GB2274714A

GB2274714A - Strain measuring device

Info

Publication number: GB2274714A
Application number: GB9302014A
Authority: GB
Inventors: Stephen Gibbs
Original assignee: Keystone Valve UK Ltd
Current assignee: Keystone Valve UK Ltd
Priority date: 1993-02-02
Filing date: 1993-02-02
Publication date: 1994-08-03
Also published as: WO1994018534A1; GB9302014D0; AU5974394A

Abstract

In an assembly comprising a (rotary) actuator (16) mounted on a body (12), e.g. of a valve fitted in a pipeline (14), and having a drive shaft (20) to actuate the valve or other driven element within the body, the actuator is mounted on the body (12) by means of a member (18), e.g. a cylindrical mounting stool, which is provided with strain gauges (28) to measure the reaction force acting on the mounting member (18) in response to the output force applied by the actuator. The assembly thus allows the direct derivation of the output torque or thrust of the actuator (16), which is equal and opposite to the reaction force measured by the strain measuring devices (28). The dimensions of the mounting member (18) may be chosen to give the same strain therein as in the drive shaft (20). The strain gauge output signal is damped to remove transients, but the undamped signal may also be used as a measure of vibration in the assembly. <IMAGE>

Description

STRAIN MEASURING DEVICE This invention relates to a strain measuring device, and also to an actuator assembly including such a device.

The invention has particular application for use in measuring torque or thrust produced by valve actuating devices.

Currently, the output torque or linear thrust of a valve actuation device is inferred or measured by various indirect methods, for example the output from pneumatic or hydraulic actuators is calculated based upon the measured or estimated pressure of the actuator operating fluid. It is also possible to measure the torque or thrust on the output drive member of an actuator using strain gauges.

This technique tends to be used only in the laboratory, rather than in the field, as it is relatively expensive to provide such strain gauges and the dimensions and operative movement of, for example, a rotating shaft, complicate the fitting, operation and connection of conventional strain gauges.

It is an object of the present invention to provide a strain measuring device which will obviate or mitigate these disadvantages.

According to one aspect of the present invention there is provided a strain measuring device comprising a mounting member including strain measuring means and arranged to provide a mounting for an actuator on a body having a driven member, the arrangement being such that the strain measuring means is capable of measuring the reaction force acting on the mounting member in response to a force applied to the driven member by the actuator.

According to a further aspect of the present invention there is provided an actuator assembly comprising an actuator having a driving member, a body having a driven member, and a mounting member including strain measuring means for mounting the actuator on the body, the arrangement being such that the strain measuring means is capable of measuring the reaction force acting on the mounting member in response to a force applied to the driven member by the driving member.

The device and assembly thus allow the direct derivation of the output torque or thrust of an actuator, which is equal and opposite to the reaction force measured by the strain measuring means. The arrangement is particularly effective for use in combination with valve actuators. The ability to measure force at the actuator output eliminates the errors associated with inferred measurement techniques, in which friction, varying efficiency and component wear can affect the accuracy of the estimated torque or thrust. Also, the location of the strain measuring means on the typically static mounting member improves reliability and facilitates connection for signal transmission.

Conveniently, the mounting member is in the form of a mounting stool or bracket.

The strain measuring means may include two or more strain sensing devices incorporated into or mounted on the mounting member. Conveniently, the strain sensing devices are of the ceramic bonded type. The strain measuring means may include a signal conditioning system to damp the output signals from the strain sensing devices and to minimise the effect of transients. If desired, undamped signals may be utilised as a measure of, for example, the vibration to which the assembly is subjected. An alarm may be provided for activation if the measured vibration rises above a predetermined safe level.

In applications where a rotatable driving shaft extends between the actuator and the driven member, the mounting member is preferably tubular and adapted to receive the shaft. It is of course desirable that the mounting member, at least at the strain measuring device location, has a predictable torque versus strain relationship, and a tubular section member fulfils this requirement. For maximum sensitivity, the strain measuring device is mounted on the exterior surface of the mounting member. The tubular member may have flanged ends to facilitate mounting to the actuator and the body.

Most preferably, the second moment of area of the mounting member at the location of the strain measuring device is selected to be the same or similar to the second moment of area of the driving member between the actuator and the driven member.

Further sensing means may be provided on the mounting member for use in measuring other forces acting on the body, for example bending moments produced by the weight of the actuator.

These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawing which shows, somewhat schematically, a valve actuator assembly in accordance with a preferred embodiment of the present invention.

The drawing shows a valve assembly 10 including a valve body 12 fitted in a pipeline 14, and a valve actuator 16. A mounting stool 18 is provided to secure the actuator 16 to an upper face of the valve body 12, the actuator 16 rotating a shaft 20 to open and close the valve. The mounting stool 18 is tubular, has a central cylindrical portion 22, and is provided with flanged ends 24, 26 which are bolted to the actuator and valve 12, 16, respectively. Strain gauges 28 are fixed to an outer surface of the cylindrical portion of the stool.

In use, the actuator 16 produces a torque on the shaft 20 to open or close the valve 12. An equal and opposite torque is applied to the stool 18, the magnitude and direction of which may be determined by- monitoring the output signals from the strain gauges 28.

By dimensioning the outside and inside diameters of the cylindrical portion 22 of the stool 18, it is possible to configure the stool 18 such that the strain measured on the outside surface of the stool is equal to the strain experienced by the shaft 20. This may be derived, for this situation, where, for a given torque on a shaft of diameter D: STRAIN = K/D4 For a tubular section of outer diameter D1 and inner diameter D2: STRAIN = K/(D14 - D24) In order to ensure a strain gauge output from the stool which is equal to the strain experienced by the shaft, for a given torque; K/D4 = K/(D14 - D24) or D4 = D14 - D24 .-. D14 = D4 + D24 Thus if an inside diameter, D2, of 35 mm is selected: D14 = 324 + 354 .-. D1 = 39.96 mm This gives a tube wall thickness of approximately 2.5 mm.

For a given shaft diameter the thickness of the corresponding tube will decrease as the inside diameter increases, for example, if an inside diameter, D2, of 40mm is selected: D14 = 324 + 404 D1 = 43.6 mm This gives a tube wall thickness of 1.8 mm.

The foregoing relationship assumes that the same material is used to produce the shaft and tube. However, if it is desirable to have a larger wall section for the tube, for example to facilitate the fitting of strain gauges, this may be achieved through use of a different material for the tube having a lower torsional modulus.

As an example, the torsional modulus (G) for steel (shaft material) is 10.5 * 106 lb / in 2, whereas for aluminium, which is a common material for mounting stools, G = 3.5 * 106.

In this case, in order to maintain the same strain for shaft and stool, the product of the respective torsional modulus and polar second moment of area (J) will be the same, that is: G1J1 = G2J2 or G1D4 = G2(D14 - D24) Hence 10.5D4 = 3.5(D14 - D24) or D14 = D24 + 3D4 For D = 32 mm and D2 = 40 mm: D14 = 404 + 3*324 D1 = 48.9 mm This gives a tube wall thickness of approximately 4.5 mm as compared with 1.8 mm for the steel tube. This increased wall section can make the fitting of strain gauge type sensors much easier.

From the above described example it will be clear that an assembly in accordance with an embodiment of the present invention provides a relatively simple and inexpensive means of sensing and transmitting actuator output torque or thrust, which is particularly suited for use in 'smart' diagnostic systems as utilised in process control. Although described above with reference to use with a valve actuator, the invention may be used in a wide range of mechanical power transmission devices. It will further be obvious to one of skill in the art that the above described embodiment is merely exemplary of the present invention, and that various modifications and improvements may be made to the above described embodiment without departing from the scope of the present invention.

Claims

1. A strain measuring device comprising a mounting member including a strain measuring means and arranged to provide a mounting for an actuator on a body having a driven member, the arrangement being such that the strain measuring means is capable of measuring the reaction force acting on the mounting member in response to a force applied to the driven member by the actuator.

2. The device of claim 1 wherein the mounting member is in the form of a mounting stool or bracket.

3. The device of claim 1 or claim 2 wherein the strain measuring means includes two or more strain sensing devices incorporated or mounted on the mounting member.

4. The device of claim 3 wherein the strain sensing devices are of the ceramic bonded type.

5. The device of any one of claims 1 to 4 wherein the strain measuring means includes a signal conditioning system to damp the output signals from the strain sensing devices and to minimise the effect of transients.

6. The device of claim 3, 4 or 5 wherein undamped signals from the strain sensing devices are utilised to provide an indication of the vibration to which the device is subjected.

7. The device of claim 6 further comprising an alarm for activation if the measured vibration rises above a predetermined level.

8. The device of any one of the preceding claims wherein the mounting member is tubular and adapted to receive a rotatable driving shaft extending between the actuator and the driven member.

9. The device of claim 8 wherein the mounting member is cylindrical.

10. The device of claim 8 or 9 wherein the strain measuring device is mounted on an exterior surface of the mounting member.

11. The device of claim 8, 9 or 10 wherein the tubular mounting member has flanged ends to facilitate mounting to the actuator and the body.

12. The device of any one of the preceding claims wherein the second moment of area of the mounting member at the location of a strain measuring device is selected to be the same or similar to the second moment of area of the driving member between the actuator and the driven member.

13. The device of any one of the preceding claims wherein further sensing means are provided on the mounting member for use in measuring other forces acting on the body.

14. An actuator assembly comprising an actuator having a driving member, a body having a driven member, and a mounting member including strain measuring means for mounting the actuator on the body, the arrangement being such that the strain measuring means is capable of measuring the reaction force acting on the mounting member in response to a force applied to the driven member by the driving member.

15. A strain measuring device substantially as described herein and as illustrated in the accompanying drawing.

16. An actuator assembly substantially as described herein and as illustrated in the accompanying drawing.