Abstract: A voice coil motor control signal is detected being coupled into a vibration sensor of a hard disk drive. The vibration sensor is used to provide a feedforward signal to a servo control system of the hard disk drive. In response to detecting the coupling, the feedforward signal is modified to prevent instability of the servo control system.

Abstract: Head disc assembly serial number information is encoded in the servo bursts recorded during servo track writing. Specifically, the HDA serial number information is written to the disc during disk manufacture as part of the Servo Track Writing process in between the normal servo frames. The serial number is preferably written on preexisting unique system (non-user) tracks in-between normal servo frames. The encoded serial number can be retrieved by increasing the sampling interval of the servo information, i.e., by over-sampling the servo information. More specifically, by writing the serial number in a servo burst exactly in the middle of two normal servo frames, the information can be retrieved by 2× over-sampling routines called by system diagnostic routines or disc drive certification routines. In addition, a disc drive apparatus having a retrievable encoded head disc assembly serial number is provided.

Abstract: An apparatus and method of reducing the size of a servo burst and thus increasing the disc surface available for data storage is accomplished by uniquely encoding track information in the gray code of a servo burst on a disc. Each track has a unique sequential track identification number assigned to it. A plurality of sets of servo sectors on each track are encoded with a modulo operation of the track identification number for the track each using a unique modulus. Preferably the sectors are grouped into two sets, i.e., even numbered sectors on each track are modulo encoded with a first modulo operation of the track identification number for the track using a first modulus and odd numbered servo sectors on each track are modulo encoded with a second modulo operation of the track identification number for the track using a second modulus. Decoding the absolute track address is accomplished by decoding the modulo numbers and comparing the even sector modulo numbers to the odd numbered sector modulo numbers.