Robotics and Autonomous Systems ( ) –
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Robotics and Autonomous Systems journal homepage: www.elsevier.com/locate/robot
A muscle-like recruitment actuator with modular redundant actuation units for soft robotics
Glenn Mathijssen a,d,∗, Joshua Schultz b,c, Bram Vanderborght a, Antonio Bicchi b,d a Department of Mechanical Engineering, Vrije Universiteit Brussel (VUB), B-1050 Brussels, Belgium b Department of Advanced Robotics, Istituto Italiano di Tecnologia (IIT), 16163 Genova, GE, Italy c Department of Mechanical Engineering, the University of Tulsa, Tulsa, OK 74104, USA d Bioengineering and Robotics Research Center, Centro E. Piaggio, Universita di Pisa, 56112 Pisa, PI, Italy h i g h l i g h t s • We present a novel design of a modular redundant actuation unit. • Solenoids with an integrated compliant coupling can be discretely activated. • Isometric experiments show the characteristics and demonstrate the repeatability. • ‘‘Designer muscles’’ can be produced by combining these units in series and parallel. • The strain rate is 21.1% and the maximum stress is 1.55 Pa. a r t i c l e i n f o
Received 22 October 2014
Received in revised form 22 June 2015
Accepted 29 June 2015
Available online xxxx
Modular motor units
Redundant actuation a b s t r a c t
Human muscles contrast sharply with traditional robot actuators in that they consist of several motor units, connected in series and parallel, which can be progressively recruited. Some roboticists have explored this idea in robotic actuators, striving for improvements such as the ability to withstand partial damage, inexpensive repeatability by discrete open loop control and the potential of modular actuators.
These systems, however, become rather complex or rely on less widely used actuation techniques such as piezo-actuators or SMAs to produce a compact implementation. This paper presents a novel design of a modular redundant actuation unit which can be combined in various combinations to form compliant actuators with varying characteristics. The actuation unit consists of discretely activated solenoids with an integrated compliant coupling. This paper presents the working principle and the physical implementation in detail. Failure of a single motor unit will merely lead to a loss in performance rather than failure of the actuator. Since eachmotor unit is discrete, neither power electronics nor control requires analog signals. Isometric experiments display the actuation characteristics and demonstrate the repeatability. The platform can be used in future work to further explore the virtues of exploiting discretization and redundancy in muscle-like control. © 2015 Elsevier B.V. All rights reserved. 1. Introduction
Reproducing the properties of biological muscle is a longstanding research effort since actuator limitations heavily influence the capabilities of robots. Stiffness properties of muscles are one of the earliest characteristics that has been studied. The seminal work of
Pratt et al. from the 1990s  showed the virtue of elastic elements ∗ Corresponding author at: Department of Mechanical Engineering, Vrije
Universiteit Brussel (VUB), B-1050 Brussels, Belgium.
E-mail address: Glenn.Mathijssen@vub.ac.be (G. Mathijssen). in series with the drive train and ever since numerous roboticists have focused on novel uses and implementations of series elastic actuators (SEA). An important extension of this work toward muscle-like actuators is the work on variable impedance actuators (VIA) [2,3]. Vanderborght et al.  provide a good reviewwith classification of actuators that can vary stiffness and damping characteristics so as to exploit and modify the natural dynamics of a system. In addition, dedicated control architectures for safety  and energy efficiency  are developed. A number of materials and techniques have been explored for their use in artificial muscles, such as pneumatic artificial muscles , electroactive polymers (EAP) [8,9] and shape memory alloys (SMA) . http://dx.doi.org/10.1016/j.robot.2015.06.010 0921-8890/© 2015 Elsevier B.V. All rights reserved. 2 G. Mathijssen et al. / Robotics and Autonomous Systems ( ) –
Fig. 1. Single degree of freedom actuation unit consisting of discretely activated elements coupled to themounting boss by a spring. This actuation unit is a building block for constructing designer muscle-like actuators with a hierarchical structure.
Such actuators are controlled by setting an activation level, or the number of active elements in the on state, a process known as recruitment.
Apart from its compliance, the hierarchical structure of skeletal muscle is also distinctive, though this aspect has received less attention. A skeletal muscle consists of multiple motor units, each consisting of a number of muscle fibers. Each motor unit can be activated through its motor neuron: as such a muscle with more motor units is able to control force output in a finer manner. The activation of motor units to produce a force is calledmotor unit recruitment and differs markedly from generating an analog signal proportional to the desired force output [11,12]. Actuators with a discrete cellular structure include the work of Dittrich , MacNair and Ueda  and Huston et al. . Both of these works present a type of actuator that is made up of numerous subactuators. This redundancy increases the robustness of these actuators. Failure of an electromechanical component will only lead to a loss in performance instead of a loss of a complete degree of freedom (DOF), since the remaining undamaged units can continue to perform the task. The cellular structure opens the possibility for modularity in the actuatorswhich represents an untried frontier in engineering. In Mathijssen et al.  the series–parallel elastic actuation conceptwas proposedwherebymultiple springs in parallel can be variably recruited bymultiple dephased intermittentmechanisms andonly onemotor. Cho et al.  introduced and validated a segmented cellular architecture of SMA wires. Ueda et al.  focused on distributed stochastic control of an actuator system consisting of many cellular SMA units. Schultz and Ueda  validated theirmultilayer strain amplificationmechanismon a camera positioning mechanism based on piezo actuators. For mathematical simplicity, each active element of the actuator in Schultz and