Showing papers on "Robust control published in 1992"

On the robust control of robot manipulators

Abstract: A simple robust nonlinear control law for n-link robot manipulators is derived using the Lyapunov-based theory of guaranteed stability of uncertain systems. The novelty of this result lies in the fact that the uncertainty bounds needed to derive the control law and to prove uniform ultimate boundedness of the tracking error depend only on the inertial parameters of the robot. In previous results of this type, the uncertainty bounds have depended not only on the inertia parameters but also on the reference trajectory and on the manipulator state vector. The presented result also removes previous assumptions regarding closeness in norm of the computed inertia matrix to the actual inertial matrix. The design used thus provides the simplest such robust design available to date. >

Model validation: a connection between robust control and identification

Abstract: The gap between the models used in control synthesis and those obtained from identification experiments is considered by investigating the connection between uncertain models and data. The model validation problem addressed is: given experimental data and a model with both additive noise and norm-bounded perturbations, is it possible that the model could produce the observed input-output data? This problem is studied for the standard H/sub infinity // mu framework models. A necessary condition for such a model to describe an experimental datum is obtained. For a large class of models in the robust control framework, this condition is computable as the solution of a quadratic optimization problem. >

Benchmark problems for robust control design

Abstract: Tih purp of this pape is to furter invesigae a Simp , yet meaningf, control problm to higkhl bue in robus coutolr desig and to demote the appicat of a risty of robus control deign methodologa The problm ha bee studied by seeal researc under a vart of [1Mor recent, this problm wa formulmaed and addrsd a benchmak problem for robut control deign 110-16]. In our flatio of the probem ertain apcts, sach a pameter ueaint with Nominal value ad nomina deeired performance, ar concrtely, whie othr aspects, such the mesnset nois model, deuita of setig time, measur of control dford, contrle coplexity, bandwt, etc., ar dft vage Each designw is thus the opportunt to emp addionl deign tradeoffs adesfred.

Accurate identification for control: the necessity of an iterative scheme

Abstract: If approximate identification and model-based control design are used to accomplish a high-performance control system, then the two procedures must be treated as a joint problem. Solving this joint problem by means of separate identification and control design procedures practically entails an iterative scheme. A frequency-response identification technique and a robust control design method are used to set up such an iterative scheme. Each identification step uses the previously designed controller to obtain new data from the plant. The associated identification problem has been solved by means of a coprime factorization of the unknown plant. The technique's utility is illustrated by an example. >

Practical Distillation Control

Abstract: Techniques and methods Methods: Rigorous modelling and conventional simulation Approximate and simplified models Plant-wide process control simulation Selection of controller structure Control structures, consistency and transformations Diagonal controller tuning Dynamic matrix control multivariable controllers Distillation expert systems Robust control case studies Experimental comparison of control structures Industrial experience with double quality control Control of distillation columns via distributed control systems Process design and control of extractive distillation Control by tray temperature of extractive distillation Distillation control in plant-wide control environment Model-based control Superfractionator control Control of vapor recompression distillation columns Heat-integrated columns Batch distillation.

Dynamic scheduling of modern-robust-control autopilot designs for missiles

Abstract: The dynamic scheduling of multiple linear-time-invariant (LTI) controllers, designed using H/sub infinity // mu techniques, for systems with widely varying plant dynamics is examined. The observer/full-state feedback structure of the LTI H/sub infinity / controller is extended to accommodate the time-varying, nonlinear dynamics problem. A procedure for handling the variations that occur between fixed operating points, in the two-Riccati equation solutions, is proposed. An example missile control problem that contains realistic and necessary performance goals and robustness constraints is included to demonstrate the viability of the approach. >

Nonlinear Quadratic Dynamic Matrix Control with State Estimation

Abstract: Quadratic dynamic matrix control (QDMC) with state estimation is presented for use with nonlinear process models. This formulation extens Garcia's nonlinear version of QDMC to open-loop unstable nonlinear processes and allows for better disturbance rejection. The modest computational requirements make it attractive for industrial implementation. The effectiveness of the approach is demonstrated by its successful application to the temperature control of semibatch polymerisation reactor

Asymptotic analysis of nonlinear stochastic risk-sensitive control and differential games

Abstract: In this paper we consider a finite horizon, nonlinear, stochastic, risk-sensitive optimal control problem with complete state information, and show that it is equivalent to a stochastic differential game. Risk-sensitivity and small noise parameters are introduced, and the limits are analyzed as these parameters tend to zero. First-order expansions are obtained which show that the risk-sensitive controller consists of a standard deterministic controller, plus terms due to stochastic and game-theoretic methods of controller design. The results of this paper relate to the design of robust controllers for nonlinear systems.

Chattering reduction of disturbance observer based sliding mode control

Abstract: The reduction of the chattering of servomotor sliding mode control is achieved by a combination of disturbance observer and feedforward control. Using the Lyapunov stability theorem, a design methodology of disturbance observer based sliding mode control is proposed, and the result is applied to a two axis SCARA robot. Through simulations and experiments, it is verified that the proposed control scheme has an advantage in less chattering with the robustness of sliding mode control. Also, the calculation of the reference angles is proposed to enhance the robustness of the proposed control. >

Robust adaptive decentralized control of robot manipulators

Abstract: The authors proposes a robust adaptive decentralized control algorithm for trajectory tracking of robot manipulators. The controller is designed based on a Lyapunov method, which consists of a PD (proportional plus derivative) feedback part and a dynamic compensation part. It is shown that, without any prior knowledge of manipulator or payload parameters and possibly under deterioration of parameter variation with time or state-independent input disturbances, the tracking error is bound to converge to zero asymptotically. In particular, the algorithm does not require explicit system parameter estimation and therefore makes the controller structurally simple and computationally easy. Moreover, the controller is implemented in a decentralized manner, i.e. a subcontroller is independently and locally equipped at each joint servoloop. To illustrate the performance of the controller, a numerical simulation example is provided. >

Control of Nonlinear Systems Using Terminal Sliding Modes

Abstract: The primary focus of this paper is development of a new approach to control synthesis for robust robot operations in unstructured environments To enhance control performance with full model information, we introduce the notion of terminal convergence, and develop control laws based upon a new class of sliding modes, denoted terminal sliders We demonstrate that terminal sliders provide robustness to parametric uncertainty without having to resort to high frequency control switching, as in the case of conventional sliders [2] In addition, stability analysis that is conducted to demonstrate terminal slider approach results in improved control performance and allows for simple robust design of control parameters Further, improved (guaranteed) precision of terminal sliders is argued for through an analysis of steady state behavior

LQG/LTR robust control of nuclear reactors with improved temperature performance

Abstract: The authors present the design of a robust controller using the linear quadratic Gaussian with loop transfer recovery (LQG/LTR) for nuclear reactors with the objective of maintaining a desirable performance for reactor fuel temperature and the temperature of the coolant leaving the reactor for a wide range of reactor powers The results obtained are compared to those for an observer-based state feedback optimal reactor temperature controller Sensitivity analysis of the dominant closed-loop eigenvalues and nonlinear simulation are used to demonstrate and compare the performance and robustness of the two controllers The LQG/LTR approach is systematic, methodical, and easy to design and can give improved temperature performance over a wide range of reactor operation >

Robust nonlinear controller design for transient stability enhancement of power systems

Abstract: A robust DFL (direct feedback linearization) nonlinear excitation controller is proposed to enhance transient stability for power systems. A robust control technique for linear systems is extended to design this controller. The design of the controller is independent of the operating point. Simulation results show that the controller has the following advantages: the fault location does not need to be known and the controller can overcome the variation of the reactance of the transmission line. Both transient stability enhancement and voltage regulation can be achieved. >

A criterion for joint optimization of identification and robust control

Abstract: A criterion for system identification is developed that is consistent with the intended use of the fitted model for modern robust control synthesis. Specifically, a joint optimization problem is posed which simultaneously solves the plant model estimate and control design, so as to optimize robust performance over the set of plants consistent with a specified experimental data set. >

Modeling and Control of Single- Link Flexible Arms With Lumped Masses

Abstract: This paper deals with the modeling and control of a special class of single-link flexible arms. These arms consist of flexible massless structures having some masses concentrated at certain points of the beam. In this paper, the dynamic model of such flexible arms is dewloped and some of the control properties are deduced. A robust control scheme to remove the effects of friction in the joins is proposed. The control scheme consists of two nested feedback loops, an inner loop to control the position of the motor and an outer loop to control the tip position. The inner loop is described in other publications. A simple fedforward-feedback controller is designed for the outer loop to driw the beam accurately along a desired trajectoty. Effects of the changes in the tip’s mass are studied. This modeling and control method is then generalized to the distributed-mass flexible beam case. Finally, experimentaf results are presented. This paper deals with the modeling and control of a special class of single-link, lumped-mass, flexible arms. These arms consist of massless flexible structures that have masses concentrated at certain points of the beam (see Fig. 1). Although the translations of these masses produce stresses in the flexible structure, their rotations do not generate any torque in the beam. Therefore, the number of vibrational modes in the structure coincides with the number of lumped masses. Book (1979) studied the case of two rigid masses connected by a chain of massless beams having an arbitrary number of rotation joints. Our problem differs from this in the sense that our structure has only one rotation joint and an arbitrary number of lumped masses. These two particular structures are studied because: Some lightweight robots and other applications can be reasonably approximated by these models. Their dynamics may be easily modeled as compared to distributed-mass flexible arms. Interesting properties for the control of flexible arms are deduced from their dynamic models. A method to control these arms is inferred from the structure of the model. The influence of changes in the tip’s mass are easily characterized. Given a distributed-mass flexible arm, there always exits a truncated dynamic model which is of the same form as the lumped-mass flexible arm model and which reproduces the dynamics of the measured variables. This allows us to generate the above mentioned control method to the case of distributed-mass flexible arms. - Contributed by the Dynamic Systems and Control Division for publication

Vibration control of a coordinate measuring machine

Abstract: A comparative study of two vibration reduction schemes, the command preshaping technique and the feedforward control technique, is presented. The objective is to drive the end-point of a coordinate measuring machine along a commanded trajectory without any oscillations at the end-point. This is achieved by designing appropriate controllers for the system based on a robust control scheme. This scheme consists of two nested feedback loops-an inner loop to control the motor position and an outer loop to control the position of the end-point. The controllers based on the two vibration reduction schemes are designed for the outer loop to make the system vibration-free. It is shown that the command preshaping scheme is a special case of the feedforward control scheme. The design method is illustrated with a numerical example. >

Robust identification of strongly stabilizable systems

Abstract: For strongly stabilizable systems for which a strongly stabilizing controller is known approximately, the authors consider system identification in the graph, gap, and chordal metrics using robust H/sub infinity / identification of the closed-loop transfer function in the framework proposed by A.J. Helmicki et al. (1990). Error bounds are derived showing that robust convergence is guaranteed and that the identification can be satisfactorily combined with a model reduction step. Two notions of robust identification of stable systems are compared, and an alternative robust identification technique based on smoothing, which can be used to yield polynomial models directly, is developed. >

Robust control of nonlinear systems: compensating for uncertainty

Abstract: We propose a new approach to robust control of nonlinear systems. The approach is indirect in the sense that we will translate a robust control problem into an optimal control problem and apply optimal control methods to solve the robust control problem. We show that the method can be applied to nonlinear systems that satisfy the matching condition.

Analysis of time-varying control strategies for optimal disturbance rejection and robustness

Abstract: Analysis of linear time-invariant systems that are controlled by time-varying controllers is given. The problems of disturbance rejection and robustness are formulated and analyzed using functional analytic methods, which reveal the properties that are common to these problems. The theoretical development presented uses the theory and techniques of nuclear operators and their relation with the duality theory of tensor products of Banach spaces. The authors show how time-varying compensation offers no advantage over time-invariant compensation for the problem of disturbance rejection over general signal spaces, in both continuous and discrete time, and for the problem of L/sub infinity / robust stabilization of time-invariant plants. In addition, another application of the theory for the problem of norm minimization subject to a norm constraint is presented. >