# Development and Control of an Electro-Hydraulic Actuator System for an Exoskeleton Robot
## 3 Design of the Hydraulic Circuit  
A hydraulic system considering a hydraulic exoskeleton robot mostly uses a unidirectional pump. This system needs a relief valve, a check valve, a pilot check valve, and a hydraulic cylinder. Because the pump is unidirectional, it needs an additional servo valve for the cylinder to change direction. So, the operation of the exoskeleton robots using the hydraulic system is divided into the active mode and the passive mode. In other words, because it is a semi-active hydraulic system, the pump rotates unidirectionally and the cylinder advances to lift the weight. However, in order for the cylinder to move backward, the flow control valve is opened to move the cylinder backward by the force of the wearer [12]. In other words, when the robot performs the sit-to-stand operation, the robot increases the strength of the wearer via the actuator. Conversely, for the stand-to-sit operation in swing phase, the actuator moves manually. The cylinder moves backward through the directional control valve while the motor continues to operate [13]. In this case, the system becomes complicated, the efficiency becomes low, and it has problems with noise, vibration, and leakage. In order to overcome these drawbacks, we developed a bidirectional pump that is applicable to the hydraulic exoskeleton robot, and we also designed a hydraulic circuit.  
Figure 7a shows the designed hydraulic circuit. The circuit consists of the bidirectional pump, two pilot-operated check valves, four check valves, two relief valves, and the oil tank. \(Q_{1}\) and \(Q_{2}\) are the actual flow rates flowing in and out of the cylinder. \(Q_{R1}\) and \(Q_{R2}\) are drained through the relief valve. For system safety, the relief valve is set so that the internal pressure does not exceed 25 bar. When a motor rotates, the pump produces pressure \(P_{1}\) and the cylinder moves forward. \(Q_{T}\) and \(Q_{P}\) are sucked through the pump from the tank, and unlike the double-rod cylinder, the cross-sectional areas \(A_{1}\) and \(A_{2}\) in the single-rod cylinder are different. So, the flow rates \(Q_{1}\) and \(Q_{2}\) are different. The pilot-operated check valve compensates for this difference in flow, allowing the cylinder to move gently [26]. The \(P_{1}\) pressure operates the pilot-operated check valve on the \(Q_{2}\) line, which results in \(Q_{2}\) being equal to \(Q_{PCV2}\) and \(Q_{PCV2}\) flowing into the tank. There are also check valves between the cylinder and the pump, so that the flow from the cylinder does not directly enter the pump. By designing the hydraulic circuit in this way, the hydraulic fluids circulate in the system but can be viewed as an open circuit. So, the mathematical modeling is simplified. Also, if the motor is not driven, the pilot-operated check valve is not opened, and the cylinder can maintain a constant force because of the check valve. Figure 7b shows the actual hydraulic circuit; the circuit was modularized with a compact size of \(130\times 100\times 70\) mm\({}^{3}\). Because of this, the hydraulic hose between the valves was eliminated. So, the circuit has been simplified, and maintenance of leaks has become easier. The oil tank was made of acrylic, and its size is \(70\times 70\times 110\) mm\({}^{3}\). The cylinder's maximum pressure was 3.5 MPa, the inner diameter was 20 mm, and the stroke length was 120 mm.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Pump Components** & **Unit** & **Value** & **Motor Parameters** & **Unit** & **Value** \\ \hline Diameter of Pump & mm & 56 & Nominal Voltage & V & 48 \\ Length of Pump & mm & 81 & No Load Speed & rpm & 5370 \\ Number of Pistons & EA & 9 & Nominal Speed & rpm & 4960 \\ Diameter of Piston & mm & 6 & Nominal Torque & mNm & 768 \\ Stroke Length & mm & 6.37 & Nominal Current & A & 9.56 \\ Volumetric Displacement & cc/rev & 1.51 & Torque Constant & mNm/A & 84.9 \\ Cam angle & degree & 12 & Speed Constant & rpm/V & 113 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The specifications of the desired pump and the motor.