High Compliant Series Elastic Actuation for the Robotic Leg ScarlETH

This paper presents the actuation system of the robotic leg ScarlETH. It was developed specifically for a quadrupedal robot and is designed to achieve fast position control as well as accurate joint torque control. It introduces strong passive dynamics to create an efficient running behavior. High spring compliance with low damping in combination with a cascaded, motor velocity based, control structure was successfully tested in simulation and experiments. Final tests with the entire leg demonstrate that the system can perform a hopping motion providing only positive actuator power

Quadrupedal robots with stiff and compliant actuation

In the broader context of quadrupedal locomotion, this overview article introduces and compares two platforms that are similar in structure, size, and morphology, yet differ greatly in their concept of actuation. The first, ALoF, is a classically stiff actuated robot that is controlled kinematically, while the second, StarlETH, uses a soft actuation scheme based on highly compliant series elastic actuators. We show how this conceptual difference influences design and control of the robots, compare the hardware of the two systems, and show exemplary their advantages in different applications. © Oldenbourg Wissenschaftsverlag

Excitation and Stabilization of Passive Dynamics in Locomotion using Hierarchical Operational Space Control

This paper describes a hierarchical operational space control (OSC) method based on least square optimization and outlines different ways to reduce the dimensionality of the optimization vector. The framework allows to emulate various behaviors by prioritized task-space motion, joint torque, and contact force optimization. Moreover, a methodology is introduced to partially excite the natural dynamics of the robot by open-loop motor regulation while the entire behavior is stabilized by hierarchical OSC. As a major contribution, the presented control strategies are tested and validated in real hardware walking, trotting, and pronking experiments using a fully torque controllable quadrupedal robot

Hybrid Operational Space Control for Compliant Legged Systems

This paper introduces the concept of hybrid operational space control, a method that unifies kinematic tracking of individual joints with an inverse dynamics task space controller for the remainder of the robot. The proposed control strategy allows for a hierarchical task decomposition while simultaneously regulating the inner forces between the contact points. At the same time it improves fast tracking for compliant systems by means of appropriate low level position controllers. Introducing StarlETH, a compliant quadrupedal robot, the applicability of the controller and the hardware is demonstrated in realtime simulations and hardware experiments. We perform static walking in challenging terrain and show how the controller can combine precise and fast position control with robust and compliant interaction with the environment

Improved Efficiency in Legged Running Using Lightweight Passive Compliant Feet

This paper investigates the mechanical benefits of employing a passive foot segment to improve energetic efficiency in legged running. The proposed lightweight design significantly reduces impact and damping losses, while simultaneously allowing for a natural-looking stance configuration. Actuator in- put and ankle spring properties were optimized in simulation and successfully tested in 2D running experiments

Towards Automatic Discovery of Agile Gaits for Quadrupedal Robots

Developing control methods that allow legged robots to move with skill and agility remains one of the grand challenges in robotics. In order to achieve this ambitious goal, legged robots must possess a wide repertoire of motor skills. A scalable control architecture that can represent a variety of gaits in a unified manner is therefore desirable. Inspired by the motor learning principles observed in nature, we use an optimization approach to automatically discover and fine-tune parameters for agile gaits. The success of our approach is due to the controller parameterization we employ, which is compact yet flexible, therefore lending itself well to learning through repetition. We use our method to implement a flying trot, a bound and a pronking gait for StarlETH, a fully autonomous quadrupedal robot

State Estimation for Legged Robots - Consistent Fusion of Leg Kinematics and IMU

This paper introduces a state estimation framework for legged robots that allows estimating the full pose of the robot without making any assumptions about the geometrical structure of its environment. This is achieved by means of an Observability Constrained Extended Kalman Filter that fuses kinematic encoder data with on-board IMU measurements. By including the absolute position of all footholds into the filter state, simple model equations can be formulated which accurately capture the uncertainties associated with the intermittent ground contacts. The resulting filter simultaneously estimates the position of all footholds and the pose of the main body. In the algorithmic formulation, special attention is paid to the consistency of the linearized filter: it maintains the same observability properties as the nonlinear system, which is a prerequisite for accurate state estimation. The presented approach is implemented in simulation and validated experimentally on an actual quadrupedal robot

Fusion of Optical Flow and Inertial Measurements for Robust Egomotion Estimation

In this paper we present a method for fusing optical flow and inertial measurements. To this end, we derive a novel visual error term which is better suited than the standard continuous epipolar constraint for extracting the information contained in the optical flow measurements. By means of an unscented Kalman filter (UKF), this information is then tightly coupled with inertial measurements in order to estimate the egomotion of the sensor setup. The individual visual landmark positions are not part of the filter state anymore. Thus, the dimensionality of the state space is significantly reduced, allowing for a fast online implementation. A nonlinear observability analysis is provided and supports the proposed method from a theoretical side. The filter is evaluated on real data together with ground truth from a motion capture system

ScarlETH: Design and control of a planar running robot

This paper introduces the mechanical design and the control concept of the Series Compliant Articulated Robotic Leg ScarlETH which was developed at ETH Zurich for fast, efficient, and versatile locomotion. Inspired by biological systems, we seek to achieve this through large compliances in the joints which enable natural dynamics, allow temporary energy storage, and improve the passive adaptability. A sophisticated chain and cable pulley design minimizes the segment masses, places the overall CoG close to the hip joint, and maximizes the range of motion. Nonlinearities in the damping and an appropriate low-level controller allow for precise torque control during stance and for fast task space position control during swing. This paved the road for the combined application of a virtual model controller for ground contact and a modified Raibert style controller for flight phase which was successfully tested in planar running. © 2011 IEEE