Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-12-14
2001-10-16
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514210
Reexamination Certificate
active
06301965
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to accelerometers and to circuitry for the control and operation of accelerometers. In particular, the present invention relates to circuitry that incorporates a digital resonance-cancelling feedback control circuit including an “idle” state for the control and operation of single-proof-mass and dual-proof-mass accelerometers. The present invention also relates to single- and dual-proof-mass microelectromechanical (MEM) accelerometers incorporating such digital feedback control circuitry.
BACKGROUND OF THE INVENTION
Sensitive microelectromechanical (MEM) accelerometers are needed for navigation and other applications that operate with a range of acceleration from less than ±10
−3
g (where the term “g” is refers to the force of gravity on earth, and is approximately equal to 9.8 meters-second
−2
) to about ±10 g or greater. Such sensitivity and enhanced dynamic range for MEM accelerometers necessitates a feedback control system that periodically re-centers a proof mass in the accelerometer.
An example of a feedback control system for a MEM accelerometer is described in a thesis by Mark A. Lemkin entitled
Micro Accelerometer Design with Digital Feedback Control
(University of California, Berkeley, 1997, available from University Microfilms). Lemkin's feedback control system is based on the generation of one of two possible states during each feedback cycle, including a “+1” state which provides a feedback voltage to urge the proof mass in one direction and a “−1” state which provides a feedback voltage to urge the proof mass in the other direction. Lemkin's feedback control system, therefore, necessarily requires that the proof mass be urged in one or the other direction during each feedback control cycle even in instances where no adjustment to the position of the proof mass is required or desirable (e.g. when the proof mass is correctly centered). This requirement that the proof mass be urged although such movement is not needed is disadvantageous since it can lead to incremental errors in navigational positioning which can accumulate over time.
The present invention provides an improvement over the two-level feedback control system of Lemkin by providing a three-level feedback control system having an “idle” state in which the position of the accelerometer proof mass is sensed and no feedback voltage is applied to re-center the proof mass during any feedback cycle wherein the proof mass is initially substantially centered (i.e. during which the acceleration of the proof mass falls below a predetermined threshold level).
An advantage of the present invention is that the accumulation of feedback errors over time are reduced, thereby improving the accuracy of the MEM accelerometer.
A further advantage of the present invention is that mechanical resonances of the proof mass are filtered out to provide a reduction in feedback errors associated with the mechanical resonances of the proof mass, thereby resulting in an improved sensitivity and stability of the feedback control system.
Another advantage of the present invention is that the MEM accelerometer and electronic feedback control circuitry can be formed on a single substrate (e.g. comprising silicon) to form a compact integrated single- or multi-axis accelerometer.
Yet another advantage is that a dual-proof-mass MEM accelerometer structure used in certain embodiments of the present invention can result in the cancellation of common-mode signals, thereby further enhancing the accuracy of the MEM accelerometer.
These and other advantages of the present invention will become evident to those skilled in the art.
SUMMARY OF THE INVENTION
The present invention relates to 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. The digital feedback control circuit comprises an amplifier (e.g. a differential amplifier) that is operatively connected to sense the motion of each proof mass and generate therefrom an amplified signal indicative of the magnitude and direction of movement of each proof mass; 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 a comparator for receiving the filtered signal and generating therefrom a digital feedback signal which is operatively coupled back to each proof mass to urge the proof mass towards its initial position. The feedback signal generated by the comparator is in one of three states during a 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 (i.e. null) and does not urge each proof mass in either direction. In the digital feedback control circuit of the present invention, the amplifier, filter and comparator are each formed from a plurality of interconnected transistors, with the transistors comprising, for example, complementary metal-oxide semiconductor (CMOS) transistors, which in some embodiments of the present invention can be formed on a common semiconductor substrate with the accelerometer structure.
The filter in the digital feedback control circuit can comprise a digital filter such as a switched-capacitor filter (e.g. a notch filter tuned to reject the resonance frequency component of the amplified signal). Additionally, the switched-capacitor filter preferably further includes at least one electronic integrator.
In some embodiments of the present invention, the accelerometer structure is a microelectromechanical (MEM) accelerometer structure formed on a semiconductor substrate. In these embodiments of the present invention, the digital feedback control circuit also comprises digital signal routing circuitry that routes electrical 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 back to the electrodes during a subsequent force-feedback time interval.
The present invention further relates to a microelectromechanical (MEM) accelerometer comprising at least one suspended proof mass formed on a semiconductor substrate (e.g. comprising silicon), with each proof mass being moveable away from an initial position in response to an applied acceleration; 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 that proof mass; a filter for receiving the electrical signal generated by the electrodes 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 electrical feedback means, electrically connected between the filter and the electrodes, for providing a feedback signal that is 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 operates digitally in response to a clock and provides the feedback signal to the electrodes, with the feedback signal during a force-feedback time interval (i.e. a predetermined number of clock cycles) having 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 (termed an “idle” state) wherein the feedback signal is nulled and does not urge each proof mass in either direction.
An amplifier (e.g. a differential amplifier) can be provided in the MEM accelerome
Campbell David V.
Chu Dahlon D.
Thelen, Jr. Donald C.
Hohimer John P.
Kwok Helen
Sandia Corporation
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