Electrical generator or motor structure – Non-dynamoelectric – Thermal or pyromagnetic
Reexamination Certificate
2001-02-09
2001-11-06
Ponomarenko, Nicholas (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Thermal or pyromagnetic
C060S528000
Reexamination Certificate
active
06313562
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to microelectromechanical (MEM) devices, and specifically to a MEM ratcheting apparatus in which motion is provided to a moveable member (e.g. a gear, stage or rack) by one or more reciprocating ratchets engaged with a plurality of indexing teeth or pins on the moveable member, with the motion being generated by an electrostatic or thermal actuator.
BACKGROUND OF THE INVENTION
Polysilicon surface micromachining adapts planar fabrication process steps known to the integrated circuit (IC) industry to manufacture microelectromechanical or micromechanical devices. The standard building-block processes for polysilicon surface micromachining are deposition and photolithographic patterning of alternate layers of low-stress polycrystalline silicon (also termed polysilicon) and a sacrificial material (e.g. silicon dioxide or a silicate glass). Vias etched through the sacrificial layers at predetermined locations provide anchor points to a substrate and for mechanical and electrical interconnections between the polysilicon layers. Functional elements of the device are built up layer by layer using a series of deposition and patterning process steps. After the device structure is completed, it can be released for movement by removing the sacrificial material in part or entirely by exposure to a selective etchant such as hydrofluoric acid (HF) which does not substantially attack the polysilicon layers.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane (e.g. a ground plane), and up to three or more additional layers of mechanical polysilicon which can be used to form functional elements ranging from simple cantilevered beams to complex systems such as an electrostatic motor connected to a gear train. Typical in-plane lateral dimensions of the functional elements can range from one micron to several hundred microns or more, while individual layer thicknesses are typically about 1-3 microns. Because the entire process is based on standard IC fabrication technology, a large number of fully assembled devices can be batch-fabricated on a silicon substrate without any need for piece-part assembly.
For various types of MEM devices, a precise control over movement or positioning is needed. Such precise movement control is difficult using present MEM motors or microengines (see e.g. U.S. Pat. No. 5,631,514 to Garcia et al which discloses a MEM engine which rotates a gear in substantially 90° increments and requires multiple complex drive signals).
The use of a reciprocating shuttle to form a wedge-type stepping motor as disclosed in U.S. Pat. No. 5,959,376 to Allen provides an improvement in precise positioning of a gear. However, the reciprocating shuttle produces an unbalanced actuation force on a hub about which the gear rotates, thereby limiting the durability and reliability of such a device.
An advantage of the present invention is a MEM apparatus is provided which provides a precise open-loop positioning of a moveable member such as a ring gear, a stage, or a rack by using a ratcheting mechanism.
Another advantage of the present invention is that a rotary MEM apparatus formed according to the present invention has actuation forces that are substantially balanced to minimize wear and thereby improve reliability.
Yet another advantage is that manufacturing tolerances (e.g. due to photomask misalignment) can be less critical compared to other MEM devices since motion of a majority of the elements in the MEM actuator of the present invention is limited to small angles and distances rather than requiring full rotation about an axis.
Another advantage of the present invention is that the MEM apparatus operates with simple drive signals, with the rotation or translation of a moveable member being precisely determinable from the drive signals.
Still another advantage of the present invention is that a relatively high torque can be provided to a moveable member (e.g. a gear, stage, or rack) without the need for any additional gears.
A further advantage of the present invention is that an electrostatic or thermal actuator for driving the moveable member can be located within an outline of the member so that no additional space is required on a substrate.
These and other advantages of the method of the present invention will become evident to those skilled in the art.
SUMMARY OF THE INVENTION
The present invention relates to a microelectromechanical (MEM) ratcheting apparatus formed on a substrate. The MEM apparatus comprises a member (e.g. a ring gear, a rotary stage, or a rack) that is moveable about a straight or curved path, with the moveable member having a plurality of indexing elements (e.g. teeth, or pins extending outward from a surface or edge of the moveable member) spaced along the path; an actuator providing reciprocating motion substantially in the direction of the path (i.e. parallel, or in-line or tangential to the path); and one or more pawls operatively connected to the actuator and engageable with the indexing elements to urge the moveable member along the path. The MEM ratcheting apparatus is preferably formed on a silicon substrate, with the moveable member preferably comprising polycrystalline silicon (also termed polysilicon). In the MEM ratcheting apparatus, the actuator can be either an electrostatic actuator or a thermal actuator.
In embodiments of the present invention employing an electrostatic actuator, the electrostatic actuator further comprises a plurality of pairs of electrostatic arms, with one electrostatic arm of each pair being stationary, and with the other electrostatic arm of each pair being moveable in response to a voltage applied across the pair of electrostatic arms. The stationary and moveable electrostatic arms of each pair of electrostatic arms can be either oriented substantially parallel to each other or can be oriented at an angle of generally less than twenty degrees with respect to each other. The plurality of moveable electrostatic arms can further be ganged together by attaching these arms to a support frame (e.g. comprising a plurality of concentric rings). The support frame can also be used to support each pawl at one end thereof.
One or more stops are preferably provided to prevent contact between the moveable electrostatic arm and the stationary electrostatic arm in each pair of electrostatic arms; and at least one restoring spring is preferably used to restore (i.e. return) the moveable electrostatic arms to a rest position in the absence of the applied voltage (i.e. when the applied voltage is reduced below a threshold value). Furthermore, the stationary and moveable electrostatic arms can include a plurality of interdigitated fingers (e.g. forming a comb actuator or a hybrid actuator as described hereinafter), with the individual fingers being either substantially straight, angled, or curved. Finally, an electrostatic shield can be provided between the stationary electrostatic arm of one pair of electrostatic arms and the moveable electrostatic arm of an adjacent pair of the plurality of electrostatic arms to reduce or eliminate an unwanted electrostatic force of attraction.
In embodiments of the present invention employing a thermal actuator, the thermal actuator further comprises a niurality of pairs of thermal arms spaced about a central axis of rotation and extending outward therefrom, with each pair of thermal arms including a hot arm and a cold arm interconnected at an end of each hot arm located distally to the central axis, the hot arm thermally expanding to a greater extent than the cold arm in response to a voltage applied across each pair of hot and cold arms, thereby incrementally rotating each pair of thermal arms about the central axis. Each pair of thermal arms is operatively connected to drive a pawl which engages the indexing element to urge the moveable member along the path.
The action of each pawl to engage the indexing elements of the moveable me
Barnes Stephen M.
Burg Michael S.
Jensen Brian D.
Miller Samuel L.
Rodgers M. Steven
Dinh Le Dang
Hohimer John P.
Ponomarenko Nicholas
Sandia Corporation
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