Conceptual Design of a Novel Variable Stiffness Actuator for Use in Lower Limb Exoskeletons Tomislav Bacek, Ramazan Unal, Marta Moltedo, Karen Junius, Heidi Cuypers, Bram Vanderborght and Dirk Lefeber Faculty of Applied Sciences, Department of Mechanical Engineering Vrije Universiteit Brussel, 1050 Brussels, Belgium Email: tomislav.bacek@vub.ac.be Abstract—A novel modular variable stiffness actuator (VSA), for use in the knee joint of lower limb exoskeletons, is presented. The actuator consists of a combination of a spindle- driven MACCEPA (Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator) and a spring acting in parallel, (dis)engaged by means of a simple on/off mechanism depending on the phase of the gait cycle. Such design approach is inspired by two clearly distinctive gait phases of a knee joint, one with a high velocity and low torque, and another one with low velocity and high torque profiles. By tackling each of these two phases separately, energy consumption and torque requirements of an active part of the actuator have been decreased, while keeping the size and the weight of the actuator at a reasonable size for use in wearable robots (WR). I. I NTRODUCTION A. Background Human musculoskeletal system is highly optimized for dynamic, versatile and efficient locomotion. Hence, mim- icking the principles underlying human locomotion imposes conflicting design requirements on the design of the actua- tors for lower limb exoskeletons. On one side, exoskeletons need to be lightweight, wearable, compliant and energy conservative. Most importantly, since these devices are used for close interaction with humans, they need to be safe. At the same time, they also need to be powerful and robust enough to meet the requirements of a demanding task, such as walking. Conventional direct-drive actuators are a good approx- imation of an ideal force source, which is often required in WRs applications [1]. However, they have relatively small torque-to-weight ratio and use high transmission to overcome this, thus being heavy, bulky, inefficient and of high output impedance. Furthermore, due to the direct transmission between the motor output and the load, these actuators are intrinsically unsafe for use in physical human- robot interaction environments. In order to overcome these drawbacks, direct-drive actuators needed to be replaced with intrinsically compliant ones [2]. The introduction of elastic (i.e. compliant) element between the motor and the load led to decoupling of the motor inertia from the load. This, in turn, together with the energy storage capability of elastic element, led to shock and impact tolerance and stable high performance force control, both necessary for safe actuation. However, passive compliance itself is not sufficient to ensure safe actuation and it should thus be complemented by the appropriate control strategies [3]. Many different compliant actuator designs have been pro- posed for use in (safe) human-robot interaction [4]. The most widely used compliant actuation principle, employed with electric drive units, is Series Elastic Actuation (SEA), ini- tially developed by Pratt and Williamson [2]. The only prac- tical alternative to electric compliant actuators in the field of assistive robotics are pneumatic actuators [5]. Nonetheless, the problem with these actuators is that, in order to achieve bidirectional actuation inherent in human musculoskeletal system, they require antagonistic setup. Also, they have slow dynamics, exhibit nonlinear characteristic, hysteresis and need pressurized air [6]. Many different realizations of the SEA principle have been proposed in the literature, such as linear [7], [8], rotary [9], [10], prismatic [11], [12] and Bowden cable-based actuators [13]. Distinctive property of these actuators is their fixed compliance, which can be a significant drawback for use in assistive devices. This is due to the fact that humans change their walking speed, running stride frequency and accommodate for surface stiffness changes by altering their joint stiffness. To account for these changes, different VSAs have been designed, including MACCEPA [14], VSA-II [15] and AwAS-II [16], just to mention few. For extensive review on VSAs principles and designs, reader is referred to [6]. MACCEPA principle has been recently used by the authors to build a modular compliant actuator for use in WRs [17], and has been successfully employed for powering all the joints of a sit-to-stance exoskeleton [18]. The actuators were designed to provide torques equivalent to 30% muscle weakness which, according to a simulation study given in [19], corresponds to a peak torque of 15 Nm. Since no high velocities are needed for sit-to-stance motion, small motors with high gear ratios were used. To reduce the cost of the entire device, identical actuator was used for all three joints. However, using the same actuator for actuating all three human joints during walking is not an optimal scenario from neither kinematic nor kinetic requirements perspective. This is due to the fact that these requirements differ significantly from one joint to another. Knee joint, in particular, is specific in that it requires high torque during the weight acceptance phase and high velocity during the swing phase of the gait cycle. From the actuation point of view, these