Design of two lightweight, high-bandwidth torque-controlled ankle exoskeletons

Abstract

Abstract-Lower-limb exoskeletons that can comfortably apply high torques at high bandwidth can be used to probe the human neuromuscular system and assist gait. We designed and built two tethered ankle-foot exoskeletons with strong lightweight frames, comfortable three-point contact with the leg, and series elastic elements for improved torque control. Both devices have low mass (< 0.87 kg), are modular and structurally compliant in selected directions, and are instrumented to measure joint angle and torque. The exoskeletons are actuated by an offboard motor, and torque is controlled using a combination of proportional feedback, damping injection and iterative learning. We tested closed-loop torque control by commanding 50 N·m and 20 N·m linear chirps in desired torque while the exoskeletons were worn by human users, and measured bandwidths greater than 16 Hz and 21 Hz, respectively. During walking trials, we demonstrated 120 N·m peak torque and 2.0 N·m RMS torque tracking error. These performance measures compare favorably with previous devices and with human ankle musculature, and show that these exoskeletons can be used to rapidly explore a wide range of control techniques and robotic assistance paradigms as elements of versatile, high-performance testbeds. Our results also provide insights into desirable properties of lower-limb exoskeleton hardware, which we expect to inform future designs.

Muscle-tendon mechanics explain unexpected effects of exoskeleton assistance on metabolic rate during walking

“…Eight healthy, able-bodied subjects (seven men and one woman; age=25.1± 5.1 years; body mass=77.5±5.6 kg; leg length=0.89±0.03 m) wore a tethered, unilateral ankle exoskeleton, capable of providing up to 120 N m of plantarflexion torque (Witte et al, 2015), while walking on a treadmill at 1.25 m s -1 . We ran two separate parameter sweeps.…”

The control algorithm of the lower limb exoskeleton synchronous gait

“…As noted earlier many researches are devoted to the bipedal gait analysis, and the results were published in many papers [5,7,8,[20][21][22]. Both theoretical and experimental methods for locomotion study were developed.…”

“…This is due to the fact that such a movement should occur both in the sagittal and frontal planes. The kinematics of such a motion is quite complicated, so the mathematical modeling of the spatial motion of two-legged robots is presented only in several works due to considerable difficulties [1][2][3][4][5][6][7][8]21]. The mathematical description of the slow (quasi-static) gait of a human in an exoskeleton, according to our analysis, is not present anywhere.…”

“…Let us consider the scheme of the interaction of the feet with the supporting surface at different instants of time, for this we use the diagram shown in Figure 2, such a diagram in the biomedical literature is called apodogram [20,21]. A thin line indicates the the absence of the contact mode of one or another foot with a support.…”

“…That's why the locomotion modeling should be considered in respect to the spatial kinematics, since the motion takes place both in the sagittal and frontal planes. This approach to the locomotion modeling appears only in some papers [1,3,7,13,15,18,21].…”