DESIGN OF SWITCHING CONTROL-SYSTEMS FOR VENTILATED GREENHOUSES

A state-space averaging method was employed in the design of a switchi ng control system for a conventional single-phase, Mo-speed ventilatio n fan with time-averaged temperature feedback. The unique state-space averaging method allows the switched, i.e., discontinuous, system to b e approximated as a continuous system; thus direct digital control des ign techniques can be applied for designing the switching controller. Dynamic simulation of the controller action and greenhouse air tempera ture indicated that the root-mean-square temperature could closely mat ch the setpoint temperature, and that the switching period affected fl uctuations about the setpoint. The validity of the designed switching controller was confirmed through field tests and compared to a convent ional staging control strategy in research greenhouses. The proposed d esign method shows promise as an alternative to conventional PID syste ms and stage controllers, and is applicable to any discontinuous switc hed system. Detailed design development is provided.