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LabsHumanoids & Human Centred MechatronicsProjects ■ Compliance exploitation for energy efficiency

Compliance exploitation for energy efficiency

This work aimed to explore the benefits of compliance in terms of energy efficiency. The elastic elements of the compliant actuators, in fact, can be used to store/release energy, and consequently reduce the work done by the motors.

link and motor trajectories
Figure 1: Link (black) and motor (green) trajectories when natural dynamics
is exploited based on the frequency in real time
The research presented in (Jafari et al., 2011) shows on AwAS (1-dof system) that energy consumption for tracking a sine-wave trajectory dramatically reduces when the resonance frequency of the system and the frequency of the periodic trajectory tracked match. Moreover, the stiffness of AwAS can be continuously regulated to tune the natural frequency of the system with a reference trajectory with varying frequency. Also in this case it was experimentally proven that when the stiffness of the actuators is controlled the system is more energy efficient (about 25% of energy saved). In fact, due to continuous adaptation of the natural dynamics, the motion amplitude of the main joint actuator is kept very small (Figure 1), which reduces the energy consumption.

These results triggered the research reported in (Moro et al., 2012). In this case, the COMAN robot (Figure 2) was used to experimentally verify the benefits of compliance for energy efficiency. The scenario now is more complex: COMAN is a multi-dof system, with compliant actuators with fixed stiffness (CompAct SEA), and the reference trajectories for walking are not simple sine-waves.
sea legs
Figure 2: The COMAN robot

A kMPs-based gait was generated (it was previously observed that the springs are more excited when the robot walks with trajectories generated by reconstruction from human kMPs than they are with engineered trajectories), and scaled in frequency to vary in a range from 0.5Hz to 1.25Hz. The overall energy consumption was measured for each of the frequencies tested, as well as the work done by the single springs.
springs to motor ratio
Figure 3: Ratio between springs and motors work.

It was shown that when the robot “walks in the resonance frequency” (i.e., the gait frequency is close to one of the main resonance frequencies of the mechanism) the springs contribute with about 15% of the work required to track the reference trajectories, by storing and releasing elastic energy (Figure 3).

Another related work (Ugurlu et al., 2012) aimed to exploit the resonance of the compliant robot COMAN in this case for improving the performance of hopping.


A. Jafari, N.G. Tsagarakis and D.G. Caldwell, “A Novel Intrinsically Energy Efficient Development of a Novel Actuator with Adjustable Stiffness (AwAS)”, IEEE Transactions on Mechatronics (accepted)

F.L. Moro, N.G. Tsagarakis, D.G. Caldwell, "Efficient Human-Like Walking for the COmpliant huMANoid COMAN based on Kinematic Motion Primitives (kMPs)", IEEE International Conference on Robotics and Automation (ICRA), Saint Paul, Minnesota, USA (2012)

B. Ugurlu, J.A. Saglia, N.G. Tsagarakis, D.G. Caldwell, Hopping at the Resonance Frequency: A Trajectory Generation Technique for Bipedal Robots with Elastic Joints, IEEE International Conference on Robotics and Automation, Saint Paul, Minnesota US, 14-18 May, 2012.

Last Updated on Thursday, 14 June 2012 10:08


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