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.
Figure 2 depicts the operation of the present and previous linear actuators and illustrates one of the advantages of the present actuator over the previous one. In the present actuator, the first and third clutches are operated in unison and are mounted on a stationary structure denoted A. The second and fourth clutches are operated in unison and are mounted on a moveable structure, denoted B, that can be driven a short distance forward or backward along the channel by a PZT. In step 1 of an operational sequence in which the slider is moved leftward, the clutches on unit A are released. In step 2, the PZT is extended to push unit B and the slider leftward. In step 3, the clutches on unit A are latched while the clutches on unit B are released. In step 4, the PZT is retracted to bring unit B rightward. Repetition of steps 1 through 4 causes the slider to move leftward in repeated small increments.
The operational sequence of the previous two-clutch actuator is similar. However, the two-clutch configuration is susceptible to tilt of the slider and a consequent large increase in drag. Hence, the primary operational advantages of the present four-point-latching design over the prior two-point-latching design are less drag and greater control robustness arising from greater stability of the orientation of the slider.
This work was done by Risaku Toda and Eui-Hyeok Yang of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-42381, volume and number of this NASA Tech Briefs issue, and the page number.