Journal of Intelligent & Robotic Systems https://doi.org/10.1007/s10846-018-0777-9 Hybrid Compliance Control for Locomotion of Electrically Actuated Quadruped Robot Edin Koco 1 · Damir Mirkovic 1 · Zdenko Kovaˇ ci´ c 1 Received: 26 June 2017 / Accepted: 19 January 2018 © Springer Science+Business Media B.V., part of Springer Nature 2018 Abstract This paper presents a new hybrid compliance control system for a electrically powered quadruped robot leg composing both of active and variable passive compliance parts. The presented control system decouples the effects of each compliance type in a manner beneficial for the overall dynamics of the robot leg system. We investigate and demonstrate how the proposed hybrid compliance control of a mechanically stiff quadruped robot leg can improve the performance of locomotion under moderate external disturbances. The observed robotic system integrates high gear ratio DC motors making the whole mechanism stiff and inconvenient for use when subjected to unknown disturbances, making this system a perfect candidate to implement compliance control. The variable passive compliance ensures the filtering of sudden impacts during locomotion while the active compliance allows lower bandwidth compliance control for locomotion purposes. Control system ensures that the joint effect of the active and variable passive compliance is fully controllable both in higher and lower frequency range. Mathematical modeling and simulation analysis is conducted in order to identify the performance of the proposed system. Finally, the system is experimentally validated from the single leg and quadruped robot perspective. Keywords Quadruped robot · Active compliance · Variable passive compliance · Hybrid compliance control · Virtual springs 1 Introduction Modern robotics is often inspired by mechanisms and concepts found in nature, primarily in the animal world. Through millions of years of evolution biological systems have developed locomotion mechanisms optimal in terms of versatility, speed, stability and energy efficiency. Bio- inspired robotics is an area with a particular focus on developing robots that emulate or simulate living biological organisms. A great example of bio-inspired robotic designs Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10846-018-0777-9) contains supplementary material, which is available to authorized users.  Edin Koco edin.koco@fer.hr 1 Faculty of Electrical Engineering and Computing, Laboratory for Robotics and Intelligent Control Systems, University of Zagreb, Unska 3, 10000 Zagreb, Croatia are ever-evolving quadruped robots due to design complex- ity, wide variety of possible gaits, complex locomotion pat- terns and problems that arise due to their inherent dynamic instability. The mentioned problems have long been tack- led by various researchers which led to different innovative quadruped robot designs over the course of 30 years. The main challenge in controlling a legged robot is the need to synchronize a large number of degrees of freedom (DOF) according to sensory information in order to ensure sta- ble locomotion over rugged terrain. The problem gets even more complex by involving the aspect of energy efficient locomotion. In order to achieve highly dynamic locomotion, a soft interaction with the robot’s surroundings is crucial. Com- pliance (whether active or passive) increases the robustness of locomotion by filtering out external disturbances. We can notice that by observing humans and animals during locomotion. Tendons and ligaments are primarily used as suspension systems to lower the peak reaction forces and extend the contact time with the ground. The muscles are used as an actuation system to provide locomotion and addi- tional mechanics which serve to modulate the compliance