In the production process of Hard Disk Drive (HDD), there is a process of writing servo signals on magnetic disks to move the head to target address. However, conventional servo track writer takes longer time to draw servo signals because of recent increase of the capacity of HDD. Therefore, they desire a low-cost method to draw servo signals. In order to resolve the problems, Self Servo Track Writing(SSTW) method which draws servo signals with its own head was already suggested. In SSTW method, there are problems to realize it. One is how to know where the absolute position of head is. The other is that written servo tracks diverge because gain of the feedback controller is higher than 0[dB] in high frequency domain. In our past work, new method for SSTW has already been suggested [1][2]. This method is essentially different from other methods, and performance is dramatically improved. However, servo tracks diverge gradually if we apply this method to real product. The purpose of this paper is to make it clear why servo tracks diverge gradually by using Hard Disk Benchmark Software, which is produced by IEEJ Specialized Committee on Servo Technology for Next Generation Mass-Storage System [3]. We concluded that the reason is in measurement noise. Furthermore, consideration for a new feedforward input is discribed.
