Identification and simulated control of greenhouse closed water supply systems

Direct feedback of drain flow is presented as a basic principle to control the supply of nutrient solutions to plants in a greenhouse. In contrast to open loop control based on the prediction of water uptake, control by means of sensor feedback of the measured drain water flow automatically compensates for fluctuations in evapo-transpiration. A pulse modulated pump keeps the drain flow at a constant set value by a feedback control action on the repetition rate and a feed forward control action on the pulse width. The water uptake by the plant is treated as a disturbance. The simulation is performed on dynamic models for three different growing systems, that are identified on the basis of noise response tests on the real supply system. Experimental results are presented, which show how the controller compensates changes in water uptake by the plant due to variations in global radiation.