Control of fluid bed tea dryers: controller performance under varying operating conditions

A study was made of the robustness of controllers for fluidized bed tea drying. Several controller configurations have been designed and studied in previous work. Tuning of these controllers is possible using a transfer function estimated from the frequency behaviour of a validated simulation model under normal operating conditions. In this work a range of operating conditions deviating from the standard conditions was studied. Controller tuning determined by the Cohen & Coon or Ziegler¯Nichols methods was not found to be robust over the range of conditions tested. A different method was developed, based on dryer modelling, to establish a range of controller settings giving minimal Integral Squared Error while maintaining adequate gain and phase margins. These settings were found to be suitable for the whole range of conditions tested. A simplification to the inferential controller, using gains only rather than complete transfer functions in the inferential estimator, was shown to be justified.