Robust control of a glass tube production process

In a special glass factory of a well-known engineering company, glass tubes are manufactured on so-called SQ (Sulvanian Quartz) furnaces. Each furnace is equipped with a MIMO control device which is used for controlling the wall thickness and external diameter of tubes. When these glass tubes are used in the production of gas-discharge lamps, the internal diameter is very important since the volume of gas to be filled in the lamp very much determines the characteristics of the lamp. It is not possible to measure the internal diameter directly. Two external diameter sensors placed vertically on top of each other and four wall thickness sensors each displaced by 90 degrees with respect to each other are used for measuring the dimensions of the glass tube. It is possible to deduce the internal diameter from the average of two external diameters and the average of four wall thicknesses. This process takes account of the fact that it is not possible for these dimensions to be measured at the same time. The particular glass tube production process studied in the paper is used for the production of three different types of tube which differ in dimensions. Specifications on these products differ significantly and some are difficult to achieve. The specifications are on the average external diameter and the average wall thickness of the tubes. The objective is to design and test a control law in the internal model control scheme for controlling the average external diameter and the average wall thickness for the three types of tube which are used for the production of gas-discharge lamps.