Combined PDM with Frequency-Temperature Profile Adaptation Control for Induction Metal Hardening

During metal hardening applications, the load parameter variation is inherently encountered. At temperature close to hardening, these parameters specifically the magnetic permeability and electrical resistivity vary. Therefore, a suitable control method is often required to allow the induction heating system operating under optimum conditions. This paper proposes on new approach dealing with the issue of implementing a frequency adaptation control without using the conventional phase-locked loop (PLL). This technique seems to outweigh the PLL in many respects particularly in induction metal hardening. This approach takes into account the equivalent electrical quantities with respect to the temperature of the workpiece during the treatment process. This strategy is combined with pulse density modulation (PDM) to conduct power control improvement which account for material resistivity increase. Applied to a full bridge series resonant inverter, the proposed method of frequency tracking is assessed through power evaluation through simulation and experimental tests during the operating frequency range of hardening process. The simulation model is performed using Matlab/Simulink. The obtained results show better performances with efficiency higher than 90%. The algorithm is implemented using an experimental setup for evaluation.