When grasping microparts extremely low gripping low forces, often in the micro to nano newton range, must be applied in order to prevent parts from being damaged and to prevent them from dislodging in the gripper. This paper presents our work in developing a force controlled microgripper and microgripping strategies using optical beam deflection techniques. The optical beam deflection sensor is based on modified Atomic Force Microscope techniques and is able to resolve forces below a nanonewton. A variety of gripper fingers made from materials with different conductivity and surface roughness are analyzed theoretically and experimentally using the force sensor. These results provide insight into the mechanics of micromanipulation, and the results are used to develop microgripping strategies. A design of a microfabricated force controlled microgripper is presented along with initial experimental results in applying various gripping forces to microparts. The results demonstrate the important role gripping force plays in the grasping and release of microparts.
It is well known that surface effect forces such as van der Waals, electrostatic, and surface tension forces dominate part interactions as part dimensions fall below approximately 100 microns. Many researchers have suggested manipulation strategies that either diminish the effect of these forces or use these forces to advantage. There is little work, however, that comprehensively analyzes, both theoretically and experimentally, the exact contributions of such phenomena as surface roughness, material properties, environmental conditions, etc. to part interactions at microscales. This paper describes our work in developing a high resolution force sensor using optical beam deflection techniques for characterizing object interactions at the microscale. The interactions among a variety of micropart shapes and materials of varying surface roughness and conductivity were analyzed under various environmental conditions. Experimental results of this analysis are presented.
For microassembly tasks uncertainty exists at many levels.Single static sensing configuration are therefore unable to provide feedback with the necessary range and resolution for accomplishing many desired tasks. In this paper we present experimental results that investigate the integration of two disparate sensing modalities, force and vision, for sensor-based microassembly. By integrating these sensing modes, we are able to provide feedback in a task- oriented frame of reference over a broad range of motion with an extremely high precision. An optical microscope is used to provide visual feedback down to micro resolutions, while an optical beam deflection technique is used to provide nanonewton level force feedback or nanometric level position feedback. Visually served motion at speeds of up to 2mm/s with a repeatability of 0.17 micrometers are achieved with vision alone. The optical beam deflection sensor complements the visual feedback by providing positional feedback with a repeatability of a few nanometers. Based on the principles of optical beam deflection, this is equivalent to force measurements on the order of a nanonewton. The value of integrating these two disparate sensing modalities is demonstrated during controlled micropart impact experiments. These results demonstrate micropart approach velocities of 80 micrometers /s with impact forces of 9nN and final contact forces of 2nN. Within our microassembly system this level of performance cannot be achieved using either sensing modality alone. This research will aid in the development of complex hybrid MEMS devices in two ways; by enabling the microassembly of more complex MEMS prototypes; and in the development of automatic assembly machines for assembling and packaging future MEMS devices that require increasingly complex assembly strategies.
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