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: 2.5). 01/2006; 25:261-281.
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

1 Bookmark
  • [Show abstract] [Hide abstract]
    ABSTRACT: Series elastic actuators are used to significant advantage in many robot designs but have not found their way into the design of haptic devices. We use a pneumatic circuit to realize both a flexible power transmission as well as the elastic element in a series elastic actuator. The pneumatic circuit effectively hides the impedance of a high friction, high mass ball-screw actuator, while a low friction, low mass pneumatic cylinder is used at the end-effector. We offer a comparative study of an impedance-control device, admittance-control device, and a device incorporating a series elastic actuator, investigating both the open-loop and the closed-loop impedance displayed to the user. While all hardware and control designs offer an ability to shape the impedance within their operational bandwidths, the series elastic design has the particular advantage of low impedance (a very compliant spring) outside of that bandwidth. Thus, a haptic device featuring series elastic actuation is capable of providing both the low impedances required in free-space and the high impedance required for rendering stiff virtual walls.
    Haptics Symposium (HAPTICS), 2014 IEEE; 02/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: Series elastic actuation in rehabilitation robotics was introduced to elastically decouple the actuator from the joint in order to improve safety and guarantee a compliant actuator behaviour. Classical control approaches of series elastic actuation usually consist of a number of cascaded control structures, where the force or torque control problem is rearranged to a translational or rotational displacement control problem. Considering the parameter tuning of PID-type cascaded control-loops, passivity of the patient-actuator interaction dynamics should be of concern and imposes parameter bounds for the controller. In order to overcome these limitations, guarantee robust stability, minimise load impedance and reduce the complexity of the classical cascaded control structure, a new control strategy is presented in this paper. The controlled plant is down-squared to yield a positive real system and augmented with frequency dependent weightings to shape the sensitivity functions. Consequently, a new ℋ2-optimal controller is designed using an additional constraint to guarantee the strict positive realness of the controller. The resulting controller is obtained by solving a LMI-system, which is of a new form for the proposed loop-shaping procedure. Additionally, orthogonality constraints of the ℋ2-problem are relaxed in the new procedure by a loop-shifting approach. The resulting controller inherently guarantees the patient-actuator interaction transfer function to be positive real, minimises the load impedance and robustly stabilises the actuator due to the dissipative controller properties.
    2014 American Control Conference - ACC 2014; 06/2014
  • Proceedings of the Institution of Mechanical Engineers Part I Journal of Systems and Control Engineering 03/2013; 228(3):138-153. DOI:10.1177/0959651813511615 · 0.78 Impact Factor

Full-text (2 Sources)

Available from
May 19, 2014