Accelerometer

• US 20030010123A1
• Filed: 07/03/2002
• Published: 01/16/2003
• Est. Priority Date: 01/13/2000
• Status: Active Grant

• Current Assignee: Atlantic Inertial Systems Ltd (Rtx Corporation)
• Original Assignee: BAE Systems Plc
• Inventors: Townsend, Kevin, Malvern, Alan R, Hopkin, Ian D, Derooij, Nicolaas F
• Granted Patent: US 6,631,643 B2
• Time in Patent Office: Days
• US Class Current: 73/514.32
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/132 with electromagnetic counterbalancing means
• CPC Class Codes: G01P 2015/0814 for translational movement of the mass, e.g. shuttle type

Abstract

The accelerometer is linear having a substantially planar plate-like proof mass (9) mounted by four or more flexible mounting legs (8) each co-planar with the proof mass (9) to a substantially planar plate-like support (10) fixedly mounted relative to the proof mass (9) and co-planar therewith. In this way the proof mass (9) is mounted for linear movement in a sensing direction in the plane containing the proof mass (9), mounting legs (8) and support (10) to sense acceleration change applied to the accelerometer. At least two spaced apart substantially planar capacitor plates (11) each co-planar with the proof mass, mounting legs and support, are provided for sensing linear movement of the proof mass (9) in the sensing direction C. The proof mass (9), mounting legs (8), support (10) and capacitor blades (11) are formed from a single sheet of silicon and the mounting legs (8) extend substantially perpendicularly to the sensing direction C. Restoring means are provided for returning the proof mass (9) in the sensing direction C towards a null position.

Claims

1. An accelerometer having

a substantially planar plate-like proof mass,

four or more flexible mounting legs each co-planar with the proof mass,

a substantially planar, ring-like support, in which the proof mass is movably mounted, which support is fixedly mounted relative to the proof mass and co-planar therewith,

with each mounting leg being connected at one end to the proof mass and connected at another end to the support so that the proof mass is mounted for linear movement in a sensing direction in the plane containing the proof mass, mounting legs and support, in response to acceleration change applied to the accelerometer, and with the mounting legs extending substantially perpendicularly to the sensing direction,

at least two spaced apart substantially planar capacitor plates, mounted in the ring-like support substantially transverse to the sensing direction with the proof mass located between the capacitor plates and with each capacitor plate being co-planar with the proof mass, mounting legs and support, for sensing linear movement of the proof mass in the sensing direction, a plurality of inter-digitated fingers in air, comprising first arrays of laterally spaced fingers extending substantially perpendicularly to the sensing direction from the support towards the proof mass and second arrays of laterally spaced fingers extending substantially perpendicularly to the sensing direction from the proof mass towards the support, with the first arrays of fingers being inter-digitated with the adjacent second arrays of fingers to provide air squeeze damping for movement of the proof mass in the sensing direction relative to the support, with the proof mass, mounting legs, support capacitor plates and inter-digitated fingers are formed from a single plate of silicon, and

restoring means for returning the proof mass in the sensing direction towards a null position.

2. An accelerometer according to claim 1, wherein the proof mass, mounting legs, support capacitor plates and inter-digitated fingers are formed by dry etching from a plate of silicon which is oriented in [111] or [100] crystal plane.

3. An accelerometer according to claim 1 or claim 2, wherein the support has a substantially rectangular ring-like shape surrounding an inner open area in which is located the proof mass which has a substantially rectangular shape, and wherein the mounting legs extend substantially perpendicularly to the sensing direction in spaced array, with at least two between the first inner wall of the support defining the inner open area and a facing first outer wall of the proof mass and with at least two between the opposing second inner wall of the support defining the inner open area and the facing second outer wall of the proof mass.

4. An accelerometer according to claim 3, wherein the mounting legs have high compliance in the sensing direction and low compliance in other directions.

5. An accelerometer according to claim 4, including a support sheet of non-conductive material on which is fixedly mounted the support and capacitor plates, with the mounting legs, proof mass and inter-digitated fingers being spaced from the support sheet.

6. An accelerometer according to claim 5, wherein the support sheet is made of glass to which the support and capacitor plates are fixedly mounted by anodic bonding.

7. An accelerometer according to claim 6, wherein the restoring means is electromagnetic.

8. An accelerometer according to claim 7, wherein the proof mass carries a thin film electrically conductive coil structure, with each mounting leg carrying at least part of a turn of the coil structure.

9. An accelerometer according to claim 3, wherein the capacitor plates are located in the inner open area of the support.

10. An accelerometer according to claim 8 and claim 9, including an apertured glass frame located on the side of the support sheet remote from the support and attached to the support sheet, and wherein the restoring means includes a magnet and two bar pole pieces located in the glass frame aperture with the pole pieces being spaced apart at opposite ends of the magnet in register with the coil structure turns carried on the proof mass.

11. An accelerometer according to claim 10, wherein the adjacent surface of the support sheet is recessed to receive part of the bar pole pieces and including a link pole piece located on the support side of the support sheet to extend between and in registry with the two bar pole pieces.

12. An accelerometer according to claim 11, including a sheet-like glass base attached to the glass frame on the side thereof remote from the support sheet to retain the magnet and bar pole pieces within the glass frame aperture, and a glass housing for the link pole piece.

13. An accelerometer according to claim 12, wherein the magnet is poled in the sensing direction.

14. An accelerometer according to claim 3, wherein first arrays of laterally spaced fingers extend substantially perpendicularly to the sensing direction from said first and second inner walls of the support towards the adjacent first and second outer walls of the proof mass and the second arrays of laterally spaced fingers extend substantially perpendicularly to the sensing direction from said first and second outer walls of the proof mass towards the adjacent first and second inner walls of the support.

15. An accelerometer according to claim 3, including at least two earth screens located within the inner open area each between the adjacent capacitor plate and the adjacent third or fourth inner wall of the support defining the inner open area, and operable to shield the capacitor plates from the support, with the capacitor plates being electrically insulated from the earth screens and with the earth screens being electrically insulated from the support on which said earth screens are mounted.

16. An accelerometer according to any one of claims 1 to 15, including means for supplying a square wave drive voltage to the two capacitor plates in anti-phase.

17. An accelerometer according to claim 16, including means for supplying a control current to the proof mass coil structure, which control current supply means includes a pre-amplifier for receiving from the proof mass coil structure a difference signal between the two capacitor plates at Alternating Current (AC) modulation frequency resulting from imbalance of the capacitor plates under acceleration, an AC demodulator for synchronously demodulating the output signal from the pre-amplifier, an integrator for integrating the output signal from the demodulator, a loop filter for ensuring stability of the output signal received from the integrator and a current driver for receiving the output signal from the loop filter and for feeding a control current to the proof mass coil structure.

18. An accelerometer according to any one of claims 1 to 15, wherein the drive means includes means for supplying a single modulation Alternating Current (AC) to the proof mass, which gives rise to an output signal to each of the capacitor plates.

19. An accelerometer according to claim 18, including means for supplying a control current to the proof mass coil structure, which control current supply means includes two pre-amplifiers, one for receiving the output signal from one of the capacitor plates and the other for receiving the output signal from the other of the capacitor plates, a differential amplifier for receiving the output signals from the two pre-amplifiers and for differencing the pre-amplifier output signals to give an output signal corresponding to the net displacement of the proof mass, and an AC demodulator for receiving the output signal from the differential amplifier and for synchronously demodulating the output signal from the differential amplifier to produce a Direct Current (DC) output signal for passage as a control current to the proof mass coil structure to return the proof mass in the sensing direction towards a null position.