ACCELERATION SENSOR

Abstract: The sensor has two sheets (18, 20) of polarized polymeric piezoelectric material secured together with their main uniaxial mode aligned with axis (16). These are connected to an electric oscillator to induce mechanical oscillation of the assembly (10). A third sheet (22) of like material is secured to the sheets (18, 20) but has its main uniaxial mode transverse to the axis (16). Twisting of the assembly due to rotational acceleration causes substantial alteration of the voltage induced in the third sheet (22), the voltage being proportional to the rate of rotational acceleration.

Description: DESCRIPTION "Acceleration Sensor" This invention relates to a device for sensing acceleration, particularly rate of rotation, and to apparat incorporating such devices. It is of course well known to sense rotational acceleration bymeans of a gyroscope, and to provide inertial platforms and the like by a plurality of gyroscopes rotating about different axes. Such apparatus is extremely accurate but is also expensive. An object of the present invention is to provide a simple and inexpensive rate sensor which has sufficient accuracy for certain applications. The invention accordingly provides a device for measuring acceleration, comprising a laminar assembly and means mounting the assembly so as to lie, in the absence of acceleration, in a reference plane; said assembly comprising a first piezoelectric member arranged for connection to an oscillator to produce oscillation of said assembly transvers to said reference plane, and a second piezoelectric member arranged to generate an electrical output when stressed in a direction oblique to the direction of said oscilllation; whereby said electrical output is a function of twisting of the assembly about the longitudinal axis of said reference plane. In a preferred form, said first piezoelectric member comprises two sheets of polarized polymeric piezoelectric material ' secured to and insulated from each other and having their main uniaxial mode aligned with said longitudinal axis said two sheets being oppositely connected to said oscillato The second piezoelectric member suitably comprises a further sheet of like material having its main uniaxial mode transve to said longitudinal axis. The material may suitably by polyvinylidene fluoride (PVDF) . A suitable form of this material is sold as KYNAR (Trade Mark) film by Pennwalt Corporation, King of Prussia, Pa. 19406, USA. From another aspect, the invention provides apparatus including a plurality of devices as defined above arranged with their said longitudinal axes mutually perpendicular, whereby movement of the apparatus in a two-dimensional plane or in three-dimensional space may be measured. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic perspective view of a device embodying the invention, Fig. 2 is an exploded schematic view of part of the device of Fig. 1, and Fig. 3 illustrates a modification. The device comprises a sheet laminate 10 clamped at its top and bottom edges by parts 12,14 of a rigid housing (the remainder of which is not shown) such that in its rest position the laminate 10 lies in a reference plane having a longitudinal axis designated at 16. As shown more clearly in Fig. 2, the laminate 10 consists of three sheets 18,20,22 bonded together. Each of these sheets is of polarised PVDF film. The first and second sheets 18 and 20 have the main uniaxial mode 19 of the film aligned with the axis 16, while the third sheet 22 has its main uniaxial mode/at right angles to the axis 16. The three sheets 18,20,22 are bonded together by any suitable insulating adhesive. The first and second sheets 18 and 20 are oppos¬ itely connected to an oscillator 24. The applied electric signal thus causes one sheet to lengthen while the other contracts and vice versa, and the laminate oscillates transverse to the axis 16 as indicated in dashed lines > in Fig. 1. The oscillator 24 suitably oscillates at the resonant frequency of the mechanical system. In this mode of operation the third sheet 22 experiences strees in a direction normal to its main uniaxial mode, and the voltage induced therein is small (a few millivolts). However, if the housing is rotated about the axis 16, as indicated by arrow 26, the oscillating laminate has- an inertia which tends to resist movement out of its initial position; this results in twisting of the laminate 10, the top and bottom edges moving with the housing but the central area lagging. Such twisting stresses the third sheet 22 in a direction oblique to the axis 16 and therefore more in alignment with its main uniaxial mode. The sheet 22 thus produces a much higher output voltage in the range of 10 - 20V. The voltafe produced is proportional to the angular acceleration. However, since the upper and lower parts of the third sheet 22 are twisted in equal and opposite directions, they produce equal and opposite voltages; the third sheet 22 is therefore composed of two separate halves connected as shown to give an output at 28. Instead of using a PVDF bimorph as the oscillating element, some other planar oscillating element could be used, such as a metal plate driven by a ceramic transducer. The device described may be modified to give an output signal which is a function of linear acceleration. This requires linear acceleration to produce twisting of the laminate while isolating the laminate from rotational movement. This may be accomplished as shown in Fig. 3 by rotatably mounting the laminate 10 within an outer body 30 on bearings 32, and mounting a mass 34 on the laminate eccentric to the rotation axis 36. By mounting a number of the above devices about . different axes in a platform, the resulting output signals representing rotational and/or linear accelerations can be integrated to define movement of the platform in a 2-dimensional plane or in 3-dimensional space. It is believed that the device of the present invention would allow bias stability of about l°/minute to be achieved. This is not sufficiently accurate for applications such as navigation. It is envisaged, however, that it will be suitable for small-scale applications such as the positioning of movable machinery especially where the part in question is returned periodically (in a time scale of a few minutes) to a fixed datum position.

Claims: _ _ - CLAIMS 1. A device for measuring acceleration, comprising a laminar assembly and means mounting the assembly so as to lie, in the absence of acceleration, in a reference plane; said assembly comprising a first piezoelectric member arranged for connection to an oscillator to produce oscillation of said assembly transverse to said reference plane, and a second piezoelectric member arranged to generate an electrica output when stressed in a direction oblique to the direction of said oscillation; whereby said electrical output is a function of twisting of the assembly about the longitudinal axis of said reference plane. 2.The device of claim 1, in which said first piezoelectric member comprises two sheets of polarized polymeric piezo¬ electric material secured to an insulated from each other and having their main uniaxial mode aligned with said longitudina axis, said two sheets being oppositely connected to the oscillator. 3. The device of claim 1 or claim 2, in which the second piezoelectric member comprises a sheet of polarized polymeric piezoelectric material having its main uniaxial mode transver to said longitudinal axis. 4. The device of claim 2 or claim 3, in which said polarized polymeric piezoelectric material is polyvinylidene fluoride. 5. A device according to any preceding claim for use as a rotation rate sensor, said laminar assembly being secured at its ends to a body whereby said electrical output is a functio of rotational acceleration of the body about the longitudinal axis of the reference plane. 6. Apparatus comprising a plurality of devices according to claim 1 arranged with their longitudinal axes mutually perpendicular, whereby movement of the apparatus in a two-dimensional plane or in three-dimensional space may

Inventors: STRACHAN JOHN SCOTT, STRACHAN, JOHN, SCOTT
Applicants: SYRINX INNOVATIONS, SYRINX INNOVATIONS LIMITED
Application year: 1985
Main classification: G01P9
IPC codes: G01C19, G01P9