A MICRO-MINI COMPUTER HIERARCHICAL CONTROL SYSTEM FOR A LABORATORY SCALE FERMENTOR

Abstract A micro-minicomputer control hierarchy has been developped for a laboratory scale fermentor. The 20-liter fermentor is used for fed-batch aerobic yeast production (Saccharomyces Cerevisiae) from carbohydrate substrates. It is fully analogically instrumented and has low-level control on variables such as temperature, pH with hardware controllers with setpoint control. Our fermentation requires a control of dissolved oxygen concentration, the control variables being the air flowrate and the agitation speed. In fed-batch a control of substrate flowrate is required. This is done using a peristaltic pump and a load cell. Other sensors include effluent gas analysers for oxygen and carbon dioxide. The control hierarchy is constituted bv a minicomnuter and two mi -crocomputers. One of the microcomputers is devoted to the control of dissolved oxygen concentration and the other to the substrate flowrate control. The microcomputers are more powerful than classical controllers as both of them contain elaborate control schemes, especially for the substrate mass flowrate control which has the advantage to avoid any pump tubing calibration although several substrates of different densities and viscosities are used. With this routine work being done by the microcomputers, the minicomputer is free to carry out higher level tasks which include advanced control schemes, data-logging, disk archiving. The optimal control strategy realizes the determination of set-poihts which are sent analogically to the hardware controllers of the fermentor and/or to the microcomputer through asynchronous lines. Dialog procedures have being established between the minicomputer and the microcomputers to permit information exchange. The minicomputer is able to interrupt at any time a microcomputer routine to send new set-points or to ask for some piece of information collected by the microcomputer . This hierarchical system has been constructed to provide a flexible and reliable control of a computer coupled fermentor. It is actually used for optimal fed-batch yeast using respiratory quotient control as optimal policy. But the techniques of hierarchical control implemented here are sufficiently versatile to be easily expanded to other fermentations and control strategies.