Capacitive detector for transducers
First Claim
1. In a method for capacitively sensing displacement in a transducer of the type having a movable member which is displaceable relative to a support structure, the steps of:
- electrically exciting a capacitive gap between said movable member and an electrode fixed in position relative to said support structure with a time varying voltage;
sensing from said excited capacitive gap a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said fixed electrode; and
amplifying the sensed time varying output signal in an amplifier having essentially zero input impedance to ground to derive an output current signal, whereby the amplitude of said output current signal is generally independent of stray input capacitance to ground of said amplifier.
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Accused Products
Abstract
Displacement of a movable transducer element is measured by exciting a capacitive gap (preferably differential capacitive gaps) with triangular waves of voltage to obtain a square current wave output into a zero input impedance amplifier. The square wave output is amplified and synchronously detected to produce a d.c. output signal proportional to displacement. The d.c. output signal is fed back to force rebalance electrodes to electrostatically rebalance the movable transducer element. The feedback signal is a measure of the force being transduced. Initial offsets in the position of the transducer element and capacitances of the capacitive gaps are compensated by signals derived from the capacitive gap excitation waveforms.
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Citations
33 Claims
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1. In a method for capacitively sensing displacement in a transducer of the type having a movable member which is displaceable relative to a support structure, the steps of:
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electrically exciting a capacitive gap between said movable member and an electrode fixed in position relative to said support structure with a time varying voltage; sensing from said excited capacitive gap a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said fixed electrode; and amplifying the sensed time varying output signal in an amplifier having essentially zero input impedance to ground to derive an output current signal, whereby the amplitude of said output current signal is generally independent of stray input capacitance to ground of said amplifier. - View Dependent Claims (2, 3, 4)
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5. In a method for capacitively sensing displacement in a transducer of the type having a moveable member which is displaceable relative to a support structure, the steps of:
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electrically exciting a pair of capacitive gaps between said moveable member and a pair of electrodes fixed in position relative to said support structure with a time-varying voltage; sensing from said excited capacitive gaps a time-varying output signal having an amplitude variable in accordance with the variable spacing between said moveable member and said pair of fixed electrodes; and amplifying the sensed time-varying output signal in an amplifier having essentially zero input impedance to ground to derive an output current signal of an amplitude which is generally independent of stray input capacitance to ground. - View Dependent Claims (6, 7, 8, 9, 10)
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11. In a transducer of the type having a movable member which is displaceable relative to a support structure, such displacement being sensed by means of a capacitive gap between said movable member and an electrode fixed in position relative to said support structure, THE IMPROVEMENT WHEREIN, said transducer includes:
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means for exciting said capacitive gap between said movable member and said electrode fixed in position relative to said support structure with a first time varying voltage; means for sensing from said exciting capacitive gap a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said fixed electrode; and means for amplifying the sensed time varying output signal in an amplifier having essentially zero input impedance to ground to derive an output current signal of an amplitude which is generally independent of stray input capacitance to ground. - View Dependent Claims (12, 13, 14)
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15. In a transducer of the type having a movable member which is displaceable relative to a support structure and having a pair of electrodes fixed in position relative to said support structure and capacitively coupled to said movable member, the improvement wherein said transducer includes:
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means for electrically exciting said pair of capacitive gaps with a pair of time varying voltages; means for sensing from said excited capacitive gaps a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said pair of fixed electrodes; and amplifier means for amplifying the sensed time varying output signal and having essentially zero input impedance to ground to derive an output current signal of an amplitude which is generally independent of stray input capacitance to ground. - View Dependent Claims (16, 17, 18, 19, 20)
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21. In a method for capacitively sensing displacement in a transducer of the type having a movable member which is displaceable relative to a support structure, the steps of:
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electrically exciting a capacitive gap between said movable member and an electrode fixed in position relative to said support structure with a time varying voltage which is cyclical having a period characterized in that in a first substantial portion of the period, the voltage changes with time at a generally constant first rate and has a first sign and during a second substantial portion of the period, the voltage changes with time at a generally constant rate generally equal to said first rate but of sign opposite to that of said first sign; sensing from said excited capacitive gap a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said fixed electrode. - View Dependent Claims (22, 23)
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24. In a method for capacitively sensing displacement in a transducer of the type having a movable member which is displaceable relative to support structure, the steps of:
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electrically exciting a pair of capacitive gaps between said movable member and a pair of electrodes fixed in position relative to said support structure with a time varying voltage; said time varying excitation voltage including a pair of voltage components for exciting said pair of capacitive gaps, each voltage component being cyclical and having a period characterized in that in a first substantial portion of the period the voltage changes with time at a generally constant first rate and of a first sign and during a second substantial portion of the period the voltage changes with time at a generally constant rate generally equal to said first rate but of sign opposite that of said first sign; and sensing from said excited capacitive gaps a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said pair of fixed electrodes. - View Dependent Claims (25, 26, 27, 28)
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29. In a method for capacitively sensing displacement in a transducer of the type having a movable member which is displaceable relative to a support structure, the steps of:
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electrically exciting a pair of capacitive gaps between said movable member and a pair of electrodes fixed in position relative to said support structure with a first time varying voltage; sensing from said excited capacitive gaps a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said pair of fixed electrodes; synchronously demodulating said output signal at the period of said time varying excitation voltage or a period harmonically related to said period of excitation voltage to derive an output signal of an amplitude which is a function of the displacement of said movable member relative to said pair of fixed electrodes; exciting a third capacitive gap between said movable member and a third electrode fixed in position relative to said support structure with said demodulated output signal to create a force on said movable member tending to urge said movable member in a direction relative to said first and second fixed electrodes so as to reduce the amplitude of said sensed time varying output signal relative to a predetermined reference level; and said third excited capacitive gap being essentially free of said first time varying excitation voltage. - View Dependent Claims (30, 31)
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32. In a method for capacitively sensing displacement in a transducer of the type having a movable member which is displaceable relative to a support structure, the steps of:
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electrically exciting a pair of capacitive gaps between said movable member and a pair of electrodes fixed in position relative to said support structure with a time varying voltage; sensing from said excited capacitive gaps a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said pair of fixed electrodes; the spacing between said movable member and each of said fixed electrodes being unequal in said capacitive gaps in the absence of a deflection of said movable member which is to be transduced such that there is an initial offset signal component of said time varying output signal produced by said unequal gaps; deriving a compensating time varying signal component from the excitation voltage used for exciting said capacitive gaps; said compensating signal component being of the phase and amplitude to cancel said initial offset signal component of said time varying output signal; and adding said compensating signal component to said sensed output signal to compensate for initial offsets in the position of said movable member relative to said pair of fixed electrodes.
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33. In a method for capacitively sensing displacement in a transducer of the type having a movable member which is displaceable relative to a support structure, the steps of:
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electrically exciting a pair of capacitive gaps between said movable member and a pair of electrodes fixed in position relative to said support structure with a time varying voltage; sensing from said excited capacitive gaps a time varying output signal having an amplitude variable in accordance with the variable spacing between said movable member and said pair of fixed electrodes; the mutually opposed areas between said movable member and said pair of electrodes being unequal in said pair of capacitive gaps such that there is an initial offset signal component in the sensed output signal due to said unequal opposed areas; and adjusting the relative amplitudes of the pair of voltages applied to said pair of capacitive gaps for electrically exciting same so that said initial offset signal component of the sensed output signal is substantially reduced to compensate for initial offsets due to unequal mutually opposed areas of said pair of capacitive gaps.
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Specification