Considering the stability of production and the balance of load the process requires that the materiel quantity should change slowly or be kept unchanged. It can be solved by controlling the liquid level of buffer container. However

the classical PID method cannot control the level of the horizontal container and the processes with phase change and disturbances. Based on practical experience a zone control method combined with fuzzy control and classical control was proposed. This method took the high limit

low limit and changing value as the basis of the input fuzzy values. It changed the classical fuzzy methods and used a new strategy to transform the fuzzy outputs and the control outputs which transformed the fuzzy outputs to the expected level values. A tuning output term was added to enhance the control accuracy when considering large deviation between practical outputs and expected outputs. This control method allowed the liquid level to fluctuate within the set range

and guaranteed a slow and stable change of materiel quantity. When the liquid level went beyond the set fluctuation range or the variable liquid level exceeded the set point

it was adjusted to ensure the stable load of the downstream unit. A practical application proved the effectiveness of the zone control method.