Manipulation of Thin Objects Using Levitation Techniques, Tilt Control, and Haptics

Levitation techniques allow delicate objects, such as silicon wafers, to be manipulated without contact. In case of electrostatic and magnetic levitation, the object can be levitated by actively controlling the electric/magnetic field based on the measured air gap between the object and the levitator. Multiple degrees of freedom are controlled by several electric/magnetic actuators. For thin flat objects, the surface area is large enough for suspending the object in vertical direction, but the side surface is too small to realize active control for lateral motions. For stability, the levitation system relies on a passive lateral restoring force that will keep the object aligned with the levitator. The drawback of this passive force is that it is far weaker than the controlled suspension force. As a result, allowable accelerations in the horizontal plane have to be limited in a manipulation task to prevent loosing the object during motion. In this paper, a solution is proposed based on compensating the acceleration by Tilt Control. For both an electrostatic and a magnetic levitation system, the significant improvement of tolerance to lateral acceleration due to feedforward tilt control is presented. It is shown that not only for automated motion but also for human operated motion, the tilt control strategy is applicable. In the human operated motion, haptic technology such as the admittance control scheme, are used to realize satisfactory manipulation.