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TABLE 3-4.
SERVO CIRCUIT FUNCTIONS (CONT'D)
Circuit Element
Function
Fine Servo
Amplifier
Provides the Fine Servo signal to the On Cylinder Detector.
This signal amplitude is proportional to
~istance
that heads
are displaced i,rom track centerline.
Scale factor is one
millivolt per microinch displacement.
If heads drift off slightly after seek is completed, track
servo Signal is no longer nUll.
This becomes fine servo
signal to drive heads back into position.
Provides the Position Error signal, via the fine gate, to the
summing amplifier during the last one-half track of the seek.
Amplitude of this signal is proportional to distance-to-go.
Phase is selected by Slope FF to be opposite in phase to ve-
locity signal.
The combination of the position error and
velocity signals controls voice coil current to bring posi-
tioner into On Cylinder position.
On Cylinder
Detector
Monitors fine servo signal when
T~l.
\ihen signal is less
than about O.3v, heads are close enough to track centerline
to be assumed to be on cylinder.
After 1.75 ms delay, On
Cylinder is generated.
If heads overshoot at end of seek
so that voltage exceeds 0.7v, delay is reinitiated.
Delay
permits carriage to settle out before controller may attempt
any read/write operations.
The On Cylinder Detector is
inhibited from triggering for 4.75 ms from the initiation
of a Seek Start.
BASIC
SeEK OPERATION
Introduction
Seek operations are initiated by a series of
control signals from the controller or by
internally-generated signals wi thin the drive
during power up conditions.
Most long seeks
may be divided into four phases (see Figure
3-20) :
1. Accelerate Phase: the voice coil re-
ceives full current to move the posi-
tioner from the current cylinder
~ds
the new cylinder.
2. Coast Phase: velocity is at its maximum
and the positioner velocity is constant.
3. Deceleration Phase: the positioner is
approaching the desired cylinder.
Its
velocity must be reduced by braking
action to prevent overshoot.
4.
S~op
Phase: the positioner is almost
at the desired cylinder.
It must be
stopped at the precise centerline of
the new data cylinder.
The logic is
~n
Fine mode to stop and hold the posi-
tioner at the new cylinder.
Refer to the various seek descripcions for
detailed information on the exact seek se-
quencing.
83318200
0
Accelerate Phase
This phase is controlled largely by the posi-
tion error Signal.
The controller loads the
difference counter with the complement of
the seek length.
For example, if the heads
are presently on cylinder 10 and must go to
cylinder 160, the seek length is 150 cylin-
ders.
In binary representation, decimal
150 is 00 1001 0110.
The complement of this
number is 11 0110 1001, or decimal 873.
At
each cylinder pulse, the counter is incre-
mented; therefore, the greater the number
in the difference counter, the fewer tracks-
to-go.
The counter is at its maximum value
when tracks to go equal zero.
This maximum
value is
10:3
(11
!
~.~
1
1111)
:'l~~
C;11 (n1
1111 1111) for the BR3E4/3E5.
The 7 low-order bits of the difference coun-
ter are applied to the Digital to Analog
(D/A) converter.
The value of these bits
indicates the position error (or tracks-to-
go) from 0 to 128.
That is, the amplitude
of the D/A converter
outp~t
is directly pro-
portional to the numner of tracks remainin'J
in the seek.
If the remaining seek length
is greater than 128, the D/A converter out-
put is at its maximum value.
3-45
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