GB2261732A - Accelerometer - Google Patents

Abstract

An accelerometer suitable for use in safety and arming units comprises a flexible beam (2) clamped at one end and provided with a pair of strain gauges (12). To facilitate connection between the strain gauge terminals and electronic circuitry, the strain gauges (12) terminate in annular tags (10) which surround a series of holes (9) drilled through the beam (2). Pins (14) mounted on a printed circuit board are inserted through the holes (9) and a laser weld (16) is made between pin (14) and tag (10). <IMAGE>

Description

ACCELEROMETER This invention relates to accelerometers of the type which comprise a flexible beam whose displacement under acceleration is detected by strain gauges which are located on the beam.

Usually the beam is clamped at one of its ends to a body portion of the accelerometer unit. The necessary electronic circuitry is usually located on a printed circuit board which is also supported by the body portion.

Connections to the terminals of the strain gauges are typically made by soldering wires between the terminals and remote pins which are located on the printed circuit board.

This wiring operation is very difficult to do because of the small dimensions of the accelerometer unit and its constituent parts. Furthermore, the soldered connections are prone to breaking owing to vibration of the unit when in use.

The use of solder requires the accelerometer unit to be oil-filled so that air, which would contaminate the soldered joints, is excluded.

This invention aims to provide an accelerometer which is easier to assemble than known devices.

Accordingly the invention comprises an accelerometer comprising a flexible beam having a pair of holes made therethrough; - and a strain gauge located on a surface of the beam, in which each end of the strain gauge terminates in an annular tag which surrounds a respective one of said holes.

For greater sensitivity to beam flexing it may be preferable to use more than one strain gauge, in which case an appropriate additional number of holes and tags are provided.

Electrical connection can be made between the annular tags and a pair of printed circuit board mounted pins by locating the pins in the holes and then welding connections between pins and tags.

Preferably the holes are placed away from the region of bending of the beam so that movement between tags and pins is kept to a minimum. Any appreciable relative movement could of course result in one or more broken connections.

The beam may be made from metal (eg stainless steel) and coated with an electrically insulating layer, prior to deposition of the strain gauge(s).

An embodiment 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 an accelerometer in accordance with the invention; and Figure 2 is a sectional view along a line A-A1 of Figure 1.

Referring to both figures, an accelerometer 1 comprises a flexible beam 2. At one of the beam's ends is fitted a weight 3. The weight 3 assists bending of the beam 2 during acceleration. At the beam's other end, a clamp 4 provided with fixing holes 5, is secured to a body portion 6.

The beam 2 is composed of a stainless steel plate 7 onto one surface of which has been deposited an electrically insulating layer of glass 8. Four holes 9 are made through the beam 2 close to the clamp 4. Around each hole 9 is deposited (onto the glass layer 8) a metal, annular tag 10.

Each tag 10 is connected via a respective metal strip 11 to an end of a pair of strain gauges 12. The strain gauges 12 are deposited onto the glass layer 8 in a region of maximum bending, whereas the holes 9 are located in a region of minimum bending of the beam 2. The tags 10, strips 11 and strain gauges 12 and the glass layer 8 are all deposited using known methods. The clamp 4 is provided with recessed channels so that the metal strips 11 cannot make electrical contact with the clamp 4.

To encourage significant bending of the beam to occur in the region of the strain gauges 12, a portion 13 of the beam 2, is reduced in thickness.

In order to make electrical connections with the strain gauges 12, each one of an array of pins 14 mounted on a printed circuit board 15 is positioned within a respective hole 9. Then a laser weld 16 is made between the tip of each pin 14 and the metal tag 10 surrounding each hole 9.

Preferably, more than one weld is effected per pin 14 so that if one should break, an electrical contact will still exist. Four welds 16 are shown on Figure 1.

It will be appreciated from the foregoing that the invention eliminates the time consuming wiring operation necessary in the fabrication of known devices. Also, because the use of solder is eliminated, oil filling is not necessary for this purpose.

The accelerometer of Figs. 1 and 2 can be housed in a metal or plastic casing, for example. Oil filling may be required for the purpose of damping unwanted beam oscillations and the invention does not preclude the use of oil filling.

The accelerometer has many applications, notably in safety and arming units and as a vehicle-mounted sensor.

Claims (7)

1. An accelerometer comprising a flexible beam having a pair of holes made therethrough, and a strain gauge located on a surface of the beam, in which each end of the strain gauge terminates in an annular tag which surrounds a respective one of said holes.

2. An accelerometer according to claim 1 comprising a body portion, and clamping means for attachment of one end of said flexible beam to said body portion.

3. An accelerometer according to claim 2 including a printed circuit board mounted on the body portion in which said printed circuit board carries a pair of pins.

4. An accelerometer according to any preceding claim in which the flexible beam is provided with a weight at one of its ends and remote from said pair of holes.

5. An accelerometer according to any preceding claim in which the flexible beam comprises a stainless steel plate, having an electrically insulating glass layer deposited on one of its faces.

6. An accelerometer according to claim 3 in which said pair of pins is located within said pair of holes and electrically connected to said annular tags by a laser weld.

7. An accelerometer substantially as hereinbefore described with reference to the drawings.