Servowriting is a manufacturing process where the data tracks and servo sectors are defined on the disk platters by creating the servo patterns so that the HDD servo system can later use them to identify the tracks and servo sectors, and to measure the relative position of the read head to the track centerline, i.e., the Position Error Signal (PES). The HDD servomechanism generates the position feedback signal by using the servo patterns as reference, which are written on the disks when no reference is present on the disks or inside the HDD. There are several conventional mechanisms used to provide the reference while the servo writing is performed. The common one is to externally write the position (servo burst) and timing (synchronization mark) information onto the disk surface by means of a costly laser-guided push-pin mechanism which can accurately move the HDD actuator arm so that the write head is positioned on the desired tracks.
To obtain adequate servo pattern quality is the key to achieve high TPI (track per inch) of the hard disk drive products, thus improvement of the servo track writer performance becomes increasingly important. As the TPI gets higher and higher, servo writing takes a longer and longer time. This requires additional investment on servo track writer which translate to increase of the product cost. One way to solve the problem is to write spirals on the servo track writer and perform in-drive fill later in the self test chamber. The latter process is called self servo writing. Since spiral writes takes much shorter time on servo write, this greatly reduces the servo track writing time, thus the servo track writer cost.
To reduce the disturbance and to improve the servo accuracy, robust and high precision learning control algorithms have been developed for single and multiple spiral writing.
- X. Chen and M. Tomizuka, “Spiral Servo Writing in Hard Disk Drives Using Iterative Learning Based Tracking Control,” in Proceedings of the 18th World Congress of the International Federation of Automatic Control (IFAC), Aug. 28-Sept. 2, 2011, Milano, Italy, Vol. 18, Part 1.