Machining accuracy enhancement using an iterative learning control approach

Error compensation is a cost-effective way to improve the machining accuracy on the conventional CNC machine tools. However, the traditional error compensation strategies usually involve the processes which are deemed to be too complex in practice. Due to its ability to achieve high performance with less modeling effort, the iterative learning control (ILC) is adopted in this research to progressively improve die machining accuracy in the repetitive machining process. The learning algorithm is developed based on the easily obtained system frequency response to ensure that the machining error decays monotonically as quickly as possible. An error correction framework is proposed where one or more separate correction interpolators are designed specifically to perform the error correction. It makes the implementation of the proposed error compensation scheme less intrusive to the existing CNC machine tool and is flexible to obtain errors from various sources. A synchronization algorithm defined along the desired cutting tool trajectory is developed so that the regular machining process and the correction actions can be harmonized accurately.; Experiments of repetitive machining of a circle on a precision horizontal CNC machine tool with an open architecture controller (OAC) are carried out. The results show that the root mean square (RMS) value of the machining error has been reduced by more than 85% in the first compensated machining cycle and is further reduced in subsequent cycles. The physical implementation of the proposed scheme on the machine tool needs very little modification to the software of the OAC and causes no interference with the regular machining process. It can be concluded that the machining accuracy enhancement scheme using the ILC approach is both technically sound and practically feasible.; Key words. Machining error compensation, iterative learning control, P-type iterative learning, repetitive machining process, correction interpolator, CNC machine tool.