Preloaded Hopping with Linear Multi-Modal Actuation Fabian G¨ unther 1 and Fumiya Iida 1 Abstract— For more dexterous and agile legged robot loco- motion, alternative actuation has been one of the most long- awaited technologies. The goal of this paper is to investigate the use of newly developed actuator, the so-called Linear Multi-Modal Actuator (LMMA), in the context of legged robot locomotion, and analyze the behavioral performance of it. The LMMA consists of three discrete couplings which enable the system to switch between different mechanical dynamics such as instantaneous switches between series elastic and fully actuated dynamics. To test this actuator for legged locomotion, this paper introduces a one-legged robot platform we developed to implement the actuator, and explains a novel control strategy for hopping, i.e. “preloaded hopping control”. This control strategy takes advantage of the coupling mechanism of the LMMA to preload the series elasticity during the flight phase to improve the energy efficiency of hopping locomotion. This paper shows a series of experimental results that compare the control strategy with a simple sinusoidal actuation strategy to discuss the benefits and challenges of the proposed approach. I. INTRODUCTION The musculoskeletal body plan of biological systems provide a number of different ways to control their bodies such as precise positioning of body parts, fast repetitive mo- tions, preloading for instantaneous high-jumps, and damped passive walking on a slope. In contrast, our robots are still severely constrained by the limitation of actuator technolo- gies as most of our legged robots are controlled through either fully actuated or completely passive joint operations. In order to relax the significant demand of new actuation technologies, a few different approaches were previously proposed to for legged robot locomotion. One of the most popular approaches is to employ the Series Elastic Actuator (SEA) in the joints of legged robots [1]. The SEA is an actuator that equips with a mechanical spring in series to mimic the viscoelastic properties of biological muscles [2]. A number of advantages have been reported for this approach, as it can be used to store kinetic energy in the mechanical springs for energy efficient motor control [3], [4], to achieve precise force control without expensive force sensors, and to filter instantaneous impacts without high control bandwidth. Even though the dynamic range of motor operation is generally limited [1], [5], the concept has also been extended to many other configurations of mechanical springs to identify how they can contribute to different dynamic motion control of robotic systems [6], [7]. *This study was supported by the Swiss National Science Foundation Grant No. PP00P2123387/1 and the Swiss National Science Foundation through the National Centre of Competence in Research Robotics. 1 F. G¨ unther and F. Iida are with Bio-Inspired Robotics Lab, Insti- tute of Robotics and Intelligent Systems, ETH Zurich, Leonhardstrasse 27, 8092 Zurich, Switzerland, fabiangu@student.ethz.ch, fumiya.iida@mavt.ethz.ch More recently, a number of researchers have been developing mechanisms to vary elasticity on the fly such that variations of mechanical dynamics can be achieved in the legged robot locomotion, for example [9], [10], [11], [12]. These actuators aim to have the benefit of both compliance when it is required but also the ability to adjust the stiffness of the system to increase the bandwidth of the actuator or to match the actuator’s resonant frequency to the task [11], [13]. Humans and animals have also been shown to regulate the stiffness of their legs to match the speed at which they are running [14]. These actuators have been implemented and tested in the legged robot locomotion to investigate specific advantages such as energy efficiency over different locomotion speeds, for example. There are also a number of different actuator technologies being developed [15] although it still requires some additional investigations to fully clarify how they can be used in legged robot locomotion and what would be the benefits. From this perspective, we have been developing an al- ternative actuation technology, the so-called Linear Multi- Modal Actuator (LMMA)[8]. Based on the small-sized dis- crete couplings that can generate relatively large holding forces, the actuator is capable of instantaneously switching between different mechanical dynamics including complete passive dynamics, to series elastic and fully actuated joint operation. While the performance of this actuator seems to be promising, it has not been clarified how this technology can be used in the context of legged locomotion and how locomotion can benefit from it. From this perspective, the goal of this paper is to propose a control strategy of one- leg hopping robot, that is, “preloaded hopping control”, that takes advantage of the discrete coupling of LMMA. The control strategy is also tested in the real-world hopping experiments to discuss the benefits and challenges of the proposed approach with respect to a conventional hopping control based on the series elastic actuator. We structure the rest of the paper as follows. Section II describes the mechanisms and designs of the LMMA and the robot platform, and the hopping control strategies are explained in Section III. Section IV shows the results of the hopping experiments, and Section V concludes the paper with some remarks on future works. II. EXPERIMENTAL PLATFORM To explore the use of the new actuation technology, we developed a planar one-legged robot (Fig. 1) that is able to accommodate the LMMA for systematic experiments. This section first introduces the basic characteristics of LMMA, 2013 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) November 3-7, 2013. Tokyo, Japan 978-1-4673-6357-0/13/$31.00 ©2013 IEEE 5847