Biomechanical engineers from TU Delft have developed a walking aid called the Achilles. The battery-powered exoskeleton adds energy to each step, but it doesn’t make walking any lighter yet.
The robot boots each contain a small 120 watt electric motor that drives a ball-screw gear behind one’s calf muscle. The lever with which it is connected to the hinge near the ankle acts like a spring and buffers the energy, just as the Achilles tendon does. When pressure sensors detect the onset of a stance, the electric engine together with the elastic lever provide a power peak that coincides with the push-off from the walker.
The mechanics of walking is a wonder of energy efficiency. When a person swings a leg forward, elastic energy is stored mostly in the Achilles tendon of the other leg. That energy is released when the standing leg’s foot pushes into the ground and the heel lifts off, propelling the body forward. To make this an efficient energy-storage it’s important that the calf muscle doesn't relax, but locks to sustain the tension over the tendon.
The Achilles robot boots deliver a peak power of 192 watt, about two thirds of the normal ankle power. More than half of the peak power comes from the spring, the rest from the electric motor. The weight of each robot has been limited to 1.5 kilogram. The walker carries a 5.2 kg backpack with batteries. The exoskeleton does contribute to the torque over the ankle, as measurements have shown, but it does not reduce the effort of walking yet.
Wietse van Dijk, who developed the Achilles walking boot together with Cor Meijneke at 3mE’s biomechanics lab, says that metabolic studies have shown that activating the robot boots did not reduce the energy expenditure of the test persons. “People do start walking differently”, he observed. “They tend to walk more on their toes as they are adapting to the system.” That might explain why walkers put in the same or more effort despite the added power from the robot boots.
Van Dijk thinks that before trying another configuration for walking exoskeletons, researchers should invest in a better model of the tendon-muscle system that propels walking. Such a mathematical model should make clear at what point additional power may be effective and energy-saving for the user. Built-in sensors on the Achilles can provide input data for such models. After all, it’s not easy to improve on a system that has been honed by evolution over millions of years.
Last year, a team from MIT, did succeed in reducing the metabolic cost of walking with a battery-powered exoskeleton. Researchers Luke Mooney and colleagues published their results in the Journal of NeuroEngineering and Rehabilitation on reaching a reduction of 5-10 % in the walking effort.
Recently, a team from Carnegie Melon University with Steven H. Collins reached a similar effect (7% less energy expenditure) with a unpowered device (batteries not included), which was basically a passive spring and hatch system in parallel with the calf muscle and the Achilles tendon.
--> Wietse van Dijk, Human-Exoskeleton Interaction, PhD thesis supervisors Prof. Herman van der Kooij and Prof. Frans van der Helm (3mE), April 21, 2015.