GB2159953A

GB2159953A - Variable resistance piezo-resistive devices

Info

Publication number: GB2159953A
Application number: GB08413939A
Authority: GB
Inventors: Aubrey Michael Crick
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1984-05-31
Filing date: 1984-05-31
Publication date: 1985-12-11
Also published as: JPS60262402A; GB2159953B; GB8413939D0

Abstract

In a variable resistance or potentiometer the resistive element is a piezo-resistive flexible silicon strip 17. Flexure of the strip 17 is effected by rotation of a cam 13 fixed to a drive shaft 12. A further reference cam 14 fixed to the drive shaft 12 abuts a support beam 19 on which the strip is mounted and provides compensation e.g. for eccentricity of the drive shaft assembly. <IMAGE>

Description

SPECIFICATION Variable resistance device This invention relates to variable resistance or potentiometer devices.

Our co-pending application No. 8312279 (J.C.

Greenwood 53) relates to a transducer for detecting relative rotational movement between first and second bodies, wherein the sensor element of the transducer comprises a flexible strip of single crystal piezo-resistive material so disposed as to be flexed by the rotational movement.

We have now found that a similar construction may be employed in the fabrication of a variable resistance or potentiometer device.

According to one aspect of the present invention there is provided a variable resistance or potentiometer device, including a flexible piezo-resistive element, and means whereby said element is subjected to a flexural force whereby the electrical resistance of the element is determined.

According to another aspect of the present invention there is provided a variable resistance or potentiometer device, including a flexible piezo-resistive element mounted on a support beam, a moveable cam assembly having a first cam surface in abutment with the element and a second cam surface in abutment with the support beam, and means for movement of said cam assembly whereby the element is deflected relative to the support beam by a distance corresponding to the relative configurations of the cam.

The device is of general application but is of particular use, e.g. in chart recorders, X-Y plotters and differential pressure recorders where a high degree of accuracy and reliability is required.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure is a schematic view of the variable resistance device. For clarity the housing in which the device is mounted is not shown.

Referring to the drawings, the device includes a double cam assembly 11 mounted on a shaft 12 typically moulded as a single layout from a plastics material. The cam assembly 11 includes an operating or 'law' cam 13, profiled to provide the desired resistance characteristic, and a substantially circular reference cam 14. Each cam bears against a corresponding cam follower 15, 16.

The sensor element of the device comprises a flexible strip 17 of single crystal silicon mounted via an integral mounting portion 18 on a moveable support beam 19. Movement of the beam 19 is provided for by a relatively thin flexible 'hinge' portion 20. Typically both the cam followers 15, 16 and the beam 19 are moulded from a plastics material. Advantageously the beam 19 and the element 17 are held in abutment against the cam followers 16 and 15 by springs 22 and 23.

The sensor element 17 is relatively doped, e.g.

with boron to form one or more piezo-resistive regions 21. Typically the element 17 incorporates a Wheatstone bridge resistor network, the values of the resistors being such that the bridge is balanced in the unstressed condition of the element 17. Contact to the resistor network 21 is effected via conductor wires 24 mounted on the support beam 19.

When the cam assembly 11 is rotated the element is flexed via the abutting cam follower 15 thus changing the values of the resistors, typically by up to 20%, producing a corresponding imbal- ance of the bridge. This imbalance signal may be amplified by an amplifier (not shown) to provide an output signal corresponding to the rotational position of the cam assembly.

Advantageously the cam 13 is profiled to provide a linear relationship between angular displacement of the cam and the resistance value of the element 17. The profile can be determined by calculation based on the strain/resistance characteristic of the particular element employed.

The transducer element 17 may be formed from single crystal silicon by masking and etching from a silicon wafer. Typically the wafer is processed to form the resistors 21 and is then masked on its front and back faces. The front mask defines the boundaries between the elements 17 and the back mask defines the mounting portion 18 of each element. The wafer is etched, first from the back face to define the flexible portion of each element, and then from the front to separate the individual elements. We have found that conventional etches provide a very uniform cut which maintains the thickness of the flexible portion of the element 17 to within one micron. Finally the contact leads 22 are applied, e.g. by nail head bonding, to contact the resistor network and the element is mounted via the mounting portion 18 on the support beam 19.

Compensation for eccentricity of the cam assembly is effected via the reference cam 14 and its cam follower 16 against which the support beam 19 abuts. When the cam assembly 11 is rotated the circular cam controls the reference beam so that the deflection of the sensor 17 corresponds to the difference in radius of the two cams. This provides automatic compensation for misalignment or eccentricity of the cam assembly.

Although the foregoing description relates to circular edge cams, other cam constructions, e.g. circular face or linear cams, can also be employed.

1. A variable resistance or potentiometer device, including a flexible piezo-resistive element, and means whereby said element is subjected to a flexural force whereby the electrical resistance of the element is determined.

2. A variable resistance or potentiometer device, including a flexible piezo-resistive element mounted on a support beam, a movable cam assembly having a first cam surface in abutment with the element and a second cam surface in abutment with the support beam, and means for movement of said cam assembly whereby the element is deflected relative to the support beam by a distance corresponding to the relative configurations of the

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Variable resistance device This invention relates to variable resistance or potentiometer devices. Our co-pending application No. 8312279 (J.C. Greenwood 53) relates to a transducer for detecting relative rotational movement between first and second bodies, wherein the sensor element of the transducer comprises a flexible strip of single crystal piezo-resistive material so disposed as to be flexed by the rotational movement. We have now found that a similar construction may be employed in the fabrication of a variable resistance or potentiometer device. According to one aspect of the present invention there is provided a variable resistance or potentiometer device, including a flexible piezo-resistive element, and means whereby said element is subjected to a flexural force whereby the electrical resistance of the element is determined. According to another aspect of the present invention there is provided a variable resistance or potentiometer device, including a flexible piezo-resistive element mounted on a support beam, a moveable cam assembly having a first cam surface in abutment with the element and a second cam surface in abutment with the support beam, and means for movement of said cam assembly whereby the element is deflected relative to the support beam by a distance corresponding to the relative configurations of the cam. The device is of general application but is of particular use, e.g. in chart recorders, X-Y plotters and differential pressure recorders where a high degree of accuracy and reliability is required. An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure is a schematic view of the variable resistance device. For clarity the housing in which the device is mounted is not shown. Referring to the drawings, the device includes a double cam assembly 11 mounted on a shaft 12 typically moulded as a single layout from a plastics material. The cam assembly 11 includes an operating or 'law' cam 13, profiled to provide the desired resistance characteristic, and a substantially circular reference cam 14. Each cam bears against a corresponding cam follower 15, 16. The sensor element of the device comprises a flexible strip 17 of single crystal silicon mounted via an integral mounting portion 18 on a moveable support beam 19. Movement of the beam 19 is provided for by a relatively thin flexible 'hinge' portion 20. Typically both the cam followers 15, 16 and the beam 19 are moulded from a plastics material. Advantageously the beam 19 and the element 17 are held in abutment against the cam followers 16 and 15 by springs 22 and 23. The sensor element 17 is relatively doped, e.g. with boron to form one or more piezo-resistive regions 21. Typically the element 17 incorporates a Wheatstone bridge resistor network, the values of the resistors being such that the bridge is balanced in the unstressed condition of the element 17. Contact to the resistor network 21 is effected via conductor wires 24 mounted on the support beam 19. When the cam assembly 11 is rotated the element is flexed via the abutting cam follower 15 thus changing the values of the resistors, typically by up to 20%, producing a corresponding imbal- ance of the bridge. This imbalance signal may be amplified by an amplifier (not shown) to provide an output signal corresponding to the rotational position of the cam assembly. Advantageously the cam 13 is profiled to provide a linear relationship between angular displacement of the cam and the resistance value of the element 17. The profile can be determined by calculation based on the strain/resistance characteristic of the particular element employed. The transducer element 17 may be formed from single crystal silicon by masking and etching from a silicon wafer. Typically the wafer is processed to form the resistors 21 and is then masked on its front and back faces. The front mask defines the boundaries between the elements 17 and the back mask defines the mounting portion 18 of each element. The wafer is etched, first from the back face to define the flexible portion of each element, and then from the front to separate the individual elements. We have found that conventional etches provide a very uniform cut which maintains the thickness of the flexible portion of the element 17 to within one micron. Finally the contact leads 22 are applied, e.g. by nail head bonding, to contact the resistor network and the element is mounted via the mounting portion 18 on the support beam 19. Compensation for eccentricity of the cam assembly is effected via the reference cam 14 and its cam follower 16 against which the support beam 19 abuts. When the cam assembly 11 is rotated the circular cam controls the reference beam so that the deflection of the sensor 17 corresponds to the difference in radius of the two cams. This provides automatic compensation for misalignment or eccentricity of the cam assembly. Although the foregoing description relates to circular edge cams, other cam constructions, e.g. circular face or linear cams, can also be employed. CLAIMS

2. A variable resistance or potentiometer device, including a flexible piezo-resistive element mounted on a support beam, a movable cam assembly having a first cam surface in abutment with the element and a second cam surface in abutment with the support beam, and means for movement of said cam assembly whereby the element is deflected relative to the support beam by a distance corresponding to the relative configurations of the cam.

3. A device as claimed in claim 2, wherein cam followers are disposed one in abutment with each said cam.

4. A device as claimed in claim 2 or 3, wherein said cam assembly is a one piece component of a plastics material.

5. A device as claimed in claim 2, 3 or 4, wherein said first cam is profiled to provide a linear relationship between angular displacement of the cam and the resistance of the element.

6. A device as claimed in any one of claims 2 to 5, wherein said support beam and flexible element are urged towards the respective cams by springs.

7. A variable resistance or potentiometer device substantially as described herein with reference to the accompanying drawing.

8. A differential pressure recorder incorporating a device as claimed in any one of claims 1 to 7.