GB2197121A - Force transducer with temperature correction - Google Patents

Abstract

The transducer comprises two sheets (10, 12) of a uniaxially oriented polymeric piezoelectric material such as polyvinylidene fluoride. The sheets (10, 12) are provided with electrodes (14, 16, 18) and are secured together face-to- face with their axes (22, 24) of piezo activity mutually orthogonal. A force applied along one of the axes (22, 24) produces a differential output, but temperature effects cause equal outputs in each sheet. <IMAGE>

Description

SPECIFICATION Force transducer with temperature correction This invention relates to a force transducer using the piezoelectric properties of peizoelectric polymeric films, with correction to reduce the effect of the pyroelectric properties of such films.

Polymeric materials having piezoelectric properties and devices using such materials are known, one example being polyvinylidene fluoride (pvdf). A problem in using such materials is that they also exhibit marked pyroelectric properties and are thus considerably affected by changes in ambient temperature. This is a particular problem when the material is used to measure a parameter which changes slowly. Prior art attempts to deal with this problem have been based on the use of a temperature sensitive element providing a compensation signal which is used to compensate the piezo output signal electronically.

An object of the present invention is to provide a transducer which is simple and cheap to produce while having a useful degree of insensitivity to changes in temperature.

Accordingly, the invention provides a transducer comprising first and second sheets of a uniaxially oriented polymeric piezoelectric material, the sheets being secured together faceto-face with their axes of piezoelectric activity mutually angled, electrode means for collecting charge generated in said sheets to provide respective first and second voltages, and means for connecting said voltages to respective inputs of a differential amplifier.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, which is a schematic illustration of a force transducer according to the invention.

Referring to the drawing, the transducer comprises two sheets 10, 12 of uniaxial pvdf.

The outer faces of the sheets 10, 12 are provided with continuous electrodes indicated at 14, 16 which may be formed for example by printing with silver ink. The inner faces are also provided with electrodes; as shown, a common electrode 18 is formed by a conductive adhesive which also serves to secure the sheets 10 and 12 together in face-to-face relationship. Alternatively, separate electrodes could be formed on the sheets and a nonconductive adhesive used to bond them together. The outer electrodes 14 and 16 are connected to the inputs of a differential amplifier 20.

It will be appreciated that the thicknesses of the elements in the drawing are greatly exaggerated for ease of illustration.

The preferred material for the sheets 10, 12 is KYNAR (trade mark) polyvinylidene fluoride film by Pennwalt Corporation, prepared by melt extrusion, uniaxial stretch orientation,and electrical polarisation.

In a uniaxial film, the voltage produced by a given deformation along the axis of piezo activity is typically five to ten times greater than that produced by the same deformation across the axis. The sheets 10 and 12 of the present invention are disposed with their axes (indicated at 22 and 24) mutually angled, most suitably at right angles as shown. Thus, if a force is applied along the axis 22 of the first sheet 10, the voltage V, produced across it will be some five to ten times greater than the voltage V2 produced across the second sheet 12, and the output of the amplifier 20 will be proportional to 0.8V, to 0.9V, and thus be a useful measure of the applied force.

Temperature variations, however, will produce equal voltages in each film giving a resultant output of nil from the differential amplifier 20.

Thus the invention provides a temperatureinsensitive force transducer of very simple construction.

Claims (5)

1. A transducer comprising first and second sheets of a uniaxially oriented polymeric piezoelectric material, the sheets being secured together face-to-face with their axes of piezoelectric activity mutually angled, electrode means for collecting charge generated in said sheets to provide respective first and second voltages, and means for connecting said voltages to respective inputs of a differential amplifier.

2. The transducer of claim 1, in which said axes are disposed at right angles to each other.

3. The transducer of claim 1 or claim 2, in which said electrode means comprises continuous conductive layers extending over substantially the whole of the major faces of the sheets, said connecting means being connected to the outer conductive layers.

4. The transducer of claim 3, in which the sheets are secured together by an electrically conductive adhesive which also acts as a common electrode for the inner faces of the sheets.

5. A force measuring device comprising a transducer in accordance with any preceding claim, a differential amplifier connected therewith, and means for applying the force to be measured substantially along the said axis of one of the sheets.