Low voltage control of MEM actuators

Details Low voltage control of MEM actuators Low voltage control of MEM actuators

2001-12-05

2003-09-16

Tso, Edward H. (Department: 2838)

Electricity: electrical systems and devices

Discharging or preventing accumulation of electric charge

Specific conduction means or dissipator

Reexamination Certificate

active

06621681

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the control of electrically activated microelectromechanical (MEM) devices, and more particularly to the control of MEM actuators in reflective microstructure positioning systems.
BACKGROUND OF THE INVENTION
Through appropriate mechanical coupling structures, one or more electrically activated MEM actuators can be utilized to adjust the orientation of a positionable reflective microstructure (e.g., a mirror) fabricated on a substrate. One example of an electrically activated MEM actuator is an electrostatic comb actuator. Electrostatic comb actuators generally include at least one stationary or fixed comb side and one moveable comb side. The fingers of the fixed and moveable combs are interdigitated with one another. The fixed and moveable combs are essentially two electrodes that are capacitively coupled with one another. Upon application of a voltage across the capacitive gap between the fixed and moveable combs of the electrostatic comb actuator, an electrostatic force is developed that attracts the moveable comb side towards the fixed comb side. Movement of the moveable comb side in response to the electrostatic force results in a displacement (i.e., change in position) at the output of the actuator. Such a displacement can be transmitted through appropriate microfabricated linkages and flexures to achieve a desired displacement of a structure that adjusts the orientation of the reflective microstructure with respect to the substrate.
The electrostatic force, and thus the amount of displacement achieved, depends on the amount of voltage that is applied across the MEM actuator. Typically, the voltages required to operate electrostatic comb actuators are many tens of volts to over a hundred volts. As may be appreciated, in order to achieve precise control of the electrostatic force generated and thus the displacement achieved, the amount of voltage applied must be controlled with precision. This requires the use of appropriate control electronics. However, due to the high voltages required, conventional, and thus low cost and widely available, control electronics (e.g., CMOS control electronics) are often not appropriate for controlling operation of the actuator.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a method and system for controlling MEM actuators such as, for example electrostatic comb actuators, using low voltages. In one desired implementation of the present invention, the voltage required to achieve the desired displacement of the actuator is divided into two components: a fixed voltage component and a variable voltage component. The fixed voltage is supplied by a fixed DC voltage source. The variable voltage is supplied by variable DC voltage source. Only the variable DC voltage source needs to be precisely controlled. By choosing an appropriate fixed voltage, the range of the variable voltage can be small enough to permit the use of inexpensive, widely available, and potentially safer low voltage control electronics. Although the method and system of the present invention are particularly suited for controlling electrostatic-type actuators, the method and system of the present invention may also be applicable to the control of other types of microfabricated actuators such as, for example, piezo-type actuators that typically require high control voltages, or thermal-type actuators though most thermal-type actuators do not require high control voltages.
According to a first aspect of the present invention, a method of controlling an electrically activated MEM actuator includes the step of applying a first control voltage to the MEM actuator. Application of the first control voltage establishes a first total control voltage across the MEM actuator, and a first position of the MEM actuator is established based upon application of the first control voltage. After the first position is established, a second control voltage is applied to the MEM actuator while still applying the first control voltage. Application of the second control voltage in conjunction with the first control voltage establishes a second total control voltage across the MEM actuator that is different from the first total control voltage, and the MEM actuator is moved from the first position to a second position based upon application of the second control voltage. In one embodiment, the MEM actuator is part of a reflective microstructure positioning system fabricated on a substrate. The MEM actuator is positionable in a plurality of positions corresponding to different total control voltages across the MEM actuator to orient the reflective microstructure in a corresponding plurality of orientations with respect to the substrate. In this regard, when the MEM actuator is in the first position, the reflective microstructure is oriented in a first orientation relative to the substrate corresponding to this first position, and when the MEM actuator is in the second position, the reflective microstructure is oriented in a second orientation relative to the substrate corresponding to this second position.
The first control voltage may be supplied using a fixed DC voltage source in the case of the first aspect, and the second control voltage may be supplied using a variable DC voltage source electrically connected in series with the fixed DC voltage source. Where the MEM actuator comprises an electrostatic comb actuator, the first control voltage may be applied to a first terminal of the electrostatic comb actuator that is electrically connected to the moveable comb side of the electrostatic comb actuator, and the second control voltage may be applied to a second terminal of the electrostatic comb actuator that is electrically connected to the fixed comb side of the electrostatic comb actuator. Upon application, the second control voltage may establish a second total control voltage that is greater than or less than the first total control voltage established upon application of the first control voltage. Where the reflective microstructure positioning system is configured for tilting the reflective microstructure with respect to the substrate, the magnitude of the first control voltage may be such that the first total control voltage biases the actuator for operation within a range of voltages corresponding to a desired operating region on a curve representing the relationship between the total control voltage applied across the MEM actuator versus a tilt angle of the reflective microstructure with respect to the substrate. Stated another way, the first total control voltage may be selected such that a relatively small range of second control voltages across the MEM actuator produces a much larger displacement than the application of the first total control voltage across the MEM actuator.
According to a second aspect of the present invention, a method of controlling a plurality of electrically activated MEM actuators includes the step of applying a common first control voltage to all of the MEM actuators. Application of the first control voltage establishes a first total control voltage across each of the MEM actuators, and first positions of each of the MEM actuators are established based upon application of the first control voltage. After establishing the first position of the MEM actuator and while still applying the first control voltage, separate second control voltages are then applied to each of a selection of the MEM actuators (i.e., one or more of the MEM actuators). Application of the separate second control voltages establishes second total control voltages across each MEM actuator in the selection that is different from the first total control voltage, and each MEM actuator in the selection is moved from its corresponding first position to a corresponding second position based upon application of the separate second control voltages. In one embodiment, the MEM actuators are incorporated in at least one reflective microstructure positioning system fabricated on a substrate. Each MEM actuator is positi

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