A Series Elastic- and Bowden-Cable-Based Actuation System for Use as Torque Actuator in Exoskeleton-Type Robots.

The International Journal of Robotics Research (Impact Factor: 2.86). 01/2006; 25:261-281. DOI:10.1177/0278364906063829
Source: DBLP

ABSTRACT Within the context of impedance controlled exoskeletons, common actuators have important drawbacks. Either the actuators are heavy, have a complex structure or are poor torque sources, due to gearing or heavy nonlinearity. Considering our application, an impedance controlled gait rehabilitation robot for treadmill-training, we de- signed an actuation system that might avoid these drawbacks. It combines a lightweight joint and a simple structure with adequate torque source quality. It consists of a servomotor, a flexible Bowden cable transmission, and a force feedback loop based on a series elas- tic element. A basic model was developed that is shown to describe the basic dynamics of the actuator well enough for design purpose. Further measurements show that performance is sufficient for use in a gait rehabilitation robot. The demanded force tracking band- widths were met: 11 Hz bandwidth for the full force range (de- manded 4 Hz) and 20 Hz bandwidth for smaller force range (de- manded 12 Hz). The mechanical output impedance of the actuator could be reduced to hardly perceptible level. Maxima of about 0.7 Nm peaks for 4 Hz imposed motions appeared, corresponding to less than 2.5% of the maximal force output. These peaks were caused by the stick friction in the Bowden cables. Spring stiffness variation showed that both a too stiff and a too compliant spring can worsen performance. A stiff spring reduces the maximum allowable controller gain. The relatively low control gain then causes a larger effect of stick in the force output, resulting in a less smooth output in general. Low spring stiffness, on the other side, decreases the performance of the system, because saturation will occur sooner. KEY WORDS—actuator design, cable transmission, exo- skeleton, impedance control, rehabilitation robotics

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