# A Novel Robust Control System Design and Its Application to Servo Motor Drive
## 1 Introduction  
Due to the simple structure and easy adjustment of control parameters, control systems designed using a PI/PD/PID controller in cascade are usually adopted to compensate for variations in reference input and external disturbances. Many turning methods, including online and offline strategies, have been developed to adjust the control parameters manually or automatically. Over the years, many commercial controllers have been designed and produced based on digital signal processing (DSP) technology, and they are functionally diverse but cheap. Nevertheless, as usual, the cascade mode is still prevalent, and the controller style is mainly still in a commercial controller's PI/PD/PID functions. Many modern control strategies are sometimes applied to commercial controller design due to the necessity of state feedback, and most of the systems are different from each other, which causes some modern controllers to be in the sense of a case study. For this purpose, we wanted to design an easy application control structure for the control system design such as the well-known commercial controllers, and the designed system is robust to the reference change and added disturbance. State feedback control has the characteristics of an easily understood and implemented structure. The system controlled by the state feedback has the properties of arbitrarily pole assignment, while the system is controllable. On the other hand, a state observer can be designed to estimate the system states and put them back to the controller if some states are not directly accessible. However, it is weak in facing external load or reference input variation. For the control end, zero steady-state error is desired for most control objects. Integral action or disturbance estimation in state feedback system designis used to overcome these drawbacks. Integral action has the property of the cascade PI control structure. Once the system's states (usually the error of the state) cannot be forced to the desired state (usually the origin) by way of the controller, steady-state errors exist. An integral control that will integrate the errors and increase/decrease the control output to force the system states back to the desired state is designed to eliminate the errors. If the resulting system is stabilizable, the system will finally reach and stop at the desired state. State feedback control with/without integral action is widely applied to motor drive systems and power converter design. Example studies include a DC motor [1; 2; 3; 4; 5; 6], a permanent magnet synchronous motor [7; 8; 9], a stepper motor [10; 11], a BLDC [12], a robot [13; 14; 15; 16], a power converter [17; 18; 19; 20; 21; 22], a magnetic levitation system [23], an unmanned surface vehicle [24; 25], landing control [13; 26], a UAV vehicle [27], and a two-tank system [28]. The system's parameters determine the default system performance using the state feedback control structure. The poles of the closed-loop system vary due to the variations in system parameters. The effects mentioned above may not cause severe deterioration to reduce steady-state performance. However, some system considerations are specified, such as minimum overshoot, rising time, settling time, or delay time. For those conditions, the system's performance may not fit the requirements. Thus, to make the system performance invariant is essential. Sliding mode control (SMC) or variable structure control (VSC) is one of the solutions to make a system invariant to the parameter variations. A system controlled by SMC with invariant property considerations can be found in [11; 15; 29]. There are two steps for the SMC controller design. First, one must determine the system performance according to the transient specifications, such as the rising time, settling time, delay time, and maximum overshoot. Some situations regarding reaching transient requirements are placing the system poles at specified locations to reach the desired performance.