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This paper presents the theoretical analysis and experimental investigation of actively controlled manipulation of a magnetic microbead using quadrupole magnetic tweezers. Bead dynamics, magnetic actuation, and visual measurement are analyzed. A feedback control law is developed and implemented to stabilize and steer the motion of the magnetic microbead. It is developed in two steps. First, an inverse model, which is associated with a lumped-parameter analytical force model, is derived to enable feedback linearization. Second, linear controllers are designed to achieve motion stabilization and manipulation of the magnetic microbead. A proportional-gain controller along with feedback linearization is implemented to establish a stable trapping of the magnetic bead to facilitate system calibration. Experiments are then performed to validate the derived inverse force model and theoretical analysis. In addition, a minimum-variance controller is designed and employed to reduce the variance of the bead's Brownian motion. The control performance in terms of variance reduction, nanostepping, and large-range steering is then experimentally demonstrated.
Date of Publication: June 2010