Figure 7 i l l ustrates the components of an adapt ive ru nour correction system in an em bedded or sector ized servo disk drive . 2 '
Re lative radial displacement i n format ion between a selected data bead and its respective data track is provided by a sa mpled- data pos ition-error signa l . This is accom p l ished b y demod ul ating the readback signal from the head w h i le the head is passing O\'er servo data fields at sector boundaries. The posit ion error signal corrects the position of t he head and is
Storage Tech nology
H EAD SE LECT R EAD/WRITE STORAGE DATA TO DATA
A N D
CI RCUITRY PROCESSING
PREAMPLI F I E R SYSTEM
SELECT
SERVO SERVO SIGNAL
DATA D E MODU LATOR
DRIVE
E R ROR RATE
SECTOR TEMPERATU R E SEN SOR
SYNC SENSOR POWER-UP OR � N EW PACK - - - - - - � H EAD SE LECT SECTOR N U M B E R STORED M I S POSITIO N CORR ECTION DATA y(n) POWER D/A A M P L I F I E R ADAPTATION OFF-TRACK
SEQU ENCER SE NSOR
M I S POSITION x(n)
COMPUTATION A/D
PROCESSOR h(n)
POSITION-ER ROR M I S POSITION CORRECTION SIGNAL S I G N A L
SERVO POSITION FEEDBACK S U M M ER
CONTROL VELOCITY FEE DBACK
C I RCUIT
VELOCITY
TRACK S E E K EST I M ATOR OR
A N D COU N T TRACK NUMBER T R A N S D U C E R C I R C U I T R Y
Figure 7 Adaptive Runout Correction System in a Disk Drive Servo
fed back through a summer to the servo compensa tor or control circu i t . This ci rcu i t provides com mands to the power amplifier responsible for driving current into the carri age actuating motor.
The position-error signal, x (n) , where n is an index correspond ing to sector number, is also fed into a ru nout or misposi tion computa tion processor with unit pu lse response , h (n) This d igital s ignal processor performs digital circular convolmion dis cussed be low .
The runout o r misposi tion correction signa l , y (n) , acts to n u l l ify the effect of ru nout for the selected data head and is generated by the proces sor . The signal is stored for i m mediate looku p in a
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sma l l random-access memory (RAM) table that con tains the stored runout or m isposi tion correction data. This signal is ideally summed back in to t he servo loop through the summer just before the servo compensator.
The adaptation sequencer provides a trigger to the runout processor and servo system to enable ru nour adaptation for the disk drive as necessary.
Signal Processing Technique
The basic algorit h m i nvolves ini tia l l y two steps : ( 1 ) measuring an i ntegral number of revolutions of sampled position-error signa l , x (n ) , and ( 2) averag-
i ng values obta i ned across si mi lar sectors on d i ffer ent revolu tions to extract the repeti tive runout com ponent only. After these steps, high-frequ ency components in the measurements w i l l sti l l exist correspond i ng ro local media defects or track anomal ies .
These high-frequency components may be e l i m i nated completely by performing digita l circular convolution with a cosine-based sequence. This approach leaves only the low-frequency sinusoidal components of the posi tion-error signa l . For example, the first Fourier or fundamental frequency compo nent of the posit ion-error signal can be extracted by convolving it with a cos ine waveform of frequency correspond ing to the rotationa l frequency, 60 Hz .
To n u l l ify the fu ndamental frequency component at the point of measurement, the processed position error signal is i njected back into the servo system . However, there may be phase lag and attenuation between the point of correction signal i njection and t he point in t he control loop where n u l l i fication i s desired - the pos i tion-e rror signa l . General ly, t h i s phase lag and attenuation a r e reasonably w e l l known, given that servo bandwidth and mecha nical resonances in the stru cture are much h igher in fre quency than the repet i tive runout compone nts. Therefore, it is reasonable to expect that a corre sponding amount of phase l ead and ga in could be appl ied to the first Fourier component of position error signal to prod uce a suitable correction si gna l . The process of i ndividual Fourier component extrac tion , phase and gai n adjustment, and rejection of other Fourier components can be easily accompl ished simultaneously. The position-error sequence can be circu larly convolved with a phase -sh ifted and scaled cosine wave sequence of appropriate frequency.
If several frequ ency components of runout need !0 be corrected , the unit pu lse response of the fi lter wou l d be t he superposition of several correspond i ng cosines. Each cos ine is at the desired freque ncy of correction and is ind iv idually phase-shifted and sca led appropriately ro compensate for the lag and attenuation of the closed-loop control system at each corresponding frequ ency. Thus, one fi lter operation can pe rform a l l of the above in one step, i ndepen dent of the number of frequ encies ro be corrected .
However, the qua l i ty of n u l l ification or amou n t of correction is l i m i ted at this point by the accuracy of the a priori estimate of closed-loop control system ga i n and phase. This l i mi ta tion is the motivation for subsequent iteration or adaptation .
13y inject i ng the most recently compu ted correct ing signal and simul taneously measu ring the remaini ng repetitive runout components in the resulting posi t ion-error signal, a second correcting
Digital Technical journal No. 8 February 1989
signal can be compu ted using the above met hod. When summed with the original, this second signal w i l l improve the ru nour correction st i l l funher. It is desirable to be able ro i terate i ndefi n i tely using this technique . The system can thus achieve the desired amount of correction or keep up with very s low ly varying re petitive runout wi thout having to stan fresh at each cali bration or adaptation i nterva l .
To ensure convergence t o a n optimal correcti ng signal after an arbi trary nu m ber of iterations , it is im portant to process the measured pos ition-error si gnal in such a way as to reject high-frequency components. I f these components were allowed to be introduced back into the servo syste m , they wou ld rend tO bu i ld up and cause divergence after several iterations. This is another prob lem that this signal processing method overcomes.