Model-based virtual sensors and core temperature observers in thermoforming applications

In this paper, we present the notion of model-based virtual sensors and virtual core temperature observers in a thermoforming process. The concept of model-based virtual sensors allows for additional surface temperature measurement points in addition to the already existing infrared sensors. This leads to improved controllability of the plastic sheet temperature as well as increased accuracy in temperature zoning, thus eliminating the use of extra infrared sensors, which significantly reduces the cost of the control system. The problem of core sheet temperature measurement is also addressed through the application of a closed-loop Luenberger core temperature observer to estimate the center-plane temperature of the plastic sheet since it is not practical to have any kind of actual core temperature measurement during the heating process. The two concepts of virtual sensors and virtual core temperature observers are then combined to form an overall observer based closed-loop control system. Finally, the functionality, performance, and robustness of the new system is investigated through simulation of an industrial-type thermoforming machine.