Passive Friction Compensation Using a Nonlinear Disturbance Observer for Flexible Joint Robots with Joint Torque Measurements

The friction and ripple effects from motor and drive cause a major problem for the robot position accuracy, especially for robots with high gear ratio and for high-speed applications. In this paper we introduce a simple, effective, and practical method to compensate for joint friction of flexible joint robots with joint torque sensing, which is based on a nonlinear disturbance observer. This friction observer can increase the performance of the controlled robot system both in terms of the position accuracy and the dynamic behavior. The friction observer needs no friction model and its output corresponds to the low-pass filtered friction torque. Due to the link torque feedback the friction observer can compensate for both friction moment and external moment effects acting on the link. So it can be used not only for position control but also for interaction control, e.g., torque control or impedance control which have low control bandwidth and therefore are sensitive to ripple effects from motor and drive. In addition, its parameter design and parameter optimization are independent of the controller design so that it can be used for friction compensation in conjunction with different controllers designed for flexible joint robots. Furthermore, a passivity analysis is done for this observer-based friction compensation in consideration of Coulomb, viscose and Stribeck friction effects, which is independent of the regulation controller. In combining this friction observer with the state feedback controller \cite{Albu-Schaeffer2}, global asymptotic stability of the controlled system can be shown by using Lyapunov based convergence analysis. Experimental results with robots of the German Aerospace Center (DLR) validate the practical efficiency of the approach

Model-Independent Control of a Flexible-Joint Robot Manipulator

Flexibility at the joint of a manipulator is an intrinsic property. Even "rigid-joint" robots, i

Control of a single-link flexible manipulator

RESUMEN: En aplicaciones de robótica es común utilizar elementos mecánicos y eslabones rígidos. Esto se realiza así especialmente porque simplifica enormemente el modelado matemático, así como la obtención de controladores dinámicos y cinemáticos. Todo esto conlleva el poder obtener manipuladores que permiten una elevada precisión en el movimiento y en el posicionamiento. Sin embargo, cada día es más frecuente que los robots interaccionen con los operadores humanos en diferentes tareas. Ejemplos de esto pueden encontrarse en las aplicaciones industriales donde los robots colaborativos tienen mucho éxito, pero también en aplicaciones médicas y de servicio a personas discapacitadas, donde un robot puede hacer tareas de atención que conlleven una interacción con la persona. Es en estos campos de interacción con las personas donde un robot que incorpore segmentos mecánicos flexibles, tales que el contacto con las personas sea totalmente inocuo, presenta un futuro de interés (además de las aplicaciones espaciales). En el presente trabajo se analizarán y diseñarán distintos controladores basados en redes neuronales, lógica difusa y control GPI con el objetivo de evaluar su funcionamiento en un sistema que incluya eslabones mecánicos flexibles.ABSTRACT: In robotics applications it is common to use mechanical elements and rigid links. This is done especially because it greatly simplifies mathematical modeling, as well as obtaining dynamic and kinematic controllers. All this leads to manipulators that allow high precision in movement and positioning. However, it is becoming increasingly common for robots to interact with human operators in different tasks. Examples of this can be found in industrial applications where collaborative robots are very successful, but also in medical and service applications for disabled people, where a robot can perform care tasks that involve interaction with the person. It is in these fields of interaction with people that a robot incorporating flexible mechanical segments, such that contact with people is completely harmless, presents a future of interest (in addition to space applications). In this work, different controllers based on neural networks, fuzzy logic and GPI control will be analyzed and designed in order to evaluate their performance in a system including flexible mechanical links.Grado en Ingeniería en Electrónica Industrial y Automátic

Robots de estructura flexible: Análisis, modelado y diseño de controladores

Las plantas industriales de producción actuales se han vuelto cada vez más adaptables, bien porque los productos se actualizan más frecuentemente, o bien para obtener una personalización a requerimiento de la demanda del consumidor. Consecuentemente, las líneas de producción requieren cambios rápidos, simples y económicos, para minimizar retrasos de producción o incremento de costos que se trasladarían al producto final [1]. Es ahí donde la robótica industrial aventaja a otras formas de automatización, porque precisamente los robots tienen la capacidad de adaptarse a modificaciones de las tareas que ejecutan, realizando cambios únicamente en su programación [2].De la misma manera, en los últimos años los robots se han integrado en aplicaciones que incluyen fusión de capacidades de sensado para facilitar la adaptabilidad. Los sensores dotan de cierta inteligencia al robot, para que esa adaptación a los cambios en su entorno de trabajo sea total o parcialmente automática. Si bien la integración de sensado en nuevas aplicaciones era baja a comienzos del milenio [3], esa tendencia se ha revertido en la actualidad por las mejoras de prestaciones y reducción de precios de los sensores, hasta casi no haber nuevas aplicaciones robotizadas sin algún tipo de sensado incorporado [4].La adaptabilidad requerida para la producción también hace necesaria la incorporación de robots a instalaciones para trabajo conjunto con personas, y eso ha obligado a modificar el diseño de los robots para garantizar que la interacción sea segura. La necesidad de trabajo colaborativo humanos-robot introdujo el modelo de robots colaborativos o cobots industriales.Los cobots se diseñan con diversas características técnicas en busca de asegurar que los riesgos en el contacto directo con humanos o con objetos que lo rodean sean minímos. Entre esas características se encuentran la construccion con materiales ligeros, bordes redondeados, pieles y sensores adicionales,de cierta manera cualidades que limitan los riesgos en caso de contacto. Eluso de materiales ligeros y deformables para la construcción de la estructura del cobot implica que la masa en movimiento es menor (que en el caso de robots) y que existe acomodamiento pasivo. Por consiguiente las posiblesconsecuencias de un choque son menores y la interacción humanomáquina es también más segura [5]. Asimismo, al poseer eslabones ligeros, se requiere menor consumo y actuadores más pequeños. Esta liviandad contribuye a que los cobots puedan ser fácilmente reubicados dentro de la planta industrial de acuerdo a la aplicación a realizar, y liberar el espacio cuando no sean usados.Por otro lado, los cobots han incorporado técnicas de programación sencillas que pueden ser realizadas sin recurrir a expertos, con programación por enseñanza o demostración - moviendo al robot manualmente- o mediante interfaces gráficas intuitivas [6] [7]. Además los cobots se pueden usar para automatizar partes de una línea de producción con cambios mínimos en el resto de la línea, proporcionando a los fabricantes pequeños y medianos, un punto de entrada económicamente viable para la automatización robótica [5].Todos estos factores mencionados redundan en que la robotización industrialutilizando cobots sea, en general, más económica

Numerical solutions for design and dynamic control of compliant robots

This work is focused on the development of numerical methods for the design and control of robots, with particular emphasis on joint elasticity. First, a general methodology is presented that is able to solve the problem of computing the inverse dynamics of a serial robot manipulator with an arbitrarily large number of elastic joints in a recursive numerical way. The solution algorithm is a generalized version of the standard Newton-Euler approach. The algorithm is presented with numerous extensions and variants, including the extension to variable-stiffness technologies and control applications. Then, an optimization framework is introduced for the design and analysis of biped walkers characterized by elastic joints, with comparative results demonstrating the scope of application of joint compliance in bipedal walking

Ansätze zur entkoppelten Regelung von mechanisch gekoppelten Doppelgelenken eines DLR-Medizinroboters

In dieser Arbeit werden die Methoden zur Modellierung, Identifikation, Regelung und Reibungskompensation von DLR-Medizinrobotern mit elastischen und differentiell getriebenen Gelenken vorgestellt. In der Praxis zeigt sich, dass bei den DLR-Medizinrobotern sowohl die Gelenkelastizität als auch die Gelenkverkopplung innerhalb des Doppelgelenks in der Modellierung und Regelung berücksichtigt werden muss. Daher ist es das Hauptziel dieser Arbeit, eine Regelungsstruktur zu entwickeln, die das gewünschte Folge- und Einschwingverhalten erreicht. Zur Regelung wird mit Hilfe der modalen Entkopplung ein MIMO – Zustandsregler für die Doppelgelenkstruktur eingeführt, der auf der Rückführung der antriebsseitigen Positionen und abtriebsseitigen Drehmomente sowie deren Ableitungen basiert. Für die medizinischen Anwendungen spielt die Positionsgenauigkeit, die durch die hohe Reibung stark beeinflusst wird, eine entscheidende Rolle. Um die Positioniergenauigkeit des Roboters zu verbessern, wird ein Reibungsbeobachter entwickelt, der eine passive Reibungskompensation ermöglicht. Da der Reibungsbeobachter nur den Momentenfehler, nicht aber den Positionsfehler integriert, wird diese Kompensation außer zur Positionsregelung auch zur Verbesserung der Qualität der Drehmoment- und Impedanzregelung eingesetzt. Für die gesamte Regelungsstruktur aus Zustandsregler und Reibungsbeobachter wird global asymptotische Stabilität des gesamten nichtlinearen Roboters nachgewiesen. Diese Regelungsstruktur wird derzeit in zahlreichen Anwendungen mit den DLR-Medizinrobotern benutzt und validiert

Adaptive Steuerung eines mehrsegmentigen Inspektionsroboters

Mehrsegmentige Roboter können verschiedene Umgebungen befahren und inspizieren. In dieser Arbeit wird eine Steuerung entwickelt, die die selbständige Durchführung komplexer Bewegungen in unstrukturierten Umgebungen ermöglicht. Vorgaben eines menschlichen Operators und Zustandsinformationen des Roboters werden in einem Regelkreis verarbeitet. Drei Sensorsysteme erfassen diesen Zustand. Die Teilkomponenten und das Gesamtsystem werden auf den Zielsystemen MakroPlus und Kairo-II evaluiert

From plain visualisation to vibration sensing: using a camera to control the flexibilities in the ITER remote handling equipment

Thermonuclear fusion is expected to play a key role in the energy market during the second half of this century, reaching 20% of the electricity generation by 2100. For many years, fusion scientists and engineers have been developing the various technologies required to build nuclear power stations allowing a sustained fusion reaction. To the maximum possible extent, maintenance operations in fusion reactors are performed manually by qualified workers in full accordance with the "as low as reasonably achievable" (ALARA) principle. However, the option of hands-on maintenance becomes impractical, difficult or simply impossible in many circumstances, such as high biological dose rates. In this case, maintenance tasks will be performed with remote handling (RH) techniques. The International Thermonuclear Experimental Reactor ITER, to be commissioned in southern France around 2025, will be the first fusion experiment producing more power from fusion than energy necessary to heat the plasma. Its main objective is “to demonstrate the scientific and technological feasibility of fusion power for peaceful purposes”. However ITER represents an unequalled challenge in terms of RH system design, since it will be much more demanding and complex than any other remote maintenance system previously designed. The introduction of man-in-the-loop capabilities in the robotic systems designed for ITER maintenance would provide useful assistance during inspection, i.e. by providing the operator the ability and flexibility to locate and examine unplanned targets, or during handling operations, i.e. by making peg-in-hole tasks easier. Unfortunately, most transmission technologies able to withstand the very specific and extreme environmental conditions existing inside a fusion reactor are based on gears, screws, cables and chains, which make the whole system very flexible and subject to vibrations. This effect is further increased as structural parts of the maintenance equipment are generally lightweight and slender structures due to the size and the arduous accessibility to the reactor. Several methodologies aiming at avoiding or limiting the effects of vibrations on RH system performance have been investigated over the past decade. These methods often rely on the use of vibration sensors such as accelerometers. However, reviewing market shows that there is no commercial off-the-shelf (COTS) accelerometer that meets the very specific requirements for vibration sensing in the ITER in-vessel RH equipment (resilience to high total integrated dose, high sensitivity). The customisation and qualification of existing products or investigation of new concepts might be considered. However, these options would inevitably involve high development costs. While an extensive amount of work has been published on the modelling and control of flexible manipulators in the 1980s and 1990s, the possibility to use vision devices to stabilise an oscillating robotic arm has only been considered very recently and this promising solution has not been discussed at length. In parallel, recent developments on machine vision systems in nuclear environment have been very encouraging. Although they do not deal directly with vibration sensing, they open up new prospects in the use of radiation tolerant cameras. This thesis aims to demonstrate that vibration control of remote maintenance equipment operating in harsh environments such as ITER can be achieved without considering any extra sensor besides the embarked rad-hardened cameras that will inevitably be used to provide real-time visual feedback to the operators. In other words it is proposed to consider the radiation-tolerant vision devices as full sensors providing quantitative data that can be processed by the control scheme and not only as plain video feedback providing qualitative information. The work conducted within the present thesis has confirmed that methods based on the tracking of visual features from an unknown environment are effective candidates for the real-time control of vibrations. Oscillations induced at the end effector are estimated by exploiting a simple physical model of the manipulator. Using a camera mounted in an eye-in-hand configuration, this model is adjusted using direct measurement of the tip oscillations with respect to the static environment. The primary contribution of this thesis consists of implementing a markerless tracker to determine the velocity of a tip-mounted camera in an untrimmed environment in order to stabilise an oscillating long-reach robotic arm. In particular, this method implies modifying an existing online interaction matrix estimator to make it self-adjustable and deriving a multimode dynamic model of a flexible rotating beam. An innovative vision-based method using sinusoidal regression to sense low-frequency oscillations is also proposed and tested. Finally, the problem of online estimation of the image capture delay for visual servoing applications with high dynamics is addressed and an original approach based on the concept of cross-correlation is presented and experimentally validated

From Underactuation to Quasi‐Full Actuation: A Unifying Control Framework for Rigid and Elastic Joint Robot

The quest for animal-like performance in robots has driven the integration of elastic elements in their drive trains, sparking a revolution in robot design. Elastic robots can store and release potential energy, providing distinct advantages over traditional robots, such as enhanced safety in human-robot interaction, resilience to mechanical shocks, improved energy efficiency in cyclic tasks, and dynamic motion capabilities. Exploiting their full potential, however, necessitates novel control methods. This thesis advances the field of nonlinear control for underactuated systems and utilizes the results to push the boundaries of motion and interaction performance of elastic robots. Through real-life experiments and applications, the proposed controllers demonstrate that compliant robots hold promise as groundbreaking robotic technology. To achieve these objectives, we first derive a simultaneous phase space and input transformation that enables a specific class of underactuated Lagrangian systems to be treated as if fully actuated. These systems can be represented as the interconnection of actuated and underactuated subsystems, with the kinetic energy of each subsystem depending only on its own velocity. Elastic robots are typical representatives. We refer to the transformed system as quasi-fully actuated due to weak constraints on the new inputs. Fundamental aspects of the transforming equations are 1) the same Lagrangian function characterizes both the original and transformed systems, 2) the transformed system establishes a passive mapping between inputs and outputs, and 3) the solutions of both systems are in a one-to-one correspondence, describing the same physical reality. This correspondence allows us to study and control the behavior of the quasi-fully actuated system instead of the underactuated one. Thus, this approach unifies the control design for rigid and elastic joint robots, enabling the direct application of control results inherited from the fully-actuated case while ensuring closed-loop system stability and passivity. Unlike existing methods, the quasi-full actuation concept does not rely on inner control loops or the neglect and cancellation of dynamics. Notably, as joint stiffness values approach infinity, the control equivalent of a rigid robot is recovered. Building upon the quasi-full actuation concept, we extend energy-based control schemes such as energy shaping and damping injection, Euler-Lagrange controllers, and impedance control. Moreover, we introduce Elastic Structure Preserving (ESP) control, a passivity-based control scheme designed for robots with elastic or viscoelastic joints, guided by the principle of ``do as little as possible''. The underlying hope is that reducing the system shaping, i.e., having a closed-loop dynamics match in some way the robot's intrinsic structure, will award high performance with little control effort. By minimizing the system shaping, we obtain low-gain designs, which are favorable concerning robustness and facilitate the emergence of natural motions. A comparison with state-of-the-art controllers highlights the minimalistic nature of ESP control. Additionally, we present a synthesis method, based on purely geometric arguments, for achieving time-optimal rest-to-rest motions of an elastic joint with bounded input. Finally, we showcase the remarkable performance and robustness of the proposed ESP controllers on DLR David, an anthropomorphic robot implemented with variable impedance actuators. Experimental evidence reveals that ESP designs enable safe and compliant interaction with the environment and rigid-robot-level accuracy in free motion. Additionally, we introduce a control framework that allows DLR David to perform commercially relevant tasks, such as pick and place, teleoperation, hammer drilling into a concrete block, and unloading a dishwasher. The successful execution of these tasks provides compelling evidence that compliant robots have a promising future in commercial applications