MEMS dual comb drive

1. A speaker device, comprising:

a planar oscillation element configured to generate an ultrasonic acoustic signal;

a shutter element configured to cover an opening that is positioned to receive the ultrasonic acoustic signal to modulate the ultrasonic acoustic signal such that an audio signal is generated;

an electrostatically actuated first comb drive coupled to the shutter element and configured to displace the shutter element in a first direction in response to a first electrical bias applied between a static comb of the first comb drive and a movable comb of the first comb drive;

an electrostatically actuated second comb drive coupled to the shutter element and configured to displace the shutter element in a second direction that is opposite to the first direction in response to a second electrical bias applied between a static comb of the second comb drive and a movable comb of the second comb drive; and

a spring element coupled to the shutter element, wherein the spring element is configured to generate a spring force on the shutter element in response to the shutter element being displaced a distance in the first direction by the first comb drive, and wherein the spring force is less than a comb drive force generated by the first comb drive to displace the shutter element the distance.

2. The speaker device of claim 1 , wherein the first comb drive is configured to displace the shutter element in the first direction by a linear translation of the shutter element.

3. The speaker device of claim 1 , wherein first comb drive is disposed on a first side of the shutter element and the second comb drive is disposed on a second side of the shutter element, and wherein the second side is an opposite side of the shutter element from the first side.

4. The speaker device of claim 1 , wherein the spring force is parallel to the first direction.

5. The speaker device of claim 1 , wherein the spring force is no greater than about one tenth the comb drive force.

6. The speaker device of claim 1 , wherein the shutter element is positioned substantially parallel to a surface of the planar oscillating element.

7. The speaker device of claim 1 , wherein the first comb drive is configured to generate a force on the shutter element in the first direction that varies as a function of a separation distance along the first direction between the movable comb of the first comb drive and the static comb of the first comb drive.

8. The speaker device of claim 7 , wherein the first comb drive is configured to generate the force on the shutter element in response to the first electrical bias applied between the static comb of the first comb drive and the movable comb of the first comb drive.

9. The speaker device of claim 8 , wherein the function of the separation distance comprises a non-linear function with respect to the separation distance.

10. The speaker device of claim 1 , wherein two or more fingers of the movable comb of the first comb drive or of the movable comb of the second comb drive, have significantly different lengths relative to each other.

11. The speaker device of claim g claim 1 , wherein two or more fingers of the static comb of the first comb drive or of the static comb of the second comb drive, have significantly different lengths relative to each other.

12. The speaker device of claim 1 , further comprising a controller that includes a second order differentiator configured to determine an acceleration of the shutter element in the first direction to cover the opening, wherein the acceleration is based on a target audio signal.

13. The speaker device of claim 12 , wherein the second order differentiator is further configured to determine an acceleration of the shutter element in the second direction to uncover the opening, wherein the acceleration is based on the target audio signal.

14. The speaker device of claim 1 , further comprising a displacement detection device configured to compare an actual displacement of the shutter element with a target displacement of the shutter element.

15. The speaker device of claim 14 , wherein the displacement detection device comprises a capacitance measurement element.

16. An microelectromechanical systems (MEMS) actuator, comprising:

a body element;

an electrostatically actuated first comb drive coupled to the body element and configured to displace the body element in a first direction in response to a first electrical bias applied between a static comb of the first comb drive and a movable comb of the first comb drive;

an electrostatically actuated second comb drive coupled to the body element and configured to displace the body element in a second direction that is opposite to the first direction in response to a second electrical bias applied between a static comb of the second comb drive and a movable comb of the second comb drive; and

a spring element coupled to the body element, wherein the spring element is configured to generate a spring force on the body element in response to the body element being displaced a distance in the first direction by the first comb drive, and wherein the spring force is less than a comb drive force generated by the first comb drive to displace the body element the distance.

17. The microelectromechanical systems actuator of claim 16 , wherein the spring force is parallel to the first direction.

18. The microelectromechanical systems actuator of claim 16 , wherein the spring force is no greater than about one tenth the comb drive force.

19. The microelectromechanical systems actuator of claim 16 , wherein the first comb drive is configured to generate a force on the body element in the first direction that varies as a function of a separation distance along the first direction between the movable comb of the first comb drive and the static comb of the first comb drive.

20. The microelectromechanical systems actuator of claim 19 , wherein two or more fingers of the movable comb of the first comb drive or of the movable comb of the second comb drive have significantly different lengths relative to each other.

21. The microelectromechanical systems actuator of claim 16 , wherein the first and second directions are substantially parallel to a direction of an acoustic signal modulated by the body element.

22. The microelectromechanical systems actuator of claim 16 , wherein the first and second directions are substantially orthogonal to a direction of an acoustic signal modulated by the body element.

23. A method to operate a microelectromechanical system (MEMS) speaker device, the method comprising:

generating an ultrasonic acoustic signal with a planar oscillation element; and

modulating the ultrasonic acoustic signal to generate an audio signal by operating a shutter element to cover an opening that is positioned to receive the ultrasonic acoustic signal,

wherein operating the shutter element includes:

electrostatically actuating a first comb drive to displace the shutter element in a first direction in response to a first electrical bias applied between a static comb of the first comb drive and a movable comb of the first comb drive; and

electrostatically actuating a second comb drive to displace the shutter element in a second direction that is opposite to the first direction in response to a second electrical bias applied between a static comb of the second comb drive and a movable comb of the second comb drive, and

wherein at least one of:

two or more fingers of the movable comb of the first comb drive have different lengths relative to each other,

two or more fingers of the movable comb of the second comb drive have different lengths relative to each other,

two or more fingers of the static comb of the first comb drive have different lengths relative to each other, and

two or more fingers of the static comb of the second comb drive have different lengths relative to each other.

24. The method of claim 23 , wherein:

the MEMS speaker device includes a spring element coupled to the shutter element,

the spring element is configured to generate a spring force on the shutter element in response to the shutter element being displaced a distance in the first direction by the first comb drive

the spring force is less than a comb drive force generated by the first comb drive to displace the shutter element the distance, and

electrostatically actuating the first comb drive to displace the shutter element in the first direction comprises generating an electrostatic force that is at least about 10 times greater than the spring force which resists displacement of the shutter element in the first direction.

25. The method of claim 23 , wherein the first comb drive is configured to generate a force on the body element in the first direction that varies as a function of a separation distance along the first direction between the movable comb of the first comb drive and the static comb of the first comb drive.