The conventional servo writing methods have some disadvantages. They need expensive external devices for positioning of the write head to write the servo patterns and require clean room environment not to contaminate the disk drive interior. Furthermore, frequent calibrations may be required. They face increasingly difficult challenges as the move towards ever increasing Track Per Inch (TPI) continues in the industry. These challenges come from different factors. Firstly, conventional servo writing requires several revolutions of the spindle to create one servo track. Therefore, the total time required to servo write an HDD increases proportionately with increasing number of tracks per surface. Because of this decrease in the throughput of servo writing, more servo writers are required to meet the production target. This in turn, requires more floor space in clean room. Secondly, for drives of smaller form factors, the jigs and fixtures for servo writing become smaller. It is challenging to design the pushpin mechanism that is small and yet sufficiently stiff.
The demand for higher throughput without increasing the production cost and process time was primary motivation for developing an alternative method of writing servo patterns called Self ServoWriting (SSW) which can be performed in an area outside expensive clean room as this method does not require any opening on the HDD's enclosure. SSW has been attractive in HDD industries because it not only potentially saves in the cost and time of mass manufacturing, but also effectively maintains the servo writing quality. SSW regenerates timing (tangential) and position (radial) information from the previously written track using the existing GMR head of the HDD itself. Hence, the external servo writers are no longer needed and servo writing does not have to be processed in the clean-room environment.
The servo system in SSW involves two control loops: position control loop timing control loop. In the position control loop, a Voice Coil Motor (VCM) is controlled to maintain the heads over the target track centerline during the reading and writing operations. And in the timing control loop, a Phase Lock Loop (PLL) is generally used to generate the servo writing clock signal. Therefore, there are two critical control objectives in SSW process with desired accuracy:
1. All the servo tracks must be perfectly concentric. If not so, the data tracks will be squeezed, resulting in loss of off-track read margin or AC track squeeze. In the position control loop,the position of the write head with respect to the track centerline is controlled by controlling the VCM. Any disturbance and eccentricity present during this process will appear as written-in Repeatable Run-Out (RRO) for the head positioning servomechanism of HDD. This written-in RRO increases TMR that must be compensated for by the servomechanism of an operational HDD. Naturally, the requirements on the accuracy in positioning the write head is more stringent in SSW than in HDD servo system.
2. The servo sectors of any track must be written perfectly radially aligned (radially coherent) with those in the adjacent tracks. If not so, reading of servo data will be degraded or corrupted. In the SSW process, this is done by controlling a Phase Lock Loop (PLL) to generate the clock signal to write the timing marks in the servo patterns. The misalignment between the written timing marks and the desired timing marks can be measured by timing jitter. Excessive timing jitter causes distortion in read back signal and warping of the servo sectors.
- F. Dong and M. Tomizuka, "An Iterative Learning Control design for Self-Servowriting in Hard Disk Drives Using L1 Optimal Control," American Control Conference, 2009.