WO1995005589A1

WO1995005589A1 - Strain gauged force transducers

Info

Publication number: WO1995005589A1
Application number: PCT/GB1994/001735
Authority: WO
Inventors: Stanley Gordon Funnell
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-08-19
Filing date: 1994-08-08
Publication date: 1995-02-23
Anticipated expiration: 1996-02-19
Also published as: AU7272794A; GB9317247D0

Abstract

A force transducer includes a rigid generally cylindrical casing from whose integrally formed end cover protrudes a load point aligned with the axis of the casing. A diaphragm is provided which includes diametrically aligned loading beams each supporting a plurality of strain gauges. These gauges are connected to transmit output signals representative of a force produced by a load applied to the transducer through the load point.

Description

Strain Gauged Force Transducers

This invention relates to force transducers and more especially to force transducers which are intended to be axially loaded.

Force transducers are usually referred to as load cells. One species of force transducer includes an axially loaded diaphragm mounted within or forming part of a rigid cylindrical casing which flexes to activate a series of strain gauges arranged as a wheatstone bridge to provide a measure of an applied load. Conventionally/ the diaphragm extends across the entire upper end of the cylindrical casing of the cell and may define a cover therefore. The strain gauges are positioned on the diaphragm in a predetermined pattern and transmit electrical or electronic output signals on flexing of the diaphragm representative of the load applied to the cell.

With conventional force transducers of the type discussed, if the load is not applied truly axially, spurious output signals can be transmitted by the strain gauges to provide an incorrect measure of the load applied.

The present invention sets out to provide an improved force transducer which does not suffer from, or at least alleviates, this particular disadvantage.

According to the present invention in one aspect there is provided a force transducer which includes a rigid generally cylindrical casing formed with an integral end cover from whose outer surface protrudes a load point aligned with the axis of the casing, the end cover defining a diaphragm which includes diametrically aligned loading beams each of which supports on its inner face a plurality of strain gauges connected to transmit output signals representative of a force produced by a load applied to the transducer through the load point.

The loading beams of the diaphragm are preferably of cruciform construction with the centre of the cruciform positioned immediately below the load point. Any uniform array of loading beams may, however, be employed. The loading beams may be machined into the outer surface of the end cover; alternatively, a fabricated end cover including loading beams may be provided. Preferably, the entire casing, end cover and loading beams are formed as an integral or fabricated unit. The strain gauges are preferably mounted on the undersides of the loading beams inside the machined body cavity of the force transducer. The body cavity can then easily be sealed without inhibiting precise movement of the loading beams.

The cover and sides of the casing are formed as a single piece with the rearward opening of the casing sealed by a closure plate. The casing may be hermetically sealed.

The casing interior may house, in addition to the strain gauges or the like, electrical circuits and other electronic components of the transducer.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which:

Figure 1 is a side view in section of a force transducer in accordance with the invention taken along line I-I of Figure 2;

Figure 2 is a plan view of the transducer illustrated in Figure 1 with back cover removed; and

Figure 3 is a circuit diagram of the features of the force transducer illustrated in Figures 1 and 2.

The force transducer illustrated in the drawings comprises a rigid cylindrical casing 1 including an integral cover 2 from which protrudes a load point in the form of a spigot, button or threaded stud 2 whose axis is aligned with the principal axis of the casing 1. The cover 2 defines a diaphragm and is formed integrally with diametrically aligned loading beams 3 which extend across the full width of the diaphragm to describe a cruciform whose centre lies immediately below the button 2. In the embodiment illustrated, the loading beams 3 are formed integrally with the diaphragm and the casing 1, cover 2 and beams 3 are formed as a single piece. In an alternative construction, the cover 2 may comprise a fabrication of a cover and separable loading beams. Positioned below the cover 2 and mounted on the underside of each beam 3 are electrical strain gauges 5 connected as a measurement circuit in the form of a wheatstone bridge 6 to a source of electrical power via a lead and connected to transmit output signals via a lead to external measurement equipment. The base of the cylindrical casing 1 is sealed by a closure plate 7. The casing 1, with the closure plate suitably secured in place by, for example welding, is hermetically sealed.

In operation, a compressive or tensile load applied to the button 2 causes the loading beam cruciform to flex to cause the strain gauges 5 to transmit electrical signals to external measuring equipment to provide a measure of the force applied to the transducer through the button 2.

The symmetrical shaping of the loading beam 3 and the consequent load sharing between the beams provides self- compensation against errors caused by input forces being misaligned with the principal axis of the transducer, thereby ensuring that all signals transmitted provide an accurate measure of the load applied to the cell. Enclosing of the vulnerable strain gauge components and circuits prevents failures due to environmental attack and the load sharing promoted by the cruciform beam structure produces repeatedly accurate output signals which are largely uninfluenced by errors caused by force inputs misaligned with the transducer axis.

From the foregoing it will be understood that the force transducer is configured as a universal load cell which can be applied across a wide range of production and mechanical engineering processes in aggressive and normal environments without degradation of accuracy. This advantage is attributable to the fact that the load sensing structure, which is designed for best accuracy, with cancellation of error, caused by non-aligned load inputs, is also homogeneously integral with a diaphragm which completely excludes the external atmosphere.

It will be appreciated that the foregoing is merely exemplary of load cells in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention.

Claims

1. A force transducer which includes a rigid generally cylindrical casing formed with an integral end cover from whose outer surface protrudes a load point aligned with the axis of the casing, the end cover defining a diaphragm which includes diametrically aligned loading beams each of which supports a plurality of strain gauges connected to transmit output signals representative of a force produced by a load applied to the transducer through the load point.

2. A transducer as claimed in Claim 1 wherein the loading beams of the diaphragm are of cruciform construction with the centre of the cruciform positioned immediately below the load point.

3. A transducer as claimed in Claim 1 or Claim 2 wherein the loading beams are formed integrally with the diaphragm.

4. A transducer as claimed in any one of claims 1 to 3 wherein the diaphragm is fabricated to include the loading beams.

5. A transducer as claimed in any one of the preceding claims wherein the entire casing, and cover and loading beams are formed as an integral or fabricated unit.

6. A transducer as claimed in any one of the preceding claims wherein the strain gauges are mounted on the undersides of the loading beams.

7. A transducer as claimed in any one of the preceding claims wherein the rearward opening of the casing being sealed by a closure plate.

8. A transducer as claimed in any one of the preceding claims wherein the casing is hermetically sealed.

9. A transducer as claimed in any one of the preceding claims wherein the casing interior houses, in addition to the strain gauges, electrical circuits and other electronic components of the transducer.