GB2230599A

GB2230599A - Accelerometer

Info

Publication number: GB2230599A
Application number: GB8908158A
Authority: GB
Inventors: Andrew John Barker
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1989-04-11
Filing date: 1989-04-11
Publication date: 1990-10-24
Anticipated expiration: 2009-04-11
Also published as: GB8908158D0; GB2230599B

Abstract

An accelerometer has a fixed structure 7 and a mass 3 connected to the structure in such a way that it moves relative to the structure along an axis under the influence of an applied acceleration. Optical sensors 1, 5; 2, 6 sense movement of each end of the mass against a restoring force in such a way as to produce an output signal representative of the acceleration. <IMAGE>

Description

ACCELERONETER This invention relates to an accelerometer, i.e. a device for sensing acceleration or deceleration. In this specification, the term 'acceleration" includes deceleration (i.e. negative acceleration). The device may produce an output representative of the acceleration which is experienced, or may produce a pulse or other signal when an acceleration greater than a predetermined level is sensed.

There is an established requirement for low cost accelerometers capable of DC operation. Commercially available accelerometers are relatively expensive; they are based on either piezo electric or piezo resistive sensing of the movement of an internal seismic mass. Use of a piezo electric accelerometer at low frequencies is hampered by increasing noise which inhibits DC operation.

Piezo resistive accelerometers can achieve DC operation, but are more expensive to employ, largely because of the need for greater sophistication in the signal processing circuitry to achieve the required stability.

According to the present invention, there is provided an accelerometer comprising a structure to be mounted on an object the acceleration of which is to be measured, a mass connected to the structure and constrained to move relative to the structure along a sensing axis against a restoring force, and optical sensors to sense the movement of the leading and trailing ends of the mass relative to the structure.

The use of an optical sensor can result in a low cost accelerometer capable of operating over a wide frequency range, including DC.

The sensing axis preferably extends between two parts of the structure, and the mass is located between the two parts of the structure. The optical sensor may comprise two sensing units, one operating on the axis on one side of the mass and the other operating on the axis on the other side of the mass such that as the mass moves in one direction, the readings from the two sensing units vary inversely.

The sensing units may each comprise a light source (for example a light emitting diode) and a photodetector. The light source may be mounted on the structure and the detector on the movable mass, or vice versa.

The mass may be mounted on a guide track coincident with the axis. Alternatively, the mass can be mounted on the end of a flexible wand which will allow the mass to move in an arc with its major component of movement lying in the axis. This has the advantage that no friction has to be overcome when the mass is moved. Because the movement is sensed at both ends of the axis, any inaccuracies resulting from non-linearity of movement relative to the axis are cancelled out.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an accelerometer in accordance with the invention; Figure 2 shows the general form of output signal obtained from the accelerometer; and Figure 3 shows an example of the output signal obtained from a laboratory demonstration of the invention.

In Figure 1, a pair of similar photodetectors 1 and 2 are bonded to opposite sides of a mass 3 attached to a spring suspension in the form of a wand-like plate spring 4 which allows the mass to move in a direction at right angles to the plane of the plate. Light sources 5, 6 in the form of light emitting diodes illuminate the photodetectors 1 and 2 respectively, and all the components are mounted on a rigid supporting structure 7 for mechanical stability. For small amplitude displacement, the mass 3 moves essentially parallel to an axis 8 through the two light sources, and this axis is referred to as the sensing axis. Alternative forms of suspension may be used which would make the movement of the mass follow the sensing axis 8 exactly.

When the accelerometer is subjected to acceleration having a component parallel to the sensing axis, then the mass 3 will move to increase the separation of one emitter/ detector pair and simultaneously reduce the separation of the other pair. These changes in separation cause the signal level from one detector to rise and that from the other sensing unit to fall. If the direction of the acceleration is reversed, so are the detector responses.

Subtraction of one detector output from the other results in an output characteristic as shown in Figure 2 remaining generally linear over a restricted range centred on zero acceleration. A similar characteristic is obtained if the difference output is displayed as the sum of the detector outputs; the resulting normalised output is partially compensated for thermal drift and ageing of the optical components.

The mass 3 and the spring 4 form a resonant mechanical system. Incorporation of an optimum degree of damping eliminates the strong peak in response near the resonant frequency and maximises the operational frequency range of the accelerometer.

An optical accelerometer as - shown in Figure 1 was assembled as a laboratory demonstration system using silicon photodiodes as the detectors 1 and 2 and gallium arsenide light emitting diodes for the light sources 5 and 6. The mass 3 was a pair of steel discs bolted together and the spring 4 was part of a hacksaw blade. All the components were mounted on a length of U-section Dural for mechanical stability.

Figure 3 shows the result obtained when the optical accelerometer was subjected to accurately known acceleration in a low frequency accelerometer calibration laboratory. In Figure 3, the ordinate is the difference between the signals obtained from the detectors 1 and 2, while the abscissa is the applied acceleration.

If preferred, the positions of the detectors 1 and 2 could be interchanged with the positions of the light sources 5 and 6 in Figure 1. Alternatively, in order to avoid the risk of failure of electrical connections following repeated flexure, the detectors 1 and 2 could be mounted alongside the light sources 5 and 6 respectively, on the fixed structure with all the components facing towards the mass 3 which would then be fitted with suitable end surfaces to reflect light from each light source onto the adjacent detector. The illumination of each photodetector would vary as before, depending on the proximity of the reflecting surface on the mass 3.

More than one light source or detector may be used either side of the mass 3 to reduce the likelihood of failure of the accelerometer.

The operational frequency range of the accelerometer may be varied by altering the stiffness of the spring 4 or by altering the magnitude of the mass 3.

The output characteristic of the accelerometer may be altered by changing the mean separation between each diode emitter (light source) and detector, or by tailoring the characteristic of restoring forces acting on the mass 3.

Instead of the leaf spring 4 shown in Figure 1, either coil or diaphragm springs could be used to restore the mass 3 to its central position in the absence of any applied acceleration.

Claims

CLaIM8

1. An accelerometer comprising a structure to be mounted on an object the acceleration of which is to be measured, a mass connected to the structure and constrained to move relative to the structure along a sensing axis against a restoring force, and optical sensors to sense the movement of the leading and trailing ends of the mass relative to the structure.

2. An accelerometer as claimed in claim 1, wherein the sensing axis extends between two parts of the structure, and the mass is located between the two parts of the structure.

3. An accelerometer as claimed in Claim 1 or Claim 2, wherein the optical sensor comprises two sensing units, one operating on the axis on one side of the mass and the other operating on the axis on the other side of the mass such that as the mass moves in one direction, the readings from the two sensing units vary inversely.

4. An accelerometer as claimed in Claim 3, wherein the sensing units each comprise a light source and a photodetector.

5. An accelerometer as claimed in Claim 4, wherein the or each light source is mounted on the structure and the detector on the movable mass.

6. An accelerometer as claimed in any one of Claims 1 to 4, wherein both the light source and the detector are mounted on the structure, and the mass is provided with a -reflecting surface.

7. An accelerometer constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

7. An accelerometer as claimed in any preceding claim, wherein the mass is mounted on a guide track coincident with the axis.

8. An accelerometer as claimed in any one of Claims 1 to 6, wherein the mass is mounted on the end of a flexible wand which will allow the mass to move in an arc with its major component of movement lying in the axis.

9. An accelerometer constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

Amendments to the claims have been filed as follows

1. An accelerometer comprising a structure to be mounted on an object the acceleration of which is to be measured, a mass connected to the structure and constrained to move relative to the structure along a sensing axis against a restoring force, and optical sensors to sense the movement of the leading and trailing ends of the mass relative to the structure, wherein the sensing axis extends between two parts of the structure, the mass being located between the two parts of the structure, and wherein the optical sensor comprises two sensing units, one operating on the axis on one side of the mass and the other operating on the axis on the other side of the mass such that as the mass moves in one direction, the readings from the two sensing units vary inversely.

2. An accelerometer as claimed in Claim 1, wherein the sensing units each comprise a light source and a photodetector.

3. An accelerometer as claimed in Claim 2, wherein the or each light source is mounted on the structure and the detector on the movable mass.

4. An accelerometer as claimed in Claim 1 or 2, wherein both the light source and the detector are mounted on the structure, and the mass is provided with a reflecting surface.

5. An accelerometer as claimed in any preceding claim, wherein the mass is mounted on a guide track coincident with the axis.

6. An accelerometer as claimed in any one of Claims 1 to 4, wherein the mass is mounted on the end of a flexible wand which will allow the mass to move in an arc with its major component of movement lying in the axis.