TRUE DIGITAL CONTROL OF CARBON DIOXIDE IN AGRICULTURAL CROP GROWTH EXPERIMENTS

摘要

The paper outlines the design of Proportional-Integral-Plus (PIP) controllers for regulating the level of carbon dioxide in a closed loop apparatus used for carbon-11 tracer experiments in plants. The system, which behaves essentially as an integrator with a four sample time delay, is quite difficult to control using conventional methods because of the rather limited pulse-width-modulated input signal. In contrast, optimal PIP designs, which exploit a special Non-Minimum State Space (NMSS) form within a Linear Quadratic (LQ) optimisation context, are able to maintain the gas concentration to within 2 ppm of the set point.