Design and characterization of a micromachined inchworm motor with thermoelastic linkage actuators

Abstract

A new micromachined inchworm motor has been designed, fabricated and characterized for microassembly applications. In order to implement inchworm motions, a pair of thermoelastic actuator is devised to have five-linkage mechanism for two-dimensional motions in tangential and normal directions. The thermoelastic actuators consist of two amplification bars and two coupling bars with four hinge springs. A forked tip located on the apex of the linkage is used to fit the teeth of shuttle mass for its tangential translation. The thermal expansions of the active bars generate the displacement of the actuator, which is then transformed into a bending of the active hinges to be finally amplified by the amplification bar. The inchworm actuator performed the operation at a step movement of 50 μm with a latch-up by the teeth fitting and a driving force of 50 μN for 0.2 μm tolerance. Keywords Micromachined Inchworm Thermoelastic Microassembly Actuator Linkage 1 Introduction For microassembly applications, an actuator should be able to translate a small workpiece with an appropriate chucking force without dropping the workpiece during operation [1] . The microchuck needs a gripping force greater than several tens of μN to grip the workpiece and a stroke longer than 50 μm. It has been suggested that electrostatic comb drivers and inchworm motors can act as driving actuators for the realization of a high precision and long stroke translation system [2–4] . Unfortunately, the driving force of electrostatic actuators is generally small and hence it is inevitable to apply a high voltage to attain a large displacement. However, thermoelastic actuator fabricated for large force and long stroke applications provides insufficient force and shows a non-rectilinear motion [5] . Recently a linear microengine was demonstrated using bent-beam electrothermal actuation, which shows sufficient driving distance but one pair of bent-beam actuator drives in one direction only [6] . In this work, a new micromachined inchworm motor has been designed, fabricated and characterized for microassembly applications with a special focus on large force, long stroke and two-directional movability conditions [7] . 2 Design of an inchworm motor 2.1 A linkage actuator for an inchworm motion In order to implement inchworm motions, a pair of thermoelastic actuator is devised to have a linkage mechanism with two-dimensional motions in tangential and normal directions. The proposed design of an inchworm motor consists of one shuttle mass, which is suspended by four leaf springs and a pair of five-linkage actuators, as shown in Fig. 1(a) [7] . The linkage mechanism provides the inchworm motor with a motion of two degrees of freedom using two active hinges, two shoulder hinges and one neck hinge. To grip the shuttle mass, additional neck hinge and the forked tip (end effector) are designed and attached as shown in Fig. 1(b) . The link between two neck hinges is small and actually works as one hinge point. Therefore, the number of hinges is apparently six but the entire thermoelastic mechanism can be considered as a five-linkage system. When an input voltage is applied to the two electrical pads of an actuator, the induced electric current flow raises the temperature of two active bars and two active hinges. This thermal expansion of active bars resulted in amplified displacement, which finally gives rise to the rotation of the amplification bar as shown in Fig. 1(c) . 2.2 Schematic of unit step motions A pair of linkage actuators is required to have two degree of freedom motions of tangential and normal directions, which make it possible to continuously drive the shuttle mass by the iteration of five unit steps as shown in Fig. 2(a)–(e) . If two amplification bars rotate in-phase, the forked tip will move leftward without the shuttle mass, rightward with the shuttle mass or return to its initial position, as shown in Fig. 2(b), (d) and (f) , respectively. On the contrary, if two amplification bars rotate out-of-phase, the forked tip moves either inward for fitting or outward for releasing the shuttle mass, as shown in Fig. 2(c) and (e) , respectively. One pair of actuators is capable of four kinds of motions such as fitting the forked tip, rectilinear driving of shuttle mass, releasing the shuttle mass, and returning to its initial position, as illustrated in Fig. 3(a)–(e) . In this way, one cycle of stepping motion enables the inchworm motor to have a large stroke by accumulating many stepping movements. The actuator can move backward when the sequence of the applied signals is reversed. To secure the actuation without slipping, teeth structure was devised at the interface between the shuttle mass and the forked tip as shown in Fig. 4(a) . In addition, the teeth may reduce power consumption by latch-up motion after the completion of actuation. The teeth can be designed to have a shape of “T” for more secure latch-up, as shown in Fig. 4(b) . 3 Theoretical analysis 3.1 Operational schemes In order to utilize the proposed thermoelastic actuator for inchworm motors, the required trajectory was calculated for one cycle of operation, as illustrated in Fig. 5 . The trajectory consists of four unit steps such as: (1) extrusion for fitting; (2) forward movement for transportation; (3) retraction for release; and (4) backward movement to initial location. The schematic diagram ( Fig. 5 ) shows the voltage necessary for right and left linkage actuators to perform inchworm motion as per the cyclic sequence. The corresponding required strokes were found to be about 5 and 2 μm in tangential and normal directions, respectively. 3.2 Estimation of the stiffness The stiffness of the actuator was theoretically analyzed assuming that dominant deformations are due to the rotation of the active hinges, as shown in Fig. 6(a) and (b) . The stiffness at the top of the forked tip is used to estimate the maximum...