A robot suit with hardware-based safety devices: Frequency response analysis of a velocity-based safety device

Safety is one of the most important issues in rehabilitation robot suits. We have designed a rehabilitation robot suit equipped with two mechanical safety devices called the 'velocity-based safety device' and the 'torque-based safety device'. The robot suit assists a patient's knee joint. The safety devices work even when the robot suit's computer breaks down, because they consist of only passive mechanical components without actuators, controllers, or batteries. The torque-based safety device stops the robot suit if it detects an unexpected high joint torque. Similarly, the velocity-based safety device stops the robot suit if it detects an unexpected high joint angular velocity. However, the mechanism for detecting the unexpected angular velocity in the velocity-based safety device may resonate with a walking cycle, because the mechanism has a bar, a rotary damper, and two tension springs, that is, the mechanism is a mass-spring-damper system. In this paper, we firstly review the robot suit equipped with the safety devices. Secondly, we analyze the mechanism for detecting the unexpected angular velocity and examine whether the mechanism resonates with a walking cycle. Finally, we present experimental results to verify the effectiveness of the velocity-based safety device.