WO1989010568A1

WO1989010568A1 - Transducer

Info

Publication number: WO1989010568A1
Application number: PCT/GB1989/000440
Authority: WO
Inventors: Dennis John Miles
Original assignee: The Secretary Of State For Defence In Her Britanni
Priority date: 1988-04-25
Filing date: 1989-04-25
Publication date: 1989-11-02
Also published as: GB8809757D0

Abstract

A transducer for acceleration measurement is fabricated from a silicon wafer and comprises three filaments (3, 4) and (5) which interconnect a support (1) with an inertial mass (2). The filaments act as hinges permitting movement of the inertial mass with respect to the support. The inertia of the mass (2), coupled with an acceleration of the support (1) in the z direction, gives rise to a compression of the filament (3) and an extension of the filaments (4) and (5). The outputs from sensing means responsive to changes in tension incorporated in the filaments (3) and (4) are differenced to give a signal related to applied acceleration. In one embodiment, the filaments are caused to vibrate by means of one of a pair of implanted heating resistors and their resonant frequencies are determined by any external forces applied thereto. The second resistor of the pair is used to detect the vibrations and both resistors form part of a closed-loop electrical circuit.

Description

TRANSDUCER

This invention relates to transducers and in particular, to miniature transducers used for measuring acceleration and having applications in vehicle navigation and guidance. One type of miniature accelerometer known for this purpose is made from silicon and comprises a tension-sensitive filament formed as a double-ended tuning fork and supported within a framework which functions as an inertial mass. However, this device has the limitation of being sensitive to temperature fluctuations and therefore requires some form of compensation. An object of the present invention is to configure a plurality of tension-sensing elements so that common mode effects such as temperature-dependent output fluctuations are virtually eliminated and therefore little or no form of compensation is necessary. The present invention therefore consists of a transducer comprising a support, an inertial mass connected thereto by at least two filaments, whereby movement of the support relative to the inertial mass in a certain direction causes a corresponding extension of one filament and compression of another, and sensing means incorporated in said two filaments for detecting said extension and compression.

The transducer may be of unitary construction and made from any elastic material, for example, silicon. Conventional photolithography and etching techniques may then be used to form the desired configuration if a silicon wafer or the like is used as a starting material. In one embodiment of the invention, the sensing means may comprise semi-conductor strain-gauges implanted in the filaments. An accurately adjusted amount of impurity can give the desired characteristics. A variation in tension in the filaments gives rise to a change in strain-gauge resistance which can be measured by a Wheatstone bridge.

In an alternative embodiment the filaments are caused to vibrate transversely. Each filament has a natural frequency of vibration whose value is a function of the dimensions of the filament, the tension applied to it and the elastic modulus and density of the material from which it is constructed. Any variation in tension in the filament will cause a corresponding change in frequency. Measurement of this frequency change can then be used to determine the magnitude of any acceleration to which the transducer has been subjected. This embodiment requires the provision of means for initiating and sustaining the vibrations of the filaments. This may be achieved, for example, by exploiting the piezo-electric effect or the thermo-mechanical properties of the transducer material. If silicon has been chosen then it is possible to sputter a piezo-electric layer of zinc oxide or barium titanate onto the filaments. This technique is known in the field of electro-acoustics.

Alternatively, the thermal expansion of a silicon transducer (which has an Inherently high thermal conductivity) can be used as a thermo-mechanical drive mechanism. This technique has hitherto been applied successfully to diaphragm pressure sensors. A heating resistor is embedded in each filament by ion implantation and an alternating current at the natural frequency of vibration of each filament Is passed through each resistor. The resulting cyclic temperature changes cause the filaments to expand and contract which results in a mechanical driving force. Vibrations of the filaments may be detected by a piezo-electric layer deposited onto each filament and connected to a frequency measuring circuit. Alternatively, the thermally induced expansion and contraction may induce strains in a second implanted resistor. If this resistor is constant-current driven, then a measurable voltage at the frequency of vibration will develop across it. The vibrating filaments may be arranged to vibrate in a plane perpendicular to the plane of the support and inertial mass or they may be double-ended tuning forks whose tines vibrate in anti-phase with one another and in the plane of the support and inertial mass. It is preferable, though not essential far the dimensions of the filaments to be such that the measured effect of the said compression and extension are substantially equal and opposite.

An embodiment of the invention will now be described, by way of example only, with reference to the drawings of which:- Figure 1 is a perspective view of the embodiment, Figure 2 is a plan view of the embodiment, Figure 3 is a cross-section along the line A-A of Figure 2; and Figure 4 is a circuit diagram of a transducer drive and measurement circuit.

The embodiment is fabricated from a silicon wafer and comprises a support 1 and an inertial mass 2 connected together by three filaments 3, 4 and 5, two of which (4 and 5) are co-planar. Filament 3 is located in the common plane of the upper face of the support 1 and inertial mass 2 and the filaments 4 and 5 are located in the common plane of the lower face of the support and inertial mass. The three filaments 3, 4 and 5 are displaced laterally from one another allowing ease of manufacture by an etching process from opposite directions of the silicon wafer. The particular configuration of the filaments ensures that the transducer has sons degree of torsional rigidity and also enables the provision of a differential output with good common mode rejection. The support 1 and inertial mass 2 are typically 0.4 mm deep and the filaments have dimensions of, typically, 1 mm × 50 μm × 50 μm.

The transducer shown in Figure 2 is sensitive to accelerations or components thereof along one axis only, shown as the Z axis (see Figure 1). An acceleration of the support 1 in the Z direction will cause the filaments 4 and 5 to be tensloned and the filament 3 to be compressed, owing to the presence of the inertial mass 2. These changes alter the natural frequency of vibration of the filaments, increasing it in the first case and decreasing it by a similar amount in the second, the magnitude of the frequency shift being a function of the acceleration applied. Sensing means incorporated in each of the filaments 3 and 4 comprise a vibrator for initiating and sustaining vibrations of each filament and a detector for monitoring the frequency of such vibrations. The sensing means for the filaments comprise, in each case, a pair of implanted heating resistors 6 and 7, one of each pair acting as the vibrator and the other, as the detector. The outputs from each detector are differenced to give a signal related to the applied acceleration. In use, the support of the trnasducer is attached to a vehicle, for example, whose accelerations are to be monitored. It can be shown that:- f₃ = f₀ (1 + kma)^½ where f ₃ is the frequency of vibration of filament 3 when subjected to an applied acceleration a, m is the mass of the inertial mass 2, f₀ is the frequency of vibration of each filament under zero accelerations (assumed for the present to be equal), and k is a known constant which is related to the dimensions of the filament and its elasticity. The factor kma is small under most circumstances as the breaking strains would normally be reached before a 10% change in frequency takes place. Hence to a close approximation, f₃ = f₀ (1 + ½ kma) Similarly, for filament 4, f₄ = f₀ (1 - ½ kma)

By measuring the frequency difference f ₃-f₄ (for a given f₀) the acceleration, a, can then be calculated, since

If the filaments 3 and 4 have different dimensions so that their natural frequencies of vibration under zero acceleration are not equal, then this effect will be, broadly, to generate a non-zero 'a' value under zero acceleration conditions. This can be simply allowed for and may indeed be beneficial in certain applications in preventing frequency lock.

The embodiment hereinbefore described requires connection to a drive and measurement circuit. A suitable circuit is shown in Figure 4.

Referring to the part of Figure 4 enclosed within dashed lines which shows an oscillator circuit 8 for sustaining and detecting oscillation of filament 3, a sense resistor R_S is supplied with a constant current (of 5 mA) from a power supply Vs via a transistor 9. A drive resistor R_D is supplied with an alternating drive voltage and a DC bias from the output of an operational amplifier 10. R_S and R_D correspond to the resistor pair 6 of Figure 2. Two more operational amplifiers 11 and 12 amplify the alternating sense voltage of approximately 5 mV peak to peak developed across the sense resistor R_S and apply it to an input of the operational amplifier 10. Two diodes 13 and 14 limit the output signal appearing on line 15 to 4 volts. The alternating drive voltage (of 4V peak to peak) supplied to the drive resistor R_D causes thermal expansion and contraction of the filament 3. In turn, this mechanical distortion of the filament induces strains in the sense resistor R_S causing its resistance to vary sinusoidally. A DC bias of 2V is applied in order that the voltage applied to the resistor swings from 0 to +4V so that only one heating pulse is generated in each cycle ie the sense voltage developed across the sense resistor R_S oscillates at the same frequency as the drive voltage. The oscillator circuit 8 has a quality factor Q of approximately 2000 and runs at, typically, 60 kHz. By comparing the frequency of the output signal 01 line 15 with that obtained from an Identical circuit connected to filament 4 a measurement of acceleration in the z direciton can be made. In this final stage, shown schematically in Figure 4, the f₃ - f₄ difference signal generated by a comparator 16 Is fed into a sealer 17 to give an output 18 indicative of the magnitude of 'a'.

A transducer embodied as shown in the drawings has a sensitivity of, typically, 20 Hz/g (where g is the acceleration due to gravity) over an operating range of ±100 g. Its differential form of output provides the advantage of nullifying any common-mode effects brought about, for example, by ambient temperature fluctuations.

In an alternative embodiment, the filament 5 of Figures 1 and 2 also incorporates a vibrator and detector similar to those of the filaments 3 and 4. This provides the transducer with a means for measuring acceleration in the y direction if desired, by comparing f₄ with f₅ (where f₅ is the frequency of vibration of the filament 5 when subjected to n applied acceleration). Furthermore, it enables any error in the z axis acceleration measurement, introduced by a simultaneous acceleration along the y axis, to be accounted for by making the following frequency comparisons f5 + -t

2 A transducer in accordance with the invention is not restricted solely to operation in conjunction with the closed loop circuit of Figure 4. Those skilled in the electronics art will be aware of possible commonplace alternative choices of components and their values which will still achieve the desired results. Furthermore, the transducer will function satisfactorily with an open-loop circuit.

The transducer may be used for navigation and guidance of vehicles although it is not limited to this application. Three single axis transducers may be strapped to the vehicle and provide output signals relating to acceleration thereof in three, for example orthogonal, directions. Such information may be processed by the vehicle's navigation and guidance computer along with additional navigational data in order to guide the vehicle on some chosen trajectory. The simplicity of the transducer's construction enables a robust and low-cost navigation and guidance system to be realised.

Although the foregoing has described the transducer as an accelerometer, it will be appreciated that it may alternatively be used as a pressure or force sensor.

Claims

1. A transducer comprising a support (1), an inertial mass (2) connected thereto by at least two filaments (3, 4) whereby movement of the support (1) relative to the inertial mass (2) in a certain direction causes a corresponding extension of one filament and compression of another and characterised by sensing means incorporated in said two filaments for detecting said extension and compression.

2. A transducer as claimed in Claim 1 in which said two filaments (3, 4) are displaced laterally from one another and are respectively located in the common planes of opposite faces of the support (1)and inertial mass (2).

3. A transducer as claimed in Claim 2 including a third filament (5) displaced laterally from the other two filaments (3, 4) and located in the common plane of one face of the support (1) and inertial mass (2).

4. A transducer as claimed in claim 3 in which the third filament (5) incorporates sensing means for detecting extension or compression of said filament (5).

5. A transducer as claimed in claim 2 or claim 4 in which the sensing means comprise semi-conductor strain gauges implanted in the filaments.

6. A transducer as claimed in claim 2 or claim 4 in which the sensing means comprise a vibrator for initiating and sustaining vibrations of the filaments and a detector for monitoring the natural frequency of vibration of each of the filaments.

7. A transducer as claimed in claim 2 or claim 4 in which the vibrator and the detector comprise a pair of piezo-electric transducers.

8. A transducer as claimed in claim 2 or claim 4 in which the vibrator and the detector comprise a pair of implanted heating resistors (6).

9. A transducer as claimed in claim 8 in which the resistor pair comprise part of a closed-loop electrical circuit (8)

10. A transducer as claimed in any preceding claim and fabricated from a silicon wafer.