Robust Controller Design for Compensation of Disturbance in Machine Tools

Kong, Mun Chun (2024) Robust Controller Design for Compensation of Disturbance in Machine Tools. Final Year Project (Bachelor), Tunku Abdul Rahman University of Management and Technology.

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Abstract

Milling machines are one of the most important machinery applied in manufacturing industries worldwide. The milling operations such as end milling, face milling and thread milling are the common machining operations for most of the materials in the manufacturing industry nowadays. Due to the ever evolving technologies and consumer demands, a high level of accuracy and precision is crucial in machining operations. However, the typical control system of the milling machines were unable to handle the undesirable nonlinear disturbances that might result in manufacturing poor precision and accuracy for the end product during the milling process. The undesirable nonlinear disturbances including friction force, cutting force or vibration that exhibit nonlinear phenomenon during machining will affect the precision and accuracy of the direct linear drive system in the milling machine. The poor performance of the control system will then result in a low quality of product such as poor surface roughness. In order to compensate for these disturbances, the suitable controller needs to be selected and applied to the control system. In this thesis, a direct drive single axis positioning system using linear motor is the examined drive system and the sinusoidal signal with the signal frequency of 0.5 Hz and 1.0 Hz is expected to be utilized to stimulate the system. Besides that, only cutting force is considered as the disturbance produced by linear cutting and the cutting forces with spindle speed of 6000 RPM, 7000 RPM and 8000 RPM are identified as the disturbance forces. The cutting force is collected using a dynamometer and then converted from time domain to the frequency domain by using Fast Fourier Transform (FFT) technique for analysis purposes. In order to compensate for the cutting force, the proposed nonlinear advanced robust controllers are Proportional-Integral-Derivative (PID) controller, Sliding Mode Controller (SMC) and Super Twisting Sliding Mode Controller (STSMC). The PID controller is the benchmark to be contrasted by SMC and ST-SMC. Apart from that, the performances of proposed controllers in compensating the cutting forces are compared in terms of maximum tracking error, improvement in tracking error reduction, chattering suppression as well as disturbance rejection efficiency. Furthermore, the stability of proposed controllers is validated through three strategies which are Bode diagram of loop system, Nyquist plot of openloop system and Lyapunov Stability Criterion. The research is fully conducted in the MATLAB Simulink environment. In conclusion, the ST-SMC is superior to the PID controller and SMC in terms of maximum tracking error (99.83%), tracking error reduction (99.89%), disturbance rejection (51.49%) and chattering suppression.