Simple fractional order controller combined with a Smith predictor for temperature control in a steel slab reheating furnace

This paper proposes a simple fractional order controller combined with a Smith predictor scheme for controlling the temperature of a steel slab reheating furnace. The dynamic model of the preheating zone of this process is obtained from an identification procedure applied in an industrial furnace. This identification procedure yields a second order plus time delay transfer function which undergoes large time delay changes. A fractional order integral controller combined with a Smith predictor is therefore designed. Simulated results compare the performances of the proposed fractional order controller with a standard PI controller, also combined with a Smith predictor, an LQR controller, and an H ∞ robust controller, in the case of the nominal process, and when the time delay varies. Four performance indexes have been used in this comparison: three related to the output performance (settling time, overshooting, and integral absolute error ( IAE )), and a fourth one related to the control effort ( TV ). The analysis of these indexes shows that the simple fractional order controller provides lower values of the compared indexes when time delay becomes much higher than the nominal value.