| Summary: | Walking on different terrains leads to different biomechanics, which motivates the development of exoskeletons for assisting on walking according to the type of a terrain. The design of a lightweight soft exoskeleton that simultaneously assists multiple joints in the lower limb is presented in this paper. It is used to assist both hip and knee joints in a single system, the assistance force is directly applied to the hip joint flexion and the knee joint extension, while indirectly to the hip extension also. Based on the biological torque of human walking at three different slopes, a novel strategy is developed to improve the performance of assistance. A parameter optimal iterative learning control (POILC) method is introduced to reduce the error generated due to the difference between the wearing position and the biological features of the different wearers. In order to obtain the metabolic rate, three subjects walked on a treadmill, for 10 min on each terrain, at a speed of 4 km/h under both conditions of wearing and not wearing the soft exoskeleton. Results showed that the metabolic rate was decreased with the increasing slope of the terrain. The reductions in the net metabolic rate in the experiments on the downhill, flat ground, and uphill were, respectively, <inline-formula><math display="inline"><semantics><mrow><mn>9.86</mn><mo>%</mo></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>12.48</mn><mo>%</mo></mrow></semantics></math></inline-formula>, and <inline-formula><math display="inline"><semantics><mrow><mn>22.08</mn><mo>%</mo></mrow></semantics></math></inline-formula> compared to the condition of not wearing the soft exoskeleton, where their corresponding absolute values were <inline-formula><math display="inline"><semantics><mrow><mn>0.28</mn></mrow></semantics></math></inline-formula> W/kg, <inline-formula><math display="inline"><semantics><mrow><mn>0.72</mn></mrow></semantics></math></inline-formula> W/kg, and <inline-formula><math display="inline"><semantics><mrow><mn>1.60</mn></mrow></semantics></math></inline-formula> W/kg.
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