# MFC/IMC Control Algorithm for Reduction of Load Torque Disturbance in PMSM Servo Drive Systems
## 1 Introduction  
Servo drive systems are commonly exploited in many areas of the modern industry where precise control over position, velocity or torque for servo motors is required. CNC machines, robotics, factory automation are just some of the fields that have been heavily influenced in the past years by the development of servomechanisms and their control algorithms.  
Permanent magnet synchronous motors (PMSM) gained broad recognition in servo drive applications due to a range of beneficial properties: Compact structure, high air-gap flux density, high power density, high torque to inertia ratio, and high efficiency. However, due to the existence of nonlinearities, structured and unstructured uncertainties, and external disturbances, there is a necessity for improvement of PMSM control schemes [1; 2; 3].  
One of the external disturbances encountered by the control system is the torque load disturbance which is closely tied to the task the PMSM is performing. The characteristics of this disturbance can be considered as either fluctuant or largely constant depending on the nature of the process and its phase. An example of such varied behavior is exhibited on a milling machine with ball-screw driven table--freely positioning the workpiece as opposed to movement while processing it (i.e., material's contact with a tool is maintained) [4]. In each case, however, it has an adverse effect on achieving the desired motion profile. Another example of a system in which torque-related external disturbances play a substantial role is a drum shearer and its transmission system. It affects the efficiency of thecutting unit and is caused by a non-homogenous coal seam with hard parcels and rocks. Dynamic load suppression, in this case, is desired to improve mining performance and to minimalize wearing of the mining shearer elements [5].  
In the literature, one can find various PMSM control techniques introduced in order to increase the robustness of PMSM servo drive control. In [6] authors introduced a cascade model predictive control (MPC) structure for a high-performance speed control of PMSM with speed ripple minimization. The inner-loop MPC provides fast feedback control action aimed at reducing the effects of disturbances, nonlinearities and model parameter uncertainty. The outer-loop MPC is embedded with a zero frequency mode for start-up and an extra frequency mode for minimizing the speed ripples in steady-state operation. In [7] authors proposed a fuzzy sliding mode controller (SMC) with a third-order load torque observer for a robust speed tracking of PMSM. The proposed observer-based fuzzy SMC method took into account the disturbance inputs representing the system nonlinearity or the unmodeled uncertainty to guarantee the robustness under motor parameter and load torque variations.  
This article discusses a method of reducing the effects of load torque disturbances in PMSM servo drive systems by using the MFC/IMC control algorithm; the algorithm itself is described in [8]. The theoretical analysis, simulations and real-world experiments indicate a greater reduction of external disturbance influence when exploiting the MFC/IMC structure as compared to commonly utilized cascade control structure containing PI controllers.  
This paper is organized as follows. In Section 2, a mathematical model of surface-mounted PMSM is introduced. Section 3 describes the linearization process of PMSM necessary to obtain a linear model for the MFC/IMC algorithm. Section 4 shows numerical analysis whereas Section 5 presents the experimental results of the MFC/IMC implementation on an actual test stand. Section 6 concludes the paper by summarizing the results and presents potential areas of application of the algorithm. Also, the advantages and disadvantages of the MFC/IMC control scheme are given along with authors' new ideas for further development of the method.