Showing papers presented at "American Control Conference in 1998"

Quadratic stabilization and control of piecewise-linear systems

Abstract: We consider analysis and controller synthesis of piecewise-linear systems. The method is based on constructing quadratic and piecewise-quadratic Lyapunov functions that prove stability and performance for the system. It is shown that proving stability and performance, or designing (state-feedback) controllers, can be cast, as convex optimization problems involving linear matrix inequalities that can be solved very efficiently. A couple of simple examples are included to demonstrate applications of the methods described.

An optimal two stage identification algorithm for Hammerstein-Wiener nonlinear systems

Abstract: An optimal two stage identification algorithm is presented for Hammerstein-Wiener systems where two static nonlinear elements surround a linear block. The proposed algorithm consists of two steps: The first one is the recursive least squares and the second one is the singular value decomposition of two matrices whose dimensions are fixed and do not increase as the number of the data point increases. Moreover, the algorithm is shown to be convergent in the absence of noise and convergent with probability one in the presence of white noise.

Non-fragile controller design: an overview

Abstract: In this paper we give an overview of nonfragile controller design for linear systems. The controller fragility problem is basically the problem of performance deterioration of a feedback control system due to inaccuracies in controller implementation. The literature on the fragility problem is reviewed and methods for designing nonfragile controllers are summarized.

Optimized s-curve motion profiles for minimum residual vibration

Abstract: A method for developing optimized point-to-point motion profiles to achieve fast motions with minimum vibration is presented. The proposed approach uses the well-known s-curve motion profiles, but optimizes the selection of the ramp-up (and ramp-down) time. The selection of ramp-up time is based on a frequency analysis that minimizes the excitation energy of the input forcing function at the system natural frequency. Simulation results on a lightly-damped system undergoing point-to-point motions demonstrate that the proposed approach decreases residual vibration by almost an order of magnitude over other approaches, even when the actual natural frequency is in error by 10%.

A stable scheme for automatic control reconfiguration in the presence of actuator failures

Abstract: The paper describes the design of an automatic control reconfiguration scheme for accommodation of actuator failures in a class of plants where the number of control inputs is larger than the number of controlled outputs. One of the main features of the proposed scheme is that the control reconfiguration is achieved automatically based only on the response of the overall system. The method is developed for the case when one or more actuators freeze in a certain position and do not respond to subsequent commands. We assumed that the information about the failure is not available to the controller. To solve this problem, a control law with adjustable parameters is introduced, and, using a convenient parametrization of the overall system, the resulting error model is expressed in a somewhat modified form as compared to those arising in standard adaptive control. The design of the corresponding adaptive laws based on Lyapunov analysis allow us to prove global stability of the overall scheme in the presence of multiple actuator failures.

Global nonlinear parametric modelling with application to F-16 aerodynamics

Abstract: Global nonlinear parameteric modeling technique is described and demonstrated. The technique uses multivariate orthogonal modeling functions generated from the data to determine nonlinear model structure, then expands each retained modeling function into an ordinary multivariate polynomial. The final model form is a finite multivariate power series expansion for the dependent variable in terms of the independent variables. Partial derivatives of the identified models can be used to assemble globally valid linear parameter varying models. The technique is demonstrated by identifying global nonlinear parametric models for nondimensional aerodynamic force and moment coefficients from a subsonic wind tunnel database for the F-16 fighter aircraft. Results show less than 10% difference between wind tunnel aerodynamic data and the nonlinear parameterized model for a simulated doublet maneuver at moderate angle of attack. Analysis indicated that the global nonlinear parametric models adequately captured the multivariate nonlinear aerodynamic functional dependence.

Control strategy for variable-speed, stall-regulated wind turbines

Abstract: A variable-speed, constant-pitch wind turbine was investigated to evaluate the feasibility of constraining its rotor speed and power output without the benefit of active aerodynamic control devices. A strategy was postulated to control rotational speed by specifying the demanded generator torque. By controlling rotor speed in relation to wind speed, the aerodynamic power extracted by the blades from the wind was manipulated. Specifically, the blades were caused to stall in high winds. In low and moderate winds, the demanded generator torque and the resulting rotor speed were controlled to cause the wind turbine to operate near maximum efficiency. A computational model was developed, and simulations were conducted of operation in high turbulent winds. Results indicated that rotor speed and power output were well regulated.

Multiobjective H/sub 2//H/sub /spl infin//-optimal control via finite dimensional Q-parametrization and linear matrix inequalities

Abstract: The problem of multiobjective H/sub 2//H/sub /spl infin// optimal controller design is reviewed. There is as yet no exact solution to this problem. We present a method based on that proposed by Scherer (1995). The problem is formulated as a convex semidefinite program (SDP) using the LMI formulation of the H/sub 2/ and H/sub /spl infin// norms. Suboptimal solutions are computed using finite dimensional Q-parametrization. The objective value of the suboptimal Qs converges to the true optimum as the dimension of and is increased. State space representations are presented which are the analog of those given by Khargonekar and Rotea (1991) for the H/sub 2/ case. A simple example computed using finite impulse response Qs is presented.

Optimal control of nonlinear continuous-time systems: design of bounded controllers via generalized nonquadratic functionals

Abstract: By using the Hamilton-Jacobi framework and sufficiency theory, this paper presents a solution of the constrained optimization problem for nonlinear systems with soft and hard bounds imposed on control. The developed concept is based on the application of a generalized nonquadratic cost, and nonquadratic return functions are applied. Necessary and sufficient conditions have been used to synthesize the bounded controllers, and sufficient conditions are applied to verify the optimality. The constrained optimization problem is solved for nonlinear systems, and the offered results extend the application of the Hamilton-Jacobi theory by using a generalized nonquadratic cost. The design procedure is reviewed in the context of motion control applications. Analytical, numerical, and experimental results are presented for a servo-system actuated by a permanent-magnet DC motor. The designed nonlinear controller is experimentally verified.

The Simplex architecture for safe online control system upgrades

Abstract: We describe the Simplex architecture, a real-time software technology which supports the safe, reliable introduction of control system upgrades while the system is running. We introduce its basic structure in control systems, discuss its fault-tolerance feature, and investigate the control issues when the technology is employed. Application of the Simplex architecture is demonstrated for a plasma-enhanced chemical vapor deposition (PECVD) system, a standard process in semiconductor manufacturing. We conclude the paper with a discussion of the potential impact that the Simplex architecture can make on future control applications.

Robust nonlinear controller for turbocharged diesel engines

Abstract: This paper addresses a problem of controlling diesel engines equipped with a variable geometry turbocharger and an exhaust gas recirculation valve. The presence of two actuators and nonlinear behavior of the system makes the problem difficult to handle using classical control designs. Instead, we employ a recently developed control Lyapunov function based design method that guarantees a robustness property interpretable as gain and phase margins. The controller has been tested in simulations and experimentally in the dynamometer test cell.

A linear matrix inequality approach to decentralized control of distributed parameter systems

Abstract: In this paper preliminary results in the use of linear matrix inequalities for the decentralized control of distributed parameter systems is presented. The class of systems being considered are those that can be expressed as multidimensional systems. It is shown that linear matrix inequalities can be used to provide tractable solutions to this problem; the conditions are in general conservative, but are computationally attractive and lead to controllers which have a decentralized structure.

Access to an instructional control laboratory experiment through the World Wide Web

Abstract: The software for "second best to being there", a distance learning application that allows a remotely-located user to conduct experiments in the Control Engineering Laboratory at Oregon State University, has been redesigned and implemented in the Java/sup TM/ programming language. This permits the experiments to be run via the World Wide Web. Access requires only a basic Web browser that runs Java and is possible from most computer platforms. User authentication, security, and automatic logging of student laboratory activity have been added, and the user interface design improved, to turn this demonstration product into courseware.

Research issues in autonomous control of tactical UAVs

Abstract: This paper summarizes the enabling technologies for an autonomous tactical unmanned aerial vehicle (UAV). Current technologies are adequate for semi-autonomous UAVs that operate in a relatively structured environment. For tactical UAVs in a rapidly changing uncertain environment the present techniques are inadequate. The essence of autonomous control is rapid in-flight replanning under uncertainty. This is cast as a large optimization or decision problem. Monolithic and decomposition techniques are discussed for the solution of large decision problems. Hierarchical decomposition is the most promising approach, but suffers from an inadequate theoretical basis. Finally, research areas are proposed to address the decomposition problem.

Robust, non-fragile and optimal controller design via linear matrix inequalities

Abstract: In this article, we introduce a robust nonfragile state feedback controller which is also optimal with respect to a quadratic performance index, using linear matrix inequalities (LMI). The uncertainties are assumed to be polytopic, both in the controller gains and the system dynamics. A numerical example is presented to demonstrate the efficiency of this method, and the controller turns out to be robust with respect to the uncertainties in the plant and the controller.

Position and attitude control of a spacecraft by sliding mode control

Abstract: Future spacecraft such as the Orbital Maneuvering Vehicle or "Remover" which tries to capture and remove large space debris objects in orbit need to be able to perform large angle and complicated position maneuvers in space. The equation of motion of a rigid body that performs attitude and translational motion (six degrees of freedom motion) is a nonlinear equation. This paper applies sliding mode control that can handle a nonlinear system to the six degrees of freedom control for such a vehicle. Its feasibility is investigated through a numerical simulation of proximity flight around a tumbling target object.

Robust resilient dynamic controllers for systems with parametric uncertainty and controller gain variations

Abstract: A feedback control design problem involving structured plant parameter uncertainties and controller gain variations is considered. Specifically, the robust fixed-structure guaranteed cost controller synthesis framework is extended to address the design of robust resilient fixed-order (i.e., full- and reduced-order) dynamic controllers for systems with structured parametric uncertainty and controller gain uncertainty. Several examples are provided which clearly demonstrate the need for robust resilient control.

Control of nonlinear systems using iterative feedback tuning

Abstract: Iterative feedback tuning (IFT) is a tuning method based on minimizing some control criterion directly from experimental data. The method was originally developed for linear time-invariant systems, but many successful applications to nonlinear system have been reported in the literature. The contribution of this paper is to provide a preliminary analysis of IFT for nonlinear systems. This analysis sheds some light on the question of which nonlinear systems IFT may be expected to work for and also gives some guidance on how to choose the user's design variables for nonlinear problems. The discussions are backed up by simulations as well as an application to a DC-servo with backlash which initially exhibits a limit cycle.

Demonstration of an automated highway platoon system

Abstract: The National Automated Highway System Consortium (NAHSC), in partnership with the Department of Transportation, demonstrated several potential AHS technologies on I-15 in San Diego in August 1997. One of the key elements in this demonstration event was a platoon system that contained a group of fully automated automobiles traveling at close spacing. The platoon system demonstrated a number of AHS features such as lane keeping, lane changing, longitudinal space and speed regulation using proved enabling technologies. The paper describes the platoon concept, the configuration of the demonstration system, the demonstration scenarios and reports the development of the platoon demonstration system and the data collected during the demonstration.

Control and steering feel issues in the design of an electric power steering system

Abstract: We address control and steering feel issues in the development of electric power assist steering (EPAS) systems. The steering system performance requirements impose natural constraints on the compensator gain, phase lag over a frequency range of driver inputs, and pole and zero locations. We develop a novel method based on a nonlinear constrained optimization procedure for fixed-structure optimal controller synthesis for EPAS. The control law obtained minimizes an objective function (H/sub 2/-norm) for a given controller structure, while accounting for requirements of steering torque amplification, steering response and steering feel.

Remote laboratory experimentation

Abstract: The aim of this work is to provide the students of both Case Western Reserve and Cooper Union Universities with remote access to the Bytronic Process Control unit, referred to as the process rig, in the Process Control and Automation Laboratory at Case Western Reserve University over the Internet. Using a web browser, the user can log-in parameters from a remote client to a LabVIEW G web server which is connected to the process rig via a PLC control module. When the server receives a request specifying a Common Gateway Interface Virtual Instrument (CGI VI), the server loads the VI and starts its execution. The CGI then reads the information that came with the request, controls the operation of the process rig in accordance with the parameters posted by the user and writes the data that is to be returned to the client. The output data files are also uploaded to an anonymous ftp-site from which the user can download them for further processing.

A multivariable laboratory process with an adjustable zero

Abstract: A novel multivariable laboratory process that consists of four interconnected water tanks is presented. The linearized dynamics of the system have a multivariable zero that is possible to move along the real axis by changing a valve. The zero can be placed in both the left and the right half-plane. In this way the quadruple-tank process is ideal for illustrating many concepts in multivariable control, particularly performance limitations due to multivariable right half-plane zeros. Accurate models are derived from both physical and experimental data and multi-loop control is illustrated.

System identification for limit cycling systems: a case study for combustion instabilities

Abstract: Presents a case study in system identification for limit cycling systems. The focus of the paper is on (a) the use of a model structure derived from physical considerations and (b) the use of algorithms for the identification of component subsystems of this model structure. The physical process used in this case study is that of a reduced order model for combustion instabilities for lean premixed systems. The identification techniques applied in this paper are the use of linear system identification tools, time delay estimation and qualitative validation of model properties using harmonic balance and describing function methods. The novelty of the paper, apart from its practical application, is that closed loop limit cycle data is used together with a priori process structural knowledge to identify both linear dynamic forward and nonlinear feedback paths.

Detection of common motor bearing faults using frequency-domain vibration signals and a neural network based approach

Abstract: Bearings and their vibration play an important role in the performance of all motor systems. In many cases, the accuracy of the instruments and devices used to monitor and control the motor system is highly dependent on the dynamic performance of the motor bearings. In addition, many problems arising in motor operation are linked to bearing faults. Thus, fault detection of a motor system is inseparably related to the diagnosis of the bearing assembly. The paper presents an approach using neural networks to detect common bearing defects from motor vibration data. The results show that neural networks can be an effective agent in the detection of various motor bearing faults through the measurement and interpretation of motor bearing vibration signals.

Robust non-fragile LQ controllers: the static state feedback case

Abstract: This paper describes the synthesis of non-fragile or resilient regulators for linear systems. The general framework for fragility is described using state-space methodologies, and the LQ/H/sub 2/ static state-feedback case is examined in detail. We discuss the multiplicative structured uncertainties case, and propose remedies of the fragility problem using a convex programming framework as a possible solution scheme. The benchmark problem is taken as an example to show how controller gain variations can affect the performance of the closed-loop system.

Interpolation of observer state feedback controllers for gain scheduling

Abstract: We propose a method of interpolating linear time-invariant controllers with observer state feedback structure in order to generate a continuously-varying family of controllers that stabilizes a family of linear plants. Gain scheduling is a motivation for this work, and the interpolation method yields guidelines for the design of gain scheduled controllers. The method is illustrated with the design of a missile autopilot using loop-shaping H/sub /spl infin// controllers.

Assessment of achievable PI control performance for linear processes with dead time

Abstract: A numerical scheme is outlined for the estimation of achievable PI control performance measured by output variance in linear processes with dead time when stochastic load disturbances are affecting the process For this numerical scheme, an approximate stochastic realization is employed for disturbance model identification, and this realized model is then used for the achievable performance prediction The approximate stochastic disturbance model realization is performed based on the feedback invariant closed-loop impulse response coefficients so that the impulse response coefficients of the realized disturbance model are approximately equal to a given feedback invariant impulse response sequence A computer simulation is carried out for the estimation of achievable PI control performance, and the result is compared to the true PI control performance bound

Solar-powered, formation-enhanced aerial vehicle systems for sustained endurance

Abstract: It has long been known that aircraft flying in close formation can achieve an overall efficiency much greater than is possible for each aircraft flying alone. When coupled with possible near-term advances in solar-electric power storage and aeronautical propulsion systems, it is theoretically possible to create a formation flight system that is capable of truly infinite endurance. This paper discusses a research effort currently underway that addresses the design and implementation of such a system. A brief overview of the system concept is given, and some of the particular problems inherent are more fully discussed. In particular, the difficulties involved in the autonomous control of such systems are presented and discussed.

On the limitations of force tracking control for hydraulic active suspensions

Abstract: This paper presents analysis of a particular force tracking control problem for rectilinear hydraulic actuators governed by a servovalve. It presents no new theory, but rather uses a revealing model reduction insight coupled with classical analysis to explain a physical phenomenon. As such, this work is an attempt to explain why a seemingly innocuous problem is more subtle than initially believed. A motivation for this problem is given along with prior attempts at a simple solution. The solution method evaluated is a common proportional-integral-derivative type of controller. It is shown that the simple PID is quite adequate for other types of control objectives such as force regulation or position tracking. However, this simple solution method is shown to be inadequate for force tracking due to fundamental limitations of the problem formulation. Therefore, more advanced control algorithms are shown to be a necessity rather than a luxury.

Command shaping control of an operator-in-the-loop boom crane

Abstract: This paper presents a control method for suppressing payload sway caused by operator commanded maneuvers, in rotary boom cranes. The crane configuration studied, consists of a payload mass that swings at the end of a spherical pendulum, which is attached to a boom capable of hub rotation and elevation. Positioning of the rotary crane is accomplished through the hub and boom angles, and the lift line length. Since the configuration of the crane affects the excitation and response of the lift line, the sway control scheme must account for the varying geometry of the system. Adaptive forward path command filters are employed to remove the components of the command signal which induce oscillation of the lift line. A real-time operator-in-the-loop simulation, is used to demonstrate results for a simultaneous three-axis maneuver.