Prototyping Millirobots Using Dextrous Microassembly and Folding

This paper discusses two processes for rapidly prototyping micromechanical systems: first microassembly, and second, laser cutting of thin sheets and folding. Sub-millimeter rigid blocks can be dextrously manipulated using two 1 DOF fingers and an XYZ micro-positioning stage. Algorithms for micro-part manipulation use open-loop compliant grasps combined with slip to align microparts, which can then be adhesively bonded. Strong, lightweight structures with low friction flexural joints can be readily laser cut, then folded. Potentially, thermally driven actuators can be simply integrated with flexural structures to build fingers for part manipulation. INTRODUCTION Microassembly provides the capability to construct 3 dimensional heterogenous microsystems by joining sensors, actuators, structures, and intelligence which are separately fabricated, and ideally available off the shelf. This paper examines applying techniques from conventional-size robotic part manipulation to manipulating sub-millimeter parts. Strategies and simple fixtures which are inherently robust without sensing can be used to manipulate micro-parts. The problem of robotic microassembly has been explored using high precision actuators and vision feedback in work by Codourey et al [1995], Feddema and Simon [1998], Kasaya et al [1998], Nelson et al [1998], and Sulzmann et al [1998]. Visionbased approaches are limited by poor depth of field of high power microscopes, cluttered views, and lack determination of contact or contact forces. In addition, it is difficult to perform several distinct operations in parallel as microscopes are quite bulky and expensive (although parallel operations can be performed with rigid pallets and fixtures [Feddema and Christensen 1999]). Alternatively, force sensor based approaches can be local and provide exact information about contact between surfaces (Zesch and Fearing [1998], Sitti and Hashimoto [1999], Zhou and Nelson [1998]). At the micro-scale, adhesion forces of surface tension, elecThis work was funded in part by NEC, ONR MURI N00014-981-0671, ONR DURIP and DARPA. †Corresponding author. trostatic and Van der Waals dominate gravitational forces (Arai et al [1995], Fearing [1995]). Recent work has shown how adhesive forces can be used to advantage during microassembly tasks by controlling contact areas and surface tension, to ensure that microparts are reliably transferred to the target surface and released from the gripper (Arai and Fukuda [1997], Miyazaki and Sato [1997], Saito et al [1999], Zhou and Nelson [1998], Zesch et al [1997]). Previous micromanipulation work has used single probes or parallel jaw grippers to manipulate parts. The parallel jaw gripper approach follows from macro-robotics where a simple gripper is used with a 6 degree-of-freedom (DOF) arm to reorient and position parts. As sub-centimeter 6 DOF micro-robot arms are not yet available, we show how macro-scale dextrous manipulation techniques can be used with much simpler mechanisms to reorient and position parts. By using gripping forces which exceed adhesion forces, we can use Coulomb friction to control part sticking and sliding. In this paper, we demonstrate how micro-parts can be dextrously manipulated in open-loop fashion using two 1 DOF fingers in the plane combined with an XY Z cartesian stage. As shown by [Fearing 1986a] and [Gopalswamy and Fearing 1989] two-finger grasps of polygons and polyhedra (respectively) will automatically slide to a stable configuration if the angle between the included faces is less than twice the friction angle. Conversely, a tangential force at one finger will cause the grasped part to roll about the opposite finger. Alternatively, rotational torques can be applied by a third finger [Fearing 1986a]. As these grasping methods do not require feedback, and are robust to initial conditions, they are well suited to the micro-domain and parallelization. Grasping methods and automatic planners using slip have been discussed further by Brost [1986], Carlisle et al [1994], Erdmann et al [1993], Goldberg [1993], Lynch [1999], Rus [1993], Rao et al [1996], Yoshikawa et al [1993], and Wiegly et al [1997]. Another method of obtaining 3D microstructures is by planar fabrication of polysilicon plates and then folding out of the plane such as Pister et al [1992], using pin hinges, and Shimoyama et al [1993], using polyimide hinges. Microrobot struc-