Search

Modal identification is a key step in modal analysis. It enables the researcher to extract modal parameters, such as natural frequency, amplitude, and damping from a given structure. There are a considerable number of techniques in the state of the art aiming to address this problem, where multi-mode approaches arise as an appealing choice due to their ability to deal with mode coupling. This tutorial paper focuses on the complex-curve fitting technique, originally conceived for an application distinct from modal analysis. It aims at guiding other researchers by providing a tutorial-like and in-depth analysis of this important method, associated with a nonlinear weighting procedure for improved precision. Additionally, this paper fills a gap on the original technique, which is limited to the ratio of two polynomials, by proposing an automatic parameter extraction technique. The original and improved methods are applied on both simulated and experimental data, highlighting the effectiveness of the proposed changes. The proposed procedure is also compared with the rational fraction polynomial method.

The Delta robot, widely used in fast pick-and-place applications with pure position control, is a parallel kinematic chain with three rotational inputs resulting in three pure translations at the end-effector. This paper proposes a complete task-space impedance control with inverse dynamics to give this robot compliant behavior, enabling it to be used in tasks involving physical interaction. For that purpose, the well-known usage of dual quaternion algebra for kinematics modeling is novelly integrated with a neural network to compose a compact representation for the forward kinematics function, that is singularity-free and suitable for real-time calculation. This network computes the forward kinematics more than 150 times faster than a numeric equation solving algorithm, with an average estimation error of less than 0.5 mm. The proposed algorithm is implemented in a rigid body simulator, and the performance of the complete system is analyzed.

This paper presents a design concept for affordable exoskeletons without renouncing to advanced interaction control modalities based on force control technologies. Affordability is reached by (1) lowering the motor requirements, thanks to a smart mechanical design, (2) lowering the sensing requirements, thanks to extensive use of microprocessor DMA, and (3) using low-cost computational platforms. A main finding of this work is that by lowering the motor requirements the exoskeleton improves its inherent force controllability.

In almost all mechatronic devices, safety is a fundamental requirement. Unpredicted system behavior resultant from control instability may potentially damage objects or even harm human users. To certify that the system will remain stable under predefined conditions is not only desirable but mandatory for systems like jet engines and wearable robots (e.g., robotic prosthesis and exoskeleton robots). The certification of control algorithms is already a standard procedure in some engineering fields, such as aviation. In robotics, however, a certification procedure is not yet traditionally incorporated in the control design. To fill this gap is an essential step towards making robots, especially those that closely interact with human beings, largely available on the market and endorsed by the public in general. This paper uses the $$\nu $$ -gap metric and the generalized stability margin to assess the stability of a closed-loop linear system, accounting for differences between plants. A novel iterative certification procedure based on these two techniques is proposed, combined with optimization techniques to reduce conservatism. The procedure is demonstrated on a real 1-DoF hydraulically actuated platform.

Although the design of legged robots may be dependent on the application, all of them share the need to safely deal with physical interaction with the environment in every step they take. Impedance controllers have been applied with success to handle contact, and some authors applied the concept of passivity to guarantee stability during the interaction. Whereas previous studies on the passivity of legged robots considered aspects such as inner force loop gains and actuation bandwidth influence on the Z-width (i.e. the range of renderable passive impedances), they did not take into account the role of the kinematic configuration of the leg on the stability of the interaction. Thus, in this work we present a systematic analysis of the effects of joint positions on the passivity conditions of a robotic leg and show that this is a very relevant aspect that may seriously affect the stability and passivity of an impedance controller. By analyzing a linearized model of the leg via its Nyquist plots and the respective Z-width diagrams, we were able to determine what joint configurations within the leg workspace are more suitable to physically interact with the environment or people.

IEEE Transactions on Robotics, 31 (6), ISSN:1552-3098, ISSN:1042-296X, ISSN:1941-0468

Fasteners assembly automation in different industries require flexible systems capable of dealing with faulty situations. Fault detection and isolation (FDI) techniques are used to detect failure and deal with them, avoiding losses on parts, tools or robots. However, FDI usually deals with the faults after or at the moment they occur. Thus, we propose a method that predicts potential failures online, based on the forces and torques signatures captured during the task. We demonstrate the approach experimentally using an industrial robot, equipped with a force-torque sensor and a pneumatic gripper, used to align and thread nuts into bolts. All effort information is fed into a supervised machine learning algorithm, based on a Convolutional Neural Network (CNN) classifier. The network was able to predict and classify the threading task outcomes in 3 groups: mounted, not mounted or jammed. Our approach was able to reduce in 10.9% the threading task execution time when compared to a reference without FDI, but had problem to predict jammed cases. The same experiment was also performed with other two additional learning algorithms, and the results were systematically compared.

This paper considers the torque control problem for robots with flexible joints driven by electrical actuators. It is shown that the achievable closed-loop tracking bandwidth using PI torque controllers may be limited due to transmission zeros introduced by the load dynamics. This limitation is overcome by using positive feedback from the load motion in unison with PI torque controllers. The positive feedback is given in terms of load velocity, acceleration and jerk. Stability conditions for designing decentralized PI torque controllers are derived in terms of Routh-Hurwitz criteria. Disturbance rejection properties of the closed system are characterized and an analysis is carried out investigating the use of approximate positive feedback by omitting acceleration and/or jerk signals. The results of this paper are illustrated for a two DoF (degrees of freedom) system. Experimental results for a one DoF system are also included.

This chapter reviews different methods for the control of legged locomotion with a special focus on bipedal locomotion. All locomotion systems are governed by complex nonlinear, hybrid dynamics, and are redundant, underactuated and often unstable, which makes their control a very challenging task. The chapter starts with a presentation of different concepts of stability and robustness of locomotion considering nominal walking situations as well as the reaction to larger external perturbations. Then, optimal control is discussed as a guiding principle of human and robot motion, and dynamic multibody system models as well as different optimization problem formulations for the generation, control and analysis of locomotion are shown. Constant or variable compliance plays an important role in biological and bio-inspired locomotion, but needs to be properly adapted in the design and control process which also can be addressed by optimal control. Next, impedance control in locomotion is discussed, looking at passive and active impedance and different approaches to emulated appropriate impedances for robots. The chapter also reviews control approaches for legged locomotion based on template models, i.e. very simple representations of the original locomotor system, with a focus using template models for the design of suitable controllers. The state of the art of passive dynamic walking robots as well as powered and almost passive dynamic robots is summarized and their achievements in terms of energy-efficiency, stability, robustness and versatility re discussed. Hybrid zero dynamics is presented as a control synthesis framework that reduces the complexity of whole-body dynamics control and allows to develop efficient controllers for dynamic walking and running motions. Finally, a control approach for locomotion based on the concept of central pattern generators is presented which helps to control locomotion of legged robots and gives insight into human movement control.

A wearable robot is constantly in contact with its user. To measure and control the human–robot interaction force is important to properly and safely perform tasks together with the wearer, such as walking and load carrying. To avoid the burdensome integration of force sensors at the human–robot attachments, we propose the use of a Kalman filter to perform data and sensor fusion to estimate the interaction force. We demonstrate the impact of real world issues on the measurement estimation and evaluate its performance in hardware. Preliminary simulation and experimental results demonstrate that good measurement prediction and control performance may be achieved with such an estimator.

Proceedings of the 2016 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2016)

This work addresses the question whether active impedance control is key to a breakthrough for legged robots. In this paper, we will talk about controlling the mechanical impedance of joints and legs with a focus on stiffness and damping control. In contrast to passive elements like springs, active impedance is achieved by torque-controlled joints allowing real-time adjustment of stiffness and damping. We argue that legged robots require a high degree of versatility and flexibility to execute a wide range of assistive tasks to be truly useful to humans and thus to lead to a breakthrough. Adjustable stiffness and damping in realtime is a fundamental building block towards versatility. Experiments with our 80 kg hydraulic quadruped robot HyQ demonstrate that active impedance alone (thus no springs in the structure) can successfully emulate passively compliant elements during highly-dynamic locomotion tasks (running 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 running trot. To the authors’ best knowledge this is the first time a flying trot was 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. An extended version of this paper has been published in IJRR in 2015 [43].

The International Journal of Robotics Research, 34 (7), ISSN:0278-3649, ISSN:1741-3176

Impedance control is a well-established technique to control interaction forces in robotics. However, real implementations of impedance control with an inner loop may suffer from several limitations. Although common practice in designing nested control systems is to maximize the bandwidth of the inner loop to improve tracking performance, it may not be the most suitable approach when a certain range of impedance parameters has to be rendered. In particular, it turns out that the viable range of stable stiffness and damping values can be strongly affected by the bandwidth of the inner control loops (e.g. a torque loop) as well as by the filtering and sampling frequency. This paper provides an extensive analysis on how these aspects influence the stability region of impedance parameters as well as the passivity of the system. This will be supported by both simulations and experimental data. Moreover, a methodology for designing joint impedance controllers based on an inner torque loop and a positive velocity feedback loop will be presented. The goal of the velocity feedback is to increase (given the constraints to preserve stability) the bandwidth of the torque loop without the need of a complex controller., 14 pages in Control Theory and Technology (2016)

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 HyQ's actively-compliant leg is able to reproduce the compliant behavior presented by an identical but passively-compliant version of the same leg.

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.

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].

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.

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.