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In the last few years carbon dioxide received increasing attention as a possible replacement for fluorocarbon-based refrigerants used within present automotive A/C system technology. R-134a is harmless to the ozone layer but the greenhouse effect is more than 1300 times higher than that of an equivalent amount of CO2. Alternative refrigerants are natural gasses such as propane and butane, however these gasses are considered explosive. With many objections raised it appears if CO2 will be the future refrigrant for automotive use. One concern with R-744 is its high operating pressure and suction/discharge pressure difference when compared to common refrigeration processes. A major problem with the CO2 cycle is the loss of effciency at high ambient temperatures. With a COP optimized control system for the expansion value based on pressure, temperature and mass flow of the refrigerant, an effective A/C system for CO2 could be deleloped. This resrach offers basic knowledge of refrigerant cycles and gives an overall view of the refrigerant change-over problem. With the results obtained from the experimental work a better understanding of the CO2 cycle and a better understanding towards effective A/C systems have been realized.

This dissertation covers the design and manufacture of an autonomous guided cart (AGC) for use in the material handling industry. General Motors South Africa (GMSA) requires a low-cost AGC for use in their Struandale plant. A budget of R35 000 per unit was proposed. The researcher, in collaboration with staff at GM, compiled a list of engineering requirements for the AGC. After research into the unique problems of the project, an examination of a previous design attempt by staff of GM Thailand, the researcher developed a new design, the subject of this report. Different solutions for each design problem were investigated before the design was finalised. A three-wheeled vehicle was designed making use of two motors in a differential-drive setup to control motion. Navigation is via a line-following mechanism, using an induction sensor-array in conjunction with a pre-laid metallic strip. To aid the design, the system was modelled to understand the different control elements at play. The researcher developed software for several aspects of the design: for the PLC controlling the system and motors; for a microcontroller that communicates with the PLC and a wireless module; for a computer server that communicates with a second wireless device, receiving information from the PLC; and a web interface to view this information. These form the SCADA integration of the project. The final product meets the GMSA specifications. It is a robot capable of towing a trolley of mass not exceeding 350 kg. While the robot is able to navigate a pre-laid route, it cannot reliably stop at marked locations. It is possible to monitor the system via a web-interface. The robot is capable of operating for an entire 8-hour shift before the batteries need to be recharged. The total cost of the prototype was R26 340.

In this work we reproduce driving techniques to mimic expert race drivers and obtain the open-loop control signals that may be used by auto-pilot agents driving autonomous ground wheeled vehicles. Race drivers operate their vehicles at the limits of the acceleration envelope. An accurate characterization of the acceleration capacity of the vehicle is required. Understanding and reproduction of such complex maneuvers also require a physics-based mathematical description of the vehicle dynamics. While most of the modeling issues of ground-vehicles/automobiles are already well established in the literature, lack of understanding of the physics associated with friction generation results in ad-hoc approaches to tire friction modeling. In this work we revisit this aspect of the overall vehicle modeling and develop a tire friction model that provides physical interpretation of the tire forces. The new model is free of those singularities at low vehicle speed and wheel angular rate that are inherent in the widely used empirical static models. In addition, the dynamic nature of the tire model proposed herein allows the study of dynamic effects such as transients and hysteresis. The trajectory-planning problem for an autonomous ground wheeled vehicle is formulated in an optimal control framework aiming to minimize the time of travel and maximize the use of the available acceleration capacity. The first approach to solve the optimal control problem is using numerical techniques. Numerical optimization allows incorporation of a vehicle model of high fidelity and generates realistic solutions. Such an optimization scheme provides an ideal platform to study the limit operation of the vehicle, which would not be possible via straightforward simulation. In this work we emphasize the importance of online applicability of the proposed methodologies. This underlines the need for optimal solutions that require little computational cost and are able to incorporate real, unpredictable environments. A semi-analytic methodology is developed to generate the optimal velocity profile for minimum time travel along a prescribed path. The semi-analytic nature ensures minimal computational cost while a receding horizon implementation allows application of the methodology in uncertain environments. Extensions to increase fidelity of the vehicle model are finally provided.

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This thesis explores the application of modem fault-detection methods to electric rail traction drives. Such drives consist of three main components, induction motors, power inverters and the control system. The power electronics are relatively simple so the scope for fault-tolerance is limited, whilst fault-detection techniques for induction motors are already well developed. There is however scope for work on the instrumentation. The thesis concentrates on the use of model-based techniques to produce a torque and flux estimator for an induction motor which is tolerant to intermittent sensor disconnections. The motors are controlled on torque and flux, these cannot be measured directly and are estimated from measurements of the applied voltages and the resulting currents. The existing estimator has poor steady-state performance at low speed and because of it's transient dynamics it is prone to sensor noise and disconnections. Induction motors have speed-dependent dynamics and the resulting state-space model has terms which are multiplied by speed, this model is strongly bilinear. Speed-dependent feedback is needed to give desirable dynamics to the state estimates. Starting from a state-space model for the induction motor, a closed-loop observer can be designed to estimate the motor states. A range of feedback methods for the observer have been considered, from gain scheduling to sliding mode techniques. These are evaluated in simulation, using a simplified model of the traction system. The simulation neglects many second order effects which would effect the real application. Using data from an induction motor test-rig the observers are shown to be able to track the motor torque during a change in operating condition. Only a limited set of data is available. The influence of parameter mis-match, noise and speed sensor errors are considered by deriving frequency domain expressions for the estimation error in the presence of uncertainty or disturbances. The effect of the observer's gain on its sensitivity to these are considered under conditions which occur in the real application. Using observer feedback to decouple sensors from the estimation a range of sensor fault-detection schemes are developed. In this way a bank of observers is designed which are independent of a different subset of sensors, this enables sensor faults to be isolated. These method are compared in simulation. A motor, inverter and instrumentation are set up, with a DSP to run an observer based sensor fault-detection scheme in real-time. This enables implementation aspects to be explored, such as discretisation, model mis-match and motor loading. These effect the detection by increasing fault-free residual or reducing the fault residual. For a each type of sensor the area of the motor operating range, where a fault is detectable is defined.

Typescript (photocopy) / x, 372 leaves : ill. (part col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Electrical Engineering, 1981

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