# Development and Control of an Electro-Hydraulic Actuator System for an Exoskeleton Robot
## 2 Design of Axial Piston Pump  
### Analysis of Gait and Calculation of the Required Flow Rate  
Analyzing a wearer's gait cycle is very important when developing an exoskeleton robot. This is because the specifications of an actuator in the robot can be determined, and a control algorithm and the intention of the wearer can be grasped. This study aims to develop an EHA system that can be applied to an exoskeleton robot that can move with a 40 kg mass. For this purpose, specifications of the pump were determined by calculating the flow rate according to the analysis. As shown in Figure 2, the gait cycle is divided into the stance phase and swing phase. The start of the stance phase is a heel strike, and during this phase, the exoskeleton robot supports the weight. This stage occupies 60% of the entire walking cycle. After that, the swing phase starts from the moment of toe-off [21]. At swing phase, the exoskeleton robot moves with the wearer according to the wearer's intentions. This phase is the section in which the robot is under force control by interaction with the wearer. The angular velocity of the knee joint is the fastest in this phase. Therefore, the hydraulic actuator on the knee joint should work accordingly. Figure 3 shows that the angle and angular velocity of each joint plotted on the sagittal plane based on the data of \"gender differences in three-dimensional gait analysis data from 98 healthy Korean adults [22].\" This gait study included people 175 cm tall, and one cycle in the stage is 1.5 s, expressed as 0% to 100%. After 60% of the walking cycle, the swing phase starts (toe-off). The angular velocity is about 210 deg/s. When the stance phase starts (at the heel strike),  
Figure 1: The composition and flow of a paper. Figure 2: The gait cycle is divided into the stance phase and swing phase. the angular velocity is about \(-225\) deg/s. The change in the angle can be seen from 10 to 65 degrees. In order to be implemented for general people, the angular velocity of the robot knee should be about 230 deg/s. Before developing an EHA system for an exoskeleton robot, we assumed that the hydraulic actuator applied only to the robot's knee and that the robot wearer's height is 175 cm tall. The knee of the exoskeleton robot is bent and stretched by the forward and backward motion of the hydraulic cylinder. The knee joint, cylinder body connecting portion, and cylinder rod connecting portion formed two triangles, as shown by the simple drawing in Figure 4a. The lengths of the links are \(l_{1}=360\) mm, \(l_{2}=88\) mm, \(l_{3}=81.2\) mm, and \(l_{4}=56.4\) mm respectively. The angle of the knee joint is \(\phi\), and we can see that the angle varies by 65 degrees (Figure 3a). The resulting change in the cylinder rod can be represented by \(dist=\sqrt{l_{1}^{2}+l_{2}^{2}+l_{3}^{2}+l_{4}^{2}+bc\cos(\alpha_{1}-\phi)}\). We were able to obtain the change of the actual length in the cylinder, and the maximum rod length of the cylinder was about 115 mm. Based on this, the cylinder with the 120 mm stroke length was applied (Figure 4b). As a result, it could be driven to the angle of the knee joint for the wearer. Figure 4: (**a**) This is the knee structure of the exoskeleton robot and its simplification. (**b**) This is the change in length of the cylinder and the speed. Figure 3: The angle and angular velocity of each joint in the sagittal plane according to the gait cycle: (**a**) is the angle, and (**b**) is the angular velocity. The displacement volume of an axial piston pump differs depending on the stroke length of the piston and the number of pistons.