GB2192718A

GB2192718A - Accelerometer or seismometer

Info

Publication number: GB2192718A
Application number: GB8617159A
Authority: GB
Inventors: John Christopher Greenwood
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-07-15
Filing date: 1986-07-15
Publication date: 1988-01-20
Anticipated expiration: 2006-07-15
Also published as: GB2192718B; GB8617159D0

Abstract

An accelerometer or seismometer element consists of a rectangular movable body (1) of silicon mounted in a gap in a frame (2) also of silicon by U-shaped beams (3), also of silicon. The frame, beams and movable body are integral. Strain gauges are diffused into the beams at the ends of their legs. Overload movement is accommodated by buckling of the U-shaped beams, which move transverse to the direction of acceleration, this movement being facilitated by a bend or buckle in the beams. <IMAGE>

Description

SPECIFICATION Accelerometer or seismometer This invention relates to accelerometer or seismometer elements, and to the manufacture thereof.

Such an element has to have a high degree of sensitivity, and a substantially linear response to an applied force. Further it should not be subject to damage due to overloads.

An object of the invention is to provide an accelerometer or seismometer element which is more successful in meeting the above criteria than are many known devices.

According to one aspect of the invention there is provided an accelerometer or seismometer element, which includes a movable body supported by at least one elastic beam such that movement of the body in response to acceleration provides an output signal representative thereof, wherein said beam is of generally laminar form and has a bend lying in the plane of the beam such that the beam can buckle elastically in response to displacement of the body in a direction transverse to the direction of movement caused by the acceleration.

According to another aspect of the invention there is provided an accelerometer or seismometer element, which includes a movable body supported in an opening in a rigid laminar body by elastic beams, wherein each said beam is generally U-shaped with the end of one leg of its U integral with a portion of the movable body and the end of the other leg of that U integral with a portion of the laminar body adjacent to said portion of the movable body. The preferred material for making the accelerometer or seismometer element is silicon.Hence the invention further provides an accelerometer or seismometer element, which includes a rectangular movable body of silicon supported in a rectangular opening in a rigid laminar body af silicon by elastic beams of silicon, the movable body, the beams and the laminar body all being integral, wherein the beams are rendered elastic by being considerably thinner than the movable body and the laminar body, and wherein each said beam is generally U-shaped with the end of one leg integral with a portion of the movable body and the end of the -other leg of the U integral with a portion of the laminar body adjacent to said portion of the movable body.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a plan view of an accelerometer or seismometer element embodying the invention, Fig. 2 is a partial view of an element such as that of Fig.1 installed on a support member, and Fig. 3 is an enlarged view of the arrangement of strain gauges used in a device such as that shown in Fig. 1.

The element shown in plan in Fig. 1 is an accelerometer made from silicon. It includes a movable body 1 which is integral with, and connected to an outer laminar frame body 2 by elastic beams formed by thin U-shaped flexible regions 3. The U-shaped regions carry strain gauges such as 4 diffused into the flexible region 3. Thus the motion of-the element 1 relative to the frame 2 is sensed by these strain gauges.

The thin flexible regions are produced by selective etching. Thus when using a p-type wafer of silicon as the starting point, the area which, in the final product, will be thin is diffused with an n-type dopant such es phosphorous. Separate electrical contacts are made to the p and n doped areas which are held at different potentials during the doping. Thus it is possible to cause the n-type materialto etch normally, but for the p-type material not to be etched at all.

Fig. 2 shows, partly in perspective and partly in section an element such as that of Fig. 1 mounted to a support member 5, which could also be of silicon. It will be- seen that the edges of the movable body 1 are bevelled and that that body 1 fits into a recess 6 in the support member 5. This recess is bevelled in a similar way to the body 1, so that it restrains it from excessive movement.

Fig. 2 also shows how relative movement of the body 1 and the frame body 2 occur, with the U-shaped regions such as 3 buckling or bending to accommodate the movement. It will be noted that "end-wise" movement of the body 1 is accommodated by such buckling, and that here the sides of the recess 6 act to limit the "end-wise" movement of the body 1.

Fig. 3 shows on an enlarged scale the ar- rangements of the strain gauges on one of the U-shaped portions, the others being similar.

We see two strain gauges 10 and 11-, one on each leg of the U, with the strain gauges set at 90 to each other. These gauges are coupled in series by conductive tracks 12 and 13, which, like the strain gauges, are diffused into the silicon of the legs of the U. Finally there are contact pads 14 and 15, connected to the strain gauge 10 and the track 73 respectively.

Setting the strain gauges at 90" is convenient for the arrangement of the pads and the tracks, and has also been found to give good results electrically.

The manner of mounting of the movable body 1 and the relative dimensions of the various parts of the element are such that the rigidity of the body 1 in the plane relative to its rigidity in the movement duration is of the order of 100:1 where the accelerations to be measured are of the order of 30 to 50G.

Thus the element can tolerate transverse accelerations of the order of 3000 to 10000G.

Devices such as described above can be used as the sensing means in accelerometers and seismometers. Accelerometers of this sort can be used in.-inertiel guidance systems, e.g.

in missiles or in aircraft navigation equipment.

The type of accelerometer or seismometer element described herein has a number of ad vantages: (a) Because it is symmetrical, unwanted in puts tend to cancel out.

(b) The design is compact because the U shaped cantilevers are folded.

(c) There is no stress stiffening with larger deflections to make the output non-linear.

(d) The arrangement of two strain gauges on each cantilever makes the design insensi tive to curvature of the beams caused by ox ide layers on the top surface. Such layers are essential to passivate the strain gauges, but tend 'to make the cantilever bend as a result of differential thermal expansion.

(e) Oyerloads in the. direction in the plane of the cantilevers, i.e. "send-wise deflections", cause a harmless buckling, which allows the seismic mass to move an appreciable distance so that overload stops can be provided with a reasonable clearance.

In one example of element embodying the invention, the movable body is 6mm long and 4mm wide, and is about 2mm thick. The body in this case is made up of a central portion:0.5mm thick, flanked in-sandwich manner to give the overall thickness of 2mm.

The U-shaped beams then have thicknesses in the range of 5 to 8 microns.

Claims

1-. An accelerometer or seismometer ele ment, which includes a movable body sup -ported by at least one elastic beam such that movement of the body in response to acceleraction provides. an output signal representative thereof, wherein said beam is of generally laminar form and has a bend lying in the plane of the beam such that the beam can buckle elastically in response to displacement of the body in a direction transverse to the direction .of movement caused by the acceleration.

2. An accelerometer or seismometer ele ment, which includes a movable body sup ported in an opening in a rigid laminar body by elastic beams, wherein each said beam is generally U-shaped with the end of one leg of its U integral with a portion of-the movable -body and the. end of the other leg of that U integral with a portion of the laminar body adjacent to said portion of the movable body.

3. An. accelerometer or seismometer ele ment,-which includes a rectangular movable body of silicon supported in a rectangular opening in a rigid laminar body of silicon by elastic beams of silicon, the movable body, the beams and the laminar body all beinginte- gral, wherein the beams are rendered elastic by being considerably thinner than the mov able body and the laminar body, and wherein -each said beam is generally U-shaped with the end of one leg integral with a portion of the movable body and the end of the other leg of the U integral with a portion of the laminar body adjacent to said portion of the movable body.

4. An element as claimed in claim 2 or 3, wherein the U-shaped beams carry strain gauges, with each said beam carrying two strain gauges, one near the end of each leg of the U.

5. An element as claimed in claim 4, wherein the strain gauges are each of zig-zag shape with the two strain gauges of a said U set at right angles to each other, and wherein the strain gauges of the same U are connected in series by conductive tracks on the U.

6. An element as claimed in claim 5, wherein the strain gauges and the associated tracks are produced by diffusion.

7. An accelerometer or seismometer element, substantially as described with reference to the accompanying drawings.