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Reliable Servo control and Off-track error protection method for HDDs

IP.com Disclosure Number: IPCOM000013822D
Original Publication Date: 1999-Dec-01
Included in the Prior Art Database: 2003-Jun-18
Document File: 6 page(s) / 123K

Publishing Venue

IBM

Related People

Hiroshi J Uchida: AUTHOR [+4]

Abstract

1. Idea of disclosure Disclosed is a new reliable servo control method for HDDs using the embedded sector servo, which protects an off-track error caused by erroneous positioning error signal (PES) value obtained from a read head, which is called "miss-read". When the miss-read occurs, the erroneous signal is injected into the servo loop and causes tracking deviation from the track center or a desired seek trajectory, which degrades the seek time. This method calculates the estimated PES value of the next sampling, in case of the miss-read, by utilizing the dynamics of the head suspension assembly (HSA) and "estimator" technique of the modern control theory. It is theoretically known that the estimated PES value finally converges into the PES value. With this technique, the PES value is quickly replaced by the estimated PES value, only when the miss-read detected. Therefore the injection of the erroneous signal into the servo loop is minimized and as results the seek performance degradation is also minimized.

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Reliable Servo control and Off-track error protection method for HDDs

1. Idea of disclosure Disclosed is a new reliable servo control method for HDDs using the embedded sector servo, which protects an off-track error caused by erroneous positioning error signal (PES) value obtained from a read head, which is called "miss-read". When the miss-read occurs, the erroneous signal is injected into the servo loop and causes tracking deviation from the track center or a desired seek trajectory, which degrades the seek time. This method calculates the estimated PES value of the next sampling, in case of the miss-read, by utilizing the dynamics of the head suspension assembly (HSA) and "estimator" technique of the modern control theory. It is theoretically known that the estimated PES value finally converges into the PES value. With this technique, the PES value is quickly replaced by the estimated PES value, only when the miss-read detected. Therefore the injection of the erroneous signal into the servo loop is minimized and as results the seek performance degradation is also minimized.

     The one of the advantages of this method is that this function is independent on the head-positioning servo control system, while a conventional estimator method is not. It means that the estimation characteristics like estimation accuracy and disturbance rejection are optimized by the parameters of the estimator which are independent from the parameters of the servo controller which define the servo stability of the servo control system. It enables us to choose optimum parameters of the estimator based on the modes of servo control and maximize the estimation performance without degrading the servo performance, such as stability and seek time.

2. Technical details This off-track protection method consists of three parts;
(1) Estimator, which calculates the estimated PES, (2) Miss-read detector, which detects the miss-read, and (3) Off-track protection controller, which controls both the PES estimator and the miss-read detector. They are implemented in the hard disk controller (HDC). Figure 1 shows the block diagram of the servo control system with the off-track protection method. (The off-track protection controller in the HDC is not shown in Fig. 1.)

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VCM C urrent

VC M skDiDriver

Head/VC M

ServoCircuit

Actuator

Servo C ontroller O utput

Servo signal/ Data

Estim ated PES

Miss-read detector

This m ethod

  PES Estimator

 Position Detection

PES

C ontroller

PES

PES

Hard Disk C ontroller

PES: Position error signal VC M : V oice coil m otor

Fig. 1 Block diagram of servo control system with off-track protection method

     Figure 2 shows the flow chart of the servo control system with the off-track protection method. At part (1), a head-positioning error signal (PES) value Y(n), is generated by the position detection circuit, where the number n is the sampling time. At part (2), the PES value is examined and the miss-read is detected by comparing a cri...