## 1 Introduction
In addition, [PERSON] [19] built a cascade controller with the LuGre model-based friction compensation, which can effectively control the tracking error of the pneumatic system under the premise that the system parameters were known. The static friction model, another category of friction models, describes friction as a function of relative velocity [20], and the Stribeck model is the most commonly used one. Over the years, the Stribeck friction model has made good progress in the servo system. In paper [21], the friction of the hydraulic actuator are well described by the Stribeck friction model whose friction parameters are identified by the proposed iterative algorithm, and the proposed scheme applies to industrial systems due to its low computational cost. Furthermore, [PERSON] [22] proposed a controller including the friction compensation based on the Stribeck friction model, and the controller achieved great stability and robustness in the hybrid brake-by-wire system. The displacement of a pneumatic actuator is difficult to control, which results in the difficulty of identifying the dynamic parameters in the dynamic friction model, therefore the use of the static friction model is more conducive to the identification and compensation in the pneumatic rotary actuator servo system. The proportional-integral-derivative (PID) control is an effective method which can dynamically reduce the output deviation of a servo system, where proportional-derivative (PD) control can reduce the system delay and is suitable for high-speed control of the proportional valve [23]. Besides, evolutionary algorithms, such as differential evolution (DE) algorithm and genetic algorithm (GA), can optimize the controller parameters [24; 25] and identify the friction parameters [26; 27], and have been used in friction feedforward compensation. At present, the evolutionary algorithm-based friction feedforward compensation has been applied to the direct-current servo system, and shows great performance [26; 27]. Therefore, the application of evolutionary algorithm-based friction feedforward compensation has broad prospects in the pneumatic rotary actuator servo field. In order to improve the position accuracy of the pneumatic rotary actuator servo system, the modeling of the system and the design of the evolutionary algorithm-based friction feedforward compensation control architecture are carried out and verified by the experiments in this paper. Among them, the linearized model of the system with Stribeck friction model and the construction of the friction feedforward compensation control architecture are described in detail. Evolutionary algorithms (DE and GA) are applied to the parameter optimization of the control architecture to further improve the system control accuracy. The control performances of four control strategies (the traditional PD control, the PD control with friction feedforward compensation without evolutionary algorithm tuning, the PD control with friction feedforward compensation based on the DE algorithm and the PD control with friction feedforward compensation based on the GA) are contrasted by the experiments which include the position tracking experiment, the positioning experiment and the stability experiment with load. The experimental results reveal that the evolutionary algorithm-based friction feedforward compensation greatly improves the position tracking accuracy and positioning accuracy, and that the DE-based case achieves better accuracy. Also, the system with the friction feedforward compensation still maintains high accuracy and strong stability in the case of load.