GB2176607A - Accelerometer device - Google Patents

Abstract

An accelerometer device comprises a support frame 11 on which a pair of flexible beam members 14,15 are mounted. The beams are colinear or parallel and are provided each with a seismic mass 16,17 at their free ends. Flexure of each beam in response to acceleration is measured by a strain gauge 18,19 formed on the beam. The gauges 18,19 may be piezo-electric devices in a Wheatstone bridge network. <IMAGE>

Description

SPECIFICATION Accelerometer device This invention relates to accelerometer devices and to inertial guidance systems employing such devices.

Accelerometer devices, e.g. of selectively etched silicon, conventionally incorporate a flexible member one surface of which is provided with a strain gauge. Deflection of the flexible member in response to an acceleration of the device produces a corresponding output from the strain gauge. A problem with devices of this type is that of mounting the device in a suitable package. It has been found that strains introduced into the device by packaging and mounting can be transmitted to the flexible portions to give false output signals.

The object of the present invention is to minimise or to overcome this disadvantage.

According to the invention there is provided an accelerometer device, including first and second parallel or colinear elastic beam members supported at one end on a mounting member, each said beam member being provided at its free end with a seismic mass whereby acceleration of the device causes flexure of the member, and wherein each said beam member is provided with a strain gauge whereby, in use, flexure of that member in response to an acceleration is measured.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a plan view of an accelerometer device; Figure 2 shows the acceleration sensing portion of the device of Fig. 1; Figure 3 shows an alternative device structure, Figure 4 illustrates a method of attaching seismic masses to the devices of Figs. 1 to 3; Figure 5 is a cross sectional view of a package in which the device of Fig. 1 and 2 is mounted, and Figure 6 is a schematic diagram of an inertial guidance system employing a plurality of devices of Fig. 1 and 2 or Fig. 3.

Referring to Fig. 1 and 2, the accelerometer device comprises a rigid support frame 11 having first and second openings 12, 13 therein and flexible beam members 14, 15 extending one into each opening. The members 14, 15 are supported on the frame 11 such that they are colinear or parallel. Each beam member 14, 15 carries at its free end a seismic mass 16, 17 whereby flexure of that beam in response to acceleration of the device in a direction perpendicular to the device plane is effected. Flexure of each beam is detected and measured via corresponding strain gauge 18, 19 disposed on each beam adjacent the support frame 11. Typically each strain gauge comprises four piezoresistors arranged in a Wheatstone bridge network. A differential output is taken from the network and cross-axis effects therefore are substantially eliminated.

Advantageously, the device of Fig. 1 and 2 is formed as an integral structure by selective etching from a body of single crystal silicon.

In this process the device configuration is defined by boron doping or by the use of an electrolytic etch stop. Typically the beams 14, 15 are defined by doping with boron to a level of about 4X10'9 atoms/cc. The silicon body is then masked and exposed to a selective etch comprising a mixture of catechol, ethylene diamine and water, or potassium hydroxide and isopropyl alcohol. The use of selective etching techniques in the fabrication of silicon structures is more fully described in our UK specification No. 1,211,496.

Fig. 3 shows an alternative, compact, device structure in which the support frame 31 has a single opening 32 into which both beams 33, 34 extend in an 'antiparallel' configuration. As before, strain gauges 35, 36 are provided one on each beam.

In the devices of Figs. 1 to 3 the beam members may be coupled to the frame via a flexible diaphragm (not shown) extending across the opening or openings. This improves the selectivity of the device. Its response to acceleration in directions other than that perpendicular to the device plane is very much reduced by this technique.

Fig. 4 shows a method whereby seismic masses may be attached to the flexible beam of the devices of Figs. 1 to 3. An opening 41 is etched in the free end of the beam 42.

Pellets 43 of a dense material, e.g. tungsten, are applied to the upper and lower surface of the beam 42 in register with the opening 41 and are secured to the beam by a solder connection 44, e.g. a glass frit solder, via the opening.

A sectional view of a device package construction is shown in Fig. 5. The package comprises a housing defined by a top plate 51 and bottom plate 52, e.g. of silicon, between which the support frame member 53 of the accelerometer device is sandwiched. The assembly is secured by an adhesive 54. Advantageously the top and bottom plate are provided with recesses 55 and 56 which receive the seismic masses 43 disposed on the device beams. This prevents excessive travel of the beams and prevents shock damage of the device.

Fig. 6 is a schematic diagram of an inertial guidance system e.g. for use in a vehicle. The system employs accelerometers 61, 62, 63 of the type shown in Figs. 1 and 2 or Fig. 3 to sense accelerations in the X, Y and Z directions respectively. The accelerometer outputs are fed each via a corresponding amplifier 64, 65, 66 to a central control unit 67. In re sponse to the signals received via the amplifiers, and e.g. to preset course information, the control unit provides output signals to X, Y and Z guidance controls 68, 69, 70 whereby the desired course may be maintained.

Claims (7)

1. An accelerometer device, including first and second parallel or colinear elastic beam members supported at one end on a mounting member, each said beam member being provided at its free end with a seismic mass whereby acceleration of the device causes flexure of the member, and wherein each said beam member is provided with a strain gauge whereby, in use, flexure of that member in response to an acceleration is measured.

2. A device as claimed in claim 1 and formed as an integral structure from single crystal silicon.

3. A device as claimed in claim 1 or 2 and comprising a support frame member having one or more openings into which the elastic beam members project.

4. A device as claimed in claim 3, wherein the beam members are coupled to the support frame via a flexible diaphragm.

5. A device as claimed in any one of claims 1 to 4, wherein each said strain gauge comprises four piezoresistors arranged in a Wheatstone bridge network.

6. An accelerometer device substantially as described herein with reference to Fig. 1 and 2 or to Fig. 3 together with Fig. 4 and 5 of the accompanying drawings.

7. An inertial guidance system incorporating a plurality of accelerometer devices as claimed in any one of claims 1 to 6.