Week 2: Robotic Exoskeleton

I came across this product that helps improve mobility for people who struggle to walk. This South Korean design appears to use rotary actuators at the joints to mimic the action of our knees and hips. I was curious what kinds of torques these pivot points experience, so I modeled the design as a simple three-bar linkage, focusing on the knee and ankle joints. The calculations indicate that the knee sees a maximum moment of about 225 Nm whenever the femur is close to horizontal, no matter the position of the tibia. This intuitively makes sense: a knee sees the most load when we are standing up from a chair or at the bottom of a deep squat. The ankle sees the same moment, though if we lean back or forward far enough then this may go up to 550 Nm.

Given this torque requirement at the knee, the power limitations, and considering that it must move at a speed of roughly 0.5 radians/sec to match human movement, a stepper motor seems an unlikely choice. Perhaps the designer used a linear actuator with a high mechanical advantage.