Precision tracking control of a horizontal arm coordinate measuring machine

Abstract

One of the major sources that affects measurement accuracy and limits the use of high motion speeds in coordinate measuring machines (CMM) is position errors caused by structural vibrations. This research aims to model the dynamics of a horizontal arm CMM by considering drive flexibility at joints and evaluates the characteristics of the system for fine motion control purposes. Design of a precision tracking controller (PTC) to perform superior tracking for enhancing the measurement accuracy and the probing speed in providing less inspection time at high motion speeds on the manufacturing cycle is carried out. A dynamic model for the CMM is developed including drive flexibilities represented with lumped springs at the joints. Due to the non-collocated nature of the control scheme in the flexible CMM dynamics, a non-minimum phase system is observed in the proposed CMM model. Using the derived CMM model with joint flexibilities, tracking motion control simulations are conducted at different probing speeds for the cases of a PI controller and a feedback PTC employed. The comparison of the PI controller with the feedback PTC is also performed. Results demonstrate that the effects of joint flexibilities on the contour error and probing speeds are significant and the PI controller is not capable of providing good accuracy during challenging tasks such as corner tracking. However, the simulation results indicated that by using the proposed feedback precision tracking controller, contour errors in corner tracking that are caused by joint flexibilities can be reduced effectively.