Monolithic accelerometer

• US 5,540,095 A
• Filed: 04/14/1995
• Issued: 07/30/1996
• Est. Priority Date: 08/17/1990
• Status: Expired due to Term

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Core, Theresa, Brokaw, A. Paul, Sherman, Steven J., Tsang, Robert W. K.
• Primary Examiner(s): Williams, Hezron E.
• Assistant Examiner(s): Oda, Christine K.
• Application Number:
  US08/422,634
• Time in Patent Office: 473 Days
• Field of Search: 73/862.61, 73/862.626, 73/718, 73/724, 73/707, 73/517 R, 73/517 B, 73/514.17, 73/514.18, 73/514.21, 73/514.32, 73/514.36, 361/283.3, 357/26
• US Class Current: 73/514.18
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81C 99/004 during manufacturing
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: G01P 2015/0814 for translational movement of the mass, e.g. shuttle type

Abstract

An accelerometer comprising a microfabricated acceleration sensor and monolithically fabricated signal conditioning circuitry. The sensor comprises a differential capacitor arrangement formed by a pair of capacitors. Each capacitor has two electrodes, one of which it shares electrically in common with the other capacitor. One of the electrodes (e.g., the common electrode) is movable and one of the electrodes is stationary in response to applied acceleration. The electrodes are all formed of polysilicon members suspended above a silicon substrate. Each of the capacitors is formed of a plurality of pairs of electrode segments electrically connected in parallel and, in the case of the movable electrodes, mechanically connected to move in unison. When the substrate is accelerated, the movable electrodes move such that the capacitance of one of the capacitors increases, while that of the other capacitor decreases. The two capacitors are connected to signal conditioning circuitry which converts this differential capacitance into a corresponding voltage. Both open loop and force-balance operation are shown.

Claims

1. A monolithic accelerometer comprising:

a. a capacitance-type acceleration sensor having first and second capacitors, each having a pair of electrodes, one of the electrodes of each capacitor being connected electrically to one of the electrodes of the other capacitor, forming a differential capacitor arrangement;

in each of the capacitors, one of the electrodes being stationary and the ether of the electrodes being movable in response to applied acceleration, the electrodes all being formed of polysilicon members suspended above a silicon substrate;

b. means for applying to the capacitors of the sensor first and second carrier signals 180 degrees out of phase with each other; and

c. means for resolving an output signal of the sensor, said means for resolving comprising;

a buffer amplifier having an input coupled to said first and second capacitors and an output; and

a demodulator having an input coupled to the output of said buffer amplifier and an output coupled to the other electrodes of said first and second capacitors, wherein the output of said demodulator is fed back to said first and second capacitors for applying force balancing voltages to said first and second capacitors.

2. A sensing apparatus comprising:

a capacitance-type, micromechanical sensor comprising a substrate and first and second capacitors in a differential capacitor arrangement, each of said capacitors having a pair of electrodes, one of the electrodes of each of the capacitors being substantially stationary with respect to the substrate and the other of the electrodes of each of the capacitors being suspended above and movable with respect to the substrate; and

a circuit comprising;

a source for applying to the capacitors of the sensor first and second carrier signals 180°

out of phase with each other, said sensor producing a sensor signal in response to said first and second carrier signals; and

a resolver responsive to the sensor signal for providing an output signal representative of the relative positions of said movable electrodes and said stationary electrodes, said circuit having a closed loop configuration and further including a force balance circuit responsive to the output signal of said resolver for applying force balancing voltages to the electrodes of said capacitors, said force balance circuit including means for applying a DC voltage to a first electrode of each of the capacitors and means responsive to the output signal for applying a feedback voltage to a second electrode of each of the capacitor, said resolver comprising;

a buffer amplifier having an input coupled to said first and second capacitors and an output; and

a demodulator having an input coupled to the output of said buffer amplifier and an output coupled to the second electrodes of said first and second capacitors, said resolver comprising;

a buffer amplifier having an input coupled to said first and second capacitors and an output; and

a demodulator having an input coupled to the output of said buffer amplifier and an output coupled to the second electrodes of said first and second capacitors, whereby the output of said demodulator is fed back to said first and second capacitors.

3. A sensing apparatus as defined in claim 2 wherein said output signal comprises a voltage.

4. A sensing apparatus as defined in claim 2 wherein said carrier signal comprise sine wave voltages of substantially equal amplitude and frequency.

5. A sensing apparatus as defined in claim 2 configured as an accelerometer, whereby said output signal is indicative of acceleration.

10. A sensing apparatus as defined in claim 2 wherein said circuit is fabricated on said substrate.

6. A sensing apparatus comprising:

a capacitance-type micromechanical sensor comprising a substrate and first and second capacitors in a differential capacitor arrangement, each of said capacitors having a pair of electrodes, one of the electrodes of each of the capacitors being substantially stationary with respect to the substrate and the other of the electrodes of each of the capacitors being suspended above and movable with respect to the substrate; and

a circuit comprising;

a source for applying to the capacitors of the sensor first and second carrier signals 180°

out of phase with each other, said sensor producing a sensor signal in response to said first and second carrier signals; and

a resolver responsive to the sensor signal for providing an output signal representative of the relative positions of said movable electrodes and said stationary electrodes, wherein said circuit has an open loop configuration.

7. A sensing apparatus as defined in claim 6 wherein said resolver comprises:

a buffer amplifier having an input coupled to said first and second capacitors and an output, and

a demodulator having an input coupled to the output of said buffer amplifier and an output supplying said output signal.

8. A sensing apparatus as defined in claim 6 configured as an accelerometer, whereby said output signal is representative of acceleration.

9. A sensing apparatus as defined in claim 6 wherein said first and second carrier signals comprise sine wave voltages of substantially equal amplitude and frequency.