Head-Disc Assembly A3; Dedicated Servo Code - HP 7936 Support Manual

Functional Description
7936 and 7937
gated by a switch which allows only sampled servo
code to reach the servo circuit. The sampled servo
signal is filtered and integrated to create a head
position error signal. This signal is fed to the servo
power amplifier which in turn supplies drive cur-
rent to the actuator, to keep the head on track.
The sampled servo circuit also generates an AGC
voltage for the sampled servo AGC amplifier in
read/write PCA-A2. The reference voltage on the
AGC amplifier is adjusted by a PROM and DAC
on the HDA to compensate for gain variations
across the surface of the disc and between dif-
f erent heads.
The position error signal from the sampled servo
circuit also drives an off-track detector. If the
head is determined to be off track , the servo con-
troller is notified, and takes corrective action.
Writing is immediately stopped when an off track
condition is detected.
At the request of controller PCA-A6, the servo
controller initiates and controls the seek operation
that moves the data heads from a known begin-
ning or present head and track location to a
desired track location. When the command to seek
is received from the controller, the servo controlJer
determines the offset and the appropriate accelera-
tion profile needed to reach the target. The servo
controller starts the seek by using the servo power
amplifier to move the actuator, at the same time
monitoring the movement of the actuator by
wa tching the track crossing inf orma tion being
received from the dedicated servo circuit. The
seek profile used is a flattop velocity profile. This
implies that the actuator accelerates at a constant
rate until it reaches a predetermined maximum
velocity, and then coasts at that maximum velocity
until the servo power amplifier is again used to
decelerate the system. When the heads are within
one-half track of target track center, the fine posi-
tion servo loop is closed. An error position signal
from the dedicated servo is used to position the
heads on servo track center. Sampled servo from
the currently selected head is then used to properly
position the head on the data track.
In response to a recalibrate command from con-
troller PCA-A6, the servo controller: a) moves the
heads, starting from an unknown velocity and
position, to the outer guard band, b) controls the
lock up of the servo PLL, c) stops the actuator
arm against the crash stop, d) settles on a track in
5-12
the guard band, e) seeks out of the guard band
until track 0 is detected,
f)
enables Start of Sector
SOS-L to the drive electronics, g) enables sampled
servo timing acquisition, and h) ends with the drive
track following on dedicated track O.
Parts of the servo system circuitry are located in
head-disc assembly A3, read/write PCA-A2, and
servo PCA-A 1. Details of this circuitry are
provided in the following paragraphs.
5-13. HEAD-DISC ASSEMBLY A3
The components of the servo system in head-disc
assembly A3 include the dedicated servo code on
the dedicated servo surface, sampled (embedded)
servo code on all of the data surfaces, servo head,
servo read preamplifier/write driver IC, head gain
reference PROM and digital-to-analog converter
(DAC), drive ID (identification) circuit, speed sen-
sor, and actuator. See figure
WII.
5-14. DEDICATED SERVO CODE. The dedi-
cated servo code is used for track-to-track seeking,
position recalibrating, as a timing reference for the
write clock when writing data, and as a reference
when the sampled servo code is written at the fac-
tory. In addition, the dedicated servo code con-
tains encoded sector marks, index marks, and guard
band marks. The dedicated servo code is divided
into three areas: an outer guard band, a data area,
and an inner guard band. See figure 5-12.
The servo code in the data area is recorded in 1.7
microsecond "cells", with each cell containing two
dibits pairs. The "in-phase" dibit pair (A/B) defines
the center of the corresponding data tracks. When
track following on the in-phase dibits, the data
heads will be on track center.
The cells on each servo track are organized into
groups of
80,
with each group corresponding to a
physical data sector on the data surfaces. The first
16 cells of each sector contain a unique pattern of
dibits which identify the beginning of a new sector
(sector mark). The in-phase dibits are deleted from
selected cells to create the zeros which allow the
servo circuitry to decode this mark.
The sector mark pattern is also used to identify the
index mark, a mark which occurs once on each