Calibration of a class of 3 DOF serial micro robotic systems through SEM vision: application to vertical AFM tip landing

When performing manipulation at small scales, the micro-robotic system and the vision system axes often aren't aligned, leading to low accuracy. To asses this issue it is possible to manually align the manipulator axes to the ones of the vision system or to calibrate the robot coordinate system with respect to the vision coordinate system. Manual alignment is a tedious procedure that requires no computation, however its performance is directly linked to the operator's experience. Calibration on the other end aims at compensating the misalignment by adjusting control laws or actuator references. In this paper a new methodology for the calibration of a class of 3 DOF serial micro-robots inside a Scanning Electron Microscope (SEM) is presented. This methodology presents the advantage of taking few assumptions about the robot, only requiring an imaging system performing an affine projection of the scene thus being applicable to a large class of systems. Moreover the proposed methodology allow the robot to move in a straight line orthogonal to the image plane, which can be used for peg-on-hole operations. The method is first tested in simulation and then is applied on a 3 DOF micromanipulator under a SEM for AFM tip landing on selected red blood cells. Theory and practice provide accurate results even in the presence of significant misalignment between robot and vision axes. Finally, some conclusions are drawn.