## 3 Development of Robot Patient's Lower Limbs
### Control Method of STS Movement  
To propose a method for controlling the STS movement of the lower limbs, the interaction between the robot's movement and the nurse's skill should be reproduced; therefore, the robot's movement must follow the external torque measured at the joint. The concept of admittance control of the compliant unit allows the dynamic relationship between the actuator velocity and the applied external forces to be shaped. This study adopted admittance control to exploit its force-sensing capability in the compliant unit and simplify the controlling mechanism. With this implementation, the inner position loop can be run rapidly, while the outer force loop is responsible for shaping the force-velocity (or force-position) relationship [52]. The STS control method was proposed as shown in Figure 7. For the postural control of the robot's lower limbs, the desired final standing position was incorporated into the planning of the motion. The joint angles (\(\theta_{hip}\), \(\theta_{knee}\), and \(\theta_{ankle}\)) were measured and incorporated into the planning of the motion as the factors for generating the motion command of the motor. In addition, the external torque (\(\tau_{external\_hip}\)) measured from the hip joint, (which is used to evaluate whether the nurse has correctly applied the skill) was adopted as the factor for computing the motor's motion command in each joint. The motion command of the motor is generated by the motion planning and sent to the profile position mode to form the trajectory. A current valve for limiting the current passing to the motor was incorporated in the position loop. The function of current limitation was implemented to reproduce the weakness of the patient's lower limbs. Motion planning in the postural control is presented in Figure 8. The objective of motion planning involves moving the joints from a sitting (\(\theta_{hip}\) = 90\({}^{\circ}\), \(\theta_{knee}\) = 90\({}^{\circ}\), \(\theta_{ankle}\) = 90\({}^{\circ}\)) to a standing position (\(\Delta\theta_{hip}\) = 0\({}^{\circ}\), \(\theta_{knee}\) = 0\({}^{\circ}\), \(\theta_{ankle}\) = 90\({}^{\circ}\)). The first step of motion planning is to measure the external force, and the program continues only if the external torque is not between the low (\(Th_{low}\)) and high thresholds (\(Th_{high}\)). Otherwise, it repeatedly reads the external torque. The required elements in motion planning include the external torque (\(\tau_{external}\)) measured at the hip joint and the absolute position of the joint angles. These values are measured and used for computing the commands that move the motor. The absolute position of each joint allows us to ensure that the STS movement proceeds as defined for the correct and incorrect applications of the skill by the nurse. The motor commands generated by the motion planning are the relative position value (\(\Delta\theta\)) and the angular velocity (\(w\)). The reading of the values and consequent generation of the motor commands is repeated until the robot reaches the final standing position. In the motion planning, \(\alpha\) and \(\beta\) were set as 1\({}^{\circ}\), representing the relative movement distance of the joint during a loop. The value of \(r\) is the ratio (1:100) of reduction gear in the hip and knee joints, and the value \(\gamma\) is the ratio of revolutions per minute (RPM) to degrees per second. Based on the interaction between the patient's movement and the nurse's skill during the STS transfer, the patient's standing speed depends on the quantity of external torque (i.e., the nurse's supporting force), which is determined by the admittance value \(Z\) (1.74 Nm - \((^{\circ}/s)^{-1}\)). Furthermore, the patient's posture moves according to the direction of the external torque. This direction is represented by either a positive or a negative value for the torque. A negative value of the external torque corresponds to a flexing of the joint, leading to a corresponding rotation (with negative angular velocity) of the joint by the motor.