Method of estimating precise piston stroke of linear compressor driven by PWM inverter

Linear compressors have been used in cooling apparatuses such as house refrigerators and portable coolers due to their high efficiency. The stroke of a free piston in a linear compressor driven by PWM inverter can be estimated by integrating the back electromotive force (EMF) of a linear permanent-magnet synchronous motor (LPMSM), which is used to drive a piston. However, it may be saturated due to dc offsets in measured motor voltage and current. The method for estimating precisely the piston stroke by using both the high-pass filter (HPF) and adaptive all-pass filter (APF) is suggested. The HPF is used for removing dc offset in the back EMF, and the resonance frequency of an adaptive APF is controlled in order to adjust the phase of back EMF to -90° under inverter frequency variations. Through simulation and the experimental results, it can be verified that the piston stroke has nearly sinusoidal waveform without any dc offset.