On Observer-Based Control of Nonlinear Systems

Filtering and reconstruction of signals play a fundamental role in modern signal processing, telecommunications, and control theory and are used in numerous applications. The feedback principle is an important concept in control theory. Many different control strategies are based on the assumption that all internal states of the control object are available for feedback. In most cases, however, only a few of the states or some functions of the states can be measured. This circumstance raises the need for techniques, which makes it possible not only to estimate states, but also to derive control laws that guarantee stability when using the estimated states instead of the true ones. For linear systems, the separation principle assures stability for the use of converging state estimates in a stabilizing state feedback control law. In general, however, the combination of separately designed state observers and state feedback controllers does not preserve performance, robustness, or even stability of each of the separate designs. In this thesis, the problems of observer design and observer-based control for nonlinear systems are addressed. The deterministic continuous-time systems have been in focus. Stability analysis related to the Positive Real Lemma with relevance for output feedback control is presented. Separation results for a class of nonholonomic nonlinear systems, where the combination of independently designed observers and state-feedback controllers assures stability in the output tracking problem are shown. In addition, a generalization to the observer-backstepping method where the controller is designed with respect to estimated states, taking into account the effects of the estimation errors, is presented. Velocity observers with application to ship dynamics and mechanical manipulators are also presented

Force Controlled Knife-Grinding with Industrial Robot

This paper investigates the application of sharpening knives using a force controlled industrial robot, for an arbitrary knife shape and orientation. The problem is divided into different parts: calibration of the knife by identifying its unknown orientation, identification of the knife blade contour and estimation of its position in the robot frame through force control, and grinding of the knife, following the path defined by the earlier identified shape, while applying the desired contact force to the revolving grinding wheels. The experimental results show that the knives can be sharpened satisfactorily. An industrial application has also been developed and tested, and it has produced a sharpening quality equal or greater to that achieved manually

Robotic Force Control using Observer-based Strict Positive Real Impedance Control

This paper presents theoretical and experimentalresults on observer-based impedance control for forcecontrol without velocity feedback. As the velocity may notavailable to measurement, which is often the case for industrialrobots, an observer was designed to reconstruct velocityin such a way that it be useful for stabilizing feedbackcontrol and to modification of the damping in the impedancerelationship. A good model of the robot joint used wasobtained by system identification. Experiments were carriedout on an ABB industrial robot 2000 to demonstrate resultson observer-based SPR feedback applied in the design.Stability was shown via a modified Popov criterion

Periodic Motion Planning for Virtually Constrained (Hybrid) Mechanical Systems

The paper presents sufficient and almost necessary conditions for the presence of periodic solutions for zero dynamics of virtually constrained under-actuated Euler-Lagrange system. This result is further extended to detect periodic solutions for a class of hybrid systems in the plane and analyze their orbital stability and instability. Illustrative examples are given

Flexible application development and high-performance motion control based on external sensing and reconfiguration of ABB industrial robot controllers

New robot applications increasingly require external sensing to accomplish robustness and performance despite the variations and uncertainties that come with the increasing demands on flexibility for manufacturing of customized products. The demands on productivity then require high performance, which for well known feedback-control reasons means short response times from external process event to reaction on the robot motor control. The need for high- performance sensor interfaces connected to the motion control without excessive buffering and computations has therefore gained increasing attention over the last decade. Apart from high sampling frequencies and short latencies, there are also engineering aspects to consider. Control engineering tools should be possible to utilize, additional computing power is better connected via real-time Ethernet connections, and all interfaces should check compatibility without sacrificing performance. Furthermore, all that should preferably coexist with the original system, superimposing the effects on external sensing while still maintaining safety and consistency of data. The reason is that the original control software is model based and built on millions of lines of C-code, and tested beyond the possibilities of a research lab. Such a system has been accomplished for control and application development using ABB robots. A short description of the system is given, and its use for application/control development is exemplified. User specific feedback control can be done easily with short sampling periods and less than 1ms of latency from sensing to motor reaction

Active Damping of a Flexible Beam

This paper describes the active damping of a flexible beamconnected to the end-effector of a robot manipulator. An observer-basedfeedback controller is developed and experimental results arepresented. Iterative feedback tuning (IFT) is applied to shape the step response

Color-Based Detection Robust to Varying Illumination Spectrum

In color-based detection methods, varying illumination often causes problems, since an object may be perceived to have different colors under different lighting conditions. In the field of color constancy this is usually handled by estimating the illumination spectrum and accounting for its effect on the perceived color. In this paper a method for designing a robust classifier is presented, i.e., instead of estimating and adapting to different lighting conditions, the classifier is made wider to detect a colored object for a given range of lighting conditions. This strategy also naturally handles the case where different parts of an object are illuminated by different light sources at the same time. Only one set of training data per light source has to be collected, and then the detector can handle any combination of the light sources for a large range of illumination intensities

Output Feedback Adaptive Control of Robot Manipulators Using Observer Backstepping

In this paper we present an observer-based adaptive control scheme for robot manipulators, for which we have both unmeasured velocity and uncertain parameters. Using the observer back-stepping method, a reduced-order adaptive velocity observer can be designed independently from the state-feedback controller, which uses damping terms to compensate the presence of the estimation error in the tracking error dynamics. The resulting closed-loop system is semiglobally asymptot-ically stable with respect to the estimation error and tracking errors. Furthermore a simulated example shows the performance of the control scheme applied to a two-link manipulator