Modeling and control of a miniature servo pneumatic actuator
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Pneumatic actuators are low-cost, safe, clean, and exhibit a high power to weight ratio. In this thesis a novel servo pneumatic system based on miniature cylinders is presented. The first cylinder investigated has a 9.5 mm bore size. Four low-cost 2-way proportional valves are incorporated to provide greater design flexibility than the traditional single 4-way servo valve solution. A nonlinear system model is developed and validated using open-loop experiments. The use of bipolynomial functions to model the valve flow rates is shown to provide a more accurate solution than the commonly used nozzle flow equations. Two multiple-input single-output nonlinear position controllers are designed using the inverse dynamics and backstepping method respectively. In addition to position control, the control designs allow a second control objective to be implemented. In the inverse dynamics controller, the chamber pressures are controlled in inner loops and the position is controlled in an outer loop. In the backstepping controller, the stability analysis includes the effects of friction modeling error and valve modeling error. In experiments with a 1.5 kg moving mass, the inverse dynamics controller produced SSE within ±0.08mm and the backstepping controller ±0.05mm. The two control laws produced maximum tracking errors of ±0.5 mm and ±0.3mm for a 1 Hz sine wave trajectory respectively. The tracking errors are shown to be 85% less than those produced by a linear controller. Experiments demonstrate that the two controllers are robust to the system operating in horizontal and vertical orientations. They are also robust to an increase of payload but not to a decrease of payload. This problem can be overcome by tuning the controller parameters for the smallest payload. The two controllers are further tested with miniature cylinders with different bore diameters and stroke lengths. The smallest cylinder tested has a 4 mm bore diameter.
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