Low inertia latching microactuator

Details Low inertia latching microactuator Low inertia latching microactuator

2001-01-17

2004-12-14

Mullins, Burton (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S036000, C310SDIG006, C335S229000

Reexamination Certificate

active

06831380

ABSTRACT:

GOVERNMENT RIGHTS IN THE INVENTION
Not applicable.
FIELD OF THE INVENTION
This invention relates to micro electro-mechanical systems (MEMS). More particularly, this invention pertains to low inertia microactuators used to operate a switch, valve, piston, or other mechanism at high rates.
BACKGROUND OF THE INVENTION
High speed, high precision actuation devices are becoming required for a proliferating number of applications, in diverse fields. In industrial applications, very precise put-and-place actuators are required in printed circuit fabrication processes. Scientific applications may require sensors with very precise resolution, which implies fine motions. Deep space astronomical observations may use multifaceted mirrors, each facet independently controlled by a precision actuator. Digital projection cameras manipulate a plurality of reflectors, in order to cast an image onto a projection screen. Drug delivery systems in medical therapeutic treatment may require valves or pistons of high accuracy. Fault detection in vehicular safety systems require devices with precise, high speed motion. Computer disk drives require the alignment of the data heads over the sub-micron data track to a sub 0.1 um accuracy. In each case, the actuator drives the member of interest to a precise position or at a precise rate.
In general, the device itself (e.g. plunger, shutter, piston) may be of arbitrarily small size and low inertia. However, the driving member, i.e. the actuator, is not. The actuator must provide the force, throw (or range), and bandwidth to accommodate the application. Particularly in the case of high speed devices, stringent design criteria are set on the physical and mechanical properties that the actuator must possess. It should have low inertia and low power requirements. For low cost applications, it should also be mechanically simple. These considerations have led to the miniaturization of familiar electromechanical devices, using photolithographic processing rather than machining bulk components. Formation of sub-millimeter scale electromechanical systems is now well known in the art, as Micro Electromechanical Systems, or MEMS.
Among the simplest MEMS actuators that can be fabricated is the cantilevered beam, a device wherein a beam of substrate material is formed by patterning the dimensions of the beam and etching a void beneath it. This technique is described in examples “Microfabrication of cantilevers using sacrificial templates,” U.S. Pat. No. 6,016,693 by Viani, et al., and “High vertical aspect ratio thin film structures,” U.S. Pat. No. 6,015,599 by Keller, et al.
The beam has a finite stiffness determined by its shape and mechanical properties, and can thereby be deflected by application of force. The amount of deflection through small angles varies linearly with the applied force, that is, the beam deflection can be characterized by a spring constant. In most cases, the force applied is electrostatic: The beam, suspended over the void and substrate, forms a parallel plate capacitor with the substrate being the opposing electrode. Actuation, or movement of the beam, results from the application of a differential charge, or voltage, between the beam the substrate.
The device to be actuated, for example a mirror, is then mounted upon the beam, and steered by the electrostatic force between the beam and the substrate.
Cantilevered actuators, while relatively simple in concept and construction, are also limited in performance. Deflection must be perpendicular to the plane of the substrate, as this plane defines the parallel plate capacitor. Additional beams, gears and bearings can translate this motion out-of-plane, as in Ho et al., in U.S. Pat. No. 5,629,918 (1997), “Electromagnetically actuated micromachined flap.” In this invention a flap, which is the moving member of the actuator, is coupled by one or more beams to a substrate and thereby cantilevered out of the plan of the substrate. While conceptually this invention allows larger motions in out-of-plane directions, the need for multiple beams and pivots seriously complicates the design and fabrication of the device, and deleteriously affects tolerances and rigidity.
Another difficulty with cantilevered actuators is that precise motion and high bandwidths require relatively stiff cantilevers. But since deflection is linearly proportional to the spring constant, a stiffer beam requires more force to achieve a certain throw. The tradeoff between stiffness, throw and bandwidth relegates cantilevers to a narrow range of applications. They are suitable for small ranges of motion, or in situations where large supply voltages are available.
Electrostatic forces are also relatively weak and provide actuation over small ranges, compared with, for example, magnetostatic forces. For this reason, magnetostatic devices are often preferred over electrostatic devices. Micromachined solenoidal magnetic actuators are known in the art, as micro-solenoid switches. Typically, a slug of magnetic material is affixed to a piston or plunger, and a coil is provided whose diameter is sufficient to admit the slug into its interior. The coil is then energized to repel or attract the slug, depending on the direction of current in the coil. The resulting linear mechanical motion is used to actuate various linear devices, such as opening and closing a switch or valve, or driving a piston.
An embodiment of a linear, solenoidal microactuator is found for example, in Guckel, et al., U.S. Pat. No. 5,644,177 (1997), “Micromechanical magnetically actuated devices.” The microactuator in this patent comprises a ferromagnetic mandrel around which a fine electrical wire conductor is wound, the mandrel further including pegs which locate and mate with corresponding receptacle holes in the stationary magnetic core.
Linear magnetic actuators are capable of higher forces and larger ranges of motion at lower driving voltages than cantilevered electrostatic actuators. They are therefore capable of actuating relatively large loads or operating against large spring constants. However, their throw is limited to the characteristic dimensions of the solenoid. Also, they operate against a spring force, required to return the moving member to the home position. This spring force requires more force or less throw, for a given energy density in the device. The spring also imparts a vibration to the device being actuated, and in general, the device is not functional until the vibration has ceased. This can add significant settling time to the switching speed.
A third design option is a rotary actuator. This device resembles a miniaturized electromagnetic motor, with a ferromagnetic material deposited on the substrate and wound with an electrical coil. Energizing of the coil induces magnetic flux in the permeable material. Generally the core is patterned with some arrangement of gaps, into each of which protrudes a driven member which interacts magnetostatically with the flux across the gap. A plurality of such elements, when driven in the proper sequence and timing, can produce a positive torque on a freely rotating member. A wide variety of designs for these magnetostatic micromotors can be found in the body of MEMS patents and publications, notably Garcia et al., U.S. Pat. No. 5,917,260 (1999) “Electromechanical millimotor;” “Surface Micromachined Microengine,”-E. J. Garcia, J. J. Sniegowski,
Sensors and Actuators
, A 48, pp. 203-214 (1995); and U.S. Pat. No. 5,631,514 “Microfabricated microengine for use as a mechanical drive and power source in the microdomain and fabrication process.”
Notwithstanding the details of the various designs, the micromotors are conceptually similar to the familiar large scale rotor/stator electromagnetic motors.
Magnetostatic micromotors can be used as rotary actuators by mounting the device of interest onto the moving member, i.e. the rotor. This concept is clearly described in Mehregany, et al. in U.S. Pat. No. 6,029,337 (2000), “Methods of fabricating micromotors with utilitarian features.” This patent describes a micromotor

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