Accelerometer

1. A sensing structure for an accelerometer, comprising:

a support and a proof mass mounted to the support by flexible legs for in-plane movement in response to an applied acceleration along a sensing direction;

the proof mass comprising a plurality of moveable electrode fingers extending substantially perpendicular to the sensing direction and spaced apart in the sensing direction;

at least four fixed capacitor electrodes, each comprising a set of fixed capacitor electrode fingers extending substantially perpendicular to the sensing direction and spaced apart in the sensing direction;

each set of fixed capacitor electrode fingers being arranged to interdigitate with the movable electrode fingers;

the proof mass being mounted to the support by at least one proof mass anchor located on a centre line of the at least one proof mass;

wherein the proof mass takes the form of an outer frame surrounding the at least four fixed capacitor electrodes, the flexible legs extending laterally inwardly from the proof mass to the proof mass anchor;

wherein the at least four fixed capacitor electrodes comprises two inner fixed capacitor electrodes, one on each side of the centre line of the proof mass, and two outer fixed capacitor electrodes, one on each side of the centre line of the proof mass;

wherein each of the two inner fixed capacitor electrodes comprises an anchored part anchored to the substrate and a cantilevered arm extending from the anchored part away from the centre line and having fixed capacitor electrode fingers mounted thereto; and

wherein each of the two outer fixed capacitor electrodes comprises an anchored part anchored to the substrate at a location that overlaps in the sensing direction with the cantilevered arm of an inner fixed capacitor electrode and a cantilevered arm extending from the anchored part away from the centre line and having fixed capacitor electrode fingers mounted thereto.

2. A sensing structure as claimed in claim 1 , wherein the at least one proof mass anchor comprises a first proof mass anchor and a second proof mass anchor, the first proof mass anchor and second proof mass anchor both being located on the centre line of the proof mass with a gap formed between them, and wherein each of the two inner fixed capacitor electrodes has its anchored part at least partially located within the gap.

3. A sensing structure as claimed in claim 2 , wherein the first proof mass anchor and the second proof mass anchor are connected together by a thin connecting structure along the centre line.

4. A sensing structure as claimed in claim 2 , wherein the first proof mass anchor and the second proof mass anchor are formed as separate structures and are electrically connected by a separate electrical connection.

5. A sensing structure as claimed in claim 2 , wherein the flexible legs are attached to the first proof mass anchor and the second proof mass anchor at inner sides thereof.

6. A sensing structure as claimed in claim 1 , wherein each set of fixed capacitor electrode fingers is mounted in cantilevered manner on a cantilevered arm of either an inner fixed electrode or an outer fixed electrode.

7. A sensing structure as claimed in claim 1 , wherein each inner fixed capacitor electrode is formed as a C-shape comprising two substantially parallel cantilevered arms extending away from an anchored part.

8. A sensing structure according to claim 1 , wherein the at least one proof mass is connected to the proof mass anchor by two or more separated pairs of flexible legs.

9. A sensing structure according to claim 1 , wherein the sensing structure is a MEMS.

10. A sensing structure according to claim 1 , wherein the support is made of glass.

11. An accelerometer comprising:

a sensing structure according to claim 1 .

12. An accelerometer according to claim 11 , the sensing structure comprising a first inner fixed electrode and a first outer fixed electrode on one side of the centre line and a second inner fixed electrode and a second outer fixed electrode on an opposite side of the centre line,

wherein the fixed electrode fingers on the first inner electrode are arranged to interdigitate with the moveable electrode fingers with an offset in one direction from a median line therebetween and the fixed electrode fingers on the second inner electrode are arranged to interdigitate with the moveable electrode fingers with a symmetrical offset in the opposite direction from a median line therebetween;

wherein the fixed electrode fingers on the first outer electrode are arranged to interdigitate with the moveable electrode fingers with an offset in one direction from a median line therebetween and the fixed electrode fingers on the second outer electrode are arranged to interdigitate with the moveable electrode fingers with a symmetrical offset in the opposite direction from a median line therebetween; and

wherein the accelerometer is arranged to drive the first outer electrode in phase with the second inner electrode and is arranged to drive the first inner electrode in phase with the second outer electrode.

13. An accelerometer according to claim 11 , wherein open loop electronics are arranged to drive the first inner electrode and the first outer electrode in anti-phase and are arranged to drive the second inner electrode and the second outer electrode in anti-phase.

14. An accelerometer according to claim 11 , wherein closed loop electronics are arranged to drive the first inner electrode and the first outer electrode in anti-phase and are arranged to drive the second inner electrode and the second outer electrode in anti-phase.