- Thursday, 03 January 2008
Fabrication is simplified and susceptibility to jamming greatly reduced.
Figure 1 depicts an experimental inchworm- type linear microactuator. This microactuator is a successor to the one described in “MEMS-Based Piezoelectric/ Electrostatic Inchworm Actuator” (NPO- 30672), NASA Tech Briefs, Vol. 27, No. 6 (June 2003), page 68. Both actuators are based on the principle of using a piezoelectric transducer (PZT) operated in alternation with electrostatically actuated clutches to cause a slider to move in small increments. However, the design of the present actuator incorporates several improvements over that of the previous one. The most readily apparent improvement is in geometry and, consequently, in fabrication: In the previous actuator, the inchworm motion was perpendicular to the broad faces of a flat silicon wafer on which the actuator was fabricated, and fabrication involved complex processes to form complex three-dimensional shapes in and on the wafer. In the present actuator, the inchworm motion is parallel to the broad faces of a wafer on which it is fabricated. The components needed to produce the in-plane motion are more nearly planar in character and, consequently, easier to fabricate. Other advantages of the present design are described below.
Whereas the previous actuator contained two clutches (denoted “holders” in the cited prior article), the present actuator contains four clutches. Each clutch includes a pair of units on opposite sides of a channel, into which the slider is inserted and along which the slider moves. Rails along the sides of the substrate prevent outward movement of the clutch units. Each clutch unit includes a rounded frictional contact that is springloaded against one side of the slider. Attached to each spring-loaded frictional contact is an electrostatic comb drive that, when energized, opposes the spring load to pull the contact away from the slider. Hence, each clutch is normally latching: the rounded frictional contacts clamp the slider from opposite sides until and unless the electrostatic comb drives are energized. The spring load is obtained by inserting the slider that is slightly wider than fabricated clutch clearance. This insertion also displaces the comb teeth to achieve very narrow (<1 μm) comb gap that is power efficient but difficult to fabricate in bulk Si structure. A low-thermal-expansion-glass lid, omitted from the figure for the sake of clarity, is placed across the rails to retain the slider in the channel.