Method for adjusting sensitivity of a sensor

• US 5,465,604 A
• Filed: 04/12/1994
• Issued: 11/14/1995
• Est. Priority Date: 08/17/1990
• Status: Expired due to Term

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Sherman, Steven J.
• Primary Examiner(s): Williams, Hezron E.
• Assistant Examiner(s): Oda, Christine K.
• Application Number:
  US08/226,771
• Time in Patent Office: 581 Days
• Field of Search: 73/1 D, 73/1 DV, 73/517 R, 73/517 B
• US Class Current: 73/1.38
• 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. A full scale sensitivity is set based on a preliminary sensitivity calculation.

Claims

1. A method for adjusting the full scale sensitivity of a capacitance-type acceleration sensor, said sensor comprising first and second capacitors, each having first and second electrodes, said first electrode of said first capacitor being coupled electrically to said first electrode of said second capacitor, forming a differential capacitor arrangement;

in each of the capacitors, a first one of the electrodes being substantially stationary and a second one of the electrodes being moveable in response to applied acceleration, whereby an accelerative force causes said moveable electrodes to move relative to said substantially stationary electrodes and alter the capacitance between each of said moveable electrodes and said corresponding substantially stationary electrodes,

resolving circuitry coupled to said moveable and substantially stationary electrodes for generating in response to movement of said moveable electrodes a signal indicative of acceleration, said resolving circuitry comprising;

means for generating first and second sinusoidal signals of equivalent amplitude and frequency and 180°

out of phase with each other;

means for supplying said first sinusoidal signal to said first capacitor,

means for supplying said second sinusoidal signal to said second capacitor,

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

a demodulator having an input coupled to said output of said buffer amplifier and an output,

an output amplifier coupled to said output of said demodulator and having an output, and

a first resistor having a first terminal coupled to said output of said output amplifier and a second terminal coupled, through a second resistor, to said first electrodes of said capacitors,

comprising the steps of;

1) applying a specified voltage at said second terminal of said first resistor,

2) measuring an output voltage at said first terminal of said first resistor resulting from step (1),

3) changing the amplitude of the carrier signals so as to simulate a displacement of said second electrodes relative to said first electrodes,

4) measuring the resulting voltage change, Δ

V₀₁, at said first terminal of said first resistor,

5) calculating a gain factor, G, based on the voltage change measured in step (4),

6) measuring a beam resonant frequency, fo,

7) calculating a capacitor gap value, between said electrodes of said capacitors for said acceleration sensor,

8) calculating a preliminary sensitivity, of said acceleration sensor, and

9) trimming said resolving circuitry based on said preliminary sensitivity so as to set a final full scale sensitivity of said acceleration sensor.

2. A method as set forth in claim 1 wherein step (3) comprises switching said carrier signals from having equal amplitudes to having unequal amplitudes.

3. A method as set forth in claim 2 wherein step (9) comprises determining a full scale output voltage of said accelerometer in response to a predetermined acceleration value.