Magnetic Levitation Servo for Flexible Assembly Automation

Abstract

Active magnetic levitation with servo control has the inherent potential to be a friction-free, backlash-free, lubrication-free, and precision multi-degree-of-freedom (DOF) actuator with the capability of multi-axis force sensing. However, magnetic levitation has been conventionally used only for contactless suspension such as magnetic bearings and magnetically levi tated vehicles. This article develops an application of magnetic levitation for actuation and force sensing and calls such a sys tem magnetic levitation servo to distinguish from conventional suspension-only systems. On the other hand, flexible assembly automation is a techni cal and engineering challenge to be accomplished promptly. For this purpose, the authors have developed a Magneti cally end-effector-levitating Instrument for a Skilled-Task- undertaking Expert Robot (MEISTER) by applying magnetic levitation servo. The MEISTER is located between a wrist and an end effec tor in a manipulator to levitate and control the end effector by magnetic forces without any mechanical contact. This friction- free and backlash-free mechanism enables precision control of multi-DOFs and permits the MEISTER to have three versatilely functional properties: (1) programmable compliance: the com pliance and viscous damping of supporting an end effector are magnetically changeable by software; (2) precision multi-DOF actuation: the position and attitude of an end effector can be accurately controlled without friction or backlash by chang ing magnetic forces; and (3) multi-axis force sensing: external forces and moments acting on an end effector can be computed by the MEISTER without any additional sensory equipments from the force equivalent equation. By using these functional properties, the MEISTER can correct a minute misalignment between a couple of mating parts to automate precision inser tion. To investigate the fundamental performance of the MEISTER in experiments, a prototype of a 5-DOF controllable MEISTER is developed. The experiments have shown the following re sults : 1. The MEISTER can be used as an advanced/active Remote Center Compliance (RCC) device. 2. The prototype can quickly position the 5-DOF of a mat ing part with 1-μm resolution. 3. It can measure a five-axis resultant force acting on a part with a resolution of about 0.04N and 0.001 N m. 4. It can successfully perform the RCC assembly method, the ideal RCC method, and the float method in precision insertion operations with the clearance ratio of 0.0004 (where a workpiece has a 20-mm diameter and the clear ance is 8 μm.)