GB1581291A

GB1581291A - Sensor device

Info

Publication number: GB1581291A
Application number: GB4543877A
Authority: GB
Original assignee: Kkf Corp
Current assignee: Kkf Corp
Priority date: 1976-12-09
Filing date: 1977-11-01
Publication date: 1980-12-10
Also published as: DE2749077C2; IT1089038B; DE2749077A1; JPS5372650A

Description

(54) SENSOR DEVICE (71) We, KKF CORPORATION, a corporation organised and existing under the laws of the state of California, United States of America having a place of business at 2936 de la Vina Street, Suite 4, Santa Barbara, California 93105, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to sensors and more particularly to simple sensors having piezoelectric material in the transducer.

Considerable effort has gone into the development of mechanisms which function to measure or locate the level of a material in a container. Typical of such devices is the apparatus taught by Mongan in United States Patent No. 3,010,318 entitled Method and Apparatus for Measuring Liquid Levels. The essence of the Mongan device includes a transmitter and a receiver with an interconnecting-waveguide running therebetween such that ultrasonic waves produced at the transmitter are passed through the wave guide to the receiver.In operation, the waveguide is positioned to pass through the liquid vapor interface and since the impedance of the waveguide will vary depending upon the amount of surface in contact with the liquid thereby affecting the amount of transmitted power which reaches the receiver, with suitable calibration of the device of the signals arriving at the receiver can be converted to liquid level position. This relatively large device uses multiple transducers, is cumbersome and expensive for many applications.

In the Fetal Heart Transducer described by J. R. Richards in United States Patent No. 3,379.901, an apparatus comprising two circular transducers mounted concentrically is described. One transducer acts as a transmitter and the other as a receiver in the transmission of sonic energy through liquids to investigate a fetal heart by studying the doppler shift in the reflected waves. The Richards' apparatus comprises two concentric piezoelectric crystals which are located in a disc assemblage and are carefully isolated physically and electrically from each other.

United States Patent No. 3,625,058 to Endress et al describes an apparatus which indicates whether or not a liquid is present.

Two parallel vibrator rods are induced to vibrate by a piezoelectric transducer and a second transducer senses the rod motion and provides an output signal which is proportional to the amplitude of such motion. When the mechanism is immersed in a liquid the vibrational frequency changes and is sensed in the receiving transducer.

Another transducer containing sensing apparatus is disclosed by Samuel et al who describe a transducer for sensing the presence of a liquid in United States Patent No.

3,825,025. The basic apparatus comprises two piezoelectric elements which are spaced apart from one another and means for inducing ultrasonic vibrations in one of the elements. The coupling between the two piezoelectric elements is a function of the level of liquid which is present in the physical space left between the two transducers and which is oriented in a vertical direction so that with suitable calibration the system can be used to determine fluid level.

A simple rugged device which can be built inexpensively and perform with great reliability such as that demanded for liquid level sensing in automobile applications is needed.

According to the present invention, there is provided a sensor device for sensing the presence of material comprising: a base having means for attaching the device to an object; a sandwich element supported by the base, capable of bending in response to an electrical input and capable of providing an electrical output in response to the bending, including: an electrically conductive material which forms a substrate, piezoelectric material coated to one side of the substrate, a drive electrode attached to the piezoelectric material to receive said electrical input, and a pickup electrode attached to the piezoelectric material to pick up said electrical output; a first electrical conductor attached to the drive electrode; and a second electrical conductor attached to the pickup electrode.

The present invention is characteristically a simple apparatus which is compact and rugged and inexpensive to manufacture.

The sandwich element can be either a cantilevered blade or a disc which is held circumferentially. The apparatus can be assembled with several sensors which use some common electronics and some individual electronics. The sensor is subjected to bending mode resonances which provide the largest mechanical displacement and in turn the largest electrical output. Also, the use of piezoelectric coating allows the resonance range to be stuffed onto the ultrasonic and near ultrasonic frequencies.

The following is a detailed description of preferred embodiments of the invention as discussed and illustrated in the accompanying drawings, wherein: Figure 1 is a cross section through a disc sensor embodiment in accordance with the present invention Figure 2 is an enlarged view of the disc element of the sensor; Figure 3 is a view of the bottom surface of the disc element; Figure 4 is a sketch of the vibrational mode of the disc element Figure 5 is a simplified wiring schematic showing a self-oscillating electronic circuit useful with the level sensor; Figure 6 is a simplified wiring diagram of an external driving oscillator circuit which is useful with the present invention; Figure 7 is a front view of a blade sensor in accordance with the present invention with the base element shown in section; Figure 8 is a side view of the apparatus shown in Figure 7;; Figure 9 is an alternate embodiment of a blade sensor; Figure 10 is a sketch of the vibrational mode of the blade element; and Figure 11 is a simplified wiring diagram showing an alternate self-oscillating electronic circuit.

One of the very practical applications of the sensor devices in accordance with the present invention is as a level sensor for fluids in the various liquid systems in an automobile. In such an application, the sensor which includes a mechanical element having a vibratory resonance in the ultrasonic range is placed at the desired lowest level position in, for example, the brake fluid system. If the sensor is exposed to liquid a characteristic electric output signal is observed, however, if the fluid drops below the position of the sensor, the acoustic load which the fluid would otherwise be exerting on the sensor is removed and is detected as a change in either the resonant frequency or the vibration amplitude of the output signal from the sensor depending on the type of electronic circuit used.

One configuration of a sensor in accordance with the present invention which uses an active element in the shape of a disc is shown in Figures 1-3. A sensor 10 is formed from a circular base 12 and a disc assembly 14. The disc assembly comprises a substrate 16, a layer 18 of piezoelectric material, a drive electrode 20 and a pickup electrode 22. A first conductor 24 is connected to the drive electrode and a second conductor 26 is connected to the pickup electrode, each conductor being electrically isolated from the other and passing through a sealant 28 which closes off one end of the circular base and holds the conductors in place. A suitable selection of the materials and shape of the components comprising the disc assembly allows the apparatus to have a bending mode charactistic such as is shown in Figure 4.A nodal ring 30 located in the disc assembly defines the preferred sites for the electrical connection between the conductors and the disc assembly. These connection points are preferred since they do not experience the extensive cyclic displacement which is experienced at other locations on the disc. As a practical matter, the drive electrode is located at the outside edge of the nodal ring and the pickup electrode is located at the inside edge of the nodal ring for the sensor shown in Figure 3.

In addition to the basic mechanical apparatus which has been described, suitable electrical circuitry is necessary to an overall sensor device. One such circuit which includes a warning lamp such as would be located on the dashboard of an automobile is shown in Figure 5. In this circuit, the sensor is placed in a feedback loop 32 with appropriate gain, frequency response, and phase shift, so that oscillation of the feedback loop occurs under stable loading conditions on the sensor. Ordinarily the system is arranged to oscillate whenever the sensor is exposed to air for example and to cease oscillating when exposed to a fluid although these conditions can be reversed in preferred circumstances.

In operation of the self-oscillating system shown in Figure 5, the gain, response and phase shift of the feedback loop are such that oscillation occurs in the circuit when the sensor is exposed only to air; no oscillation occurs when the sensor is exposed to a liquid. A feedback amplifier 34 drives a voltage level sensor 36 which determines the level of the output voltage due to the oscillation condition. The voltage sensor controls a lamp switch 38 which in turn activates a warning lamp 40 for those conditions when the fluid is not in contact with the sensor.

In some applications multiple sensor units are used in combination and a single drive oscillator powers several sensors. This results in less critical individual amplifier circuits since the amplifiers do not have to be tuned. Such a circuit is shown in Figure 6 wherein the output from a sweeping oscillator 42 causes a frequency modulated oscillator 44 to undergo a range of periodic frequency changes in output signals which includes the resonant frequency of the sandwich under no load conditions and excludes the loaded resonance. The frequency range over which the signals from the sweeping oscillator is modulated is made sufficient to include the range of resonant frequencies which will occur when the sensor is subject to an error load, the range being due to variations in such considerations as manufacturing tolerances and temperature, etc.Although the externally driven system in Figure 6 is described for use with multiple sensors, the concept is applicable to a single sensor device.

An alternative approach to the externally driven frequency-sensitive system is shown in Figure 11. The circuitry required does not include the sweeping oscillator 42 or the frequency modulated oscillator 44 and is therefore simplar and less expensive. The output from the feedback loop 32 is passed through a Schmitt trigger 46 for the purpose of squaring up the pulse profile. These signals are then used to trigger a one shot multivibrator 48 to produce pulses of a uniform width and amplitude. The pulses are next passed through a pulse integrator 50 which adds the individual pulses to form a continuous signal level which increases with increasing pulse rate and decreases with decreasing pulse rate and which is matched against a reference voltage in a level comparer 82.In those circumstances in which the comparer determines that the output from the pulse integrator is high, which condition occurs when the vibration frequency is above a predetermined value as is the case when the fluid level drops. the lamp switch 36 is activated to turn on the warning lamp 38.

A sensor device in accordance with the present invention in a particular blade con figuration is shown in Figures 7 and 8. The transducer is a single bender of monomorph device. If an input electrical signal is applied across the piezoelectric layer 18 between the drive electrode 20 and the substrate 16, a mechanical force is set up across the piezoelectric layer causing the layer to bend.

Since the piezoelectric material is sandwiched between the substrate and the two electrodes, the motion of the piezoelectric material is experienced by the entire assemblage. As a practical matter the blade assembly is caused to resonate by subjecting it to an electrical signal of alternating current and the resonance in turn produces an output voltage between the pickup electrode 22 and the substrate due to the presence of the piezoelectric material. The blade assembly tends to bend in an arc having a length L which is the length of the substrate 16 which projects from the base 12. The length as well as the thickness, density and Young's modulus of the blade assembly are determinative of the frequency of the resonant bending mode of the blade.

Rather than have the assembly bend in an arc along its lengthwise axis, judicious selection of the ratio between the length L and the width W of the substrate with consideration being given to the support means can cause the assembly to undergo a crosswise bending mode such as is illustrated in Figure 10 with suitable suppression of other competing mode. Care must be taken to keep the longitudinal and transverse bending modes separated or a situation can be created in which the two modes cancel each other. Under these circumstances the anticipated resonant motion will not occur and thus no output signal will be realized.

In the blade configuration, the sensor is expected to come into direct contact with the material which is being measured and therefore a barrier is required to protect the piezoelectric material particularly for certain liquid materials. For example, if the liquid is electrically conductive, direct shorting between the electrodes can occur. Also, some materials may react deleteriously with the piezoelectric itself. Therefore, a thin coating is often applied to the entire exposed surface of the blade assembly for protection thereby preventing deterioration of operating characteristics. The coating or the film must be thin enough to minimize the tendency of the coating to damp the sandwich thereby reducing or even eliminating vibration and also hard enough to avoid viscous damping which can involve energy storage in the coating.Some epoxy materials have been found particularly satisfactory in this regard. For example, in one instance the blade device was heated to approximately 300"F (149"C) and then immersed in a powdered epoxy. The device was kept in contact long enough to allow the epoxy to melt due to the preheating, and then the blade was allowed to cool in the air.

A suitable protective coating of several mils (25.41l) usually two to three mils (50.8 to 76.2eel) in thickness was formed.

Another embodiment of the blade sensor in accordance with the present invention is shown in Figure 9. The drive electrode 20 and the pickup electrode 22 are physically arranged to produce the blade electrical characteristics which are different from those for the device shown in Figures 7 and 8.

Although this invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.

WHAT WE CLAIM IS: 1. A sensor device for sensing the presence of material comprising: a base having means for attaching the device to an object; a sandwich element supported by the base, capable of bending in response to an electrical input and capable of providing an electrical output in response to the bending, including: an electrically conductive material which forms a substrate piezoelectric material coated to one side of the substrate, a drive electrode attached to the piezoelectric material to receive said electrical input, and a pickup electrode attached to the piezoelectric material to pick up said electrical output; a first electrical conductor attached to the drive electrode; and a second electrical conductor attached to the pickup electrode.

2. The invention according to Claim 1 wherein the sandwich element has a resonant bending mode in the range of the ultrasonic frequencies.

3. The invention according to Claim 2 wherein the surface area of the drive electrode is greater than the surface area of the pickup electrode.

4. The invention according the Claim 3 wherein the piezoelectric material is ceramic.

5. The invention according to Claim 1 wherein the piezoelectric layer is applied as a slurry coating.

6. The invention according to Claim 1 wherein the base is electrically conductive.

7. The invention according to Claim 1 including further an electronic circuit connected electrically to the sandwich element by the first and second conductors wherein the circuit includes: means for generating an input electrical signal which is applied to the piezoelectric material between the driving electrode and the substrate to cause the sandwich element to bend; and means for receiving and processing an electrical output signal generated in the piezoelectric material by the bent condition between the pickup electrode and the substrate.

8. The invention according to Claim 7 wherein the input electrical signal is an alternating current signal which causes the sandwich element to vibrate resonantly.

9. The invention according to Claim 8 including further means for indicating a change in the resonant frequency of the sandwich element.

10. The invention according to Claim 9 wherein the electronic circuit which includes an amplifier, a voltage level sensor, a lamp switch and a warning lamp is a selfoscillating circuit.

11. The invention according to Claim 9 wherein the electronic circuit which includes a sweeping oscillator, an FM oscillator, an amplifier, a voltage level sensor, a lamp switch and a warning lamp is an external driving oscillator circuit.

12. The invention according to Claim 9 wherein the electronic circuit which includes an amplifier, a Schmitt trigger, a multivibrator, a pulse integrator, a level comparer, a lamp switch and a warning light is a selfoscillating circuit.

13. The invention according to Claim 7 including further an electrically insulating material which bonds the sandwich element to the base.

14. The invention according to Claim 13 including further a third electrical conductor which connects the flat substrate directly to the electronic circuit.

15. The invention according to Claim 1 wherein the sandwich element is in the form of a circular disc which is circumferentially retained within the base.

16. The invention according to Claim 15 wherein the drive electrode is substantially annular.

17. The invention according to Claim 16 wherein the pickup electrode is substantially circular.

18. The invention according to Claim 1 wherein the piezoelectric material adjacent to the drive electrode is physically separated from the piezoelectric material adjacent to the pickup electrode.

19. The invention according to Claim 1 wherein the sandwich element is in the form of a blade which is cantileveredly supported from the base.

20. The invention according to Claim 19 including further a protective coating over

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. epoxy to melt due to the preheating, and then the blade was allowed to cool in the air. A suitable protective coating of several mils (25.41l) usually two to three mils (50.8 to 76.2eel) in thickness was formed. Another embodiment of the blade sensor in accordance with the present invention is shown in Figure 9. The drive electrode 20 and the pickup electrode 22 are physically arranged to produce the blade electrical characteristics which are different from those for the device shown in Figures 7 and 8. Although this invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention. WHAT WE CLAIM IS:

1. A sensor device for sensing the presence of material comprising: a base having means for attaching the device to an object; a sandwich element supported by the base, capable of bending in response to an electrical input and capable of providing an electrical output in response to the bending, including: an electrically conductive material which forms a substrate piezoelectric material coated to one side of the substrate, a drive electrode attached to the piezoelectric material to receive said electrical input, and a pickup electrode attached to the piezoelectric material to pick up said electrical output; a first electrical conductor attached to the drive electrode; and a second electrical conductor attached to the pickup electrode.

the exposed surfaces of the piezoelectric material.

21. The invention according to Claim 20 including further means for indicating a change in the vibrational amplitude of the sandwich.

22. A sensor device for sensing the presence of material substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.