Total control of fast coordinate measuring machines

Abstract

The design of CMM has undergone a radical change with the aim of improving speed and measurement accuracy. This is achieved by substituting the analogue component with high speed digital processors and the mechanical components by high power electronic devices. The control system is, however, hardly changed. Requirements of speed and accuracy should be simultaneously satisfied. To ensure the total control, four interacting feedback loops, (current, velocity, position and vibration loops) for each arm, and one feedforward loop, (profile loop) need to be designed, implemented and tuned. This gives a total of 15 control loops for the three arms. Unlike the conventional control system where the controlled variable is available for feedback, the probe position is difficult and expensive to measure. The paper develops an optimal control such that the speed and the measurement accuracy are maximised while the tuning and commissioning time are minimised. The proposed control design scheme uses a hierarchical approach to decompose the system into smaller subsystems. The current and velocity loops are firstly optimised. It is then shown that the MIMO control problem can be decoupled into three SIMO control problems. A two DOF optimal controller is developed to control probe position and arm vibration. The feedforward controller is automatically generated by introducing the model of the profile. Simulation and experimental results are given.