Small-signal modeling and active damping of resonant electromagnetic levitation melting system with experimental verification

It is well-known that levitation melting type induction heating system, fed by resonant inverter, is a complex and nonlinear dynamical system, in which high-frequency electrical-side dynamics and low-frequency mechanical side dynamics are strongly coupled. Such systems are typically operated at resonant electrical frequency in order to both stabilize the open-loop dynamics and maximize the power transfer. While stable, resulting open-loop dynamics is typically nonlinear and highly underdamped, depending on both floating object vertical position and operation frequency. Oscillative behavior of high-temperature work piece being melted is obviously undesired due to safety issues. In order to address the issue, this paper first derives small-signal operation point dependent linearized dynamics of the system, based on the recently established envelope modeling methodology. Then, model reduction is carried out within bandwidth of interest. It is shown that both linearized control-to-output and disturbance-to-output transfer functions may accurately be represented by second-order dynamics with operating point dependent gains, natural frequencies and damping factors within the bandwidth of interest. Once derived, reduced system representations are employed in active damping feedback controller design, in which operation frequency rather than one of mechanical-side variables is utilized as feedback signal, making the proposed algorithm sensorless. It is shown that suggested methodology influences only the damping factor of original system while leaving both gain and natural frequency unaltered and is capable of eliminating oscillative response throughout the whole operation range. Simulations and experimental results are shown to validate both reduced-order linearized dynamics and the proposed active damping methodology.