Drive method for MEMS devices
View Patent ↗Embodiments of exemplary MEMS interferometric modulators are arranged at intersections of rows and columns of electrodes. In certain embodiments, the column electrode has a lower electrical resistance than the row electrode. A driving circuit applies a potential difference of a first polarity across electrodes during a first phase and then quickly transition to applying a bias voltage having a polarity opposite to the first polarity during a second phase. In certain embodiments, an absolute value of the difference between the voltages applied to the row electrode is less than an absolute value of the difference between the voltages applied to the column electrode during the first and second phases.
1. A method of driving an interferometric modulator, comprising:
establishing a first charge differential across a first conductive plate and a second conductive plate of said interferometric modulator wherein said first and second conductive plates are separated by a variable gap distance, wherein said first charge differential is greater than the midpoint of a hysteresis window of the interferometric modulator and is below an actuation threshold of the interferometric modulator;
isolating said first and second conductive plates for a first duration, the first and second conductive plates maintaining a charge differential that is within the hysteresis window during the first duration, wherein the first duration is based on a predetermined time constant associated with the interferometric modulator; and
decreasing said first charge differential to a second charge differential being less than said first charge differential and wherein said second charge differential corresponds to a second value of said variable gap distance.
2. The method of claim 1 , further comprising isolating said first and second conductive plates for a second duration after decreasing said first charge differential.
3. The method of claim 1 , wherein the first and second conductive plates are isolated for at least 0.01 of a second.
4. The method of claim 1 , wherein the first and second conductive plates are isolated for at least 0.5 of a second.
5. The method of claim 1 , wherein the first and second conductive plates are isolated for at least 2 seconds.
6. The method of claim 1 , wherein establishing said first charge differential comprises coupling said first conductive plates to a reference voltage source.
7. The method of claim 1 , wherein said first charge differential causes an initial attractive force between said first and second conductive plates that is larger than a second attractive force corresponding to said second value of said variable gap distance.
8. A method of driving an interferometric modulator, comprising:
establishing a preliminary known charge state with respect to a first conductive plate and a second conductive plate of a variable capacitor wherein said first and second conductive plates are separated by a variable gap distance;
establishing a first charge differential across said first and second conductive plates to force said first and second conductive plates toward each other, wherein said first charge differential is greater than the midpoint of a hysteresis window of the interferometric modulator and is below an actuation threshold of the interferometric modulator;
isolating said first and second conductive plates for a first duration, the first and second conductive plates maintaining a charge differential that is within the hysteresis window during the first duration, wherein the first duration is based on a predetermined time constant associated with the interferometric modulator; and
decreasing said first charge differential to a second charge differential being less than said first charge differential and wherein said second charge differential corresponds to a second value of said variable gap distance.
9. The method of claim 8 , wherein the first and second conductive plates are isolated for at least 0.01 of a second.
10. The method of claim 8 , wherein the first and second conductive plates are isolated for at least 0.5 of a second.
11. The method of claim 8 , wherein the first and second conductive plates are isolated for at least 2 seconds.
12. The method of claim 8 , wherein establishing said first charge differential comprises coupling said first conductive plate to a reference voltage source.
13. The method of claim 8 , wherein said variable capacitor is controlled by a voltage control circuit.
14. The method of claim 8 , wherein said variable capacitor is controlled by a charge control circuit.
15. A charge control circuit, comprising:
a power supply; and
a switch circuit configured to convey a pulse charge from said power supply onto an interferometric modulator to isolate said interferometric modulator for a determined duration, wherein the pulse charge establishes a first charge differential across the interferometric modulator that is greater than a midpoint of a hysteresis window of the interferometric modulator and is below an actuation threshold of the interferometric modulator, the interferometric modulator maintaining a charge differential that is within the hysteresis window during the determined duration, wherein the determined duration is based on a predetermined time constant associated with the interferometric modulator.
16. The circuit of claim 15 , wherein the switch circuit comprises a tri-state circuit.
17. The circuit of claim 15 , wherein the switch circuit electrically isolates the plates for at least 0.01 of a second.
18. The circuit of claim 15 , wherein the switch circuit electrically isolates the plates for at least 0.5 of a second.
19. The circuit of claim 15 , wherein the switch circuit electrically isolates the plates for at least 2 seconds.
20. A micro-electromechanical system, comprising:
an M-row by N-column array of a micro-electromechanical cells, wherein each of said cells includes a micro-electromechanical device (MEM device) having a variable capacitor formed by a first conductive plate and a second conductive plate separated by a variable gap distance; and
a switch circuit configured to apply a selected voltage level across first and second conductive plates of a variable capacitor of said MEM device to cause a charge differential across said variable capacitor, wherein said charge differential is greater than the midpoint of a hysteresis window of said MEM device and is below an actuation threshold of said MEM device, and wherein the charge differential is decreased over time, and wherein the switch circuit is further configured to isolate the first and second conductive plates for a first duration, the first and second conductive plates maintaining a charge differential that is within the hysteresis window during the first duration, wherein the first duration is based on a predetermined time constant associated with the MEM device.
21. A charge control system, comprising:
means for establishing a first charge differential between first and second conductive plates of an interferometric modulator, wherein said first charge differential is greater than the midpoint of a hysteresis window of the interferometric modulator and is below an actuation threshold of the interferometric modulator; and
means for isolating said first and second conductive plates for a first duration, the first and second conductive plates maintaining a charge differential that is within the hysteresis window during the first duration, wherein the first duration is based on a predetermined time constant associated with the interferometric modulator.
22. The method of claim 1 , further comprising sensing capacitance between the first conductive plate and the second conductive plate to deteimine the variable gap distance.