## 1 Introduction  
In some high-latitude regions, especially in North America (United States and Canada), Europe (Russia, Belarus and Ukraine) and Asia (China), the temperature is already below freezing point when corn comes into maturity. After corn is frozen, its kernels' mechanical properties are changed. Particularly after a snowfall, corn ears are covered with snow, as shown in Figure 1. Frost is formed on the surface of corn ears, and kernel gaps are filled with frosts. Frozen corn kernels are often damaged by low temperatures [1], which reduces corn quality and causes mildew [2]. The threshing loss rate is improved with a higher kernel splash speed, as the collision coefficient restitution of frozen kernels is increased [3]. Therefore, the mechanical harvesting of frozen corn, especially threshing devices, is also in urgent need of reducing its loss rate and damage rate. Both the loss and damage rate were directly affected by the process parameters of a longitudinal axial threshing cylinder for corn harvesters [4; 5; 6]. The drum speed (rotational speed of threshing cylinder, rpm), feed rate (weight of corn material inputting the threshing cylinder per unit time, kg/s) and concave clearance (radial distance between the outside of threshing drum and the inside of concave, mm) were considered to be the main characteristics of corn combine harvester process parameters [7; 8; 9; 10]. [PERSON] et al. [11] established a numerical model of corn threshing cylinder adopting 3-d discrete element method (DEM). The effect of the feeding rate and drum speed on the threshing rate was verified, but this method cannot detect the kernel loss rate or damage rate. [PERSON] et al. [12] found that threshing loss was significantly impacted by concave clearance applied response surface method and central composite design. The threshing loss ranged from 1.2% to 14.06%, as concave clearance ranged from 10 mm to 30 mm. [PERSON] et al. [10] discovered that corn kernel damage was related to kernel moisture content, and the threshing loss was mainly affected by drum speed. A factorial experiment was carried out under a completely randomized design. The results demonstrated that the damage rate was positively correlated with moisture content. The loss rate decreased from 4.939% to 3.242% when the drum speed decreased from 400 rpm to 200 rpm. [PERSON] et al. [13] adjusted the corn material feed rate based on the designed PID controller of corn harvester forward speed. The field experiment results showed that the lowest loss rate was 0.52%, as the feed rate ranged from 2.82 kg/s to 3.10 kg/s. However, there were few reports on how to optimize the three process parameters (i.e., drum speed, feed rate and concave clearance) together for a corn combine harvester. Additionally, the performance parameters of longitudinal axial threshing cylinders for frozen corn have not been optimized synchronously. In order to get the exact optimal model and improve the accuracy and efficiency of the optimization, new methods have been applied in other fields. NSGA-II was improved on the basis of NSGA and used for multi-objective optimization. By using the fast non-inferior hierarchical sorting mechanism, the Pareto (optimal solution) front can be approached very well. At the same time, by introducing the crowding degree operator, the dispersion uniformity of the Pareto optimal solution can be guaranteed. The experimental design, ANOVA and optimization algorithm by NSGA-II were combined to optimize the process parameters to save energy consumption during injection molding [14]. Sheet stamping process parameters were optimized by the multi-objective method [15]. Posterior multi-objective optimization by genetic algorithm was used to optimize cutting parameters in turning [16]. The optimal forming quality was obtained through the simulation experiments and multi-objective optimization for the precision forging process parameters [17]. The optimal PID tuning was obtained by various multi-objective optimization algorithms for the paper machine [18].