PWM-based measurement interface for a micro-machined electrostatic actuator

• US 6,933,873 B1
• Filed: 12/29/2003
• Issued: 08/23/2005
• Est. Priority Date: 11/01/2000
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Clark, William, Horsley, David, Conant, Robert
• Primary Examiner(s): Williams, Howard L.
• Application Number:
  US10/747,875
• Time in Patent Office: 603 Days
• Field of Search: 341/148, 341/152, 324/661, 324/663
• US Class Current: 341/148
• CPC Class Codes: G01R 31/2829 Testing of circuits in sensor or actuator systems testing of apparatus for measuring electric or magnetic variables G01R35/00; testing of indicating or recording apparatus G01D; in airbag systems B60R21/0173; checking gas analysers G01N33/007; monitoring or fail-safe circuits for electromagnets H01F7/1844
• CPC Class Codes: H02N 1/002 Electrostatic motors

Abstract

Methods and apparatus for varying and measuring the position of a micromachined electrostatic actuator using a pulse width modulated (PWM) pulse train are disclosed. One or more voltage pulses are applied to the actuator. In each of the pulses, a voltage changes from a first state to a second state and remains in the second state for a time t_pulse before returning to the first state. The position of the actuator may be varied by varying the time Δt_pulse. A position of the actuator may be determined by measuring a capacitance of the actuator when the voltage changes state, whether the time t is varied or not. An apparatus for varying the position of a MEMS device may include a pulse width modulation generator coupled to the MEMS device an integrator coupled to the MEMS device and an analog-to-digital converter coupled to the integrator. The integrator may measure a charge transferred during a transition of a pulse from the pulse generator. The integrator may include an amplifier, an integrator capacitor, a hold capacitor, a compensation voltage generator and three switches. The hold capacitor and integrator capacitor may be coupled to a MEMS device. The integrator capacitor, hold capacitor, and compensation voltage generator may be selectively coupled to the amplifier by two of the switches. The MEMS device and hold capacitor may be selectively coupled to ground by a third switch.

Claims

1. A method for measuring a position of a micromachined electrostatic actuator, comprising:

applying one or more voltage pulses to the actuator wherein a voltage changes from a first state to a second state and remains in the second state for a time Δ

t_pulse before returning to the first state;

measuring a capacitance of the actuator when the voltage changes state;

and determining a position of the actuator from the capacitance.

2. The method of claim 1, wherein the capacitance is measured by integrating a current to the actuator with an integrator and converting the integrated current to a digital word with an analog-to-digital converter (ADC).

3. The method of claim 2, wherein the integrator measures charge transferred during a transition of one or more of the voltage pulses.

4. The method of claim 2, wherein the time Δ

t_pulse is greater than or equal to the sum of time-delay of the integrator Δ

t_s and a conversion time of the ADC Δ

t_ADC.

5. The method of claim 1, wherein a frequency of the one or more voltage pulses is above a mechanical bandwidth of the actuator.

6. The method of claim 1, wherein the time Δ

t_pulse is varied by modulating the duty cycle of a fast pulse train with a slower base-band signal.

7. The method of claim 1 wherein each of the one or more pulses is guaranteed high at a beginning of the time Δ

t_pulse and guaranteed low at the time Δ

t_pulse.