Research And Simulation On The Key Technology For Self-servowriting Of Hard Disk Drive

Servo information is essential for hard disk drives. The position information of magnetic head related to magnetic tracks is computed by means of sampled servo information. Precise servo information can boost position precision of magnetic head and enhance the signal to noise rate of read back signal. The traditional servo information writting process is completed in clean room which costs much and maintained difficultly. The writing process needs expensive servo-writing equipments which can provide precise position information. With the rapid progress of recording density, more advanced servo-writing equipments and more clean room time are needed. Self servo-writing technology can make hard disk drives write servo information with their own read and write mechanism. It can improve manufacturing efficiency of hard disk greatly.The advanced technologies and new trends that deployed in hard disk drives are discussed first. The components and characteristics of servo systems for hard disk drivers are discussed. The factors that affect servo system performance are also analyzed. The mechanism and argrithms of servo system and self servo-writing are discussed and compared. It shows that position precision and resistance ability to outer disturbance are the key factors to the design of controller in self servo-writing process. Three kinds of controllers have been designed: lead-lag controller, PID controller and robust controller. H∞controller is the most suitable candidate for self servo-writing process. It can maintain good ability to resist disturbance and high position precision.The process of self servo-writing involves machinary and electronics. The writing of servo information relies on read/write channel of hard disk drive. The auto gain control loop and timing loop have been discussed in detail. Because ervo information tradeoff is big and the sampling band limits, dual frequencies servo coding method is proposed,utilizing the powerful processing ability of R/W channel. Since the bit error rate indicates the servo performance, R/W chanenel may enhance position precision. State machine sequence of self servo-writing has been proposed. The sequence can show the working state of channel in self servo-writing process.The radial error propagation process of self servo-writing is discussed in detail in the paper. The process of radial error accumulation is analyzed. It shows noise accumulation and position reliance on last track shape is the key elements of radial error propagation. The separating head structure can affect the self servo-writing process. The effects of skew angel of magnetic head can't be ignored. In order to analyze the effects of skew angel to self servo-writing process in quantitative approach, a method which tests the off-track capability of head is proposed. The method controls the move of head flying over the disk by firmware and analyzes the width change of projection of the read head, write head and erase band. The experiment shows skew angel can change off-track capability of magnetic head and change the radial error propagation of radial error of self servo-writing.The paper shows the effect of skew angel needs to be considered in self servo-writing process for the change of skew angel can change the radial error in self servo-writing. It proposed a radial error constrain method which takes the effect of skew angel. The absolute radial error and relative radial error can be constrained simultaneously. The radial error can be constrained in two error removal process. Research shows the error constrained process of self servo-writing is very similar to iteration control process. The paper proposes a method which uses iteration learning control theory to design correction signal to self servo-writing loop. And the correction process can be used to conditions which precision model of plant can't be got. The convergence of iteration process can be proved by theory and experiments.Timing error propagation is another main factor to be considered. The method and process of timing recovery are discussed in the paper. The timing error propagation process and its mechanism are also analyzed. It shows separating read/write head can lead to time delay and the no parallelism of read/write head can lead to the rotation of trigger pattern. A method of timing mark propagation is proposed which needs little computation working load, maintains bit synchronous and resists almost all kinds of systematic and random errors.A self servo-writing process method is proposed in the paper with radial error and timing error propagation consideration. The all aspects are proposed in detail especially the key steps of servo information and timing information writing and the written sequences of this information. The good and effective servo information can be provided by this means. It makes some sense to the research of servo information self-servowriting in hard disk drive.