Z. Jamaludin, Disturbance compensation for machine tools with linear motor drives, 2008

Abstract

Machining processes have evolved significantly over time in order to adapt to the increasing demand for speed, accuracy, and efficiency. This evolution or paradigm shift has created new and highly critical challenges. This thesis aims at addressing some of these issues, namely the compensation of the effect of friction and cutting forces on the accuracy of the machining process.

Issues regarding friction effects in machining process have been studied extensively in the past and various techniques and ideas have been proposed and validated. Simple linear feedback control techniques such as PI, PID, or cascade control alone are insufficient to compensate the nonlinear friction behaviour. In this thesis compensating elements are proposed, developed and validated that can be added to existing feedback controllers in order to improve accuracy. Friction-model-based and friction-model-free approaches are considered to supplement the cascade P/PI position controller. The compensation performance is measured based on the magnitude of the “quadrant glitch” – a product of highly nonlinear complex friction behaviour near zero velocity or motion reversal identified by the appearance of “spikes” at each quadrant of a circle. In this thesis, the recently developed Generalized Maxwell-slip (GMS) friction model is used as a feedforward element in combination with the well-known and widely applied inversemodel- based disturbance observer for friction compensation on a linear drive based xy feed table of a high-speed milling machine. This combined approach almost completely compensates all friction effect.

Besides friction forces, the effect of cutting forces on machining accuracy is significant. Several techniques described in literature are studied and their applicability to compensate cutting forces in machining process is evaluated. First, the application of the inverse-model-based disturbance observer is further extended to cutting force compensation. However, its performance is critically influenced by the limited bandwidth of a low pass filter often referred to as the Q-filter that is necessary to preserve the system stability. Second, cutting forces are estimated from the force balance acting on the drive using a Ferraris relative acceleration sensor measurement. The bandwidth is again restricted by a stability preserving low-pass filter, similar as for the inverse-model based disturbance observer.

Finally, a method that is renown for its excellent compensation of periodic disturbance signals is applied, namely, the repetitive controller (RC). A repetitive controller is developed for the considered linear drive based xy feed table.

To validate the performance of this RC, an actual cutting process is performed on the test setup. It is shown that the developed RC is able to compensate almost completely the tracking errors introduced by the cutting forces. The repetitive controller, when combined with the previous friction compensation elements such as the GMS friction model feedfoward and the disturbance observer, almost completely removed the cutting forces during an actual cutting process.

This thesis has successfully demonstrated that the tracking performance of a machine tool can be increased significantly by adding dedicated compensation elements to the simple and widely used cascade P/PI position controller. However, further studies are desired to include adaptive measures in both friction and cutting forces compensation using the advanced GMS friction model and the RC. This will ensure a robust friction compensation approach to changing friction behaviour and characteristics over time due to the influence of lubrication, heating and etc. An adaptive RC will compensate against changes in the cutting conditions, for example, changes in the spindle speed, tools diameter, tracking speed, and etc.

Order Code

Code: 08D09