# Development and Control of an Electro-Hydraulic Actuator System for an Exoskeleton Robot
## 1 Introduction  
Exoskeleton robot has been developed in various fields of industrial and rehabilitation training applications. These robot systems are largely divided into muscle strengthening and muscle assisting systems, according to the purpose of use. Muscle-assisted exoskeleton robots are applied to rehabilitation training and welfare fields for paralyzed patients and elderly people to provide muscular strength. In other words, these exoskeleton robots assist people to walk without help. So, the actuator in the exoskeleton robot needs only power to support the weight of a wearer. Therefore, it can be operated and powered by most electric actuators [1; 2; 3; 4]. Muscle-strengthening exoskeleton robots are used in industrial applications to carry heavy loads that are difficult to perform using human strength. They are classified into electric and hydraulic type considering their use in the robot. In the case of the electric type, it is a structure in which a motor and a reducer are combined, and it operates the robot by the power of the motor. A large reducer with a large gear ratio is installed additionally for high power, and each joint has a complicated structure [5; 6; 7]. However, this increases the noise, volume, and weight of the robot as well as its development costs. Also, the range of motion of the joint is limited and the wearer feels uncomfortable. Because the power is small compared to the hydraulic type, it is not applicable to the muscle strengthening exoskeleton robots. On the other hand, hydraulic types can produce higher power than the electric types and are inexpensive. Hydraulic systems consist of several hydraulic parts, such as servo valves, which control the unidirectional pump and flow rate [8; 9]. However, hydraulic systems are complicated and thus difficult to maintain. Because the hydraulic pump is unidirectional, it must continue to operate, regardless of whether or not the hydraulic actuator operates. Because of this problem,noise and vibration occur. Also, it consumes energy constantly, which lowers the energy efficiency. However, this has been studied in the applications of muscle-strengthening robots. This is because hydraulic pump robots are more powerful than electric type robots [9; 10; 11]. Most muscle strengthening exoskeleton robots use conventional hydraulic systems because of the high power ratio of torque and mass, and these systems use a unidirectional pump and a control valve. Because the pump cannot change direction, when the robot strengthens a muscle during a Sit to Stand in stand phase, an actuator is driven in active mode, but in other cases of a stand to sit in the swing phase, it is operated in passive mode. That is, for the wearer to bend the knee, a controller opens solenoid valves and the wearer has to bend the knee joint of the robot. This makes the wearer feel tired and has the disadvantage of the slow response [12; 13; 14]. Such conventional hydraulic systems are complicated because of solenoid valves and have problems such as leakages. In addition, many gear pumps apply to exoskeleton robots. The gear pumps have the limitation of high-speed rotation and have the disadvantage of large noise and vibration instead of a simple structure. It is also difficult to produce high pressures because of the many internal leakages [15; 16; 17]. Recently, in order to solve the problems of the existing hydraulic systems and electric system, an electro-hydraulic actuator (EHA) which combines electric and hydraulic systems was replaced [18]. This system has more power and higher energy-efficiency than the electric type. Moreover, the system is equipped with a bidirectional pump, and the speed and position of the hydraulic actuator can be controlled through a motor attached to the pump. This EHA provides a high level of power, has the advantages of a conventional hydraulic system, and solves the disadvantages of the hydraulic and electric types. Compared with the hydraulic system, the EHA system does not need a directional control valve.