Efficient position encoding in hard disk drive using dual frequency servo bursts

This paper presents a new scheme to encode the position information in hard disk drives. The conventional method uses amplitudes of 4 servo bursts, placed sequentially along the track (down-track direction), are used to find the displacement between the head and the centre of the track. Each of these bursts consists of equidistant, parallel magnetic transitions, forming a specific offset pattern in the direction of disk radius (cross-track). The read-back signal from each servo burst contains series of alternating Lorentzian pulses at fixed intervals. The amplitudes of the burst signals are functions of displacement between the head and the centre of the track. The demodulator finds the magnitude of read-back pulses for each burst, and estimates the position error signal (PES) from there. Since all the bursts use the same pattern of magnetic transitions, they are separated from each other in the down-track direction. This leaves empty spaces between burst patterns, increasing the servo overhead or the percentage of storage area used for servo information. The proposed servo pattern uses two different frequencies for two servo bursts, making it possible to place them at the same angular location. The read-back burst signals are contributions of the servo patterns having two different frequencies. The demodulator extracts the amplitude of each of these frequency components, which is then used for PES calculation. Several detection rules are used to extract the amplitude of each frequency, and the results are compared. The effectiveness of the scheme is shown using experimental results.