# Assist-as-Needed Controller of a Rehabilitation Exoskeleton for Upper-Limb Natural Movements
## 3 Experimental Result
### Evaluation of AAN Controller  
To verify the effectiveness of the proposed controllers, we let the volunteer wear FREE for two sets of tasks. As shown in Figure 1, the volunteer's upper limb is connected to the FREE through the upper arm attachment module and the end-effector grip. The experimental tasks include head touching and cleaning desk. During the head touching task, the volunteer is instructed to move his right hand from a specified starting point to head. In the cleaning task, two reference points are placed in front of the volunteer. He was required to control the hand to move reciprocally between these two points, while ensuring the trajectory remains as close as possible to the horizontal plane (xy-plane). In the two sets of experiments, we use the ZMC and AAN controllers to control joint torques of FREE, respectively. The trajectory of the volunteer's right hand in two sets of tasks is shown in Figure 8. The trajectory is presented by collecting markers placed on the wrist of the volunteer using an optical motion capture system. As shown in Figure 8, after receiving the instruction to perform an action, the trajectories of the actions completed by both the volunteer and FREE under different controllers are almost identical. ZMC serves as the baseline controller for AAN. The objective of this section is to verify that the AAN can adjust the assistive torque in the intended direction based on the ZMC by using two different controllers to assist users in performing similar motions. The data of the two F/T sensors at upper arm and forearm were converted into the base coordinate system. This can intuitively reactor the physical interaction between the volunteer and FREE. \(F_{x}\), \(F_{y}\), \(F_{z}\), \(T_{x}\), \(T_{y}\), and \(T_{z}\) of each interactive point in the two groups are recorded for quantitative HRI force and torque. Figure 9 shows the complete situation of HRI force/torque in each set of experiments. Table 3 shows the average absolute value of interaction force/torque in each group of experiments. Figure 7: Comparison of three friction models. The tested joint was J2. Figure 8: Trajectories of the volunteers’ hands in the Cartesian coordinate system in both experimental groups. (**a**) head touching. (**b**) cleaning. The orange solid line represents hand trajectory generated by ZMC controller, the blue dashed line represents hand trajectory generated by the AAN controller. Figure 9: Human–robot interaction force/torque at the two connection points of the upper limb in the world coordinate system during the head-touching and cleaning tasks. The used controller is indicated at the bottom of each column. The first row represents the upper limb interaction force/torque, and the second row represents the wrist interaction force/torque. (**a–d**) represents upper limb sensor data during two task under two controller. (**e–h**) represents upper limb sensor data during two task under two controller. Figures 9a,c,e,g display the active force/torque exerted by the user to drive the robot's movement under the ZMC controller. In both task groups, the absolute values of the active forces exerted by the volunteer are all less than 8 N, and the torques are less than 5 Nm. The maximum upper limb force in the x-direction is \(-\)7.76 N (Figure 9a), and the maximum upper limb torque in the y-direction is 4.58 Nm (Figure 9a). The maximum wrist force in the y-direction is \(-\)7.71 N (Figure 9g), which corresponds to the characteristic motion of the hand primarily along the y-direction during the cleaning task. The maximum wrist torque in the x-direction is 1.11 N:m (Figure 9a). Table 3 presents the average absolute values of the interaction force/torque. The average value of the interaction force is less than 4 N, and the average torque value is less than 2 Nm. These data validate that under the ZMC controller, the user can have the FREE device follow their movement at a low interaction level. Figures 9b,d,f,h show the interaction force/torque between the volunteer and FREE under the AAN controller.