Modelling and proportional-integral-plus control design for free air carbon dioxide enrichment systems

Proportional-integral-plus (PIP) control is employed to maintain gas concen tration in a small-scale free air carbon dioxide enrichment (FACE) system. FACE systems are designed to produce controlled concentrations of elevated carbon dioxide, or other atmospheric gases, enabling plant growth experimen ts to be carried out for in situ vegetation without the use of chambers or other enclosures. Current FACE systems employ control algorithms based on c lassically derived two- or three-term control laws with manually tuned para meters. However, small FACE plots are more susceptible to turbulent eddies than larger scale systems, making control of concentration particularly dif ficult. The research described in the present paper employs data from plann ed FACE experiments to develop PIP control algorithms exploiting model-base d predictive control action. Initial trials utilizing this approach yield g ood results for a small-scale FACE system operating in an uncut arable mead ow. (C) 2000 Silsoe Research Institute.