| Volume 25 Issue 3 - Publication Date: 1 March 2006 |
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| A Series Elastic- and Bowden-Cable-Based
Actuation System for Use as Torque Actuator in Exoskeleton-Type Robots |
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| J.F. Veneman, R. Ekkelenkamp,
R. Kruidhof, F.C.T. van der Helm, and H. van der Kooij
Biomechanical Engineering, BMTI University of Twent, P.O. Box 217,
7500 AE Enschede, The Netherlands |
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| 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. |
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| Multimedia Key |
= Video |
 = Data |
= Code |
= Image |
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Extension |
Type |
Description |
| 1 |
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Example
1: Moving view of the actuator. (0.9 MB) |
| 2 |
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Example
2: Actuator in use; control subjective is horizontally a zero
force and vertically a fixed position. (5.3 MB) |
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