MACCEPA-based Series-Parallel Elastic Actuators (SPEA) for the next generation of robotic co-workers

Robots are still outperforming humans regarding their efficiency and dexterity. On the other hand, however, muscles are more efficient and have a higher power to weight ratio than electric motors. As a consequence, among others, manipulators for human robot interaction in industry or service applications have a low payload to weight ratio and a low energy efficiency. Therefore, we developed a novel actuation concept which we name Series-Parallel Elastic Actuation (SPEA). The concept enables variable recruitment and locking of multiple springs in parallel. In this paper the latest MACCEPA-based SPEA is presented and first results are shown. Servomotors are generally used for industrial robots as they make the joint’s mechanical impedance very high, often considered as infinite, so they are ideal for precise tracking with a high bandwidth. As a consequence, however, industrial robots are unsafe for human-robot interaction and are positioned in cages or secured by safety light curtains and sensors. The next generation of robots will strongly collaborate with humans, implying new requirements such as safety and energy-efficiency. For example manipulators for assistance technologies in industrial settings, also known as co-workers, are currently a vastly emerging technology. Multiple manipulators, of which some are listed in Table I, are being developed in research projects or are already commercially available. Mekabot’s A2 compliant robot arm and Rethink Robotics’ Baxter are rather exeptional in Table I since these are the first commercialized manipulators which are driven by complaint actuators, also known as Series Elastic Actuators [1]. The limited torque to weight ratio of current actuators, stiff or compliant, however limits the payload to weight ratio of these manipulators. This can be seen from Table I. On the contrary, the average specific power density of a mammalian skeletal muscle (0.05 W/g) is an order or magnitude lower compared to electric motors (0.5 W/g) [2]. The maximum energy efficiency of an electric motor (>80%) is also higher compared to a muscle (<40%). Despite this higher power density and maximum efficiency, the electric motors are not yet able to better All authors are with the Robotics & Multibody Mechanics Research Group, Faculty of Mechanical Engineering, Vrije Universiteit Brussel, 1050 Elsene, Belgium. http://mech.vub.ac.be/robotics. Glenn Mathijssen is with the Robotics & Multibody Mechanics Research Group at the Vrije Universiteit Brussel and with Centro E. Piaggio of the University of Pisa. This work has been funded by the European Commission as a part of the ERC-grant SPEAR (no.337596). 1 Corresponding author: [email protected] Stiff actuators ABB’s dual arm concept FRIDA 2x0.5 kg/20 kg Universal Robots UR5 5 kg/18.4 kg Yaskawa’s SDA robots 5 kg/110 kg Kawada NextAge 2x1.5 kg/28 kg Kuka lightweight LWR4+ 7kg/16 kg