Showing papers in "IEEE Control Systems Magazine in 1999"

Basic problems in stability and design of switched systems

Abstract: By a switched system, we mean a hybrid dynamical system consisting of a family of continuous-time subsystems and a rule that orchestrates the switching between them. The article surveys developments in three basic problems regarding stability and design of switched systems. These problems are: stability for arbitrary switching sequences, stability for certain useful classes of switching sequences, and construction of stabilizing switching sequences. We also provide motivation for studying these problems by discussing how they arise in connection with various questions of interest in control theory and applications.

An Internet-based real-time control engineering laboratory

Abstract: Describes the design of generic virtual instruments used for real-time experimentation at Polytechnic University's control engineering laboratory in a remote-access environment. These instruments can be freely downloaded and the remote user can access the laboratory facilities from anywhere at any time. Our Internet-accessed remote laboratory is based on a client/server computer configuration. The server, situated near the experiment, transfers to it the received command signals transmitted by the client. The client locally computes the command signal based on the reference waveform and the transmitted system response. The remote user can select the transmission protocol, switch between asynchronous and synchronous sampling, use either a batch or a recursive data transfer mode, and view the experimental testbed. Our approach is distinct from others in that it offers more flexibility and responsibility to the client side, since the remote user compiles and executes the controller locally. Issues concerned with network reliability, dynamic delays caused by Internet traffic, concurrent user access, and limited computing power have been addressed. The designed set of experiments is the first step toward our remote control laboratory.

Friction compensation for an industrial hydraulic robot

Abstract: A model based friction compensation scheme using a novel dynamical friction model was implemented on an industrial Schilling Titan II hydraulic robot. Off-line estimation of parameters was carried out, using the results of two kinds of experiments. These experiments were done independently at each joint. A nonlinear PI type controller was used in the inner torque loop to improve its performance. The complete control scheme has shown to substantially improve the position precision in regulation and tracking. Higher precision applications can be performed by the hydraulic robot with this controller.

An experimental comparison of several nonlinear controllers for power converters

Abstract: This article summarizes the results of an experimental comparison of nonlinear control algorithms on a DC-DC power converter of the boost type. It compares five algorithms, including the linear design, with respect to their ease of design and their closed-loop performance. For all these algorithms local asymptotic stability of the desired equilibrium is ensured. The motivation of the present study is not to illustrate the validity of these theoretical results, but to test their performance when confronted with a real physical application where situations not predicted by the theory will arise. The behavior of the schemes is compared with the following basic criteria: transient and steady state response to steps and sinusoidal references, attenuation of disturbances in the power supply, and sensitivity to unknown loads. Particular emphasis is placed throughout on the flexibility provided by the tuning parameters to shape the responses. Even though this issue is not always appreciated in theoretical studies, we have found it of prime importance in experimentation.

Using the Web in your courses: what can you do? what should you do?

Abstract: This paper gives an overview on the topic of control education on the Web. Accompanied by a Web page, the material is partly tutorial, enabling readers to step in at their current levels and move forward in their Web usage. For those readers who have not yet made use of the Web in their courses, we will demonstrate models of Web sites for consideration to suggest what can be done, and offer introductory steps for implementation. Experienced Web users and developers may use some of the more sophisticated models, identify what it means to manage a course with a Web site, improve their own design, and hear some tips on the hurdles to avoid. Specific applications to the control field are discussed, including software demonstrations and virtual and remote labs. In the end, it is hoped that readers will find information to move them a step forward from their current level.

Terrain navigation using Bayesian statistics

Abstract: The performance of terrain-aided navigation of aircraft depends on the size of the terrain gradient in the area. The point-mass filter (PMF) described in this work yields an approximate Bayesian solution that is well suited for the unstructured nonlinear estimation problem in terrain navigation. It recursively propagates a density function of the aircraft position. The shape of the point-mass density reflects the estimate quality; this information is crucial in navigation applications, where estimates from different sources often are fused in a central filter. Monte Carlo simulations show that the approximation can reach the optimal performance, and realistic simulations show that the navigation performance is very high compared with other algorithms and that the point-mass filter solves the recursive estimation problem for all the types of terrain covered in the test. The main advantages of the PMF is that it works for many kinds of nonlinearities and many kinds of noise and prior distributions. The mesh support and resolution are automatically adjusted and controlled using a few intuitive design parameters. The main disadvantage is that it cannot solve estimation problems of very high dimension since the computational complexity of the algorithm increases drastically with the dimension of the state space. The implementation used in this work shows real-time performance for 2D and in some cases 3D models, but higher state dimensions are usually intractable.

Sliding Mode Motion Control of Nonholonomic Mobile Robots

Abstract: As nonholonomic mobile robots have constraints imposed on motions that are not integrable, i.e. the constraints cannot be written as time derivatives of some function of the generalized coordinates, advanced techniques are needed for the tracking control. In the paper a robust control law is proposed for trajectory tracking of nonholonomic wheeled mobile robots. The state variables of the mobile robot are represented in polar coordinates, and the dynamic equation of the system is feedback-linearized by a computed-torque method. A novel sliding mode control law is derived for asymptotically stabilizing the mobile robot to a desired trajectory. It is shown that the proposed scheme is robust to bounded system disturbances. Simulation examples and experimental results are provided to show the effectiveness of the accurate tracking capability and the robust performance of the proposed controller.

LPV techniques for control of an inverted pendulum

Abstract: The authors present a comprehensive application of linear fractional transformation and polytopic control techniques to the control of an arm-driven inverted pendulum, The particular interest of this application lies in the fact that all ingredients of the design problem have to be taken into account; from the specifications up to the constraints inherent to real-world implementations. In this context, it has been shown that currently available synthesis methodologies, such as /spl mu/ and LPV techniques, may fail to provide acceptable answers, A major obstacle is undoubtedly the implementation constraint that puts hard limitations on the controller dynamics, These limitations are generally difficult to handle within the usual formulation of LPV control techniques. It has been shown that a suitable extension of these techniques including LMI region constraints on the closed-loop dynamics can overcome this difficulty. When implementable, it has been observed that LPV controllers outperform fixed /spl mu/ controllers both in robustness and performance. These observations were confirmed by simulations but more importantly by a number of records on the physical experiment.

Bilateral matched impedance teleoperation with application to excavator control

Abstract: A bilateral teleoperation controller is presented in the article. The master and slave manipulators are controlled in impedance mode, with their target impedances adjusted in a dual manner to match high or low impedance environments. The adjustment rules presented use the relative sizes of forces and velocities to simply interpolate between low and high impedance controllers. The method was justified by the success of dual-hybrid teleoperation and is demonstrated to work using a simulator driven by a maglev force-feedback joystick. Experimental results using the same maglev force-feedback joystick to control an excavator are also presented, demonstrating through a typical leveling task that this method can work well in practice. The ratio of forces to velocities for deciding the master and slave impedances seems to be an effective method for dealing with extreme environment conditions, namely, free motion or hard constraint.

Supervisory hybrid systems

Abstract: Supervisory hybrid systems are systems generating a mixture of continuous-valued and discrete-valued signals. This systems paradigm is particularly useful in modeling applications where high-level decision making is used to supervise process behavior. Hybrid system methodologies are also applicable to switched systems where the system switches between various setpoints or operational modes to extend its effective operating range. Hybrid systems, therefore, embrace a diverse set of applications. There has been considerable activity in the area of hybrid systems theory, and this article provides an introduction to some of the basic concepts and trends in this emergent field. The term hybrid refers to a mixing of two fundamentally different types of objects or methods. The paper deals with supervisory hybrid systems. Supervisory hybrid systems are systems that combine discrete event and continuous-valued dynamics. The article is organized as follows. We first provide an example of a hybrid system, to be used throughout as a pedagogical tool illustrating various concepts in hybrid systems theory. We then discuss modeling frameworks for hybrid systems, paying specific attention to the hybrid automaton. Not only may the system have a hybrid character, but the specifications on desired system behaviors may also be hybrid. The article also discusses specification logics that express system requirements on both the discrete and continuous states of the system. The article continues with a survey of current methods and concepts used to verify or validate desired system behaviors, and concludes with a survey of current methods for hybrid control system synthesis.

Modeling and control of compressor flow instabilities

Abstract: Compressors are widely used for the pressurization of fluids. Applications involve air compression for use in aircraft engines and pressurization and transportation of gas in the process and chemical industries. The article focuses on two commonly used types of continuous flow compressors: the axial compressor, where the gaseous fluid is processed in a direction parallel to the rotational axis, and the radial or centrifugal compressor, where the pressurized fluid leaves the compressor in a direction perpendicular to the rotational axis. In these machines, the entering fluid is pressurized by first accelerating it via the kinetic energy imparted in the rotors and then converting the kinetic energy into potential energy by decelerating the fluid in diverging channels. Toward low mass flows, stable operation of axial and radial compressors is constrained by two aerodynamic flow instabilities: rotating stall and surge. The article gives an overview of the current state of modeling and control of these instabilities.

Posture control of a cockroach-like robot

Abstract: As legged robots become more animal-like, it is likely that these robots will have many complex limbs with redundant degrees of freedom (DOF). This is especially true when we desire them to move like an animal. Animals are capable of spontaneous and non-stereotyped locomotion, such as turning, swaying, twisting, deliberately falling, jumping, climbing, and running, Therefore, it becomes difficult to provide joint space trajectories, in real-time, for these complex movements when many limbs are simultaneously involved, and when some or all of these limbs contain redundant DOF. When locomotion takes place rapidly, it has been suggested that there is a feedforward control component that involves a proactive, higher level computation in the nervous system. We suggest and demonstrate an intuitive and computationally simple algorithm for controlling the posture of a complex, multileg robot with many redundant DOF. The algorithm avoids inverse kinematics by issuing feedforward force commands to both maintain static posture and generate body motion. In so doing, it is also shown that the multileg mechanics of postural control can be reduced to a simple center-of-pressure representation, or equivalently, an instantaneous virtual leg model.

A new control strategy for DLR's multisensory articulated hand

Abstract: Concerns a multisensory 12-DOF 4-finger hand in which all actuators are integrated in the palm or the fingers. It was necessary to use a specially designed miniaturized linear actuator. For telemanipulation our first approach was to make the hand follow the desired states commanded from higher levels, e.g., dataglove. This requires accurate tracking, compliance in the constrained environment, and smooth transition between these two modes. The tracking control problem is to design a control scheme which generates the appropriate control signal so as to ensure that the joint angle follows any specified reference trajectory as closely as possible. Compliance is needed for fine manipulation and also for the protection of the hand. Transition control is essentially the problem of making the energy conversion more effective and smooth. We propose a parallel torque/position control strategy for this task. The robust trajectory tracking in free space is implemented by sliding mode control; the compliant motion in the constrained environment is realized by using impedance control; and a parallel observer based on contact torques and system states has been built to determine the switch between these two kind of controllers for the transition control from or to contact motion.

A multidisciplinary flight control development environment and its application to a helicopter

Abstract: CONDUIT, a computational facility for aircraft flight control design and evaluation, has been developed and demonstrated. CONDUIT offers a graphical environment for integrating simulation models and control law architectures with design specifications and constraints. This tool provides comprehensive analysis support and design guidance to a knowledgeable control system designer. Combining the easy-to-use graphical interface, the preprogrammed libraries of specifications, and the multiobjective function optimization engine (CONSOL-OPTCAD) in a single environment, CONDUIT offers the potential for significant reduction in time and cost of design, analysis, and flight-test optimization of modern flight control systems. A case study application to a complex rotary-wing flight control problem was presented. The baseline RASCAL UH-60 control system, as provided by the flight control contractor, was evaluated versus the ADS-33D handling-quality specifications. The selectable system gains were optimized to meet all system performance and handling-quality specifications. CONDUIT successfully exploited the tradeoff between forward loop and feedback dynamics to significantly improve the expected handling qualities and stability robustness, while reducing crossover frequency and minimizing actuator activity. The tradeoff studies showed the effect of increasing design margin (overdesign) on closed loop performance and actuator activity. Design margins exceeding 7.5% led to rapidly increasing actuator energy and saturation, resulting in shortened fatigue life of rotor control.

Enhancing undergraduate control education

Abstract: Education must be both conceptual and experiential. Abstract concepts are elegant and powerful, but learning is always enhanced by direct experience, concrete examples, and real-world relevance. Control theory and much of control education is highly conceptual. In fact, control engineering tends to be the least tangible of all subjects in the engineering curriculum. In the hope of tipping the balance from the conceptual to the experiential, I offer the following modest suggestions. These suggestions encompass modeling, control, technology, and cultural issues. By discussing these issues in an undergraduate control course, the instructor can emphasize some of the more practical aspects of the subject. My hope is that these suggestions will enhance the teaching and appreciation of a rich and intellectually exciting subject.

Design and implementation of a hybrid control strategy

Abstract: Hybrid controllers for a double-tank system and a heating/ventilation process have been designed and implemented. Both simulations and real experiments are presented. It is shown that a hybrid controller consisting of a time-optimal controller combined with a PID controller gives very good performance. The controller is easy to implement. It gives, in one of its forms, guaranteed closed loop stability.

Robust computer control of an inverted pendulum

Abstract: This article illustrates the design of a robust digital controller using a blend of state space and frequency response methods. The specific application is that of balancing an inverted pendulum on a moving cart while controlling the cart position. This work investigates issues regarding robust stability and disturbance attenuation. Furthermore, the effectiveness of a simple nonlinear compensation scheme for counteracting the Coulomb friction is studied. The controller design is based on an approximate discrete-time linear model of the pendulum-cart system. Design calculations and analysis are carried out using Matlab/toolboxes and M-files developed by the author.

Tailoring theory to practice in tactical missile control

Abstract: The tactical missile industry makes an excellent example of the need to bridge the gap between theory and practice in control systems. Many of today's highly successful missiles employ only the most simple classical algorithms, such as variants of proportional navigation guidance and PID control. The undeniable effectiveness of these techniques has been proven many times over, and the tactical missile industry has therefore had little use for many of the theoretical advancements made in the last two decades of control research. Recently, however, this situation has begun to change. Emerging threats and new operational constraints have created the need for a variety of new weapon systems that will eventually replace yesterday's workhorses in future conflicts. Even existing systems can expect significant upgrades as both performance requirements and available on-board computing power continue to increase. The article covers the following aspects: industry requirements; enabling technologies; fixed-structure control; parameter optimisation methods; LPV control; nonlinear dynamic inversion; and integrated guidance and autopilot design.

Shared control framework applied to a robotic aid for the blind

Abstract: Demonstrates how the shared discrete event control system is applied to the Robotic Cane, a system which aids the visually impaired navigate through obstacle rich environments. It is also shown how the control framework operates to avoid obstacles with or without human interaction. During experiments the cane effectively helped the user to maneuver through an indoor environment. With a little practice the user could follow the direction in which the cane steered. This shows that the control mapping was defined effectively. The only command that was not very intuitive was the left and brake command used during a weak conflict. However, a different kinematic solution of the cane could achieve the turning and braking as desired. In times of strong conflict between the user input and the autonomous control command the human remained in control. The audio warning during these conflicts was effective in alerting the user.

Control of two industrial manipulators rigidly holding an egg

Abstract: Based on the framework of general constrained robots, a delicate experiment with control of two heavy-duty industrial robots rigidly holding a raw egg is reported. The system possesses smooth transition behavior from free motion to constrained motion when working with unilateral constraints, since a unified no-switching control law is used. This experiment indicates that the robots have the capability to perform very delicate operations with the aid of advanced control algorithms and force feedback. It also reveals potential applications to human-centered robotics.

From Wiener to hidden Markov models

Abstract: The authors investigate common properties of 3 types of filters obtained by considering various stochastic models; Wiener filters, Kalman filters and hidden Markov model (HMM) filters. Unifying features which particularly stand out are the forgetting of old data and of initial conditions, and protection from round-off error effects' overpowering the calculations. They differentiate the concept of fixed-lag smoothing from filtering, and expose the comparative advantages and disadvantages. Once again, there are common properties which allow a unified viewpoint. We focus especially on characterizations of a maximally effective smoothing lag, and identification of the SNR circumstances under which smoothing is especially beneficial. The motivation is the processing of data from an array of acoustic sensors towed by a submarine.

Enhancing classical controls education via interactive GUI design

Abstract: This column presents a graphical and somewhat automated tool for performing the iteration when using classical control design methods. The graphical user interface is an easy way for students to see the effect of the gain and the compensator pole-zero locations on the closed loop step response. Thus, the tool presented extends the capabilities of classical control design methods as they are currently presented in textbooks. Three examples are given that demonstrate the use and effectiveness of this tool. Furthermore, the Bode plot and the root locus windows are tied together in such a way that students can see how changes to the root locus affect the open loop Bode plot and vice versa.

Vision-based control of an air hockey playing robot

Abstract: We demonstrate an integrated sensing and control design for high-speed ballistic manipulation in two dimensions. We show the effectiveness of a vision-based switched control scheme for precision impulsive manipulation. Vision-based hybrid control is a promising avenue of research for fully autonomous interaction between robots and complicated environments. Much work remains to be done in both computer vision and hybrid systems. In the context of robot air hockey, important future areas to investigate include the problems of modeling and identification of the environment dynamics, in particular, the table friction and puck spin. We show how the puck spin significantly influences its trajectory and subsequently our ability to perform accurate trajectory estimation and prediction.

Precise position control of robot arms using a homogeneous ER fluid

Abstract: Semi-closed-loop control of a robot achieves only the control of the angular position of the motor, so it is not clear whether the end-effector is precisely positioned or not. Closed-loop control would improve positioning accuracy, but industrial robots which have elasticity in their driving systems might become unstable easily because of the noncollocated positional relationship between sensors and actuators. This study, which is aimed at realizing high-speed and high-precision closed-loop control, uses an intelligent electrorheological fluid (ER fluid). In the next section, a brief introduction is given with respect to two types of ER fluids and the development of ER dampers. Following this is a description of the experimental apparatus, which consists of an ER damper and a one-link robot arm. A discussion of its mathematical model follows. In addition, the effects of an ER damper to the resonance/antiresonance characteristics of 2-inertia systems are discussed. Next, the design of the hierarchical control system is discussed. Firstly, the design of the inner loop controller for motor velocity control is presented. As an outer loop controller, an H/sup /spl infin// controller is obtained by using a mixed sensitivity design method of robust control theory. Then it is shown that the ER damper makes the design of a high gain controller possible while maintaining robust stability. Finally, the effectiveness of the proposed control method is demonstrated by experiments.

Development of a flight test system for unmanned air vehicles

Abstract: In this paper, an integrated system for the design, development, and testing of guidance, navigation, and control algorithms for unmanned air vehicles was presented. Extensive use was made of commercial off-the-shelf hardware. This kept costs relatively low and made the system easily scaleable. Sophisticated autocode tools allowed a two man team to write, test, and maintain thousands of lines of error-free real time code. In a single unified environment, the avionics system was designed, simulated, and tested incorporating hardware-in-the-loop and used to control an aircraft in flight. Results of application to a rapid flight test prototyping system (RFTPS) for a prototype unmanned aircraft named Frog are presented.

QMotor 2.0-a real-time PC based control environment

Abstract: This paper describes QMotor 2.0, a QNX based single-processor software environment that allows the implementation of real-time control programs on standard Intel processor based personal computers (PCs). The control program, as well as the development tools and graphical user interface, can all execute simultaneously on the PC due to the deterministic response of the operating system. This architecture replaces the traditional multiprocessor Host/DSP board architecture used in control applications. Advantages of a single-processor system include reduced cost and complexity, as well as increased flexibility and upgradability. Since its development, QMotor 2.0 has been used successfully in all of the control experiments, including motor and robot control, active magnetic bearing experiments, web handling, and vibration control in flexible structures.

Temperature control of a solar furnace

Abstract: An application of an automatic control strategy to a solar plant for material treatment has been shown. Since these kinds of plants are manually operated by skilled operators, the development of an automatic control strategy aimed at achieving adequate results throughout the wide range of operating conditions, under which these plants operate, represents an important improvement towards facilitating operation and obtaining desired performance. The control scheme presented is based on a PI controller which incorporates feedforward compensation, an anti-windup mechanism and actuator slew rate constraint handling, both in fixed and self-tuning configurations. It has been developed and applied to the control of a solar furnace for samples of different materials under extreme temperature profiles. Different plant results have been shown and both advantages and drawbacks of the scheme have been commented on.