Includes bibliography. / This dissertation describes the application of two multivariable frequency domain techniques in order to design controllers for a “flotation plant”. A flotation plant simulator was designed and constructed at the University of Cape Town. The design of the multivariable controllers was based on a linear time invariant model (in s-domain) developed for the simulator. The two frequency domain techniques, Characteristic Loci (CL) and Inverse Nyquist Array (INA), were implemented in the form of CAD packages. The INA CAD package had already been written at the university but the CL CAD package had to be developed before the design of the controllers could proceed.

Includes bibliographical references.

Electrical and Electronic Engineering / This dissertation investigates the application of digital image processing techniques in the development of a machine vision system that is capable of characterising the froth structures prevalent on the surface of industrial flotation cells. At present, there is no instrument available that has the ability to measure the size and shape of the bubbles that constitute the surface froth. For this reason, research into a vision based system for surface froth characterisation has been undertaken. Being able to measure bubble size and shape would have far reaching consequences, not only in enhancing the understanding of the flotation process but also in the control and optimization of flotation cells.

Includes bibliographical references. / Industry often requires the sorting of one material from another. Although the detection of desired (or undesired) elements is well advanced, the mechanical ejection or removal of particles is fairly underdeveloped. Agriculture and mining applications have used air jets and water jets to eject particles ranging in weight from a few grams to several hundred. With the current trends in mechanization leading towards higher processing speeds, these traditional methods have been found to be unsuitable: they have slow turn-on and turn-off response times, leading to a high volume of material being ejected with the target. Higher processing speeds will lead to even greater amounts of waste material being ejected thus producing even lower yield concentrations. Thus the need for a quick response time, repetitive, impulse ejection mechanism in the sorting industry is apparent. A kinematic analysis of the required ejection mechanism blast strength shows that the required force depends on the target mass, the required deflection angle the force application angle and the force duration. Acoustical techniques in air are unsuitable as ejection force mechanisms. A water jet is proposed to meets these requirements. This water jet is caused by an electrical discharge in a liquid cavity. This produces a weak shock wave which is focused by the cavity to a nozzle where a slug of water is emitted. The cavity is an elliptical cavity of height h, with the electrodes mounted end on at the first focus and a reflecting cone and nozzle at the second focus. The propagation of weak shock waves in the elliptical cavity is studied theoretically and numerically - using a finite difference simulation program. The reflected converging wave is shown to depend on the cavity eccentricity and the wall admittance. The resulting converging shock wave has an asymmetrical pressure distribution. This analysis is used in the design of a prototype water jet generator. The electrical discharge circuit used for the production of shock waves in the cavity is analysed and the physical discharge process of electrical to shock energy conversion reviewed. Conditions for the maximisation of this transfer correspond to large water gap resistances, high voltages and low circuit inductances. Experiments on the prototype generator show that the transient water jet slug energy is relatively low. High speed photographic techniques reveal that the jet velocity is of the order of 30 m/s. Published results show much higher jets speeds are possible. The operation of the electrical discharge circuit is found to critically influence the water jet performance - electrical measurements show that the circuit is a sub-optimum, underdamped RLC circuit. The cone / nozzle operation is also shown to have a marked effect on performance. The nozzle in particular requires optimisation. The prototype in its present form is not suitable for use in an ejection system. Although the pulse length, rise time and channel spread of the device are suitable, the blast strength is not sufficient for deflection of the heaviest range of particles. Optimisation of the electrical circuit and increased energies will increase the blast strength.

Includes bibliography. / Uninterruptible power supplies (UPS) maintain a constant supply of power to a critical load. The distinguishing features are a fixed voltage and frequency, low harmonic content and the ability to supply the load for a period of time after the incoming supply has failed. The document begins by identifying the typical power-line disturbances and their effect on various types of equipment. Together with the power conditioners used to suppress these disturbances, various UPS standby, line-interactive and on-line configurations are introduced. Next, the possibility of modifying a locally manufactured UPS to meet the design specifications is investigated. The performance of the system under load is evaluated, and forms the basis for the following decision. Due to the large number of modifications required and the inflexibility of local UPS topology, an alternative topology is adopted. In the new topology a power factor corrector, constructed around a DC to DC boost converter, interfaces with the incoming AC line. It delivers a half sinusoidal current into the DC bus. An IGBT inverter using sinusoidal unipolar pulse-width modulation regenerates the AC load voltage after filtering through a LC-filter. The DC bus voltage ripple is reduced by synchronising the inverter load and power factor corrector current while the battery pack maintains the DC bus during a power failure. The power factor corrector employs a dedicated analogue controller chip while a MCS-51 microcontroller generates the inverter PWM, provides the remote monitoring facilities, battery charging and performs general support tasks. The total harmonic distortion of the input current is measured at less than 4% while the power factor remained above 0.99 over the entire load range. The ripple regulator reduced the DC bus voltage ripple without any noticeable effect on the load. Under maximum load, the steady state output voltage is maintained during the -20%, +10% variation in the incoming line voltage. However, the transient response fails to meet the 5% design specification. A 0-100% load step results in a 7% drop in the output voltage while the loss of the load results 10% jump in voltage. System efficiency is measured at 85%. It is the lack of processing power, precluding the use of floating point or an optimal control algorithm, which ultimately compromises the performance of the system. It is recommended that the microcontroller be replaced with a 16-bit processor or digital signal processor to provide the extra computational power needed to optimise the UPS response. To improve the voltage regulation, it is recommended that the control include an inner current loop while the switching frequency should be increased to reduce the energy storage in the output filter. Further adjustments and refinements to the topology are suggested in the final chapter.

Bibliography: leaves 105-108. / This thesis investigates the problem of electric long-tenn load forecasting based on weather conditions (specifically temperature) and also investigates load forecasting by segmenting customers according to their pattern of use using clustering techniques in order to produce an effective long-tenn load forecast.

Includes bibliography. / New equipment for producing and following the fatigue process, in metals, has been developed. The specimens, which must be in the shape of tuning forks, are resonated at high amplitudes. This is achieved by attaching small, powerful, samarium-cobalt magnets to the ends of the tynes, enabling them to be driven efficiently by a "U"-core electromagnet. A small, piezoceramic strain gauge provides a method of picking up the vibrations. To maintain resonance, the signal is used in a positive feedback loop, which incorporates an analogue multiplier to provide AGC. This also keeps the amplitude constant at any desired level, throughout the duration of an experiment.

This thesis set out to improve on previous methods of digital control of a three phase fully controlled thyristor bridge, using phase control of the thyristors, and to use this bridge for the purpose of controlling the speed of a separately excited D. C. machine, under the supervision of a small digital computer. Interfacing equipment for this purpose was designed and built. Various methods of speed detection were considered, and a simple digital tachometer designed and built. This tachometer was able to feed a digital number, related to the speed of the machine, to the computer, and was accurate to better than 1%. A direct digital control system was then implemented. The behaviour of the system was first studied under open loop conditions in order to determine the parameters of the system, and to discover any shortcomings in it. The behaviour of the closed loop system was then studied, using z-transform analysis, at sampling rates of from twice to eight times the natural frequency of the machine. Initially, the computer was used as an integral controller. The behaviour of the mathematical model was compared to that of the actual system, and the correlation found to be very good. The control system was then compensated, using compensation routines based on the z-transform analysis, and the behaviour of various compensated systems studied at sampling rates varying from 0,4 to 1 second. Again, the actual system response was found to correspond closely to the designed behaviour. Finally, a dual-machine control system, on a time sharing basis, was implemented, and a degree of interrelation between the speeds of the two machines introduced. The control algorithms developed were able to provide speed control to better than l% accuracy, with a 5% or less overshoot in the step response, a delay of one sampling period but no overshoot in the ramp response, and satisfactory behaviour under load transient conditions. The time, and memory, utilization was such that such algorithms could be extended to the control of several machines, using the single computer, if sufficient input and output facilities were available.

Includes bibliographical references. / This thesis aimed to design a measurement system to obtain B/A readings for different liquids using the isentropic phase method. The resulting system uses a sinusoidal wave source that operates on less than 20kPa to pressurize a sample liquid of less than 0.2 ml. The advantage of the system is that it can obtain measurements using relatively few parameters of the liquid, requiring only density and sound speed, and that it can obtain a result in a relatively short amount of time (less than 15 minutes after system has reached thermal equilibrium).

Bibliography: leaves 109-113. / Various transducers have been developed turbulent shear in the ocean. The airfoil probe with it's piezoceramic beam encapsulated in a rubber airfoil has the simplest electronic processing system. However, the beam is very fragile and displays unpredictable thermal effects. An airfoil probe with semiconductor strain gauges on an aluminium cantilever beam has been developed as an alternative to the piezoceramic beam. The probe was calibrated by exciting it with a known shear generated by the water flow from an oscillating nozzle. During the calibration the thermal sensitivity of the probe was established to be -1.7 % °C-1• The probe, along with it's high gain, low noise processing system, is fitted to a tethered free-fall vehicle. A solid state data logger situated in the vehicle is used to record the data generated by the turbulence probe and a pressure transducer. Field trials at St.Helena Bay and Hout Bay showed that the sensitivity of the system is 20 dB lower than that of similar systems using piezoceramic beams. The system is able to resolve turbulent dissipation levels above 10-8 W kg-1, making it useful in regions characterised by a typical mixed layer (dissipation level of 10-W kg-1); however, it's sensitivity is not adequate for deep sea measurements where dissipation levels may be as low as 10-10 W kg-1.