Micromanipulation contact transition control by selective focusing and microforce control

A fundamental requirement of micromanipulation is to control the impact force and subsequently the contact force in the transition of the micromanipulator end-effector from noncontact to contact state. This is especially important in protecting fragile microstructures and preventing undesirable motion. This paper proposes a method of using the integration of selective focusing and microforce control to achieve fast transition control while minimizing impact force. The method is applied to contact transition in microassembly pick-and-place operations. The initial long-range approach motion of the end-effector towards its target is controlled based on focus measures computed from images captured through a microscope. When the end-effector comes into focus near the target, the system switches to microforce control to minimize impact force and to regulate the contact force. An optics model for microscope focusing is proposed to characterize the dynamic behavior of the end-effector images during the approach motion. The connection between this model and the scale-space theory of computer vision is emphasized. Three different focus measures are tested and compared in performance. The proposed method has been experimentally verified to be able to achieve fast transition control with minimal impact force.