# Development and Control of an Electro-Hydraulic Actuator System for an Exoskeleton Robot
## 2 Design of Axial Piston Pump
### Design of the Piston Pump  
The EHA system for an exoskeleton robot is composed of a motor, a hydraulic pump, a hydraulic cylinder, an oil tank, and a hydraulic circuit (hydraulic lines, hydraulic valves, etc.). Depending on the performance of the hydraulic pump, the operation of the robot is affected seriously. In this study, the hydraulic pump is a bidirectional swash plate piston pump, and it is designed based on the required flow rate according to the wearer's gait cycle. Piston pump has excellent characteristics in the high-pressure region and is suitable for high-speed rotation. This is widely used in hydraulic systems that can vary the output capacity and produce high power [23]. Piston pumps are also divided into swash plate type axial pumps and bent axis type axial pumps, according to the field of use. Therefore, it is applied in various fields and researched and developed [24; 25]. Such a piston pump can overcome the limitations of the gear pump. However, piston pump is complex and has many components. First of all, we developed a piston pump to compensate for the disadvantage of the existing piston pump and to apply it to the exoskeleton robot and the EHA system. The pump was compacted by integrating the pump housing and valve plate, and was simplified by integrating the cylinder block, spherical washer, and spline connection. The structure of the piston pump is shown in Figure 5, and the diameter and length are 56 mm and 81 mm, respectively. The hydraulic pump mainly consists of a drive shaft, a valve plate, a cylinder block, pistons, piston shoes, and a swash plate. The swash plate is at a 12 degree fixed angle. The nine holes in the cylinder block are formed at angles of 40 degrees. Through Equations (1) and (2), the stroke distance of the piston was designed to be 6.38 mm, and the displacement volume was about 1.5 cc. The suction and discharge of the hydraulic pump change according to the rotation direction of the motor. First, when the motor rotates the drive shaft of the pump, the cylinder block connected \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**z** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** \\ \hline \(\varepsilon\) [\%] & 4.98 & 14.0 & 2.53 & 7.81 & 1.53 & 4.97 & 1.02 & 3.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pulsation rate according to the number of pistons. to the drive shaft rotates. At this time, the piston shoe connected to the piston and ball socket joint moves along the surface of the cam, and the piston moves up and down in the cylinder block. As a result, the hydraulic pressure is produced, and then the pump starts sucking and discharging the hydraulic fluids. Because the valve plate and the cylinder block rotate in surface contact, they are made of different materials in consideration of friction. The cylinder block is made of brass and the valve plate is made of iron. Furthermore, when the piston pushes the hydraulic fluids, the oil film is formed between the cylinder block and the valve plate, so that the cylinder block gently rotates. For the same reason, the piston was made of brass, and the hole was made in the piston and shoe to produce an oil film. The valve plate and the cylinder block contact each other, and the pistons with shoes and the cam are shown in Figure 6b (the inside of the proposed pump). When the exoskeleton robot supports a mass of 40 kg, the internal pressure of the cylinder becomes 12.73 kgf/cm\({}^{2}\). At this time, the force F on one piston can be obtained as follows:  
\[F=PA\tan\alpha+\mu PA=(\mu+\tan\alpha)PA. \tag{9}\]  
The piston area A is 0.283 cm\({}^{2}\), and the cam angle \(\alpha\) is 12 degrees.