Robust fractional-order temperature control of a steel slab reheating furnace with large time delay uncertainty

This paper proposes a new strategy to control the temperature of a steel slab reheating furnace based on fractional-order controllers. A second order plus time delay model of the preheating zone of this process is used, which was obtained from an identification procedure applied in an industrial furnace. It is shown that this process experiences very large time delay changes. A fractional-order integral controller (FI) combined with a Smith predictor is designed, which is robust to changes in such time delay. Simulated results of a standard PID controller, PI controller combined with a Smith predictor and the proposed FI controller also combined with a Smith predictor are compared. Seven performance indexes have been used in this comparison: four related to the output performance (settling time, overshoot, ISE and IAE), and three related to the control signal (maximum amplitude, control effort TV and fuel consumption). The analysis of these indexes show that the fractional-order controller exhibits the most robust behaviour (lowest indexes averaged in all the range of time delay variation) for ranges that include large time delays. Then the robustness features of the FI controller combined with a Smith predictor outperform the other integer order controllers both in the time response to a setpoint change and in the control effort aspects.