## 2 Mechanism  
### Principle of the 5-DOF Upper-limb Exoskeleton  
The structure of the human arm is complex, so the joints and segments of a human arm are usually simplified into a 7-DOF kinematic system [19], as shown in Figure 1. The shoulder joint can be modeled as a 3-DOF ball-and-socket joint: flexion/extension \((Z_{2})\), abduction/adduction \((Z_{3})\) and internal/external-rotation \((Z_{1})\); The wrist joint can be modeled as a 3-DOF ball-and-socket joint: palmar flexion/dorsiflexion \((Z_{6})\), ulnar deviation/radial deviation \((Z_{7})\) and medial rotation \((Z_{5})\); The elbow joint can be modeled as a 1-DOF hinge joint with \((Z_{4})\)[20; 21; 22]. The motion range of the joints is shown in Table 1[23]. Here, the ball-and-socket wrist joint with 3-DOF is primarily used to ensure the flexibility of the end-effector. Because the exoskeleton mainly performs lifting, pulling and pushing of a load during material handling, it does not require the end-effector to have the same DOFs as the hand. To further simplify the structure, only the wrist internal/external-rotation is retained. The elbow joint is considered a 2-DOF joint, i.e., flexion/extension and pronation/supination [24]. Based on the above analysis, a hydrodynamic 5-DOF upper-limb exoskeleton was designed, as shown in Figure 2. The 5 DOFs are as follows: shoulder flexion/extension \(\left(\theta_{2}^{5}\right)\), shoulder adduction/abduction \(\left(\theta_{1}^{5}\right)\), shoulder internal/external-rotation \(\left(\theta_{3}^{5}\right)\), elbow flexion/extension \(\left(\theta_{4}^{5}\right)\) and elbow pronation/supination \(\left(\theta_{5}^{5}\right)\). While operating the 5-DOF upper-limb exoskeleton, the operator can drive the exoskeleton through the handle, the exoskeleton upper-arm and forearm fit perfectly with the upper-arm and the forearm of the human body, and a strap between the human and exoskeleton is not required. We measured the motion range of the upper-limb of human body when performing the typical movements of material handling (raising up and lateral lifting), and after appropriately increasing and rounding to the value of the range, the motion range of the 5-DOF upper-limb exoskeleton was obtained. The motion range of the 5-DOF upper-limb exoskeleton is described in Table 2.  
The 5-DOF upper-limb exoskeleton that has similar degrees of freedom to human body is a typical anthropomorphic upper-limb exoskeleton. The power-assisted principle of the 5-DOF upper-limb exoskeleton shoulder and elbow is the same as that of the XOS2 [2; 3], the Body Extender system [4; 5; 6] and the HEXAR [1] when carrying loads. Thus, the workspace, the joint torque and the power consumption of the anthropomorphic upper-limb exoskeleton are obtained by analyzing the 5-DOF upper-limb exoskeleton. \begin{table}
\begin{tabular}{c c} \hline \hline
**Motion** & **Range** \\ \hline Shoulder flexion (Flex.)/extension (Ext.) \((Z_{2})\) & \(180^{\circ}/50^{\circ}\) \\ Shoulder adduction (Add.)/abduction (Abd.) \((Z_{3})\) & \(180^{\circ}/0^{\circ}\) \\ Shoulder internal (Int.)/external-rotation (Ext. Rot) \((Z_{1})\) & \(90^{\circ}/90^{\circ}\) \\ Elbow flexion (Flex.)/extension (Ext.) \((Z_{4})\) & \(0^{\circ}/145\) \\ Palmar flexion/dorsiflexion \((Z_{6})\) & \(90^{\circ}/70^{\circ}\) \\ Ulnar deviation/radial deviation \((Z_{7})\) & \(55^{\circ}/25^{\circ}\) \\ Wrist pronation (Pron.)/supination (Sup.) \((Z_{5})\) & \(90^{\circ}/90^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Motion range of 5-DOF upper-limb exoskeleton. Figure 1: Kinematic model of the upper-limb of the human body.