The purpose of this work is to investigate the efficiency of wearable assistive devices under different load-carried walking. We designed an experimental platform, with a lightweight ankle-assisted robot. Eight subjects were tested in three experimental conditions: free walk with load (FWL), power-off with load (POFL), and power-on with load (PONF) for different levels of force at a walking speed of 3.6 km/h. We recorded the metabolic expenditure and kinematics of the subjects under three levels of load-carried (10%, 20%, and 30% of body mass). We define the critical force, where at a certain load, the robot inputs a certain force to the human body, and with the assistance of this force, the positive effect of the robot on the human body exactly compensates for the negative effect. The critical forces from the fit of the assistive force and metabolic cost curves were 130 N, 160 N and 215 N at three different load levels. The intrinsic weight of our device increases mechanical work at the ankle as the load weight rises, with 2.08 J, 2.43 J and 2.73 J for one leg during a gait cycle. With weight bearing increasing, the ratio of the mechanical work input by the robot to the mechanical work output by the weight of the device decreases (from 0.904, to 0.717 and 0.513), verifying that the walking assistance efficiency of such devices decreases as the weight rises.
Lower limb assistive robots have a wide range of applications in medical rehabilitation, hiking, and the military. The purpose of this work is to investigate the efficiency of wearable assistive devices under different weight-bearing walking conditions. We designed an experimental platform, with a lightweight ankle-assisted robot weighing 5.2 kg and carried mainly on the back. Eight subjects were tested in three experimental conditions: free walk with load (FWL), power-off with load (POFL), and power-on with load (PONF) for different levels of force at a walking speed of 3.6 km/h. We recorded the metabolic expenditure and kinematics of the subjects under three levels of weight-bearing (equal to 10%, 20%, and 30% of body mass). The critical forces from the fit of the assistive force and metabolic depletion curves were 130 N, 160 N and 215 N at three different load levels. The intrinsic weight of our device increases mechanical work at the ankle as the load weight rises, with 2.08 J, 2.43 J, 2.73 J for one leg during a gait cycle. The ratio of the mechanical work input by the robot to the mechanical work output by the weight of the device decreases (0.904, 0.717, and 0.513 with different load carriages), verifying that the walking assistance efficiency of such devices decreases as the weight rises. In terms of mechanical work in the ankle joint, our results confirm that the efficiency of the ankle-assisted walking robot decreases as weight bearing increases, which provides important guidance for the lightweight design of portable weight-bearing walking robots.
Background The construction of lightweight robots poses one of the major challenges in the field of active robots since bearing the weight of an active robot significantly increases metabolic cost. However, few studies have achieved a substantial reduction in the robot weight. The primary reason is that the weight of the actuator, which comprises the main weight of the robot, is limited by the specific power, power requirements and assisting efficiency. Methods In this paper, we propose a new method that is utilizing the energy harvesting function of the Achilles tendon to improve the assistance efficiency of ankle robots to reduce the weight of the actuator and we design a novel ankle robot to test the validity of the method. The robot works with the ankle plantar flexor at 43%-60% of the gait cycle and has no other effects on the joints or the tendon of the lower limb. Healthy subjects were recruited to test the prototype in three conditions: free walking, power-on walking, and power-off walking. Data on the robot assisting power, metabolic cost and kinematics in different conditions were collected and analyzed. Result The results showed that the ankle robot can deliver forces at the controlled assistance timing. The average assisting power of 0.0650±0.0054 W/kg per leg resulted in an 8.7±8.1% and 19.0±6.4% net reduction in metabolic cost in power-on walking compared to free-walking and power-off walking, respectively. Conclusion Compared with the results of some of the best research, our initial result supports the validity of the method. This method can help to reduce the weight of active robots and the technical innovative method to determine the assistance timing more accurately and the novel design of the ankle robot can provide a reference for future research. To the best of our knowledge, this method is the first to use the human physiological structure to optimize the design of active robots.
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