Thiago Boaventura

Dynamic torque control of a hydraulic quadruped robot

Legged robots have the potential to serve as versatile and useful autonomous robotic platforms for use in unstructured environments such as disaster sites. They need to be both capable of fast dynamic locomotion and precise movements. However, there is a lack of platforms with suitable mechanical properties and adequate controllers to advance the research in this direction. In this paper we are presenting results on the novel research platform HyQ, a torque controlled hydraulic quadruped robot. We identify the requirements for versatile robotic legged locomotion and show that HyQ is fulfilling most of these specifications. We show that HyQ is able to do both static and dynamic movements and is able to cope with the mechanical requirements of dynamic movements and locomotion, such as jumping and trotting. The required control, both on hydraulic level (force/torque control) and whole body level (rigid model based control) is discussed.

Towards versatile legged robots through active impedance control

Robots with legs and arms have the potential to support humans in dangerous, dull or dirty tasks. A major motivation behind research on such robots is their potential versatility. However, these robots come at a high price in mechanical and control complexity. Hence, until they can demonstrate a clear advantage over their simpler counterparts, robots with arms and legs will not fulfill their true potential. In this paper, we discuss the opportunities for versatile robots that arise by actively controlling the mechanical impedance of joints and particularly legs. In contrast to passive elements such as springs, active impedance is achieved by torque-controlled joints allowing real-time adjustment of stiffness and damping. Adjustable stiffness and damping in real-time is a fundamental building block towards versatility. Experiments with our 80 kg hydraulic quadruped robot HyQ demonstrate that active impedance alone (i.e. no springs in the structure) can successfully emulate passively compliant elements during highly dynamic locomotion tasks (running, jumping and hopping); and that no springs are needed to protect the actuation system. Here we present results of a flying trot, also referred to as a running trot. To the best of the authors’ knowledge this is the first time a flying trot has been successfully implemented on a robot without passive elements such as springs. A critical discussion on the pros and cons of active impedance concludes the paper. This article is an extension of our previous work presented at the International Symposium on Robotics Research (ISRR) 2013.

Stability and performance of the compliance controller of the quadruped robot HyQ

A legged robot has to deal with environmental contacts every time it takes a step. To properly handle these interactions, it is desirable to be able to set the foot compliance. For an actively-compliant legged robot, in order to ensure a stable contact with the environment the robot leg has to be passive at the contact point. In this work, we asses some passivity and stability issues of the actively-compliant leg of the quadruped robot HyQ, which employs a highperformance cascade compliance controller. We demonstrate that both the nested torque loop performance as well as the actuator bandwidth have a strong influence in the range of virtual impedances that can be passively rendered by the robot leg. Based on the stability analyses and experimental results, we propose a procedure for designing cascade compliance controllers. Furthermore, we experimentally demonstrate that the HyQs actively-compliant leg is able to reproduce the compliant behavior presented by an identical but passively-compliant version of the same leg.

On the role of load motion compensation in high-performance force control

Robots are frequently modeled as rigid body systems, having torques as input to their dynamics. A high-performance low-level torque source allows us to control the robot/environment interaction and to straightforwardly take advantage of many model-based control techniques. In this paper, we define a general 1-DOF framework, using basic physical principles, to show that there exists an intrinsic velocity feedback in the generalized force dynamics, independently of the actuation technology. We illustrate this phenomena using three different systems: a generic spring-mass system, a hydraulic actuator, and an electric motor. This analogy helps to clarify important common aspects regarding torque/force control that can be useful when designing and controlling a robot. We demonstrate, using simulations and experimental data, that it is possible to compensate for the load motion influence and to increase the torque tracking capabilities.

Torque-control based compliant actuation of a quadruped robot

In the realm of legged locomotion, being compliant to external unperceived impacts is crucial when negotiating unstructured terrain. Impedance control is a useful framework to allow the robot to follow reference trajectories and, at the same time, handle external disturbances. To implement impedance control, high performance torque control in all joints is of great importance. In this paper, the torque control for the electric joints of the HyQ robot is described and its performance assessed. HyQ is a quadruped robot which has hybrid actuation: hydraulic and electric. This work complements our previous work, in which the torque control for the hydraulic joints was addressed. Subsequently, we describe the implementation of an impedance controller for the HyQ leg. Experimental results assess the tracking capability of a desired Cartesian force at the end-effector under the action of external disturbances. Another set of experiments involves the tracking and the shaping of different desired stiffness behaviors (stiffness ellipses) at the foot.

Control of a hydraulically-actuated quadruped robot leg

This paper is focussed on the modelling and control of a hydraulically-driven biologically-inspired robotic leg. The study is part of a larger project aiming at the development of an autonomous quadruped robot (hyQ) for outdoor operations. The leg has two hydraulically-actuated degrees of freedom (DOF), the hip and knee joints. The actuation system is composed of proportional valves and asymmetric cylinders. After a brief description of the prototype leg, the paper shows the development of a comprehensive model of the leg where critical parameters have been experimentally identified. Subsequently the leg control design is presented. The core of this work is the experimental assessment of the pros and cons of single-input single-output (SISO) vs. multiple-input multiple-output (MIMO) and linear vs. nonlinear control algorithms in this application (the leg is a coupled multivariable system driven by nonlinear actuators). The control schemes developed are a conventional PID (linear SISO), a Linear Quadratic Regulator (LQR) controller (linear MIMO) and a Feedback Linearisation (FL) controller (nonlinear MIMO). LQR performs well at low frequency but its behaviour worsens at higher frequencies. FL produces the fastest response in simulation, but when implemented is sensitive to parameters uncertainty and needs to be properly modified to achieve equally good performance also in the practical implementation.

DESIGN AND SCALING OF VERSATILE QUADRUPED ROBOTS

The rough terrain mobility of legged robots is expected to exceed the performance of their wheeled or tracked counterparts. To fully take advantage of the legs, such robots need to be versatile by achieving highly dynamic motions at the same time as careful navigation over rough terrain. Highly dynamic robots need to be designed to be fast and strong enough to run and jump. A dynamic robot needs to be light and powerful at the same time. Two requirements that are conflicting and therefore have to be traded off. In this work we present a tool that helps quadruped robot designer to better select and size joint actuators for various robot sizes. We use the squat jump as characteristic motion of a highly dynamic robot and estimate required joint torque and velocity in relation to maximum jump height, body mass and leg segment length.

Modeling of a novel 3-way rotary type electro-hydraulic valve

Hydraulic actuation is characterized by fast dynamics, high power density, high stiffness, large output force/torque, and in recent years it has become an increasing attractive form of robot actuation. Hydraulic control valves that not only provide the interface between hydraulic power element and actuators, but also receive feedback signal and adjust the system output accordingly, are key components of hydraulic actuation systems. This paper presents a novel 3-way rotary type electro-hydraulic valve which is driven by a DC motor. The valve is mainly composed of a rotary spool, a bush and a body. The operating principle is presented in details and a mathematical model is developed. In particular, the drag torque that majorly affects the valve performance is analyzed based on the theory of fluid mechanics.

Performance Assessment of Digital Hydraulics in a Quadruped Robot Leg

ABSTRACT cylinders and/or This paper presents an investigation of the performance of digital hydraulic actuation in robot applications. The research compares two different hydraulic actuation systems, utilizing servo and digital hydraulic valves, developed to drive leg one of a hydraulic quadruped robot (HyQ). Comparisons between the two systems for position tracking, required flow rate and system efficiency are discussed. Results show that digital hydraulic systems can be a valid alternative to servo valves in terms of position tracking, and show that digital valves can greatly improve system performance in the form of reduced required flow rate and improved overall system efficiency. INTRODUCTION Electric motors are the typicalactuation method employed in robotics because of their low cost and the large availability of sizes and specifications. although electric motors However, are simple and accurate to control their performance is limited by several factors. Including that their available torque is small relative to their size and weight, and they often require a gearbox and gears introduce backlash and reduce that driveefficiency. Alternatively, hydraulic actuators have relatively faster a response, and a higher force/torque-to-weight ratio thanelectric actuators. Hydraulic actuation has been employed in a wide range of robotic systems, namely the exoskeleton system BLEEX [1-2], Raytheon SARCOS [3], SARCOS hydraulically actuated humanoid robot CB [4], legged robots Kenken [5], BigDog [6-8], Petman [9], KITECH and POSTECH Korean Quadruped Robot [10] andthe hydraulic quadruped HyQ [11 -13].

Coupled systems analyses for high-performance robust force control of wearable robots

A wearable robot is constantly in contact with its user. To properly and safely perform tasks together with the wearer, such as walking and load carrying, it is important that the robot is able to control its joint torques. To enhance the performance of torque/force controllers, feedforward controllers such as velocity and friction compensation are commonly used. Although such controllers are able to enhance the torque closed-loop bandwidth, they can also significantly reduce the systems robustness. For coupled systems, such as wearable robots, the soft human skin and the compliance of the human/robot attachment pose additional challenges to the performance and stability of such controllers. In this paper we investigate the robustness issues associated with the force control on coupled systems, performing thorough analyses of the torque loop sensitivity, including how the attachment stiffness and the human impedance may influence it. Based on these analyses, we propose two potential control solutions that may improve both the disturbance attenuation and torque reference tracking on wearable robots.