A Guideline for Humanoid Leg Design with Oblique Axes for Bipedal Locomotion

The kinematics of humanoid robots are strongly inspired by the human archetype. A close analysis of the kinematics of the human musculoskeletal system reveals that the human joint axes are oriented within certain inclinations. This is in contrast to the most popular humanoid design with a configuration based on perpendicular joint axes. This paper reviews the oblique joint axes of the mainly involved joints for locomotion of the human musculoskeletal system. We elaborate on how the oblique axes affect the performance of walking and running. The mechanisms are put into perspective for the locomotion types of walking and running. In particular, walking robots can highly benefit from using oblique joint axes. For running, the primary goal is to align the axis of motion to the mainly active sagittal plane. The results of this analysis can serve as a guideline for the kinematic design of a humanoid robot and a prior for optimization-based approaches

Using Elastically Actuated Legged Robots in Rough Terrain: Experiments with DLR Quadruped bert

This paper addresses walking and balancing in rough terrain for legged locomotion in planetary exploration as an alternative to the commonly used wheeled locomotion. In contrast to the latter, where active balancing is not necessary, legged locomotion requires constant effort to keep the main body stabilized during motion. While common quadrupedal robots require to carefully plan motions through torque control and force distribution, this paper presents an approach where elastic elements in the drive train function as an intrinsic balancing component that allows to ignore inaccuracies in the locomotion pattern and passively accommodate for terrain unevenness. The approach proposes a static walking gait algorithm, which is formulated for a general quadrupedal robot, and a hardware foot design to support the locomotion pattern. The method is experimentally tested on an elastically actuated quadrupedal robot

From Space to Earth - Relative-CoM-to-Foot (RCF) control yields high contact robustness

This paper introduces the Simplest Articulated Free-Floating (SAFF) model, a low-dimensional model facilitating the examination of controllers, which are designed for freefloating robots that are subject to gravity. Two different stateof-the-art control approaches, namely absolute CoM control accompanied by an assumption about the foot acceleration, and a controller combining absolute CoM and foot control objectives, are shown to yield exponential stability in the nominal case, while becoming unstable if the foot contact is lost. As an improvement over the state of the art, the so-called Relative-CoM-to-Foot (RCF) controller is introduced, which again yields exponential stability nominally, while preserving a BIBO stable behavior even in case of a complete contact loss. The controller performance is validated in various simulations

Dynamic Bipedal Walking by Controlling only the Equilibrium of Intrinsic Elasticities

This paper presents a methodology for controlling dynamic bipedal walking in a compliantly actuated humanoid robotic system. The approach is such that it exploits the natural leg dynamics of the single and double support phase of the gait. The present approach avoids to close a torque control loop at joint level. While simulation implementations of torque based walking for series elastic actuator (SEA) humanoids display very promising results, several robustness issues very often appear in the experiments. Therefore we introduce here a minimalistic controller, which is based on feedback of control input collocated variables, with the only exception of zero joint torque control. Reshaping of the intrinsic elasticities by control is completely avoided. In order to achieve a coordinated movement of swing and stance leg during single support phase, an appropriate one-dimensional manifold of the motor positions is designed. This constrained behavior is experimentally shown to be compatible with the intrinsic mechanical oscillation mode of the double support phase. The feasibility of this methodology is experimentally validated on a human-scale, anthropomorphic bipedal robotic system with SEA actuation

Optimal and Robust Walking using Intrinsic Properties of a Series-Elastic Robot

Series-Elastic Actuators (SEA) have been proposed as a technology to build robust humanoid robots. The aim of this work is to generate efficient and robust walking for such robots. We present a combined approach which exploits the system dynamics through optimization based trajectory generation and a robust control scheme. The compliant actuator dynamics are explicitly modeled in the optimal control problem. For local stabilization, a passivity based tracking controller distributes the required control forces onto the available contacts. Additionally, a predictive control scheme for step adaptation is presented, which provides feasible contact points in the future. Using a reduced model, this combines efficient walking with robustness against model or environment uncertainties and external disturbances

Passive Impedance Control of Robots With Viscoelastic Joints Via Inner-Loop Torque Control

This article presents passive impedance control of flexible joint robots (FJRs) via inner-loop torque control of elastic joints. However, according to our theoretical analysis, the torque control methods of series elastic actuators (SEAs) are often limited by the fact that the acceleration signals are amplified by the control gains. Since the acceleration signals are often affected by differentiation noise, the analysis may become invalid in practice. To alleviate this limitation, we propose the use of the so-called series viscoelastic actuator (SvEA), which significantly reduces the acceleration amplification. Consequently, in contrast to the SEA case, the theoretical analysis of an SvEA-based FJR is valid in real implementations. We would like to highlight the fact that the theoretical analysis (more specifically, passivity analysis) is performed for nonlinear robot dynamics without linearization. As a result, the passive impedance controller can be realized more robustly with enhanced inner-loop torque control

Enhancing joint torque control of series elastic actuators with physical damping

This paper presents that the joint torque control capability can be enhanced by adding physical damper to a series elastic actuator (SEA). Joint torque tracking of standard SEA has known limitations that the torque dynamics has an relative order of two, and, as a consequence, the torque controller often requires acceleration feedback when the desired torque is defined by a function of velocity (for example, compliance control). This limitation can be removed by introducing physical damping, reducing the relative degree of torque dynamics by one. Based on this observation, we design a robust controller using the disturbance observer technique. The resulting control law is given by a feed-forward term combined with PI control. The proposed controller is verified in simulation and experiment

Analyzing the Performance Limits of Articulated Soft Robots Based on the ESPi Framework: Applications to Damping and Impedance Control

In situations of harsh impacts, damping injection directly on the link of an articulated soft robot is challenging and usually requires high actuator torques at the moment of impact. In this work, we discuss the underlying reasons and analyze the performance limitations arising in the implementation of basic impedance elements, such as springs and dampers, through the elastic structure preserving impedance (ESPi) control framework. Using the insights obtained, we present a way to design impedance controllers with a damping design based on dynamic extensions. Inspired by the design of shock absorbers and the muscle-tendon model, the presented damping layout requires substantially smaller actuator torques in situations where the robot is subject to harsh impacts. The implementation is facilitated through the ESPi control framework resulting in a physically intuitive impedance design. The resulting closed-loop system can be interpreted as an interconnection of passive Euler Lagrange systems, which again, yields a passive system. The design's passive nature ensures stability in the free motion case and enables the robot to interact robustly and safely with its environment. The work focuses on robotic systems with no inertial coupling between the motor and link dynamics. Experimental results, obtained with the presented design on a dedicated series elastic actuator (SEA) test bed, are reported and discussed