Mechanical Design and Control of 3-DOF Active Scanning Probe Using Parallel Link Mechanism

Abstract

A 3-DOF active scanning probe using a Parallel Link Mechanism (PLM) is proposed. It employs a small touching force control and can be used for small position sensing devices which works within sizes of few millimeters. The device is attached to the tip of coordinate measuring machines or numerical-control machine tools and used as a scanning probe. In this paper, the characteristics of the mechanism, optimum design, and a control system of the PLM are introduced. The kinematics and differential relations of the PLM are derived. An optimum design for the PLM is proposed as that in which the mechanism’s manipulability is equal in all directions. This implies that the force and position sensitivities of the PLM share an isotropic relationship. A prototype of the PLM is developed. The PLM is constructed on three voice coil motors that are vertically aligned on a base plate, and a sensing stylus is fixed on an end plate. Universal joints with ball bearings are developed for reducing the joint resistance. Position and force control methods for the scanning probe are introduced. For stable scanning motion, a disturbance observer-based mechanical impedance control is formulated. An experimental system for measuring the position of the probe using image processing is developed. It is found that the standard deviation of the positioning error along the z direction is less than 3 µm, whereas those along the x and y directions is larger because of the tilt of the end plate caused by alignment errors of the PLM’s mechanical parts.