Nonlinear Fault Detection for Hydraulic Systems

One of the most important areas in the robotics industry is the development of robots capable of working in hazardous environments. As humans cannot safely or cheaply work in these environments, providing a high level of robotic functionality is important. Our work in this area focuses on a fault detection method known as analytical redundancy, or AR. In this paper we discuss the application to a hydraulic servovalve system of our novel rigorous nonlinear AR technique. AR is a model-based state-space technique that is theoretically guaranteed to derive the maximum number of independent tests of the consistency of sensor data with the system model and past control inputs. Conventional linear AR is only valid for linear sampled data systems. However, our new nonlinear AR (NLAR) technique maintains traditional linear AR’s mathematical guarantee to generate the maximum possible number of independent tests in the nonlinear domain. Thus NLAR allows us to gain the benefits of AR testing for nonlinear systems with both continuous and sampled data

Advanced Control Strategies for Mobile Hydraulic Applications

Mobile hydraulic machines are affected by numerous undesired dynamics, mainly discontinuous motion and vibrations. Over the years, many methods have been developed to limit the extent of those undesired dynamics and improve controllability and safety of operation of the machine. However, in most of the cases, today\u27s methods do not significantly differ from those developed in a time when electronic controllers were slower and less reliable than they are today. This dissertation addresses this aspect and presents a unique control method designed to be applicable to all mobile hydraulic machines controlled by proportional directional valves. In particular, the proposed control method is targeted to hydraulic machines such as those used in the field including construction (wheel loaders, excavators, and backhoes, etc.), load handling (cranes, reach-stackers, and aerial lift, etc.), agriculture (harvesters, etc.), forestry, and aerospace. For these applications the proposed control method is designed to achieve the following goals: A. Improvement of the machine dynamics by reducing mechanical vibrations of mechanical arms, load, as well as operator seat; B. Reduction of the energy dissipation introduced by current vibration damping methods; C. Reduction of system slowdowns introduced by current vibration damping methods. Goal A is generally intended for all machines; goal B refers to those applications in which the damping is introduced by means of energy losses on the main hydraulic transmission line; goal C is related to those applications in which the vibration attenuation is introduced by slowing down the main transmission line dynamics. Two case studies are discussed in this work: 1. Hydraulic crane: the focus is on the vibrations of the mechanical arms and load (goals A and B). 2. Wheel loader: the focus is on the vibrations of the driver\u27s seat and bucket (goals A and C). The controller structure is basically unvaried for different machines. However, what differs in each application are the controller parameters, whose adaptation and tuning method represent the main innovations of this work. The proposed controller structure is organized so that the control parameters are adapted with respect to the instantaneous operating point which is identified by means of feedback sensors. The Gain Scheduling technique is used to implement the controller whose set of parameters are function of the specific identified operating point. The optimal set of control parameters for each operating point is determined through the non-model-based controller tuning. The technique determines the optimal set of controller parameters through the optimization of the experimental machine dynamics. The optimization is based on an innovative application of the Extremum Seeking algorithm. The optimal controller parameters are then indexed into the Gain Scheduler. The proposed method does not require the modification of the standard valve controlled machine layout since it only needs for the addition of feedback sensors. The feedback signals are used by the control unit to modify the electric currents to the proportional directional valves and cancel the undesired dynamics of the machine by controlling the actuator motion. In order for the proposed method to be effective, the proportional valve bandwidth must be significantly higher than the frequency of the undesired dynamics. This condition, which is typically true for heavy machineries, is further investigated in the research. The research mostly focuses on the use of pressure feedback. In fact, although the use of position, velocity, or acceleration sensors on the vibrating bodies of the machine would provide a more straightforward measurement of the vibration, they are extremely rare on mobile hydraulic machines where mechanical and environmental stress harm their integrity. A comparison between pressure feedback and acceleration feedback alternatives of the proposed method is investigated with the aim to outline the conditions making one alternative preferable over the other one (for those applications were both alternatives are technically viable in terms of sensors and wiring reliability). A mid-sized hydraulic crane (case study 1) was instrumented at Maha Fluid Power Research Center to study the effectiveness of the proposed control method, its stability and its experimental validation. Up to 30% vibration damping and 40% energy savings were observed for a specific cycle over the standard vibration damping method for this application. The proposed control method was also applied to a wheel loader (case study 2), and up to 70% vibrations attenuation on the bucket and 30% on the driver\u27s cab were found in simulations. These results also served to demonstrate the applicability of the control method to different hydraulic machines. Improved system response and a straightforward controller parameters tuning methodology are the features which give to the proposed method the potential to become a widespread technology for fluid power machines. The proposed method also has potential for improving several current vibration damping methods in terms of energy efficiency as well as simplification of both the hydraulic system layout and tuning process

Controlador em cascata com adaptação de parâmetros para robôs hidráulicos

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2010Este trabalho trata do problema de controle de robôs hidráulicos que utilizam válvulas proporcionais direcionais com centro supercrítico realizando seguimento de trajetória de posição e tem enfoque na compensação de zona morta destas válvulas. Os robôs hidráulicos apresentam grande capacidade de carga e um grande potencial de aplicação na indústria. Isto se deve a elevada relação força/dimensão dos atuadores hidráulicos e a capacidade que possuem para responder de forma rápida aos sinais de comando. No entanto, os robôs hidráulicos oferecem algumas dificuldades ao controle. Há, por exemplo, acoplamento de dinâmicas, dificuldades da estimativa de parâmetros do modelo matemático, variação de alguns parâmetros durante a operação e comportamento não-linear provocado pelo atrito no cilindro hidráulico e pela zona morta das válvulas proporcionais direcionais de centro supercrítico. Neste trabalho, uma estratégia de controle em cascata é aplicada com objetivo de superar estas dificuldades. Esta estratégia tem como característica a divisão do modelo matemático do robô hidráulico em subsistemas e permite a aplicação de técnicas de controle não-linear para superar as dificuldades inerentes de cada subsistema. Tanto o atrito no cilindro hidráulico quanto a zona morta da válvula provocam os erros de seguimento de trajetória do robô. O atrito influi na dinâmica do movimento no subsistema mecânico que tem como entrada a força produzida no cilindro pelo subsistema hidráulico e como saída a posição angular dos elos do robô. Os erros de seguimento de posição dos elos provocados pelo atrito podem ser reduzidos através de sua compensação direta no subsistema mecânico utilizando observadores de atrito baseados em modelos dinâmicos. A zona morta retarda a abertura da válvula gerando erros significativos de seguimento de força no subsistema hidráulico. Sua compensação pode ser realizada através de uma função inversa da zona morta. Assim, apresenta-se um controlador em cascata capaz de compensar a dinâmica da válvula e sua zona morta, a dinâmica da força hidráulica, do atrito e do movimento dos elos do robô. Apresentam-se, também, leis de adaptação de parâmetros para a função inversa da zona morta. A implementação destas leis adaptativas tem como objetivo principal a redução dos erros de seguimento no subsistema hidráulico através da compensação da zona morta e, consequentemente, a redução dos erros de seguimento de posição angular dos elos do robô. Mostra-se, através da análise de estabilidade por Lyapunov e de forma experimental, que os erros resultantes do seguimento de trajetória convergem para um conjunto residual mesmo quando o controlador em cascata não realiza a compensação do atrito e da dinâmica da válvula, mas utiliza as leis de adaptação e a compensação de zona morta propostas neste trabalho. Os resultados teóricos e experimentais permitem concluir que a compensação de zona morta também pode compensar, de forma indireta, outras dinâmicas como a do atrito.This work addresses the problem of controlling hydraulic robots using overlapped proportional valves which perform trajectory tracking, with special focus on the dead-zone compensation of these valves. Hydraulic robots present great load capability and a huge potential of application in the industry. This is due to the high torque/size ratio of the hydraulic actuators and its ability to respond quickly to control signals. However, hydraulic robots introduce some difficulties to the control. There are, for example, dynamic coupling, problems to estimate the mathematical model parameters, changes in some parameters during operation and a nonlinear behavior mainly caused by friction in the hydraulic cylinder and the dead-zone of overlapped proportional valves. In this work, a cascade control strategy is applied to surpass these difficulties. This strategy is based on the division of the hydraulic robot mathematical model into subsystems and allows the application of nonlinear control techniques to overcame the inherent difficulties in each subsystem. Both the friction in the hydraulic cylinder and the dead-zone of the valve cause tracking errors in the robot trajectory. Friction influences the motion dynamics in the mechanical subsystem which has as input the force produced in the cylinder by the hydraulic subsystem and as output the angular position of the robot links. Tracking errors of the links positions caused by friction can be reduced through its direct compensation in the subsystem using mechanical friction observers based on dynamic models. The dead-zone slows the opening of the valve causing significant force tracking errors in the hydraulic subsystem. The compensation can be accomplished through an inverse function of the deadzone. Thus, this work presents a cascade controller able to compensate the dynamics of the valve and the dead-zone, the dynamics of the hydraulic force, friction and motion of the robot links. Adaptation laws for the dead-zone inverse function are also presented. The adaptation laws implementation has the main goal to reduce tracking errors of the hydraulic subsystem through the dead-zone compensation and consequently the reduction of angular position tracking errors of the robot. It is shown through Lyapunov stability analysis and through experimental ents that position tracking errors converge to a residual set even when the cascade controller does not perform compensation for friction and valve dynamics, but using only the adaptation laws and dead-zone compensation proposed in this work. Theoretical and experimental results showed that the dead-zone compensation may also compensate, indirectly, other dynamics such as friction

Metodología de diseño de manos robóticas basada en los estados de su sistema accionador

La mano humana es una de las herramientas más asombrosas de la naturaleza, tanto que no ha podido ser superada en ningún aspecto hasta el momento. Siendo el principal medio por el cual se ha creado y construido, directa o indirectamente, todo lo artificial que actualmente nos rodea, es natural pensar de que gran parte de la comunidad científica relacionada con la robótica dedique grandes esfuerzos por imitarla. En la actualidad se puede realizar un extenso catálogo de manos robóticas desarrolladas y todas buscan resolver un determinado comportamiento de la mano humana, aún así, éstas se pueden dividir en tres grupos bien definidos: las pinzas robóticas, las cuales se caracterizan por su aplicación industrial en tareas de agarre firme de elementos específicos y por su robustez, precio y vida útil; por otro lado, están las manos robóticas subactuadas en las que se buscan mecanismos cada vez más complejos que hagan disminuir la cantidad de actuadores y la complejidad de su sistema de control a favor de mejorar la funcionalidad de las pinzas robóticas en lo que se refiere a extender su capacidad de agarre a objetos con formas y tamaños cada vez más diferentes; y finalmente encontramos las demás manos robóticas en las que su objetivo es la experimentación de un determinado comportamiento de la mano humana más centrada en las tareas de manipulación. Esta tesis propone una metodología de diseño de manos robóticas desde un punto de vista particular, que es el de los estados que puede ofrecer su sistema de accionamiento, teniendo en cuenta la capacidad de combinarlos y hacerlos independientes. Los elementos móviles que componen una mano robótica son accionados por un actuador o conjunto de actuadores. El sistema accionador es el órgano principal que da vida a un determinado sistema robótico como una mano robótica, por lo tanto es preciso identificar la capacidad que tiene el mismo de hacer que ese movimiento pueda generar tareas cada vez más complejas. La forma de identificar esta capacidad se resume en los estados y la calidad de los mismos que el sistema accionador puede ofrecer. Esta metodología de diseño se basa fundamentalmente en este concepto y que si bien en este trabajo es aplicado a manos robóticas, puede ser extendido a cualquier sistema robótico que disponga de un sistema accionador y de esta forma optimizar sus recursos no sólo a nivel funcional, sino también en el ahorro de energía. En el transcurso de este trabajo se han diseñado dos manos robóticas con esta metodología y se ha realizado un ensayo de viabilidad técnica de un actuador capaz de ofrecer un número finito de estados mayor a los tres que ofrece actualmente cualquier actuador. Estos diseños han demostrado que este tipo de metodología puede ofrecer una alternativa para la optimización del sistema accionador de una mano robótica. Por otro lado, la misma también puede ser aplicada a cualquier tipo de mano robótica y para cualquier aplicación y servir como una herramienta útil para el análisis del diseño de las manos robóticas actuales y buscar puntos de optimización para futuros desarrollos