Proefschrift Universiteit van Amsterdam. / Met een samenvatting in het Nederlands.

Conventional control systems express control solutions by means of expressions, usually mathematically based. In order to completely express the control solution, a vast amount of data is required. In contrast, knowledge-based solutions require far less plant data and mathematical expression. This reduces development time proportionally. In addition, because this type of processing does not require involved calculations, processing speed is increased, since rule process is separate and all processes can be performed simultaneously. These results in improved product quality, better plant efficiency, simplified process, etc. Within this project, conventional PID control has already been implemented, with the control parameter adjustment and loop tuning being problematic. This is mainly due to a number of external parameters that affects the stability of the process. In maintaining a consistent temperature, for example, the steam flow rate varies, the hot well temperature varies, the ambient may temperature vary. Another contributing factor, the time delay, also affects the optimization of the system, due to the fact that temperature measurement is based on principle of absorption. The normal practice in industry to avoid an unstable control condition is to have an experienced operator to switch the controller to manual, and make adjustments. After obtaining the desired PV, the controller is switched back to automatic. This research project focuses on eliminating this time loss, by implementing a knowledge-based controller, for intelligent decision-making. A FLC design tool, which allows full interaction, whilst designing the control algorithm, was used to optimize the control system. The design tool executed on a PC is connected to a PLC, which in turn is successfully integrated into the process plant.

The thesis examines diverse methods of damping the librational motion of earth-orbiting satellites. Starting with passive stabilization, two classical mechanisms for energy dissipation are studied, for performance comparison, when executing librations in the orbital plane. The first model, consisting of a sliding mass restricted to relative translational motion with respect to the main satellite body, establishes the suitability of various approaches to the problem in circular orbit. In this case, numerical and analog methods do not readily yield information on the influence of parameters and approximate methods are found to be particularly helpful. Butenin's method based on averaging techniques predicts the response of the satellite with good accuracy for small damping constant while the exact solution to the linearized equations provides optimum damper characteristics for motion in the small. A comparison of the sliding mass damper model with a damper boom mechanism involving only relative rotational displacements, is then performed for equal equilibrium inertias of the damping devices. It indicates that, for optimum transient tuning, the damper boom would have a better time-index while the sliding mass would lead to smaller steady-state amplitudes for low eccentricity orbits. A numerical example using GEOS-A satellite data illustrates the outcome of the study when applied to physical situations. A stability analysis is also included which uses Routh and Lyapunov approaches to determine the domain of parameters leading to asymptotic stability, as well as numerical methods to define the bounds on stable initial disturbances: it is found that for most practical applications, the stability contour in circular orbit is close to that of the undamped case. How-ever, for eccentric trajectory, the amount of damping critically affects asymptotic stability. The next model, which involves active stabilization, uses solar radiation pressure to achieve planar librational control of a satellite orbiting in the plane of the ecliptic. This is obtained by adjusting the position of the center of pressure with respect to the center of mass through a controller depending on a linear combination of librational velocity and displacement. The motion in circular orbit is; first investigated through the W.K.B. method. Although the approximate equation involves an infinity of turning points, only a few of them are required to evaluate the damped behaviour of the system. A comparison of the analytical results with a numerical integration of the exact equation of motion shows good agreement only over a limited range of parameters and, therefore, the latter is used to complete the study for circular and elliptic cases. The concept leads to great versatility in positioning a satellite at any angle with respect to the local vertical. Also, high transient ; performance is observed about local vertical and horizontal and the dichotomous property of good transient associated with poor steady-state inherent to passive damping can be avoided by selecting appropriate controller parameters. An example is included which substantiates the feasibility of the configuration. Finally, the attention is directed towards the influence of gravity torques on the stability of damped axisymmetric dual-spin satellites. The nutation damper mounted on the slowly-spinning section is of the pendulum type. For this section rotating at orbital angular rate, application of the Kelvin-Tait-Chetaev theorem indicates that the asymptotic stability region reduces basically to the mainly positive stable spin region of the undamped case. However, some care is required depending upon the shape and natural frequency of the damper. If the damper section rotates at a much higher rate than the orbital one, torque-free motion need only be considered for short term pre-dictions. Stability charts corresponding to this case, given for comparison, emphasize the effect of gravity. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

It is assumed that processes to be identified or controlled can be described by linear or non-linear differential equations with unknown coefficients aᵢ. For on-line identifications a model is constructed to have the same form of differential equation as the process, but with adjustable parameters αᵢ replacing the aᵢ. The parameters αᵢ are adjusted in a steepest descent fashion; they are shown to converge to the aᵢ as long as an adjustment gain K(t) is non-negative, and not identically zero. An approximate analysis is carried out to determine the best constant K which gives the fastest identification. Optimal control theory is introduced to find the best piecewise continuous K(t) in the interval 0 ≤ K(t) ≤ Kmax . From the exact solution in a special case, a switched suboptimal K which always gives fast identification is determined. Identification schemes are developed for processes which include an unknown non-linearity that, can be assumed to be piecewise linear. An adaptive control, system is developed to control processes with unknown time-varying coefficients. The system is shown to be stable as long as the process inverse is stable; the process need not be stable. Systems to control linear and non-linear unstable processes are designed and simulated. The limitations of the adaptive system are determined, and compared with the limitations of conventional feedback systems. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

An investigation is made into the approximate synthesis of optimal feedback controllers from the maximum principle necessary conditions. The overall synthesis can be separated into two phases: the computation of optimal open-loop controls (control programs) and trajectories from the necessary conditions, and the processing of this data to obtain an approximate representation of the optimal control as a state function. A particular technique for approximating the optimal feedback control from the optimal open-loop controls and trajectories is proposed and examined in Part I of the thesis. Parameters in a prechosen suboptimal controller structure are computed such that a sum of integral square deviations between the suboptimal and optimal feedback controls is minimized. The deviations are computed and summed over a certain set of trajectories which "cover" the system operating region. Experimentation with various controller structures is quite feasible since the controller parameters are computed by solving linear algebraic equations. Examples are given to illustrate the application of the technique and ways in which suitable controller structures may be found. If general purpose functions are to be used for this purpose, piecewise polynomial functions are recommended and techniques for their use are discussed. The synthesis method advocated is evaluated with respect to control sensitivity and instrumentation and compared to alternative procedures. Part II is concerned with the computation of optimal control programs, the most time consuming numerical task in the synthesis procedure. A new numerical optimization technique is presented which extends the function space Newton-Raphson method (quasilinearization) to a more general terminal condition. More significantly, a generalized Ricatti transformation is employed, and as a consequence, the integration of the unstable coupled canonical system is eliminated. Examples are given as evidence of the improved numerical qualities of the new algorithm. This method is one example of a class of algorithms, defined and developed in the thesis, called second variation methods. Some methods in this class have previously appeared in the literature but they are developed in the thesis from a unified point of view. The recognition of this class allows the relationships between the various methods to be seen more clearly as well as allowing techniques developed for use in one algorithm to be used in others. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Methods for studying the behaviour of on-off feedback systems, with the emphasis on steady-state periodic phenomena, are presented in this thesis. The two main problems analyzed are (1) the determination of the periods of self and forced oscillations in single-, double-, and multiloop systems containing an arbitrary number of on-off elements; and (2) the investigation of the asymptotic stability in the small of single-loop systems containing one on-off element which may or may not have a linear region of operation. To study the periodic phenomena in on-off systems, methods of determining the steady-state response of a single on-r-off element are first described. Concepts pertaining to the steady-state behaviour are then introduced: in this respect it has been found that generalizations of the concepts of the Hamel and Tsypkin loci and also of the phase characteristic of Neimark are useful in the study of self and forced oscillations. Both the Tsypkin loci and the phase characteristic concepts are used to determine the possible periods of self and forced oscillations in single- and double-loop systems containing an arbitrary number of on-off elements; these concepts are also applied to multiloop systems. On-off elements containing a linear region of operation, called a proportional band, are then described: both the transient and periodic response are presented. An approximate method for determining the periodic response is given. The concept of the Tsypkin loci is used to determine the possible periods of self and forced oscillations in a single-loop system containing one on-off element with a proportional band. The asymptotic stability in the small, or local stability, of the periodic states of single-loop systems containing one ideal on-off element has been considered by Tsypkin. In this thesis, Tsypkin's results have been generalized to include the cases of on-off elements containing a proportional band. The stability of such systems is determined by the stability of equivalent sampled-data systems with samplers having finite pulse widths. Finally, this stability problem is solved by a direct approach, one that makes use of the physical definition of local stability; the results obtained by this method agree with those derived by the sampled-data approach. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The research developed force-motion strategies and subsequent force and position control algorithms, using a PUMA 560 robot arm and its original controller. A task decomposition methodology has been developed that enables a mechanical assembly task to be subdivided into a series of executable subtasks. By applying this methodology to the assembly of a hydraulic gear pump, a library of special purpose, task oriented, subtask programs were created. Most of these programs, though derived for a pump assembly task, are applicable (when used with appropriate parameters) to other assembly tasks. Most of the algorithms require force/torque sensory information that is supplied by a JR³ wrist force sensor. The force control algorithms use that data and system compliance in order to produce new position instructions that are transferred to the controller of the arm. The logic of the control law and system behaviour when contacting the environment, were checked, using the dynamics and compliance of a simplified structure of a robotic arm and its wrist sensor. A demonstration of the pump assembly task, using the arm, force sensor, controller and the derived library algorithms is an integral part of the thesis. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The feasibility of utilizing the environmental forces for three-axis librational damping and attitude control of spinning satellites is investigated in detail. An appreciation of the environmental influence is first gained through a librational dynamics study of spinning, axisymmetric, cylindrical satellites in the solar radiation pressure field. The highly nonlinear, nonautonomous, coupled equations of motion are analyzed approximately using the method of variation of parameters. The closed form solution proves to be quite useful in locating periodic solutions and resonance characteristics of the system. A numerical parametric analysis, involving large amplitude motion, establishes the effect of the radiation pressure to be substantial and destabilizing. Next, a possibility of utilizing this adverse influence to advantage through judiciously located rotatable control surfaces is explored. A controller configuration for a dual-spin spacecraft is analyzed first. The governing equations, in the absence of a known exact solution, are solved numerically to evaluate the effect of system parameters on the performance of the control system. The available control moments are found to be sufficient to compensate for the rotor spin decay, thus dispensing with the necessity of energy sources maintaining the spin rate. The controller is able to damp extremely severe disturbances in a fraction of an orbit and is capable of imparting arbitrary orientations to a satellite, thus permitting it to undertake diverse missions. The development of an efficient yet structurally simple controller configuration is then considered. A logical approach for solar controller design is proposed which suggests a four-plate configuration. Its performance in conjunction with a bang-bang control law is studied in detail. The utilization of maximum available control moments leads to a substantial improvement of the damping characteristics. Attention is then focussed on using the earth's magnetic field interaction with onboard dipoles for attitude control. Magnetic torquing, however, is unable to provide first order pitch control in near equatorial orbital planes. The shortcoming is overcome by hybridizing the concepts of magnetic and solar control. Two magnetic controller models, employing a single rotatable dipole or two fixed dipoles, are proposed in conjunction with a solar pitch controller. The system performance is evaluated for a wide range of system parameters and initial conditions. Although high spin rates lend considerable gyroscopic stiffness to the spacecraft, the controllers continue to be quite effective even in the absence of any spin. Even with extremely severe disturbances, damping times of the order of a few orbital degrees are attainable. As before, the concept enables a satellite to change the desired attitude in orbit. The effectiveness of the controllers at high altitudes having been established, the next logical step was to extend the analysis to near-earth satellites in free molecular environment. A hybrid control system, using the solar pressure at high altitudes and the aerodynamic forces near perigee, is proposed. The influence of important system parameters on the bang-bang operation of the controller is analyzed. The concept appears to be quite effective in damping the satellite librations. Both the orbit normal and the local vertical orientations of the axis of symmetry of the satellite are attainable. However, for arbitrary pointing of the symmetry axis, small limit cycle oscillation about the desired final orientation results. Finally, the time-optimal control, through solar radiation pressure, of an unsymmetrical satellite executing planar pitch librations is examined analytically. The switching criterion, synthesized for the linear case, is found to be quite accurate even when the system is subjected to large disturbances. Throughout, the semi-passive character of the system promises an increased life-span for a satellite. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

No description available.

A project report submitted to the Faculty of Engineering, University of Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg 1997. / This project report describes the design of a fuzzy logic controller for automatic generation control (AGC) in Eskom in 1995 and the process of re-optimisation of the fuzzy logic controller in 1997. The main purpose of the AGC controller is to determine the shortfall or surplus generation of electricity for South Africa. The difficulties associated with optimising the original AGC controller, the design,implementation and optimisation of the fuzzy controller are described in detail. [Abbreviated Abstract. Open document to view full version] / AC2017