GB2196120A - A resonant element force sensor in a housing affording temperature and pressure compensation - Google Patents

Abstract

A force sensor using a mechanically resonant element 13 includes two diaphragms 11, 12 connected by a rigid link 15 to prevent changes in pressure affecting the resonant element. The link may be a tubular member around the element 13 and this is connected to end caps 16, 17 on the diaphragms 11, 12. The element 13 is suggested at one end by a rigid arm 14. Optical fibres 18, 19, 20 provide pulsed and continuous light to excite the element 13 into resonance and to supply a detector 24 which measures the light modulation caused by the vibrating element 13. A processor 25 converts the detector signal into a value for the force F applied to the diaphragm and element 13. In another arrangement the arm 14 supports two resonant elements and the fibre bundles 18-20 are duplicated. A ball and socket connection between the or each element 13 and the end cap is also disclosed. <IMAGE>

Description

SPECIFICATION Force sensor This invention relates to force sensors of the type including a mechanically resonant element.

An example of such a force sensor is described in UK Patent Application No.

2,146,120A. From which figure 1 is extracted. In such a sensor a mechanically resonant element 1 changes its resonant freguency in response to changes in applied force in the direction of the arrow. The resonant element is driven into resonance photo thermally by light pulses along an optical fibre 2 and is optically read by a light beam transmitted along an optical fibre 3, reflected from and modulated by the movement of the resonant element 1 and then received by an optical fibre 4.

A problem with such force sensors is that changes in external parameters, such as temperature and atmospheric pressure can cause force to be applied to the resonant element.

The effects of this force will be indistinguishable from the effect of an applied force which it is intended to measure.

This invention was made while attempting to produce a force sensor that is less sensitive to such external influences.

This invention provides a force sensor comprising a mechanically resonant element enclosed within a sealed housing, and means for using resonation of the element to sense a force applied to it, the housing having a first wall portion which is displaceable so that a force to be sensed can be applied from outside the housing to the element and a second displaceable wall portion connected to the first wall portion by a link so that a force applied directly to the first wall portion by a pressure difference between the inside and outside of the housing is at least partially compensated for by a second force applied by the pressure difference to the second wall portion and thence via the link to the first wall portion.

Improved isolation of the sensor output from the effects of ambient pressure changes can be obtained by including a second mechanically resonant element enclosed within the sealed housing and means for using resonation of the second element to sense a force applied to it, the second wall portion being displaceable so that a force to be sensed can be applied from outside the housing to the second element.

Each displaceable wall portion is preferably a diaphragm but other possibilities such as pistons could be envisaged.

The link between the two displaceable wall portions may be a tube surrounding the or each resonant element. This is considered advantageous because it allows the forces to be applied symmetrically to and between the diaphragms.

In a preferred construction the or each resonant element is attached to its diaphragm by a ball and socket arrangement. This prevents forces acting on the diaphragm from causing undesired twisting or bending of the element.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 2 shows a cross sectional view of a force sensor constructed according to the invention.

Figure 3 shows a cross sectional view of another force sensor constructed according to the invention giving improved compensation.

Figure 4 shows a cross sectional view of a joint used in the force sensors of figures 2 and 3.

Equivalent parts have the same reference numerals throughout.

In figure 1 a sealed casing is made up of a solid cylindrical wall 10, the two ends of which are sealed by diaphragms 11 and 12 of equal size.

A resonant element 13 is mounted between a rigid support 14 connected to the wall 10 and the centre of the diaphragm 11. The resonant element 13 is such that its resonant frequency alters when stress is applied to it so that the stress it is under can be calculated from its resonant frequency.

The diaphragms 11 and 12 are linked by a rigid tube 15 which surrounds the resonant element 13. The central sections 16 and 17 of diaphragms 11 and 12 respectively are thickened to form end caps for the tube 15.

Three optical fibres, 18, 19 and 20 are used to drive and read the sensor they connect the sensor to a remote site 21. At the remote site 21 a pulsed light source 22 generates a series of light pulses that are passed down fibre 18 and drive the resonant element 13 into resonance photo-acoustically. A second light source 23 sends continuous wave light along fibre 19 to illuminate the resonant element 13. Light reflected from the element 13 then passes along fibre 20, this reflected light is modulated by the movements of the element 13. The modulated light from the fibre 20 is received by an optical detector 24 which supplies an electrical signal corresponding to the modulated light to a processor 25.

The processor 25 calculates the force that has been applied to the element 13 from this signal.

In operation the sensor is used to detect force applied to the diaphragm 11, this force is represented by the arrow F. Although this would usually be a force such as weight it could be generated by a differential pressure applied to the two diaphragms. The sensor is mounted so that this force is applied to the central section 16 of the diaphragm 11 to avoid damage to the diaphragm 11.

If there is a pressure difference between the inside and outside of the casing a force will be produced on each of the diaphragms 11 and 12, because the two diaphragms are of equal size these two forces will be equal in magnitude and due to the tube 15 linking the two diaphragms, will cancel one another out.

Thus ideally there will be no net force acting on the resonant element 13 due to the pres sure difference.

Any forces acting on the casing, due to differential thermal expansion or the way in which the sensor is secured for example, can only be transmitted to the element 13 through the diaphragm 11, and because the diaphragms are flexible any such forces will be greatly attenuated.

If forces are applied that are not normal to the diaphragm 11 the two diaphragms linked by a tube will not twist, so only the component of force normal to the diaphragm 11 will be transmitted to the element 13 Referring to figure 3 another design of force sensor is shown. A sealed casing is formed by a solid cylindrical wall 10 and two diaphragms 11 and 12 as before. A tube 15 connects the thickened central sections 16 and 1 7 of diaphragms 11 and 12.

A resonant element 1 3A is mounted between the centre of the diaphragm 11 and a rigid support 14 connected to the wall 10 and a second, similar, resonant element 138 is mounted between the centre of the diaphragm 12 and the support 14.

Element 13A is driven and read by pulsed light source 22A, continuous wave light source 23A and optical detector 24A along optical fibres 18A, 1 9A and 20A while element 138 is driven and read by pulsed light source 228, continuous wave light source 23B and optical detector 24B along optical fibres 18B 198 and 20B in the same way as was described earlier.

The eiectrical signals from optical detectors 24A and 24B are processed by a processor 26.

In operation the sensor is used to detect forces applied to diaphragm 11, these forces are represented by the arrow F.

The tube 15 linking the two diaphragms causes any forces due to pressure differences inside and outside of the casing to cancel, additionally it causes the element 13B to be stretched by the same amount as the element 13A is compressed. Because of this only signals corresponding to pairs of forces of equal magnitude acting in opposite senses on the two elements 13A and 13B can correspond to applied forces. So only signals of this type are used by the processor 26 to calculate the applied force. Thus any forces transmitted to the elements 13A and 13B from the casing via the diaphragms 11 and 12 will be ignored by the processor 26 unless they act on elements 13A and 13B with equal magnitude and opposite senses and the sensitivity of the sensor to forces acting on the casing will be further reduced.

Figure 4 shows a connection between the mechanically resonant element 13 and the diaphragm 11.

A ball 27 is formed on the end of the resonant element 13. The end cap 16 contains an internally threaded recess 28 with a lip 29 around one end. The ball 27 is held between the lip 29 and an anti-rotation washer 30 by a securing screw 31.

The screw 31 is then covered over by a layer of epoxy resin 32 to ensure a gas tight seal.

A similar arrangement, without the epoxy resin 32, is used to secure the oither end of the resonating element 13 to the support 14.

If two elements are used they are both secured similarly.

The anti-rotation washer 30 ensures that when screw 31 is tightened to hold the element 13 in place no torque is transmitted to the element 13 because this would effect the resonation of the element and make the sensors measurements unreliable.

The element 13 and ball 27 can also rotate within the recess 28, so movement perpendicular to the element 13 will not effect the resonation of the element 13. As a result of this small movements of the diaphragms perpendicular to the element 13 will not cause any change in the resonation of the element 13.

Although diaphragms are preferred it would be possible to use some other form of moveable sealed wall portion such as a piston.

A resonating element driven and/or read in a way other than that described such as piezo-electric, capacitive or electromagnetic systems could be used in a force sensor embodying the invention.

Claims (7)

1. A force sensor comprising a mechanically resonant element enclosed within a sealed housing, and means for using resonation of the element to sense a force applied to it, the housing having a first wall portion which is displaceable so that a force to be sensed can be applied from outside the housing to the element and a second displaceable wall portion connected to the first wall portion by a link so that a force applied directly to the first wall portion by a pressure difference between the inside and outside of the housing is at least partially compensated for by a second force applied by the pressure difference to the second wall portion and thence via the link to the first wall portion.

2. A force sensor as claimed in claim 1 and additionally comprising a second mechanically resonant element enclosed within the sealed housing and means for using resonation of the second element to sense a force applied to it, the second wall portion being displaceable so that a force to be sensed can be applied from outside the housing to the second element.

3. A force sensor as claimed in claim 1 or 2 in which each displaceable wall portion is a diaphragm.

4. A force sensor as claimed in claim 1, 2 or 3 in which the link between the two displaceable wall portions is a tube surrounding the or each resonant element.

5. A force sensor as claimed in any preceding claim in which the or each resonant element includes a ball which is secured in a securing structure arranged to prevent forces acting perpendicularly to and rotationally about the element being transmitted to the element.

6. A force sensor substantially as shown in and substantially as described with reference to figure 2 of the accompanying drawings.

7. A force sensor substantially as shown in and substantially as described with reference to figure 3 of the accompanying drawings.