Modelling, identification and control of thermal deformation of machine tool structures, part 5: experimental verification

摘要

Machining accuracy is more often governed by thermal deformation of the machine structure than by static stiffness and dynamic rigidity. Since thermally-induced en cannot completely be eliminated at the design stage, the use of control andcompensation systems is an inevitable course of action. Existing control systems are based on different approaches; the use of empirical compensation function, and on-line execution of numerical simulation models. To overcome the limitations of thesemethods, a new control system has recently been proposed by the authors. This system, which is based the concept of generalized modelling, incorporates a realtime inverse heat conduction problem IHCP solver to estimate the transient thermal load appliedto the structure. V this information, the relative thermal deformation between the tool and the workpiece estimated and used as a feedback control signal. In previous parts of this series, computer simulation test cases were carried out to examine thedynamic response, accuracy stability of the system.In the present study, the performance of various components of the control system specifically, the IHCP solver, the thermal deformation estimator, and the feedback controller are verified experimentally using a three-component structure. The resultsshowed that the derived generalized thermoelastic transfer functions and algorithms indeed quite accurate in predicting and controlling the transient thermoelastic response behaviour of a predominantly linear structure. The results showed that the IHCP so is inherently stable even when the temperature measurements are contaminated with random errors. The excellent computational efficiency of the integrated system is shown to be well suited for real-time control applications involving multi-dimensionalstructures, achieving a control cycle of less than 0.5 second. The experimental results showed that in real structures higher modes can be present, and therefore, a fourth order deformation model should be used to improve the prediction accuracy. Theproposed PID con system, with feedforward branches, was capable of reducing thermal deformations of order of 200μm to levels below ±8μm. These results also demonstrated the effectiveness of artificial heat sources as a control actuation mechanism, inspite of their inherent limitations, namely, thermal inertia, coupledness, and unidirectionality.