Electrical generator or motor structure – Non-dynamoelectric – Charge accumulating
Reexamination Certificate
2000-06-09
2002-02-26
Tamai, Karl (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Charge accumulating
C318S116000
Reexamination Certificate
active
06351054
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a driving device and method for driving electrostatic actuators. More particularly the invention relates to a driving device and method using a compounded AC signal.
BACKGROUND OF THE INVENTION
Electrostatic actuators have become selected and are the solution of choice for actuators that employ low power, operate at high speed, require low cost to produce, and are of small size. These devices present significant advantages: over thermal devices by requiring much less power; over electromagnetic devices using less power and having smaller size; or piezoelectric actuators that have a higher cost and have a much smaller amplitude of motion.
To date, however, there are no commercially available electrostatic actuators. Of particular concern are electrostatic actuation in the presence of dielectrically isolated electrodes, where specific problems are incurred.
In electrostatic actuators, the desired displacement is the result of the attractive electrostatic force generated by the interaction between a distribution of opposite sign charges placed on two bodies, one of which is moveable. For the purposes of this invention, these two bodies are known as actuator plates. The actuator plates are placed apart by a predetermined distance. The charge distribution is then generated by applying a potential difference between two conductive electrodes that are part of the actuator plates. The actuator will be in the ON state or mode when a potential difference is applied between the electrodes and the two plates move toward each other. The actuator will be in the OFF state when the electrodes are at the same potential (shorted).
A DC voltage is theoretically capable of inducing the “ON” state of the actuation cycle. Practical problems, however, limit the application of a DC voltage for the actuation of some electrostatic actuators. In real devices, DC driving shows memory effects, such that the behavior of the actuators depends strongly on the history of the actuation process. DC driving also induces stiction (1) through charges injected in the dielectric at the dielectric/metal electrode interface, and (2) through charges trapped at the dielectric/air interface.
Using an AC signal for inducing the ON state of an electrostatic actuator is also possible and it can remove the drawbacks of the DC driving.
Sine wave AC drive has been considered and found to have some detrimental properties. Sine wave AC drive does not generate the steady level of electrostatic pressure required in many applications. The displacement of the actuator plate/plates tends to follow the sine wave applied voltage, resulting in an undesired vibratory motion when the AC sine wave voltage is applied. To overcome this drawback a multiphase/multielectrode driving scheme can be used. This method results however in complicated structures and limited force. Square wave AC driving can provide high and steady electrostatic forces. However, it can also produce premature stiction that adversely affects performance sooner than would permit many practical devices to operate with reasonable life expectancy.
One family of patents describes fluid control employing micro miniature valves, sensors and other components using a main passage between one inlet and exit port and additionally a servo passage between inlet and outlet ports. The servo passage is controlled by a control flow tube such that tabs are moved electrostatically. U.S. Pat. No. 5,176,358 to Bonne et al teaches such a fluid regulating device, while divisional U.S. Pat. Nos. 5,323,999 and 5,441,597 relate to alternative embodiments.
The actual electrostatic device is only briefly described in the above patents, wherein at least one tab formed as part of a dielectric layer moves toward and away from an aperture upon activation of a means for varying the potential of at least one electrode associated therewith to generate an electrostatic force.
The above referenced patents identify another family of patents for further information on microvalves using electrostatic forces. The pending U.S. patent application referred to in those first discussed patents has matured into U.S. Pat. No. 5,082,242 to Bonne et al. This patent describes a microvalve that is an integral structure made on one piece of silicon such that the device is a flow through valve with inlet and outlet on opposite sides of the silicon wafer. The valves are closed by contact with a valve seat where surfaces must be matched in order to avoid degradation of valve performance. Two patents, U.S. Pat. Nos. 5,180,623 and 5,244,527 are divisional patents relating to the first patent. These patents generally describe operation of the electrostatic valve as being driven by various kinds of voltage sources. Specifically, the valve is said to operate as a two position valve with fully open and fully closed positions by applying a DC voltage between electrodes. Also, operation as a proportional control valve is disclosed as being effected by applying a voltage proportional to the voltage necessary to close the valve. Finally, These patents describe operation of the valve with a pulse width modulated voltage signal to modulate gas flow through the valve.
In some electrostatic actuators, the actuator plates have to come in intimate contact during the normal operation cycle. These actuators are sometimes referred to as touch-mode electrostatic actuators. In order to prevent electrical shorting during the touch phase of the operation cycle, the conductive electrodes are isolated from each other by dielectric layers. In order to get the maximum work from a specific device, large electric fields are usually developed between the two conductive electrodes. The non-linear character of the electrostatic attraction results in a snapping action, where the actuator plates move toward each other with accelerations as high as 10
8
g and speeds that exceed 10
3
m/sec. After the impact, the free surfaces of the actuator plates are pushed against each other by the large electrostatically generated pressure. This operation mode creates the possibility of very large mechanical impact and strong interaction forces being developed between the actuator plates. Some of these forces can continue to act after removal of the potential difference between the actuator plates. In some cases, these forces are stronger than the restoring forces available for bringing the electrodes in their original position. In such a case, the two electrodes remain temporarily or permanently attached and the actuator stops functioning as intended and desired. This condition is sometimes referred to as ‘stiction.’
The main forces producing stiction in electrostatic actuators are surface interaction forces (solid bridging, Van der Waals forces, hydrogen bonds) and electrostatic forces produced by charges permanently or temporarily trapped into the dielectrics. To reduce the surface interaction forces, two approaches may be used. The first, reducing the contact area, requires more sophisticated structures and gives up some of the available electrostatic force. The second, reducing the surface energy of the layers in contact, has not yet been successfully demonstrated for devices based on that concept.
During the lifetime of an actuator, the large mechanical impact and the large electrostatic pressure can gradually increase the real contact area between the actuator plates and enhance the surface interaction forces.
It would be of great advantage to the art if this buildup could be reduced or avoided altogether.
It would be another great advance in the art if an improved driving method for electrostatic actuators could be provided for use with any actuator and configuration of the physical components thereof.
Other advantages will appear hereinafter.
SUMMARY OF THE INVENTION
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention comprises an actuator drive producing a compounded AC signal with three differ
Cabuz Cleopatra
Cabuz Eugen I.
Honeywell International , Inc.
Shudy, Jr. John G.
Tamai Karl
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