Robust Controller Design for CNC Servo Motors Against the Variation of External Loading via the H-infinity/QFT Approach

In industrial applications servo motors are applied to different loads and varied control performance is thus unavoidable. Under such circumstances, engineers usually re-design or re-tune controller parameters to meet certain design specifications without reliable and consistent control performance or motion precision. In robust controller design, the quantitative feedback theory (QFT) is adopted by applying both the loop-shaping design and the feedforward controller in its feedback loop with a suitable order determined from experience. On the other hand, H-infinity controller is designed in frequency domain but is directly calculated in time domain with the controller type without restriction in selecting its order. However, H-infinity controller only deals with sensitivity without meeting control performance as in QFT design. In this study, the combined approach by using both the H-infinity algorithm and the QFT controller with better robustness is applied to the servo motor control design; in addition, the QFT feedforward controller in loop-shaping is included to lead to desirable control performance. Experimental results show that by applying the present H-infinity/QFT approach, the similar system responses under different external loading on the CNC servo motor can be successfully obtained to prove the feasibility of the present method in industrial automation.