Prevention of charge accumulation in micromirror devices through bias inversion
View Patent ↗Methods and apparatus are provided for preventing charge accumulation in microelectromechanical systems, especially in micromirror array devices having a plurality of micromirrors. Voltages are applied to the micromirrors for actuating the micromirrors. Polarities of the voltage differences between mirror plates and electrodes are inverted so as to prevent charge accumulation.
1. A method of operating a micromirror device that comprises a movable mirror plate and an electrode formed on a substrate for driving the mirror plate, the method comprising:
applying a first voltage to the mirror plate and a second voltage to the electrode such that voltage difference between the mirror plate and the electrode drives the mirror plate to rotate relative to the substrate;
applying a third voltage to the mirror plate, and a fourth voltage to the electrode such that the voltage difference between the mirror plate and the electrode drives the mirror plate to rotate relative to the substrate, wherein difference between the third voltage and the fourth voltage has an opposite polarity to that between the first voltage and the second voltage;
wherein the first voltage and the second voltage are applied in response to a first subsequence of a sequence of actuation signals, and the third voltage and the fourth voltage are applied in response to a second subsequence of the sequence of actuation signals; and
wherein the actuation signal corresponds to an ON state of the micromirror, wherein the ON state is defined as a state such that the micromirror reflects light into a projection lens for producing a bright pixel of an image on a display target.
2. The method of claim 1 , wherein the actuation signal corresponds to an OFF state of the micromirror, wherein the OFF state is defined as a state such that the micromirror reflects light away from a projection lens for producing a dark pixel of an image on a display target.
3. The method of claim 1 , wherein the first subsequence and the second subsequence are interleaved.
4. The method of claim 1 , wherein the second subsequence is determined such that a predetermined number of applications of the first and second voltages is between two consecutive applications of the third and fourth voltages.
5. The method claim 1 , wherein the second subsequence of the sequence of the actuation signals has a frequency more than a predetermined frequency, wherein the frequency is defined as the number of actuation signals in the subsequence per second.
6. The method of claim 5 , wherein the critical frequency is determined in accordance with a perceptual ability of human eyes.
7. The method of claim 1 , wherein the fourth voltage is zero.
8. The method of claim 1 , wherein the step of applying the third voltage and the fourth voltage further comprises:
grounding the electrode.
9. The method of claim 1 , wherein the step of applying the third voltage and the fourth voltage further comprises:
grounding the mirror plate.
10. The method of claim 1 , wherein the third voltage has an opposite polarity to the first voltage.
11. The method of claim 1 , wherein the fourth voltage has an opposite polarity to the second voltage.
12. The method of claim 1 , wherein the difference between the first voltage and the second voltage is from 15 volts to 80 volts.
13. The method of claim 1 , wherein the difference between the first voltage and the second voltage is from 25 volts to 50 volts.
14. The method of claim 1 , wherein the difference between the first voltage and the second voltage is around 30 volts.
15. The method of claim 1 , wherein the difference between the third voltage and the fourth voltage is from 15 volts to 80 volts.
16. The method of claim 1 , wherein the difference between the third voltage and the fourth voltage is from 25 volts to 50 volts.
17. The method of claim 1 , wherein the difference between the third voltage and the fourth voltage is around 30 volts.
18. The method of claim 1 , wherein the first voltage and the second voltage are from 0 to 100 volts.
19. The method of claim 1 , wherein the first voltage and the second voltage are from 0 to 50 volts.
20. The method of claim 1 , wherein the first voltage and the second voltage are around 30 volts.
21. The method of claim 1 , wherein the third voltage and the fourth voltage are from 0 to 100 volts.
22. The method of claim 1 , wherein the third voltage and the fourth voltage are from 0 to 50 volts.
23. The method of claim 1 , wherein the third voltage and the fourth voltage are around 50 volts.
24. The method of claim 1 , wherein the second subsequence of the sequence of the actuation signal has a frequency higher than 30 Hz.
25. The method of claim 1 , wherein the rotation of the mirror plate driven by the voltage difference between the third voltage and the fourth voltage is along a rotation direction that is the same as that of the mirror plate driven by the voltage difference between the first voltage and the second voltage.
26. The method of claim 1 , wherein the application of the first voltage and the second voltage and the application of the third voltage and the fourth voltage are performed alternatively.
27. The method of claim 1 , wherein the application of the first voltage and the second voltage and the application of the third voltage and the fourth voltage are performed once per video frame.
28. The method of claim 1 , wherein the application of the first voltage and the second voltage and the application of the third voltage and the fourth voltage are performed once per time interval determined by a time interval between two consecutive color segments of a color wheel used by the display system in producing a color image.
29. The method of claim 1 wherein the application of the first voltage and the second voltage and the application of the third voltage and the fourth voltage are performed once per time interval determined by a wave-segment of a pulse-width-modulation waveform used in producing the grayscale of the image or the video frame.
30. The method of claim 1 , wherein the application of the first voltage and the second voltage and the application of the third voltage and the fourth voltage are performed at the beginning of displaying the image or the video frame.