## 1 Introduction  
Rice, one of the major grain crops in China, is mostly harvested in a mechanized manner with combine harvester in the country [1]. The thresher device is classified into two types, i.e., tangential flow type and longitudinal axial flow type based on the confugration of the threshing drum. The tangential flow threshing device adopts the configuration with single or multiple drums arranged in the horizontal direction, which features simple structure and compactness; however, limited by the space in the horizontal direction, the threshing drum should not be too long and the threshing performance is far from perfect; while the longitudinal axial flow threshing device adopts the configuration with single or multiple drums arranged in the longitudinal direction, which lenghthens the drum (the area for threshing and separation) to gain relatively good threshing performance without widening the machine body; however, the grains suffer from repeated threshing processings, which leads to relatively high damage rate and power consumption. Most of the existing large-scale combine harvesters adopt the configuration with combination of the tangential flow drum and longitudinal axial flow drum, which utilize the tangential flow drum to separate the grains easy for being threshed and then achieve re-threshing separation of the material with the longitudinal axial flow drum, thus achieving relatively good threshing performance [2]. As a key component of the combine harvester, the performance of thresher device controls the operation performance, efficiency, and adaptability of the entire machine. The threshing gap refers to the clearance formed between the threshing element of the drum and the concave grid. As per the research, threshing gap plays an important role in the operation performance of the thresher device. The actual feed rate fluctuates greatly due to the different biomechanical characteristics and growth densities based on different varieties of rice during field harvesting. As the outcome, the performance of the combine harvester in terms of threshing and separation is decreased [3; 4; 5; 6; 7]. In that case, the adjustment of threshing gap according to different operation conditions on a timely basis should be implemented to make sure that the operation of the combine harvester is stable, thus improving the adaptability of the combine harvester in terms of harvesting [8; 9]. Research in large quantities have been implemented by the researchers to investigate the method to change the threshing gap. There are two major adjustment methods, one of which is by means of changing the position of the concave grid. For example, hinging two ends of the concave grid with four mounting seats, placing the middle part of the concave grid on the cross beam, adjusting the cross beam by moving the connecting rod mechanism downwards or upwards, resulting in the consequent fall or rise of the middle part of the concave grid and increase or decrease of threshing gap; or hinge one end of the concave grid with the rack, and hinge the other end with the piston rod of the hydraulic cylinder, and achieve the adjustment of the clearance of the concave grid by means of expansion of the piston rod of the hydraulic cylinder. The adjustment mechanism for concave adjustment type is installed at the bottom or outside of the thresher device. While the combine harvester is in operation, the concave grid will be kept still and available for adjustment of the threshing gap by means of real-time changing of the concave grid position; the operation is simple and convenient [10; 11; 12]. Alternatively, the threshing drum adjustment type is available to adjust threshing gap by changing the diameter of the drum. The existing threshing drum, in most cases, has its threshing racks welded to several connecting bases which are then fixed onto the supporting wheel. Due to the fact that the drum keeps rotating during the operation of the combine harvester, the drum diameter can only be adjusted by stopping the combine harvester first.