A servo system for displacing a head relative to a disk of a hard disk drive. The servo system comprises a dual-stage actuator, a calibration signal generator, and a calibration factor generator. The calibration signal generator generates a calibration signal having a calibration portion. The calibration factor generator generates a calibration factor. The servo system operates in operating and calibration servo modes. In the operating mode, the system defines first and second servo loops. The calibration factor is used as one of the parameters of the second servo loop. In the calibration servo mode, the calibration signal forms the second control signal, and the calibration factor generator generates the calibration factor based on movement of the head before any substantial response of the third servo loop to the calibration portion.