On stability and passivity of haptic devices characterized by a series elastic actuation and considerable end-point mass

Series elastic actuators have considerable potential in rehabilitation robotics. However, the reflected mass of the motor and considerable robot´s end-point mass, both linked by an elastic element, result in a potentially unstable coupled mechanical oscillator. Since rehabilitation devices are in constant contact with patients, safety concerns and consequently the devices´ stability are very important. In this study, the conservative conditions that guarantee the stability of the haptic device (with a considerable end-point mass and driven by a series elastic actuator) were established. We have shown that sufficient damping should be presented in parallel to the spring in order to achieve the passivity of the haptic device. Theoretical results were confirmed in an experimental evaluation on previously developed rehabilitation device.