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Three readback signal detection methods are presented for real-time head disk spacing variation measurement under vibration conditions. The proposed PW50 parameter estimation method is not restricted by the isolated readback pulse signal requirement and, thus, can be used for spacing variation measurement in normally operated hard disk drives, especially for low-frequency variations. The thermal signal detection method, on the other side, is more effective for high-frequency spacing variation measurement. Amplitude detection gives a rough but simple way to study the head disk spacing change. Theoretical analysis and experimental results are given to illustrate the effectiveness of the methods.