GB2130373A - Accelerometer device - Google Patents

Abstract

An accelerometer or seisometer element comprises a movable body 11 mounted via two symmetrically disposed elastic beams 14 in an opening in a support body 13. Movement of the body is restricted by the provision of an elastic membrane (16) or by the provision of projections 51 on the body 11 which projections, in use, engage corresponding recesses 62 in a housing 61, Figure 6). Slots 17 are provided to reduce the stiffness of the coupling of beams 14. Strain gauges (15, Figure 2) are provided on the beams 14 to measure the movements of body 11. The element 11 may be formed as an integral structure by selective etching of a silicon body doped with boron. <IMAGE>

Description

SPECIFICATION Accelerometer device This invention relates to accelerometer or seisometer devices and to methods of fabricating such devices.

An accelerometer or seisometer device must provide a high degree of sensitivitytogetherwith a substantially linear response to an applied force.

Conventional devices having these characteristics are however somewhat delicate and are subject to damage under overload conditions. Attempts to overcome this problem usually involve stiffening of the device with consequent loss of sensitivity and, in some cases, of linearity.

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

According to one aspect of the invention there is provided an accelerometer or seisometer element, including a rigid laminar body having an opening therein, and a movable body supported in said opening via one or more elastic beams mounted on or integral with the laminar body, wherein the structure is provided with means whereby displacement of the movable body from a rest position is restricted.

According to another aspect of the invention there is provided an accelerometer or seisometer device, including a housing, and a transducer element mounted within the housing, wherein the transducer element comprises a rigid laminar body a single crystal silicon having an opening wherein a body movable in response to an accelerating force is supported by one or more elastic beams, wherein said body has one or more pyramidal projections extending perpendicular to the plane of the laminar, and wherein the housing has recesses in registration with the projections and in which the projections are engagableto limit travel of the body.

Advantageously the element is formed as an integral structure by selective etching of a body of resilient material, e.g. silicon.

In a preferred embodiment the laminar body is provided with slots adjacent the coupling points of the elastic beams to the body. This provides a degree of mechanical decoupling and avoids excessive stiffness of the arrangement.

Embodiments of the invention will not be described with reference to the accompanying drawings in which: Figure lisa plan view of the accelerometer or seisometer element; Figure 2 is a view of the underside of the element of Figure 1; Figures 3 and 4 show two sectional views of the element of Figures 1 and 2 supported in a mounting frame; Figure 5 is a plan view of a modified transducer construction; Figure 6 is a cross-sectional view of the transducer of Figure 5 mounted in a housing; and Figure 7 is a cross-sectional view of a further modified transducer construction.

Referring to Figures 1 and 2, the accelerometer or seisometer element includes a movable element 11 supported in an opening 12 in a laminar body 13 via two symmetrically disposed elastic beams 14. Movement of the element 11 relative to the plane of the body 13 is detected by strain gauges 15 disposed on the beams 14.

Movement of the element 11 in directions parallel to the plane of the supporting body 13 is inhibited by an elastic membrane 16 whereby the element 11 is coupled to the body 13. This ensures that oscillation of the movable element 11 in response to an accelerating force is confined to a single mode in a direction perpendicular to the plane of the body 13.

The arrangement is thus responsive only to that component of an accelerating force that is in a direction normal to the plane of the arrangement.

Advantageously the element 11 has an opening whereby a loading seismic mass 31 (Figure 3) may be attached to the element of enhance sensitivity and to reduce the resonant frequency.

Advantageously the body 13 is provided with slots 17 adjacent the regions at which the beams 14 are attached to the body 13. This provides a degree of mechanical decoupling between the beams 14 and the body 13 and by reducing the stiffness of the coupling, provides enhanced sensitivity of the arrangement to an accelerating force.

Figures 3 and 4 show cross-sectional views of the element of Figures 1 and 2 mounted between a pair of rigid support frame members 32 and provided with a loading mass 31. In Figure 3 the section is taken along the plane X-X of Figure 1, and in Figure 4 along the plane Y-Y of Figure 1. As shown in Figures 3 and 4 the loading mass is of two-part construction and comprises a body portion 311 having a threaded boss 312 which boss protrudes through the opening in the movable element 11 and receives a lock nut 313. Typically the body portion 13 of the element is secured to the frame members 32 by frit bonds 33.

The frame members 32 are spaced such that the elastic beams 14 are free to move therebetween, the frame members providing a limit stop against excessive deflection of the beams 14. Typically the maximum displacement of the beams 14 is from 10 to 20 microns.

Advantageously the arrangement shown in Figures 3 and 4 is mounted in a sealed housing (not shown) which may be evacuated or filled with an inert gas or with an inert liquid to provide damping.

The transducer element shown in Figure 5 and 6 provides for control of extreme overloads. In this construction the movable element 11 is supported by two elastic beams 14 as before but is provided with a protruberance 51 in the form of a truncated pyramid formed integral with the element 11. In use the transducer element is mounted in a housing 61 (Figure 6) having a truncated pyramid recess 62 in which the protruberance 51 is located. The protruberance 51 and recess 62 are spaced sufficiently to allow small displacements of the transducer element but excessive travel in any direction is prevented by abutment of the element and housing. Advantageously the element 11 has pyramidal protruberances on both its upper and lower surfaces.

Preferably the transducer element of Figure 5 and 6 is formed from single crystal silicon by a selective etching process.

The selective etching process in single crystal silicon produces structures bounded by flat planes, with included angles of 90 in the plane of the wafer and 35Q 12', normal to the wafer, uniquely defined by the crystallography of the material. It is therefore possible to form the protruberances or projections 51 on the seismic mass 11 in the shape of a truncated square pyramid. The angles of the pyramid are determined by the crystal structure and are invariant. If the housing 61 against which the mass abuts is also made of silicon and has a pyramidal recess 62 etched into it similar two the projection, the two will nest accurately together under overload and the device will be effectively clamped under high load conditions against X, Y, Z and rotational forces.

A modification of this structure is shown in Figure 7 in cross section. In this arrangement the movable element 71 supported on arms 72 is formed in the shape of a hollow truncated pyramid. The structure is clamped between two housing members 73,74 one of which (73) has a projection 75 engaging recess 76 in the element 71. The other housing member has a pyramidal recess 77 for receiving the truncated pyramidal member 71. The structure is self aligning during assembly and provides a high degree of overload projection whilst allowing free movement of the transducer element under normal conditions.

In a preferred embodiment the transducer element is formed as an integral structure by selectively etching a body of single crystal silicon.

Typically a silicon body is selectively doped with boron to a level of at least 4 x 1019 atoms/cc in certain regions that will ultimately comprise the finished device. The wafer is then etched e.g. with a mixture of catechol, ethylene diamine and water or a mixture of potassium hydroxide, isopropyl alcohol and water. Such etch compositions have been found to be chemically selective when employed with boron doped silicon. There is an abrupt change in etch rate from that normalfor undoped silicon to substantially zero at a boron doped interface so that the configuration of unetched regions is defined precisely by their boron doping profiles. Typically a single crystal silicon body is doped with boron through a mask in those areas where etching is not required and is then subjected to the etching treatment to remove only the undoped material.In some cases a plurality of masking, doping and etching stages will be required.

Such techniques are more fully decribed in our published specification No. 1 211 496 (J.C. Greenwood 6). In a modification of the process some parts of the silicon body are prevented from etching by boron doping. Other parts are protected from the etch by resistant layers typically of silicon dioxide or silicon nitride.

Although only a single device has been described it will be clear to those skilled in the art that a plurality of such devices may be fabricated simultaneously e.g. on a single semiconductor wafer, the wafer subsequently being subdivided by conventional techniques to form the individual transducer devices.

In a further construction (not shown) the transducer body may be provided with pyramidal recesses which engage corresponding truncated pyramidal projections on the housing which the transducer is mounted.

Claims (9)

1. An accelerometer or seisometer element, including a rigid laminar body having an opening therein, and a movable body supported in said opening via one or more elastic beams mounted on or integral with the laminar body, wherein the structure is provided with means whereby displacement of the movable body from a rest position is restricted.

2. An element as claimed in claim 1, and comprising an integral structure formed from an elastic material.

3. An element as claimed in claim 2, wherein the elastic material is single crystal silicon.

4. An element as claimed in claim 1,2 or3, wherein openings are provided in the laminar body adjacent the elastic beams whereby mechanical coupling between the two bodies is reduced.

5. An element as claimed in any one of claims 1 to 4, wherein strain gauges are provided on said beams.

6. An element as claimed in any one of claims 1 to 5, wherein said movable body has an opening for receiving a loading mass.

7. An accelerometer or seisometer element substantially as described herein with reference to the accompanying drawings.

8. An accelerometer or seisometer device, including a housing, and a transducer element mounted within the housing, wherein the transducer element comprises a rigid laminar body of single crystal silicon having an opening wherein a body movable in response to an accelerating force is supported by one or more elastic beams, wherein said body has one or more pyramidal projections extending perpendicular to the plane of the laminar, and wherein the housing has recesses in registration with the projections and in which the projections are engagableto limit travel of the body.

9. An accelerometer or seisometer incorporating an element as claimed in any one of claims 1 to 7.