Exoskeletons and Wearable Robots

An exoskeleton is a robot worn on the body. Lower-body exoskeletons can help people walk farther with less effort, or help people with weak muscles walk at all. The key challenge is timing: the motor must push in the same direction the person's muscles are already pushing, at the same moment. Help at the wrong time makes walking harder.

Assist peak timing (% of the gait cycle) Assist strength Assist duration

Assistive power

The ankle does most of its positive work in push-off, at about 50 to 60 percent of the gait cycle, measured from heel strike. An exoskeleton adds torque $\tau_{\text{exo}}(t)$. It helps when its power has the same sign as the ankle's motion:

$$P_{\text{exo}}(t) = \tau_{\text{exo}}(t)\,\dot\theta_{\text{ankle}}(t), \qquad \text{net help} = \int_{\text{cycle}} P_{\text{exo}}\,dt$$

What to notice

Line the assist peak up with push-off, near 52 percent, and almost all of the assist is positive work that helps the person forward, shown in green. Shift it earlier or later and part of it pushes against the motion, shown in red, which actually makes walking harder. Research exoskeletons tune this timing for each person, sometimes automatically with human-in-the-loop optimization, which measures the wearer's breathing to find the best settings.