DUAL AND TRIPLE AXIS ACCELEROMETERS

1 . An accelerometer comprising:

a frame;

a first proof mass suspended from the frame by one or more flexures to move relative to the frame along a first axis;

a first resonant element assembly fixed between the frame and the first proof mass, wherein movement of the proof mass along the first axis relative to the frame exerts a strain on the first resonant element that affects the resonant behaviour of the first resonant element assembly;

a second proof mass suspended from the frame by one or more flexures to move relative to the frame along a second axis,

a second resonant element assembly fixed between the frame and the second proof mass, wherein movement of the proof mass along the second axis relative to the frame exerts a strain on the second resonant element that affects the resonant behaviour of the second resonant element assembly;

wherein the second proof mass surrounds the first proof mass and the first resonant element assembly.

2 . An accelerometer according to claim 1 , wherein a centre of mass of the first proof mass is substantially coincident with a centre of mass of the second proof mass.

3 . An accelerometer according to claim 1 or 2 , wherein the first proof mass and the second proof mass are coplanar and lie in a plane defined by first and second axes.

4 . An accelerometer according to any one of the preceding claims, wherein the first and second axes are orthogonal to one another.

5 . An accelerometer according to any one of the preceding claims, wherein the first proof mass and second proof mass are of substantially equal mass.

6 . An accelerometer according to any one of the preceding claims, wherein the accelerometer is a micro electrical mechanical systems (MEMS) device.

7 . An accelerometer according to any one of the preceding claims, wherein the frame, first proof mass, second proof mass and flexures are formed from a single piece of semiconductor material.

8 . An accelerometer according to claim 7 , wherein the resonant element assemblies are formed from the single piece of semiconductor material.

9 . An accelerometer according to any one of the preceding claims, comprising a third resonant element assembly fixed between the frame and the first proof mass, wherein movement of the proof mass along the first axis relative to the frame exerts a strain on the third resonant element that affects the resonant behaviour of the third resonant element assembly.

10 . An accelerometer according to claim 9 , wherein the third resonant element assembly is substantially identical to the first resonant element assembly and is fixed on an opposite side of the first proof mass to the first resonant assembly in the direction of the first axis.

11 . An accelerometer according to any one of the preceding claims, comprising a fourth resonant element assembly fixed between the frame and the second proof mass, wherein movement of the proof mass along the second axis relative to the frame exerts a strain on the fourth resonant element that affects the resonant behaviour of the fourth resonant element assembly.

12 . An accelerometer according to claim 11 , wherein the fourth resonant element assembly is substantially identical to the second resonant element assembly and is fixed on an opposite side of the second proof mass to the second resonant assembly in the direction of the second axis.

13 . An accelerometer according to any one of the preceding claims, wherein one or more of the resonant element assemblies comprises a pair of coupled resonant elements.

14 . An accelerometer according to any one of the receding claims, comprising one or more electrodes adjacent the first proof mass or second proof mass and spaced from the first proof mass or the second proof mass along a third axis orthogonal to the first axis and the second axis.

15 . A gravimeter comprising an accelerometer according to any one of the preceding claims.