Electrical generator or motor structure – Non-dynamoelectric – Charge accumulating
Reexamination Certificate
1999-08-31
2003-12-02
Tamai, Karl (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Charge accumulating
Reexamination Certificate
active
06657359
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to electrostatic actuators and more particularly to micromachined electrostatic actuators.
BACKGROUND OF THE INVENTION
With the advent of micromachining techniques, there has been renewed interest in electrostatic actuators. Electrostatic actuators achieve high energy densities and can be manufactured using straightforward manufacturing techniques. Electrostatic actuators have been used to position optical devices, to operate switches, and to turn small gears. For advanced data storage devices and other applications, micromachined actuators that have a large travel, whose positioning can be controlled with great precision, and that operate in response to a low actuation voltage are needed. These requirements are not met by known micromachined electrostatic actuators.
A micromachined electrostatic actuator that satisfies some of the above requirements is described by Trimmer and Gabriel in
Design Considerations for a Practical Electrostatic Micro
-
Motor
, SENSORS AND ACTUATORS, Vol. 11, pages 189-206 (1987) and in U.S. Pat. No. 4,754,185. These documents describe an electrostatic actuator in which a grounded moveable silicon substrate or “rotor” is moved relative to a fixed silicon substrate or “stator.” The stator has several sets of electrodes on its surface, one of which is held at a voltage different from ground in order to position the rotor. Stepped motion is provided by setting the pitches of the stator and rotor electrodes in a vernier relationship. The rotor electrodes all having the same voltage, i.e., ground potential, significantly eases fabrication of the device.
However, the electrostatic actuator described by Timmer and Gabriel does not meet all of the requirements set forth above. For example, an actuation voltage of approximately 100 V is required to exert a force on the rotor in the direction parallel to the plane of the rotor surface (an “in-plane force”) in the range of forces required to operate an advanced memory device. This actuation voltage is outside the range of voltages that can be controlled using conventional MOS integrated circuits. Moreover, the in-plane force is accompanied by an out-of-plane force perpendicular to the plane of the rotor. The out-of-plane force attracts the rotor towards the stator and is as much as ten times greater than the in-plane force.
The large attractive out-of-plane force places significant constraints on the suspension used to maintain the spacing between the rotor and stator. For conventional-size electrostatic actuators, spacers, bearings and lubricating layers may be used to support the rotor against the attractive force. However, for micro-scale structures, it is more difficult to provide an effective way of maintaining the spacing between the rotor and stator without large frictional forces that adversely affect operation.
Folded beam flexures are most commonly used in micromachined devices to support the rotor above the stator. Advanced data storage applications require actuators that can travel 25 &mgr;m laterally while maintaining the rotor-stator spacing to an accuracy of 0.1 &mgr;m. If the ratio of the out-of-plane force to the in-plane force is near 10, as in the electrostatic actuator described by Timmer and Gabriel, then a 2 &mgr;m-wide beam flexure would need to be at least 100 &mgr;m tall to have sufficient out-of-plane stiffness. Such a structure is extremely difficult to fabricate using conventional processing.
A first approach to mitigate the effects of the out-of-plane attractive forces in micromachined devices is to use two stationary electrode plates on opposite sides of a movable plate. By selecting the appropriate electrode configuration, it is possible to levitate the moving plate at a relatively stable position between the two stationary plates. However, this approach requires exacting process control during fabrication and/or assembly.
A second known approach applicable to micromachined devices is to use the weight of the movable substrate to counteract the attractive force. However, since this approach does not work if the electrostatic actuator is tilted, its usefulness is significantly restricted.
In both of the approaches discussed above, the rotor electrodes are all held at a single voltage. Macro-scale electrostatic actuators are known that have three or more voltages present on both the stator and rotor. One approach using a three-phase oscillating voltage pattern is described in U.S. Pat. No. 5,534,740 of Higuchi et al. This approach can produce a very large in-plane force. However, the large in-plane force is accompanied by a large out-of-plane force about four times greater than the in-plane force. Oscillating voltages of approximately 200 volts are required to generate an in-plane force of sufficient magnitude to overcome friction in the suspension elements. Therefore, this approach will not conveniently scale to a micromachined device because of the large out-of-plane force and the requirement to connect three oscillating voltages to the rotor electrodes. Making electrical connections to a moveable rotor is difficult, particularly for a micromachined rotor, so it is desirable to minimize the number of voltages present on the rotor electrodes. In addition, the way in which the voltages vary with time should be made as simple as possible.
Some conventional electrostatic actuators provide precise position control and a large range of travel, but cannot simply be scaled for use in micromachined electrostatic actuators. This is because these actuators operate with actuation voltages greater than those that can be controlled using conventional MOS integrated circuits, generate an out-of-plane force that is too large relative to the in-plane force, and require too many electrical connections to be made to the rotor. What is needed is an electrostatic actuator and a way controlling an electrostatic actuator that provides precise positioning and that can be controlled using conventional CMOS integrated circuits. What is also needed is such an electrostatic actuator that can be fabricated using micro-machining techniques that employ processing similar to that used to make integrated circuits.
SUMMARY OF THE INVENTION
The above requirements are met by a micromachined electrostatic actuator having a structure that will be described in more detail below. An alternating voltage pattern is imposed on electrodes located on opposed electrode surfaces of both the rotor and the stator. The actuator provides a significantly lower out-of-plane force for a given in-plane force. The actuator will provide an in-plane force in the range of forces required in an advanced memory device when driven with actuation voltages in the range that can be controlled using conventional MOS integrated circuits. The actuator can be manufactured using micromachining techniques that employ processing similar to that used to make integrated circuits.
The electrostatic actuator includes a stator having a first linear array of electrodes disposed along an opposed surface and a rotor having a second linear array of electrodes disposed along an opposed surface opposite the opposed surface of the stator. The opposed surfaces of the stator and rotor are spaced apart by a spacing d. The rotor is supported relative to the stator to allow to rotor to move in the in-plane direction, parallel to the opposed surfaces. Initially, an alternating voltage pattern is imposed on the electrodes on both the rotor and stator. For example, a first voltage level is applied to every other electrode in each array, and a second voltage level, different from the first voltage level, is applied to each electrode adjacent the electrodes at the first voltage level. By introducing a local disruption into the alternating voltage pattern on the stator, the rotor can be moved in the in-plane direction by a precise distance.
The alternating voltage patterns will not by themselves reduce the out-of-plane-attractive force to a level comparable with the large in-plane force. To reduce the out-of-plane fo
Hoen Storrs
Taussig Carl
Hardcastle Ian
Tamai Karl
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