Advances in control of inductive heating by introducing model based control techniques

An important step in processing of SSM is the inductive reheating of the raw material. Using conventional technologies the process behavior is unsatisfactory in terms of reproducibility and disturbance rejection. But reproducibility is a hard requirement for quality control in the forming process that follows the reheating in the commonly used SSM process flow. One reason for this problem is the lack of consideration of disturbances in todays control structures where often open loop systems are used. Therefore in this paper a modern control structure is presented, that uses a model of the system to calculate control inputs. First, in this article we describe the derivation of a finite element method (FEM) model of the inductive heating process. For validation the results of simulations with this model are shown in comparison to real plant data. This model is too complex for online calculations and has to be simplified for use in a controller application. Based on this simplified model a model based controller is introduced. The material properties needed for this model can be derived by parameter identification. The article focuses especially on the question of applicability of advanced control designs in a production environment where sensor data, necessary for these controllers are difficult to obtain. Therefore the derived simplified model will be used to estimate the temperature field during reheating based on only little online data. The precision of this estimation will be discussed by comparing the results with real plant data. For optimizing the SSM process a method for adjusting the reheating time is shown which is able to adapt the reheating process to a varying process flow. The closed loop performance of the controller is discussed.