WO1989010567A1

WO1989010567A1 - Accelerometer

Info

Publication number: WO1989010567A1
Application number: PCT/GB1989/000441
Authority: WO
Inventors: Dennis John Miles; Sidney George Smith
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: GB8809755D0; GB9023236D0; GB2238874A; EP0412106A1; JPH03503932A; GB2238874B

Abstract

A transducer for acceleration measurement is fabricated from a silicon slice and comprises four filaments (3, 4, 5 and 6) which interconnect a central support (1) with an inertial mass (2). The filaments vibrate in a common plane, their resonant frequencies being determined by any external forces applied thereto. Vibrations are sustained and detected by means of a pair of thermal resistors (RS, RD) implanted in each filament. The embodiment is configured as a 2-axis accelerometer for use in vehicle navigation and guidance systems. Piezo-electric means for sustaining and detecting vibrations of the filaments may be used instead of the thermal resistors (RS and RD).

Description

ACCELEROMETER

This invention relates to transducers and, in particular, to miniature transducers adapted for use as accelerometers for 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 acceleration along one axis only. Furthermore, the output from the transducer is sensitive to temperature fluctuations and therefore requires some form of compensation.

Two objects of the present invention are, firstly, to provide a transducer capable of measuring accelerations along two or three axes and secondly, 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 temperature compensation is necessary.

The present invention therefore consists of a transducer comprising a central support, an annular inertial mass co-planar with the central support and connected thereto by at least two filaments, whereby movement of the central support relative to the inertial mass causes a corresponding extension of one filament and compression of the other, 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 be used to form the desired configuration if a silicon wafer or the like is used as a starting material.

It is preferable, though not essential, for the filaments to have similar dimensions.

A transducer in accordance with the invention which is configured to have two filaments diametrically opposed on either side of the central support may be used as a single-axis accelerometer. Accelerations along an axis which connects the two filaments will result in a stretching of one filament and a compression in the other owing to the effect of the inertial mass. The sensing means incorporated in each of the two filaments may be arranged to give a differential output indicative of the magnitude of the applied acceleration.

A transducer in accordance with the invention may be made sensitive to components of acceleration along two orthogonal axes by arranging three or more filaments, each incorporating sensing means, around a central support. An embodiment comprising four equally-spaced filaments is preferred due to the inherently high common mode rejection of this configuration. This embodiment will be described in detail hereinafter. It may be desirable to limit the relative rotation of the central support and inertial mass or their relative movement along an axis perpendicular to their common plane, particularly in the case where only two filaments are provided. In such a case, one or more filamentary tie-bars linking the support with the inertial mass but not incorporating sensing means may be distributed around said support.

A transducer in accordance with the invention may include a plurality of tension-sensitive filaments which are configured so that the transducer is sensitive to components of accelerations along three orthogonal axes. Such a configuration will be described in greater detail hereinafter. 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. This type of sensor may be used with either a single or multi-axis transducer configuration. 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. Either type of drive or sensing means, piezo-electric or resistive, may be used with either a single axis or multi-axis transducer configuration. 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 are arranged to vibrate in anti-phase with one another and in the plane of the support and inertial mass. Alternatively, the transducer may have deep but narrow filaments arranged to vibrate in the plane of the support.

A transducer comprising a central support and annular inertial mass requires attachment to an accelerating body at one mounting point only (on the central support). In contrast, a transducer having an outer support and a central inertial mass would require multiple mounting points and if, for example, thermal expansion were to occur at one point and not another, a differential strain between filaments would be produced which would be seen as an erroneous acceleration. This disadvantage is not inherent in the present invention.

A further advantage arises from the differential form of transducer output which nullifies any common-mode effects brought about, for example, by ambient temperature fluctuations. Some embodiments of the invention will now be described by way of example only with reference to the drawings of which:-

Figure 1 is a plan view of a first embodiment of the invention;

Figure 2 is a cross-section along the line II-II shown in Figure 1; Figure 3 is a plan view of a second embodiment; Figure 4 is a plan view of a third embodiment; Figure 5 is a cross-section along a line V-V of Figure 4; and Figure 6 is a circuit diagram of a transducer thermal drive and measurement circuit suitable for use with any of the above embodiments.

Referring to Figs 1 and 2, a transducer for measuring components of acceleration along two orthogonal axes (x, y) comprises a support 1 connected to an inertial mass 2 by four equally-spaced filaments 3, 4 ,5 and 6. The support, mass and filaments are etched from a silicon slice of thickness approximately 0.4 mm, the outer diameter of the transducer being approximately 10 mm and the filaments having dimensions of typically 1 mm × 50 μm × 50 μm. In use, the central support of the transducer is attached to a vehicle, for example, whose accelerations are to be monitored. If, for example, the host vehicle were to be accelerated along the x axis, then owing to the presence of the inertial mass the filament 6 would be tensioned and filament 4 compressed.

These changes would alter the natural frequency of vibration of the two 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.

It can be shown that:f₆ = f₀ (1 + kma_x)^½ where f₆ is the frequency of vibration of filament 6 when subjected to an acceleration a_x exerted in the x direction, m is the mass of the inertial mass 2, f₀ is the frequency of vibration of the filaments under zero acceleration and k is a known constant which is related to the dimensions of the filament and its elasticity. The factor kma_x 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, f₆ = f₀ (1 + ½ kma_x) Similarly, for filament 4, f₄ = f₀(1 - ½ kma_x) if the filaments 4 and 6 have equal dimensions.

By measuring the frequency difference f₆-f₄ (for a given f₀) a can then be calculated, since

If filaments 4 and 6 have different dimensions so that their frequencies of vibration under zero acceleration are not equal, then the effect of this will be, broadly, to generate a non-zero a_x value under zero acceleration conditions. This can be simply allowed for and may indeed be beneficial in certain applications in preventing frequency lock.

All four filaments are forced to vibrate transversely and in a plane perpendicular to that of the support by a drive resistor R_D which is one of a pair of heating resistors implanted into each of the four filaments (see Fig 1). This frequency of vibration is monitored by the second of the resistor pair, a sense resistor R_S for each of the four filaments. By connecting R_D and R_S to a suitable drive and measurement circuit, accelerations in two axes may be detected and measured. Each filament requires connection to a separate circuit.

A second embodiment of the invention is shown in Figure 3. A support 1, inertial mass 2 and four filaments 7, 8, 9 and 10 are made as before from a single silicon slice having a thickness of typically 0.4 mm and a diameter of 10 mm. The filaments 7-10 are configured as double-ended tuning forks and have the same thickness as the inertial mass in order to restrict vertical movement thereof. The tines of each of the filaments are forced to vibrate In anti-phase with one another and in the plane of the support 1 by electrically driven resistors (not shown) implanted in each filament. Sense resistors implanted in the filaments enable any change in the natural frequency of vibration brought about by accelerations applied along the x or y axes to be detected.

A third embodiment is shown in Figures 4 and 5. It permits sensitivity to accelerations in three orthogonal axes. This embodiment is similar in construction to the first embodiment having a central support 11, an annular inertial mass 12 and four filaments p, q, r and s but incorporating two extra filaments m and n. The filament m includes an implanted drive and sense resistor pair (not shown), which, in conjunction with the resistors (not shown) implanted in the filament p enables meaaurement of accelerations in the z direction. The filaments p and q and their associated resistors also permit measurement of acceleration in the x direction and filaments r and s plus their associated resistor pairs (not shown) permit measurement of acceleration in the y direction.

As a further alternative to the afore-mentioned embodiments, a transducer may be composed of a mixture of plate-like filaments and tuning forks.

Each of the embodiments hereinbefore described requires connection to a drive and measurement circuit. A suitable circuit is shown in Fig 6. Its operation in conjunction with the first embodiment will be described below. Referring to the part of Fig 6 which shows an oscillator circuit 13 for sustaining and detecting oscillation of filament 4, the sense resistor R_S is supplied with a constant current (of about 5 mA) from a power supply Vs via a transistor 14. The drive resistor R_D is supplied with an alternating drive voltage and a DC bias from the output of an operational amplifier 15. Two more operational amplifiers 16 and 17 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 15. Two diodes 18 and 19 limit the output signal appearing on line 20 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 4. 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 13 has a quality factor Q of approximately 2000 and runs at, typically, 60 kHz. By comparing the frequency of the output signal on line 20 with that obtained from an identical circuit connected to filament 6, a measurement of acceleration along the x axis can be made. In this final stage, shown schematically in Fig 6, the f₆ - f₄ difference signal generated by the comparator 21 is fed into a sealer 22 to give an output 23 indicative of the magnitude of a_x. By using two additional oscillator circuits connected to filaments 3 and 5 accelerations along the y axis may be measured in a similar fashion.

A transducer embodied as shown in any of Figures 1 to 5 has an x axis and a y axis sensitivity of approximately 20 Hz/g (where g is the acceleration due to gravity) over an active range of ±100 g. The embodiment of Figure 5 has a higher sensitivity along its z axis of about 100 Hz/g. The sensivitity of any of the embodiments may be enhanced by employing the known technique of frequency multiplication using, for example, a phase-locked-loop.

A transducer in accordance with the invention is not restricted solely to operation In conjunction with the closed loop circuit of Fig 6. 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, any of the transducers described herein 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. A three-axis transducer or alternatively a two-axis transducer together with a single axis transducer may be strapped to the vehicle and provide output signals relating to accelerations thereof in three 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 central support (1), an annular inertial mass (2) co-planar with the central support and connected thereto by at least two filaments (3, 5), whereby movement of the central support (1) relative to the inertial mass (2) causes a corresponding extension of one filament and compression of the other, and characterised in that sensing means are incorporated in said two filaments (3, 5) for detecting said extension and compression.

2. A transducer as claimed in Claim 1 and having one or more filamentary tie-bars linking the central support (1) with the inertial mass (2) in order to limit relative rotation of the central support and inertial mass or their relative movement along an axis perpendicular to their common plane.

3. A transducer as claimed in Claim 1 or Claim 2 in which the sensing means comprise semi-conductor strain gauges implanted in the filaments.

4. A tranducer as claimed in Claim 1 or Claim 2 in which said sensing means comprise a vibrator for initiating .and sustaining vibration of said filaments and a detector for monitoring the natural frequency of vibration of said filaments.

5. A transducer as claimed in Claim 4 in which the vibrator (R_D) and detector (Rs) comprise heating resistors.

6. A transducer as claimed in Claim 4 or Claim 5 in which at least two filaments are arranged to vibrate in a plane perpendicular to the common plane of the support (1) and inertial mass (2).

7. A transducer as claimed in any of Claims 4 to 6 in which at least two filaments are in the form of double-ended tuning forks (7, 9) whose tines are arranged to vibrate in anti-phase in the common plane of the support (1) and inertial mass (2).

8. A transducer as claimed in any of Claims 4 to 7 in which at least two filaments are deep and narrow and arranged to vibrate in the plane of the support (1).

9. A transducer as claimed in any of Claims 1 to 7 including two filaments (p, m) located close to one another and in the common plane of opposite faces of the support (1), and inertial mass (2) whereby an acceleration perpendicular to said planes causes an extension of one filament and a compression of the other and sensing means incorporated in said two filaments for detecting said extension and compression and arranged to give a differential output indicative of the magnitude of said acceleration.

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