Displacement amplification structure and device

Details Displacement amplification structure and device Displacement amplification structure and device

2000-09-08

2003-05-06

Fenstermacher, David (Department: 3682)

Machine element or mechanism

Control lever and linkage systems

Variable output force

C074S108000, C267S165000, C310S331000

Reexamination Certificate

active

06557436

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to the field of mechanisms which modify the displacement or force generated by an actuator. More specifically, the present invention relates to a core structure, and devices having a plurality of these core structures, that relies on the elastic deformation of its constituent elements to transmit forces and motion from an input to an output. In one specific embodiment, the preset invention relates to the field of microelectromechanical (MEM) systems and, in particular, to a pivotless structure formed by surface micromachining processes for use in combination with a MEM actuator (such as an electrostatic comb actuator, a capacitive-plate electrostatic actuator) or a thermal actuator to modify a displacement or force provided by the MEM actuator.
BACKGROUND OF THE INVENTION
Historically, engineered devices are designed to be strong and stiff and engineered systems are usually assembled from discrete components. In nature, however, designs are strong and compliant and natural systems develop as one connected whole.
The limitation of many currently available actuators, and in particular smart material actuators, is their small stroke and power. Currently, piezoelectric actuators have a high bandwidth, but low strain, whereas shape memory alloy (SMA) actuators have relatively high strain, but extremely low bandwidth. The use of the above-type actuators in a system design must therefor involve a stain versus weight versus bandwidth tradeoff. In an effort to amplify their displacement, some investigators have pursued the stacking of multiple actuators in different configurations. Although modest displacement amplification can be achieved in this manner, these arrangements are often cumbersome and impose a heavy penalty by way of voltage requirements.
In the formation of many types of microelectromechanical (MEM) devices, a motive forcer or actuator is required. The previous used electrostatic comb actuators have generally consumed a large portion of the die on which the MEM device is formed (e.g. up to ⅔ of the die size). Further, the die size is constrained by available steppers used for the photolighographic processes of the MEM fabrication process. As a result, the size and complexity of MEM devices is presently constrained by the size of the actuator used.
Reducing the size of the motive source can alleviate this problem and leave more space on the die for MEM devices of increased complexity and functionality. A smaller-sized electrostatic comb actuator, however, will produce a correspondingly smaller range of displacement (i.e. a smaller actuation stroke) which can be, for example, about 5 microns or less. Thermal actuators and capacitive-plate electrostatic actuators generally provide a much larger force than is available with electrostatic comb actuators. However, this larger force is generated over a short actuation stroke of typically 0.25-2 &mgr;m. Such a short actuation stroke is insufficient for driving many types of MEM devices including ratchet-driven gears, stages or racks; or microengines such as that disclosed in U.S. Pat. No. 5,631,514 to Garcia et al.
From the above it is seen that there is a need in a MEM device for a mechanism that multiplies the range of displacement from a short-stroke actuator and provides an increased range of displacement that is sufficient for actuating a particular MEM device. This will allow the use of compact electrostatic comb actuators or, alternately, capacitive-plate or thermal actuators, allowing the formation of MEM devices of increased complexity and functionality within a give die size.
Employing mechanical means to modify (amplify or attenuate or reorient) an output displacement or force is not new. Perhaps the simplest displacement-amplifying device is a lever arm moving about a pivot joint. A lever arm is shown in FIG.
1
. The use of a pivoted displacement-modifying device, however, may be undesirable in certain situations. For example, if a linear output is desired, devices utilizing a pivoting lever arm provide movement d
2
at the output end of the lever arm which is arcuate. Play in the pivot joint, which is limited by fabrication tolerances, can render a lever arm undesirable, as can a required high geometric advantage, which would require extreme length in L
2
. The play in the pivot joint of a MEM device can be substantial compared to the range of displacement of a short-stroke actuator. For example, a thermal actuator can provide a range of displacement that is only 0.25 &mgr;m, when heated from room temperature to about 400° C. This is comparable to the actual amount of play in MEM pivot joints and as such the use of a displacement-multiplying device having a pivot joint would not be suitable for use in increasing the range of displacement of a thermal actuator. Additionally, an arcuate output displacement can complicate the design of the MEM device.
Another variety of a displacement modifying strategy is a rigid link system.
FIG. 2
illustrates a crimping mechanism employing a conventional rigid link displacement modifying strategy. When operated in one direction the mechanism amplifies displacement and when operated in the opposing direction it amplifies force.
Compliant mechanisms are a relatively new class of devices that utilize elasticity or compliance elements to transmit motion and/or force. They can be designed for any desired input-output force displacement characteristics, including specified volume/weight, stiffness and natural frequency constraints. A “compliant mechanism” is defined herein as a structure that exploits elastic deformation to achieve a force/displacement transformation, with the displacement being changed (e.g. increased) relative to an input force provided to the same end of the compliant structure. In one direction of operation, an output displacement is increased relative to an input displacement; and an output force is correspondingly decreased relative to an input force. In the other direction of operation, the opposite effect occurs with the output force being increased relative to the input force; and with the output displacement being decreased relative to the input displacement. As flexure is permitted in these mechanisms, they can be readily integrated with unconventional actuation schemes including the above mentioned actuators. Compliant mechanisms can be made from any ductile material such as nylon, aluminum, steel, nickel-titanium alloy, etc.
More specifically, compliant mechanisms are single-piece flexible structures that deliver the desired motion by undergoing elastic deformation as opposed to rigid body motions of conventional mechanisms. As compliant mechanisms are thus far known in the art, they have been limited to the replacement of mechanical hinges with flexural joints (living hinges). One such mechanism having a living hinge flexural joint is illustrated in FIG.
3
. However, simply replacing mechanical joints with flexural joints has the disadvantages of being a source of stress concentration leading to early fatigue failure (as a result of the flexural joints being unusually thin and resultant stress concentrations), being a source of significant efficiency loss due to the large localized strain energy loss at each of the flexural joints and can be difficult to manufacture.
Compliant mechanisms having flexural joints are seen to exhibit what is herein referred to as “lumped compliance”. Lumped compliance results because the thin flexural joints localize strain energy losses where flexing in the device occurs, at each of the flexural joints. Between the flexural joints, these structures generally operate in a rigid manner.
FIG. 3
b
illustrates in cross-section a lumped compliant device as might be used in a precision instrument.
The present invention proposes a deviation from the known art of compliant mechanisms. More specifically, the present invention proposes a compliant mechanism which lacks the flexural joints of the prior art. A compliant mechanism according to the present invention

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