MIMO CONTROL OF CONVEYOR-BELT DRYING CHAMBERS

The development of MIMO control systems for individual conveyor-belt drying chambers was studied for the case of industrial units used for the moisture removal from wet raisins. The process was considered to be a 2x2 open-loop system in which material moisture content and temperature at the exit of the drying chamber were to be controlled. Its dynamic behavior was investigated via digital simulation of the corresponding process mathematical model which involves 6 state variables. The effect of several manipulated and load variables on process outputs was explored by examining the corresponding responces obtained when the input variables were step forced into the non-linear dryer model simulator around the operational point studied. Reliable transfer functions for each interaction module were produced, based on simulated data and process basic constants. The best control configuration was selected by deriving the RGA values of each one that was based on its frequency information. The proposed scheme, used steam and fresh air flowrates as manipulated variables. This scheme was found to imply insignificant interactions between material moisture content and temperature loops. The suggested pairing was material moisture content controlled by steam flowrate and fresh air flowrate controlling material temperature. Single-loop PI-feedback controllers were installed in each loop and tuned by Ziegler-Nichols techniques. The closed-loop system performance was examined by suitably introducing step changes to both set-points as well as to process disturbances. The overall control system performance proved to be quite satisfactory.