# Mechanical Structural Design and Actuation Technologies of Powered Knee Exoskeletons: A Review
## 3 Actuation Design
### Conventional Driving Actuators  
The conventional driving models of powered knee exoskeletons are mainly linear hydraulic actuators, pneumatic cylinder and electric actuators. As shown in Figure 4a, [PERSON] et al. [55] designed a unidirectional variable stiffness hydraulic actuator for load-carrying knee exoskeleton, which can mimic the efficient passive behavior of the knee joint and provide active assistance in locomotion. In order to provide mobility and independence to the elderly and the disabled, [PERSON] et al. [56] developed a prototype of power-assisted knee joint exoskeleton. Its man-machine conversation system was equipped with a hydraulic actuator specially designed to realize backdrivability and improve the discomfort caused by the errors between human and robot motion. Besides, [PERSON] et al. [57] introduced an active knee exoskeleton utilizing \begin{table}
\begin{tabular}{l l l} \hline \hline  & **Simplified Structure** & **Complex Structure** \\ \hline \multirow{4}{*}{construction of knee components} & (a) a hinge with single degree of freedom; & (a) multi-bar mechanism; \\  & (b) modular actuator with rotary motion; & (b) a pulley and several rolling cams; \\  & & (c) self-adapting joint with Schmidt \\  & & coupling, guide rail, bearing, etc.; \\ \hline \multirow{4}{*}{advantages} & (a) Easy to design and light in weight; & (a) little or no effect on kinematics; \\  & (b) kinematic analysis is very simple; & (b) adapt to the knee joint motion; \\  & (c) high accuracy of mechanical motion; & (c) suitable for highly recovery patients; \\ \hline \multirow{4}{*}{disadvantages} & (a) alignment and adjustability are difficult; & (a) require complex mechanisms; \\  & (b) usually alter normal walking kinematics; & (b) can be bulky and high inertia; \\ \cline{1-1}  & (c) may cause discomfort due to undesired & (c) kinematic analysis is difficult; \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the simplified structure and complex structure of powered knee exoskeletons. a pneumatic cylinder for the purpose of assisting the wearer while walking, as shown in Figure 4b, the experimental results show that the assistant strategy performed well in gait interference. To provide power assistance for leg swing and prevents knee buckling during stance, [PERSON] et al. [58] proposed a pneumatically operated gait orthosis which was actuated by pneumatic cylinders. Nevertheless, according to the working principle of hydraulic cylinder or pneumatic cylinder, the fluid with a certain pressure was applied on the piston rod to generate strong power output, but it required the device with a large power and had a low efficiency [59]. Therefore, these types of assistive wearing devices mainly depended on sufficient power to support the wearer with great assistance, but they had obvious defects in the overall weight and drive flexibility. The electric actuator, as another conventional driving model, has the advantage of controllability that can be used for the precise movement of exoskeletons. Therefore, the exoskeleton of a knee joint can be powered by a motor in combination with other transmission mechanisms, such as gears, ball screws, Bowden cables, and belts [60,61]. In order to provide walking assistance for people with knee joint disorders, [PERSON] et al. [62] designed an electromyography sensor-based knee exoskeleton actuated by a linear actuator, as shown in Figure 4c. In the bond graph model of fixed causality, the actuator with a linear DC motor lies in the electrical domain, whereas the gearbox, lead screw, and four-bar mechanism stay in the mechanical domain. To seek out an ideal actuator with a high torque, a high efficiency, and a small size for the knee exoskeleton, [PERSON] and [PERSON] [63] developed a new type of actuator with direct current (DC) motors and batteries, which could provide continuously varying torque through two belts alternately deflected by cams. [PERSON] et al.