vibrational forces acting upon the sensor; and means for electrically series connecting said primary and secondary sensing elements in opposition between said first and second sensor output terminals to provide a sensor output having a reduced contribution due to the shock and vibration forces acting upon said sensor.

2. The sensor of claim 1, further comprising means for distributively loading said secondary sensing element with a mass.

3. The sensor of claim 2, wherein said primary sensing element is loaded with a particular mass distribution, and said loading means distributively loads said secondary sensing element with substantially the same mass distribution.

4. The sensor of claim 3, wherein said loading means is a loading member.

5. The sensor of claim 4, wherein said loading member is positioned adjacent to the opposite side of said secondary sensing element from that side supported by said substrate.

6. The sensor of claim 4, wherein said loading member is formed of plastic.

7. The sensor of claim 4, wherein said secondary sensing element has first and second electrodes thereon, each positioned between said sensing element and a respective one of said substrate and said loading member.

8. The sensor of claim 7, wherein said primary sensing element has first and second electrodes thereon, each positioned substantially parallel to one another.

9. The sensor of claim 8, wherein the electrodes on said primary and secondary sensing elements are planar and are all substantially parallel to one another.

10. The sensor of claim 9, wherein the physical quantity sensed is electrical current.

11. The sensor of claim 10, further including at least one conductor for carrying the current to be measured, and positioned parallel to the electrodes on said primary sensing element.

1 2. The sensor of claim 11, further including at least one insulative member disposed between said at least one conductor and an adjacent one of said first and second electrodes.

1 3. A piezoelectric sensor substantially as herein described with reference to the accom Danvina drawn.