# State of the Art: Bipedal Robots for Lower Limb Rehabilitation
## 3 Rehabilitation Application
Unlike the wearable exoskeleton robot, rehabilitation equipment is generally larger and heavier in quality. Rehabilitation equipment usually requires more sensors to obtain human motion data, and have specialized data processing equipment. Although rehabilitation robots can provide better service as well as a rehabilitation effect, they are expensive and cumbersome. So, rehabilitation robots can only exist in hospitals or large rehabilitation centers. Toyota demonstrated the rehabilitation equipment Welwalk-1000 in 2017 and will lease the rehabilitation system to various medical institutions by the end of the year [74]. The patient's thighs, knees, ankles, and feet are fixed to the equipment and then the patient walks on a treadmill. In the course of walking, the internal sensor detects the action and continually adjusts the robot. This system contributes to the recovery of patients with slow progress, and training can accelerate rehabilitation. Simultaneously, the system helps the therapist to monitor the patient's progress and better grasp the patient's condition. LOKOMAT (Figure 6a) and LokoHelp (Figure 6b) have a function similar to Welwalk-1000. Being bulky and hard to carry are their common shortcomings. * Other Emerging Exoskeleton Robots  
To minimize the burden on the human body due to the weight of the lower limb auxiliaries, exoskeleton robots with soft structures have been proposed by researchers. The advantages of the soft material include the weight and the ability to overcome the constraints of a rigid structure. The soft exoskeleton can better adapt to the deformation of the human body during walking. However, how to effectively drive a soft material and pass the force on to the human body becomes a challenge. In 2013, Harvard University developed an exoskeleton robot Exosuit [76] (Figure 7a). The robot employs a flexible design that allows the wearer to remove the constraints of the rigid material and move more naturally. In realizing the assistive walk function, the robot weighs only 7.5 kg. The video of the stroke on the treadmill proves that it can improve a person's ability to walk  
Figure 6: Rehabilitation equipment. (**a**) LOKOMAT; (**b**) LokoHelp. (reproduced with permission from [75].) When the soft exoskeleton is worn, the patients no longer need crutches, and their footsteps become faster and more confident. The unpowered exoskeleton robot is a new concept. Similar to passive walking in bipedal robots, robotic joints do not require a driver, using the body's own gravity or spring potential to assist people to walk. Although the principle of the unpowered exoskeleton robot is simple and does not require a control system, the design must consider the constraint of the degree of freedom, gait planning, and force analysis. Researchers from Carnegie Mellon University and the University of North Carolina designed an unpowered exoskeleton system, Exoboot [77] (Figure 7b). This exoskeleton system uses carbon fiber material, so it is very light at about 500 g. The use of springs to store energy reduces the energy consumed in walking by nearly 7%. Advanced exoskeleton robots have many advantages over traditional exoskeleton robots. However, the research on advanced exoskeleton robots has just started and remains a good research prospect. Table 2 summarizes and compares the characteristics of different exoskeleton robots.