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Fast seek control using a model following servo for minimum residual vibration

IP.com Disclosure Number: IPCOM000015330D
Original Publication Date: 2002-Jan-01
Included in the Prior Art Database: 2003-Jun-20

Publishing Venue

IBM

Abstract

1. Idea of disclosure

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Fast seek control using a model following servo for minimum residual vibration

1. Idea of disclosure

     Disclosed is a control method for a hard disk drive (HDD) to achieve fast seek movements using a model-following servo, which cancels out the residual vibration due to mechanical resonance without increasing the seek time.

     A bang-bang type control is known to be optimal to quickly move an object from one place to the other. However such fast seek movements easily excite the residual vibrations in the head-actuator system in HDDs. This increases the settling time and worsens the positioning accuracy. Therefore, in order to minimize the residual vibration, conventional methods shape the rise/fall time of the voice coil motor (VCM) current by applying a notch filter or a low pass filter. However, this results in increasing the seek time although the residual vibration is minimized. Since in general the high frequency component of the VCM current is required to move a head-actuator system quickly, there is a trade-off between fast seek movement and the residual vibration problem, and this makes it difficult to improve the seek time. However the disclosed method cancels out the residual vibration not by increasing the seek time, but by making the best use of the superior error rejection capability of the servo control loop. The disclosed methods consists of the following. 1) A high-bandwidth feedback servo system in which the head-actuator resonance must be stabilized. 2) A model of the actuator including the actuator resonance mode 3) A feedforward control signal generated by the model to move the head-actuator system along a desired trajectory. 4) The reference trajectory modified by subtracting the residual vibration created by the head-actuator resonance mode as excited by the seek motion.

2. Technical details
2.1. Introduction

     Figure 1 shows a block diagram of the conventional model-following control method. The variables P, C, F, Pm are a head-actuator system (plant), a feedback controller, a control signal generator, and a plant model, respectively. On seeking, the feedback controller makes the plant P follow the trajectory of the plant model Pm even when there is model error between the plant model Pm and the plant P. Therefore this is called the "model-following control method". If there is no model error, the feedback control output Ufb will always be zero, because the input signal, i.e. the difference between the head position y and the reference head position yr, will always be zero. In that case, we only need to consider the plant model to get a desired seek trajectory, because the head position y of the plant P is the same as the reference head position yr of the plant model Pm, and the feedback controller does not affect the head position y since the input yr-y is always zero. Otherwise it remains difficult to force the plant to follow the desired trajectory, especially during the seek-settling, because the feedback contr...