GB2215053A - Electro-mechanical oscillating transducer devices - Google Patents

Abstract

A transducer, e.g. for the sensing and measurement of acceleration, includes an oscillatory structure comprising three parallel filaments (15, 16, 17) coupled to a seismic mass (13). The frequency of oscillation of the filaments corresponds to the magnitude of displacement of the mass in response to an accelerating force. The filaments are maintained in oscillation in a mode such that the centre filaments (16) oscillates in antiphase with the two outer filaments (15, 17) to provide a balanced arrangement. Piezoresistive devices are implanted at the ends of the filaments for both driving and frequency sensing. The mass (13) is also connected to the support frame (11) by hinges (14) and the structure is made by etching a single piece of silicon. <IMAGE>

Description

TRANSDUCER DEVICE.

This invention relates to transducers, and in particular to electromechanical transducers in which a sensing element is, in use, maintained in a state of vibrational resonance.

Transducers in which the sensor element includes a mechanically resonant structure are finding increasing use, particularly in the field of inertial navigation on sensors of force or acceleration. The magnitude of the force acceleration is determined from a corresponding change in the frequency of the resonant structure. One example of such a device is shown in our published specification No. 21623214 (J.C. Greenwood et al 59-7X) which describes a transducer in which the resonant structure includes a double ended tuning fork.

The arrangement is such that, when subjected to an acceleration, a torsional force is applied to the resonator structure thus causing a corresponding change in the resonant frequency.

Whilst this device has proved satifactory in use, the power requirements for the maintenance of oscillation are somewhat critical and difficulty has been experienced in confining oscillation to a single balanced mode.

According to the present invention there is provided a transducer, including an oscillatory structure whose natural resonant frequency is a function of a stress applied to the oscillatory structure, wherein the oscillatory structure includes three parallel resilient filaments, and wherein the transducer includes means for maintaining the filaments in a state of oscillation in a mode such that two of said filaments oscillate in phase with each other and in antiphase with the third filament, said oscillation being at a frequency corresponding to said stress.

An embodiment of the invention will now be described with reference to the accompanying drawings in which Fig. 1 is a plan view of a transducer structure; Fig. la shows in detail the ocsillatory part of the transducer of Fig. 1; Fig. 2 is a sectional view of the transducer structure of Fig. 1; Fig. 3 shows in schematic form a drive circuit for maintaining oscillation of the transducer structure of Figs. 1 and 2.

and Fig. 4 is a schematic diagram of an inertial navigation and guidance system employing transducer structures as shown in Figs. 1 and 2.

Referring to Figs. 1 and 2 the transducer includes a rigid frame 11 having an opening 12 within which a seismic mass 13 is supported on flexible hinge members 14. In the absence of an applied force, the equilibrium position of the seismic mass 13 is in the plane of the frame 11. Movement of the seismic mass 13 is restrained by three parallel filaments 15, 16, 17 whereby the seismic mass is coupled to the frame. The filaments are disposed on a plane parallel to but distinct from that of the hinge members 14.

The filaments 15, 16 and 17 are all of substatially equal thickness, but the width of the centre filament 16 is preferably twice that of the two outer filaments 15 and 17. Each filament has a diffused or implanted piezo-resistor, 18, 19, 20 respectively, disposed at the region of the junction of that filament with the frame 11. The piezo-resistors provide a means of driving the filaments into ocsillation and of providing output signals whereby the frequency of that oscillation may be determined.

The transducer structure of Figs. 1 and 2 may be formed on an integral structure from a body of single crystal silicon by selective etching. Typically, a body, i.e. a wafer, of single crystal silicon is masked on both its upper and lower surfaces and is then exposed to an anisotropic etch. Etching may be performed with a mixture of potassium hydroxide, isopropanol and water, or with hydrazine hydrate. The thickness of hinge members 14 and of the filaments 15, 16 and 17 is determined by timing the etching process and termination exposure to the etchant when the desired thickness has been obtained. The etch produces an inwardly tapered cut. Thus, the mask dimensions are determined from the wafer thickness to obtain the desired dimensions of the finished transducer structure. Preferably the piezo-resistors are diffused or implanted prior to the etching process.

Fig. 3 shows a schematic circuit whereby the filaments 15, 16 and 17 are maintained in oscillation in the desired mode. The oscillatory system is driven by current pulses applied to the piezo-resistor 19 disposed on one end of the centre filament 16. This piezo-resistor is biased with current from a current source I1 and is supplied with current pulses from amplifier AMP via capacitor C1. Feedback signals at the frequency of oscillation are fed to the input of the amplifier from the piezo-resistors 18 and 20 associated with the outer filaments 15 and 17, these piezo-resistors being coupled in parallel and biased from a further current source I2. We have found that the mechanical coupling between the filaments is such that, by driving the centre filament, the two outer filaments oscillate in synchronism, but in antiphase, with the centre filament.The phase difference presented by the series combination of the amplifier AMP and the capacitor C1 maintains this 1800 phase relationship between the oscillating filaments.

The oscillatory mode described above is the preferred mode as the structure is in a condition of dynamic balance. The mass displacement of the larger central filament 16 is couterbalanced by the opposite mass displacements of the outer filaments 15 and 17.

This ensures that a high quality factor is provided and that the structure is relatively insensitive to transient noise s'ignals.

In use, subjection of the transducer structure has an acceleration having a component perpendicular to the plane of the structure causes a corresponding displacement of the seismic mass 13. This causes a change in tension within the filaments 15, 16 and 17 resulting in a change in resonant frequency that provides a measure of the magnitude of the accelerator.

It will be appreciated that, whilst oscillation of each filament at its fundemetal frequency is preferred, oscillation of the filaments at harmonic frequencies is also envisaged.

The transducer structure of Figs. 1 and 2 is of particular application to inertial navigation,'guidance systems, although it is not of course limited to this application. An inertial guidance system is shown schematically in Fig. 4 of the accompanying drawings.

Typically three transducer devices 41, 42, 43 are employed for sensing acceleration in three mutually perpendicular directions. The output of each transducer, comprising an oscillatory signal corresponding to acceleration in the respective direction, is fed to a central processor unit 44. Preferably each transducer output is amplified by a respective amplifier 45. In response to the input signals from the transducers, and from stored navigational information. The central processor unit 44 provides control signals to direction controls 46, 47 and 48 whereby guidance in the three mutually perpendicular directions may be affected.

The navigation/guidance system of Fig. 4 is compact, robust and of relatively low cost. It is thus particuarly suitable for use as a guidance system in airborne weapons. The system may also be employed in an aircraft.

It will be appreciated that although the transducer structure has been described with particular reference to its uses as an accelerometer, it is not limited to this application but may be used, for example, as a sensor of pressure or strain.

Claims (6)

1. A transducer, including an oscillatory structure whose natural resonant frequency is a function of a stress applied to the oscillatory structure, wherein the oscillatory structure includes three parallel resilient filaments, and wherein the transducer includes means for maintaining the filaments in a state of oscillation in a mode such that two of said filaments oscillate in phase with each other and in antiphase with the third filament, said oscillation being at a frequency corresponding to said stress.

2. A transducer, including a rigid laminar support structure having an opening within which a seismic mass is supported by a flexible hinge coupled to the support structure, first, second and third resilient parallel filaments coupled to the support structureand the seismic mass such that movement of the mass about the hinge causes a corresponding variation of tension in the filaments, and means for maintaining the filaments in a state of oscillation in a plane perpendicular to the support structure and in a mode such that two of said filaments oscillate in phase with each other and in antiphase with the third filament, said oscillation being at a frequency corresponding to the seismic mass relative to the support structure.

3. A transducer as claimed in claim 1 or 2, and comprising an integral structure formed from single crystal silicon.

4. A transducer as claimed in claims 1, 2 or 3, wherein the mass of said third filament is substantially equal to the sum of the masses of said two filaments.

5. A transducer substantially as described herein with reference to and as shown in Figs. 1 to 3 of the accompanyning drawings.

6. An inertial navigation/guidance system incorporating one or more transducers as claimed in any one of claims 1 to 5.