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1. Introduction
Wearable exoskeleton is an assist device which can amplify the strength and reduce human effort through the use of external power supply or human power. It has been widely investigated all over the word in the past decades (Young and Ferris, 2016) and have been used in various fields. In 2000, DARPA began to invest a lot of money in the program of exoskeleton for human performance augmentation, which attempted to enhance the capabilities of ground soldiers (Garcia et al., 2002). Researchers from Israel, Japan, Russia, et al. have developed their exoskeletons for medical usage of rehabilitation or assistance to disability individuals (Bogue, 2015). The exoskeletons used in industry area to reduce workers’ physical labor were also widely reported.
The wearable exoskeletons can be divided into active, semi-active and passive ones. Currently, most researchers having been concentrating on active or semi-active exoskeleton. However, the active joints usually lead to complex system, which have to rely on accurately controlled actuator and external power supplies. The Berkeley Lower Extremity Exoskeleton had actuated joints at the hip, knee and ankle for both lower limbs, which allowed soldiers to carry heavy loads over long distances (Kazerooni and Steger, 2006), and its major limitations were large mass, complex operation and external energy requirements. Yu et al. developed an underactuated exoskeleton with an active motor in the knee joint for walking and load-carrying assist (Yu et al., 2016); they achieved force reduction in the gastrocnemius and rectus femoris when climbing with load. However, the active knee joint made their system complicated, which decreased its reliability and practicability.
The passive exoskeleton has no active actuator to enhance people’s strength, but it can also help wearer save energy by elastic material such as springs. The advantages are obvious; it is lighter, cheaper, more durable and user-friendly. Researchers from Carnegie Mellon University designed a passive ankle exoskeleton to assist ambulation (Collins et al., 2015). Their device recycled the negative work applied by the ankle joint during walking to assist the pedaling locomotion. It can reduce energy consumption during level walking activity by 7.2 per cent. Their research was published in Nature, which revealed the fact that we could take advantage of passive exoskeleton to optimize the human locomotion...