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

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 designed 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 elastic 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 bandwidths were met: 11 Hz bandwidth for the full force range (demanded 4 Hz) and 20 Hz bandwidth for smaller force range (demanded 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.

A cable-based series elastic actuator with conduit sensor for wearable exoskeletons

“…Devices for the lower limb benefit from the workspace being notably limited, as gait is repetitive and fairly uncomplicated in a kinematic sense. Therefore, we see successful implementation of designs such as the LOPES, where bulky but light structures can be added that allow for robust force control [11], [13]. Soft exosuits developed for the lower limb are highly effective and do manage to keep an extremely low profile that interferes minimally with activity, but they are not able to provide the same level of forces as rigid exoskeletons [10].…”

“…One example of a cable driven hand exoskeleton uses the principle of series elastic actuation to measure and control forces applied to the individual finger joints in addition to a DC motor with cable-conduit transmission design [14]. Similar series elastic actuation is also seen in some lower limb devices, such as LOPES et al [11], [13].…”

“…For soft exosuit applications, series elastic actuators provide a stable means of force and impedance control given the intrinsic compliance of Bowden cable transmissions and textiles-based robotuser interface points. Series elastic actuation has previously been combined with cable-conduit actuators in rigid robotic devices [13], [14] and recently some designs by Cappello et al for a non-linear SEA for an elbow exosuit have been proposed but not fully validated [15].…”

Influence of Internal Oscillations on Force Sensing in Coil Springs

“…Force sensors would be applied at the ends of the coil springs, so their outputs are F i and F o as in (7).…”

“…This requires additional deflection sensor elements, e.g. encoders [2], [6] or potentiometers [7], [8], [9]. Ideally, coil spring deflection is measured directly, for example by placing linear potentiometers.…”