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SPEAR: Series-Parallel Actuators for Robotics

Actuators are key components for moving and controlling a mechanism or system. However, the torque and energy efficiency of the actuators are insufficient, much lower than a human. There are several applications (prostheses, exoskeletons, running robots) where the unavailability of suitable actuators hinders the development of well-performing machines with capabilities comparable to a human.

The SPEAR project develops radically new solutions to develop a new generation of actuators to go beyond the current stiff servomotors and the Variable Stiffness Actuators (VSA). The proposed solution is based on the novel Series-Parallel Elastic Actuator (SPEA) concept, which is inspired by the series-parallel organisation of the muscle fibres in a human muscle.

Series-Parallel Elastic Actuator

In a stiff actuator or a VSA all the torque that is generated at the joint, goes through the motor and consequently consumes electrical current (since torque is proportional to current). Moreover electric motors are rarely used at optimal efficiency in robotic applications since the motor is used in its complete operating range. A stiff motor consists of an electric motor and a gearbox, a VSA of an electric motor, a gearbox and a mechanical spring with a mechanism and motor to adapt the stiffness.

An SPEA consists of one or several electric motors, several springs arranged in series and/or parallel and locking mechanisms to adapt the topology of the actuator. As such the majority of the loads do not pass the motor and power can be optimally distributed to minimize energy consumption apart from realizing the required output loads. Several studies investigated the energy efficiency from a fundamental point of view.


SPEAR developed and validated several novel topologies of such redundant actuators and developed control algorithms to do control allocation so the energy efficiency of the overall system is drastically improved. Two demonstrators are under development being a hand-arm system powered by modular SPEA actuators and a transtibial prosthesis.

Self-Healing actuators

Since such actuators are very complex mechanical system and hence difficult to repair. Moreover robotic systems are typically dimensioned to be able to withstand occasional extreme loads, instead of being designed based on their performance tasks. This over-dimensioning results in heavy and oversized robotic systems. In SPEAR we work on the ambitious breakthrough to develop a material-oriented solution by implementing self-healing (SH) materials for actuators. Several demonstrators of self-healing actuators are under development.

More information on this subject is available right here.


Series and Parallel Elastic Actuation: Influence of Operating Positions on Design and Control
Beckerle, P., Verstraten, T., Mathijssen, G., FURNéMONT, R. G., Vanderborght, B. & Lefeber, D. 25 Oct 2016 In : IEEE/ASME Transactions on Mechatronics.

Towards self-healing actuators:a preliminary concept
Terryn, S., Brancart, J., Mathijssen, G., Verstraten, T., Van Assche, G. & Vanderborght, B. 16 May 2016 In : IEEE Transactions on Robotics. 32, 3, p. 736-743 8 p., 1552-3098

Series and Parallel Elastic Actuation: Impact of Natural Dynamics on Power and Energy Consumption
Verstraten, T., Beckerle, P., Furnémont, R., Mathijssen, G., Vanderborght, B. & Lefeber, D. 2 May 2016 In : Mechanism and Machine Theory. 102, p. 232-246 14 p.

Bi-directional Series-Parallel Elastic Actuator and overlap of the actuation layers
Furnemont, R., Vanderborght, B., Mathijssen, G., Lefeber, D. & Verstraten, T. 26 Jan 2016 In : Bioinspiration & Biomimetics. 11, 1, p. 1-26 26 p., 016005

Energy Consumption of Geared DC Motors in Dynamic Applications: Comparing Modeling Approaches
Verstraten, T., Furnemont, R., Mathijssen, G., Vanderborght, B. & Lefeber, D. 13 Jan 2016 In : IEEE Robotics and Automation Letters. 1, 1, p. 524 - 530 7 p.

A muscle-like recruitment actuator with modular redundant actuation units for soft robotics
Mathijssen, G., Schultz, J., Vanderborght, B. & Bicchi, A. Dec 2015 In : Robotics and Autonomous Systems. 74, Part A, p. 40-5011 p.

Modeling and dimensioning of geared DC motors for energy efficiency: Comparison between theory and experiments
Verstraten, T., Mathijssen, G., Furnemont, R., Vanderborght, B. & Lefeber, D. Sep 2015 In : Mechatronics. 30, p. 198-213

Development of a self-healing soft pneumatic actuator: a first concept
Terryn, S., Mathijssen, G., Brancart, J., Lefeber, D., Van Assche, G. & Vanderborght, B. 1 Aug 2015 In : Bioinspiration & Biomimetics. 10, 4, 17 p., 046007

Lock your robot: A review of locking devices in robotics
Plooij, M., Mathijssen, G., Cherelle, P., Lefeber, D. & Vanderborght, B. 12 Mar 2015 In : IEEE Robotics and Automation Magazine.22, 1, p. 106-117 12 p.

Variable recruitment of parallel elastic elements: Series-Parallel Elastic Actuators (SPEA) with dephased mutilated gears
Mathijssen, G., Lefeber, D. & Vanderborght, B. 4 Mar 2014 In : IEEE/ASME Transactions on Mechatronics. 20, 2, p. 594-602 9 p.

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