Microelectromechanical accelerometer with resonance-cancelling control circuit including an idle state

• US 6,301,965 B1
• Filed: 12/14/1999
• Issued: 10/16/2001
• Est. Priority Date: 12/14/1999
• Status: Expired due to Term

• Current Assignee: National Technology & Engineering Solutions Of Sandia LLC (Honeywell International Inc.)
• Original Assignee: Sandia Corporation (Martin Marietta Corporation)
• Inventors: Thelen, Donald C. Jr., Chu, Dahlon D., Campbell, David V.
• Primary Examiner(s): KWOK, HELEN C
• Application Number:
  US09/461,062
• Time in Patent Office: 672 Days
• Field of Search: 735/141.7, 735/141.8, 735/142.1, 735/143.2, 735/143.5, 738/626.1, 738/626.26, 361/280, 361/283.3
• US Class Current: 73/514.18
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: H03H 19/004 Switched capacitor networks

Abstract

A digital feedback control circuit is disclosed for use in an accelerometer (e.g. a microelectromechanical accelerometer). The digital feedback control circuit, which periodically re-centers a proof mass in response to a sensed acceleration, is based on a sigma-delta (ΣΔ) configuration that includes a notch filter (e.g. a digital switched-capacitor filter) for rejecting signals due to mechanical resonances of the proof mass and further includes a comparator (e.g. a three-level comparator). The comparator generates one of three possible feedback states, with two of the feedback states acting to re-center the proof mass when that is needed, and with a third feedback state being an “idle” state which does not act to move the proof mass when no re-centering is needed. Additionally, the digital feedback control system includes an auto-zero trim capability for calibration of the accelerometer for accurate sensing of acceleration. The digital feedback control circuit can be fabricated using complementary metal-oxide semiconductor (CMOS) technology, bi-CMOS technology or bipolar technology and used in single- and dual-proof-mass accelerometers.

Claims

1. A digital feedback control circuit for use with an accelerometer structure having at least one proof mass which is moveable away from an initial position in response to an acceleration provided thereto, comprising:

(a) an amplifier operatively connected to sense the motion of each proof mass and generate therefrom an amplified signal indicative of the magnitude and direction of the movement of each proof mass;

(b) a filter connected to the amplifier to receive the amplified signal and generate therefrom a filtered signal substantially free from a resonant-frequency component in the amplified signal resulting from a mechanical resonance of each proof mass; and

(c) a comparator to receive the filtered signal and a voltage reference signal and generate therefrom a digital feedback signal which is operatively coupled back to each proof mass during a force-feedback time interval to urge the proof mass towards the initial position, the feedback signal being in one of three states during the force-feedback time interval, including a first state wherein the feedback signal urges each proof mass in one direction, a second state wherein the feedback signal urges each proof mass in the opposite direction, and a third state wherein the feedback signal is substantially equal to zero and does not urge each proof mass in either direction.

2. The digital feedback control circuit of claim 1 wherein the amplifier comprises a differential amplifier for generating an amplified output signal in response to the motion of a pair of proof masses in the accelerometer.

3. The digital feedback control circuit of claim 1 wherein the amplifier includes an offset trim to zero out the amplified signal when the proof mass is at rest, thereby calibrating the accelerometer structure.

4. The digital feedback control circuit of claim 1 wherein the filter comprises a digital filter.

5. The digital feedback control circuit of claim 4 wherein the digital filter comprises a switched-capacitor filter.

6. The digital feedback control circuit of claim 5 wherein the switched-capacitor filter is a notch filter programmable to reject the resonance frequency component of the amplified signal.

7. The digital feedback control circuit of claim 5 wherein the switched-capacitor filter further includes at least one electronic integrator.

8. The digital feedback control circuit of claim 1 wherein the comparator has an odd number of levels greater than or equal to three.

9. The digital feedback control circuit of claim 8 wherein the comparator is a three-level comparator.

10. The digital feedback control circuit of claim 9 wherein the amplifier, filter and three-level comparator are each formed from a plurality of interconnected transistors.

11. The digital feedback control circuit of claim 10 wherein the transistors comprise complementary metal-oxide semiconductor (CMOS) transistors.

12. The digital feedback control circuit of claim 1 wherein the accelerometer structure is a microelectromechanical (MEM) accelerometer structure formed on a semiconductor substrate.

13. The digital feedback control circuit of claim 12 further comprising digital signal routing circuitry that routes electrostatic signals from a plurality of electrodes adjacent to each proof mass to the amplifier during a position-sense time interval, and routes the digital feedback signal to the electrodes during the force-feedback time interval.

14. The digital feedback control circuit of claim 12 wherein the amplifier, filter and comparator are each formed on the semiconductor substrate containing the MEM accelerometer structure.

15. A microelectromechanical (MEM) accelerometer comprising:

(a) at least one suspended proof mass formed on a semiconductor substrate, with each proof mass being moveable away from an initial position in response to an applied acceleration;

(b) electrodes located on the substrate proximate to each proof mass to capacitively generate an electrical signal indicative of the magnitude and direction of the acceleration of the proof mass;

(c) a filter for receiving the electrical signal generated at the electrodes by the acceleration of each proof mass and generating therefrom a filtered signal having a resonance-frequency component of the electrical signal produced by a mechanical resonance of each proof mass removed; and

(d) electrical feedback means, electrically connected between the filter and the electrodes, for providing a feedback signal generated from the filtered signal to urge each proof mass towards its initial position after movement in response to the acceleration, the electrical feedback means operating digitally in response to a clock and providing the feedback signal to the electrodes during a force-feedback time interval, with the feedback signal during the force-feedback time interval being in one of three states, including a first state wherein the feedback signal urges each proof mass in one direction, a second state wherein the feedback signal urges each proof mass in the opposite direction, and a third state wherein the feedback signal is nulled and does not urge each proof mass in either direction.

16. The MEM accelerometer of claim 15 further including an amplifier connected between the electrodes and the filter to amplify the electrical signal prior to filtering.

17. The MEM accelerometer of claim 16 wherein the amplifier further includes an offset trim to zero out the amplified electrical signal when the proof mass is at rest, thereby calibrating the accelerometer.

18. The MEM accelerometer of claim 15 wherein the filter comprises a digital filter.

19. The MEM accelerometer of claim 18 wherein the digital filter comprises a switched-capacitor filter.

20. The MEM accelerometer of claim 19 wherein the switched-capacitor filter is a notch filter programmable to reject the resonance-frequency component of the electrical signal.

21. The MEM accelerometer of claim 19 wherein the switched-capacitor filter further includes at least one electronic integrator.

22. The MEM accelerometer of claim 15 wherein the electrical feedback means comprises a comparator having an odd number of levels greater than or equal to three.

23. The MEM accelerometer of claim 22 wherein the electrical feedback means comprises a three-level comparator.

24. The MEM accelerometer of claim 15 wherein a clock frequency is in a range of 1-100 MHz.

25. The MEM accelerometer of claim 15 wherein the filter and electrical feedback means are formed on the semiconductor substrate.

26. The MEM accelerometer of claim 25 wherein the filter and the electrical feedback means each comprise complementary metal-oxide semiconductor (CMOS) or bipolar circuitry.

27. The MEM accelerometer of claim 15 wherein each proof mass is located, at least in part, within a cavity formed in the semiconductor substrate.

28. A microelectromechanical (MEM) accelerometer comprising:

(a) a pair of suspended proof masses formed on a semiconductor substrate, with each proof mass being moveable in the same direction away from an initial position in response to an applied acceleration;

(b) electrodes located on the substrate proximate to each proof mass to capacitively generate an electrical signal indicative of the magnitude and direction of the acceleration of the proof mass;

(c) a differential amplifier connected to the electrodes to amplify a difference component in the electrical signal generated by the pair of proof masses, thereby generating an amplified electrical signal;

(d) a filter for receiving the amplified electrical signal from the differential amplifier and generating therefrom a filtered signal having a resonance-frequency component of the electrical signals produced by mechanical resonances of the proof masses removed; and

(e) electrical feedback means, electrically connected between the filter and the electrodes, for providing a feedback signal generated from the filtered signal to urge each proof mass towards an initial position after movement in response to the acceleration, the electrical feedback means operating digitally in response to a clock and providing the feedback signal to the electrodes during a force-feedback time interval, with the feedback signal during the force-feedback time interval being in one of three states, including a first state wherein the feedback signal urges each proof mass in one direction, a second state wherein the feedback signal urges each proof mass in the opposite direction, and a third state wherein the feedback signal is nulled and does not urge the proof masses in either direction.

29. The MEM accelerometer of claim 28 wherein the substrate comprises silicon.

30. The MEM accelerometer of claim 28 wherein the differential amplifier includes an offset trim to zero out the amplified electrical signal when the proof masses are at rest, thereby calibrating the accelerometer.

31. The MEM accelerometer of claim 28 wherein the filter further includes at least one electronic integrator.

32. The MEM accelerometer of claim 28 wherein the filter comprises a digital filter.

33. The MEM accelerometer of claim 32 wherein the digital filter comprises a switched-capacitor filter.

34. The MEM accelerometer of claim 28 wherein the electrical feedback means comprises a comparator having an odd number of levels greater than or equal to three.

35. The MEM accelerometer of claim 34 wherein the electrical feedback means comprises a three-level comparator.

36. The MEM accelerometer of claim 28 wherein the differential amplifier, filter and electrical feedback means are formed on the semiconductor substrate.

37. A method for operating an accelerometer to sense an acceleration of a proof mass therein, comprising steps for:

(a) sensing a movement of the proof mass from an initial position due to the acceleration, and generating an electrical signal containing information about the position of the proof mass at particular instants in time;

(b) filtering the electrical signal to remove a resonant-frequency component thereof arising from a mechanical resonance of the proof mass and thereby generating a filtered signal;

(c) comparing the filtered signal to a voltage reference signal and generating a feedback signal comprising digital voltage pulses having one of three states during a force-feedback time interval, including a first state for urging the proof mass in a forward direction, a second state for urging the proof mass in a reverse direction, and a third state for providing a null signal and thereby urging the proof mass to remain in place; and

(d) generating an indication of the acceleration from the feedback signal.

38. The method of claim 37 further including a step for amplifying the electrical signal prior to the step for filtering the electrical signal.

39. The method of claim 38 further including a step for zeroing out the amplified electrical signal with an amplifier offset trim when the proof mass is at rest.

40. The method of claim 38 wherein the step for filtering the electrical signal to remove the resonant component comprises filtering the electrical signal with a notch filter.

41. The method of claim 40 wherein the notch filter comprises a digital filter.

42. The method of claim 41 wherein the digital filter comprises a switched-capacitor filter.

43. The method of claim 37 wherein the step for comparing the filtered signal to a voltage reference signal comprises comparing the filtered signal with a three-level comparator.

44. The method of claim 37 wherein the step for comparing the filtered signal to a voltage reference signal comprises comparing the filtered signal with a comparator having an odd number of levels greater than or equal to three.

45. The method of claim 37 wherein the accelerometer is a microelectromechanical (MEM) accelerometer and the feedback signal is electrostatically coupled to the proof mass to urge the proof mass to return to the initial position as needed.

46. The method of claim 37 wherein the step for generating the indication of the acceleration from the feedback signal comprises counting the digital voltage pulses.