Anti-capture method and apparatus for micromachined devices

• US 8,917,099 B2
• Filed: 05/23/2011
• Issued: 12/23/2014
• Est. Priority Date: 06/11/2008
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Hollocher, David C., Samuels, Howard R.
• Primary Examiner(s): Nguyen, Tung X
• Assistant Examiner(s): Astacio-Oquendo, Giovanni
• Application Number:
  US13/113,548
• Publication Number:
  US 20110221454A1
• Time in Patent Office: 1,310 Days
• Field of Search: 324660-662, 735/141.7
• US Class Current: 324/661
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 3/0008 Structures for avoiding electrostatic attraction, e.g. avoiding charge accumulation
• CPC Class Codes: G01C 19/56 Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces
• 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

A MEMS device has a movable beam, a differential capacitor with a movable electrode that moves in response to the displacement of the movable beam and that is disposed between two stationary electrodes, and a voltage circuit for applying a first voltage to the first stationary electrode, second voltage to the second stationary electrode, and a third voltage to the moveable electrode. The MEMS device also has a monitor operably coupled with the movable beam to monitor the displacement of the movable beam. In some embodiments, the monitor may monitor the distance between the movable electrode and at least one of the stationary electrodes. The MEMS device further has a voltage reducing circuit operatively coupled with the monitor, the movable electrode, and the stationary electrodes. The voltage reducing circuit reduces the differential between the third voltage and the voltages on the stationary electrodes when the monitor detects that the displacement of the movable beam is greater than or equal to a threshold value.

Claims

1. An electrode driver for providing a drive signal to controllably provide an electrostatic force between a stationary electrode fixed to a substrate of a micromachined device and a movable electrode of a movable beam of the micromachined device, which movable beam is movably suspended above the substrate, the electrode driver comprising:

a detector configured to detect excess displacement of the movable beam relative to the substrate by sensing whether the beam displacement, relative to the substrate, exceeds a threshold distance, the detector having a detector output configured to produce a disable signal in response to such excess displacement; and

a driver circuit configured to produce a plurality of electrode drive signals to the stationary electrode and comprising a driver circuit input, and an electrode driver output, the driver circuit input electrically coupled to the detector output and configured to receive the disable signal, and the electrode driver output electrically coupled to the stationary electrode, the driver circuit configured to withhold electrostatic force to the stationary electrode in response to receipt of the disable signal from the detector at the driver circuit input.

2. The electrode driver of claim 1, wherein the detector configured to detect excess displacement of the movable beam relative to the substrate is configured to detect whether a gap between an electrode fixed to the substrate and an electrode on the movable beam is less than a threshold.

3. The electrode driver of claim 1, wherein the driver circuit comprises a first transistor coupled between a first voltage source and the electrode driver output, a second transistor coupled between a second voltage source and the electrode driver output, and a third transistor operably coupled between the electrode driver output and a third voltage source, the third voltage source also coupled to the movable electrode.

4. The electrode driver of claim 1, wherein the driver circuit comprises a switch having an output electrically coupled to the stationary electrode, a first input coupled to a finger driver, and a second input coupled to a common voltage, the switch under control of the disable signal such that:

the stationary electrode is coupled, via the switch, to the common voltage when the disable signal indicates that the beam displacement, relative to the substrate, exceeds the threshold distance, and

the stationary electrode is coupled to the finger driver in the absence of such a disable signal.

5. An electrode driver for providing a first drive signal to a first stationary finger affixed to a substrate and for providing a second drive signal to second stationary finger affixed to the substrate, the electrode driver comprising:

a detector configured to detect excess displacement of the movable beam relative to the substrate by determining whether the beam displacement, relative to the substrate, exceeds a threshold distance, the detector having a detector output configured to produce a disable signal in response to such excess displacement; and

a driver circuit configured to receive the disable signal, and to set the second drive signal identical to the first drive signal in response to the disable signal,

so as to reduce electrostatic forces exerted by the first stationary finger and the second stationary finger on a beam finger, which beam finger extends from a beam movably suspended from the substrate such that the beam finger is disposed between and movable with respect to the first stationary finger and the second stationary finger, and forms a differential capacitor with the first fixed finger and the second fixed finger.

6. The electrode driver of claim 5, wherein a driver circuit is configured to set the second drive signal and the first drive signal identical to a bias voltage on the beam finger, such that the voltage on the first stationary finger and the second stationary finger and the beam finger are identical.

7. The electrode driver of claim 5, wherein the detector configured to detect excess displacement of the movable beam relative to the substrate is configured to detect whether a gap between a finger fixed to the substrate and a finger on the movable beam is less than a threshold.

8. The electrode driver of claim 5, wherein the driver circuit further comprises:

a second switch having an output electrically coupled to the second stationary finger;

a second driver input coupled to a second finger driver; and

a second disable input coupled to the common voltage, the second switch under control of the disable signal such that the second stationary finger is coupled, via the second switch, to the common voltage when the disable signal indicates that the beam displacement, relative to the substrate, exceeds the threshold distance, and is coupled to the second finger driver in the absence of such a disable signal.