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Positioner Motion
'rhe Forward Latch (I1S6 and I160) ANCed with
the Seek Latch (I164, I166) gates the invert-
ed output of the O/A converter (position
error signal) into the desired velocity func-
tion generator.
(A
Reverse FF enable would
have gated an uninverted position error sig-
nal. )
Since the seek length is greater
than
127 tracks. the
D/A
converter output is
held at a
fixed
level by the
~>128
siqnal
from the Difference counter.
Receipt of
the
Seek Start signal also caused a Start Seek
siqnal to occur.
Start Seek clears the Fine
!'P,
so the output of the desired velocity
function generator is gated through the
coarse gate to the summing amplifier.
Since
the carriage is stationary, no velocity sig-
nal exists to balance the position error,
and forward motion of the positioner begins.
With the position error signal clamped at
maximum, the power amplifier output (and
VOice coil positioner current) will
be
max-
imum and the positioner will continue to
accelerate.
As
the positioner moves forward,
outputs .from the track servo head are pro-
cessed to derive a cylinder pulse as each
cylinder is crossed.
Each pulse increases
the content of the difference counter by one.
As accelaration continues, the velocity sig-
nal opposes the position error signal
by
an
increasing amount.
'rhe input to the summing
amplifier drops off, finally becoming zero
when these opposing siqnals are equal.
Wi th
a nulled input to the summing amplifier,
voice coil current is zero.
During this
phase, the positioner coasts along the 60
ips plateau with the power amplifier provid-
ing only enough output voltage to compensate
for the back emf of the moving voice coil
pasitioner.
When the tracks
rema~n~ng
in the seek become
less than
128
tracks to go, the O/A clamp
is disabled for the remainder of the seek
.(except the last track).
As each track is
crossed, the O/A converter output steps down
by a precise and linear amount.
So that the
position error provided at the desired
velocity function generator input is not
also stepped, the integrator clamp gates the
velocity integrator on between each cylinder
pulse.
The resulting integrator sawtooth
output is added to the O/A converter output.
This removes the step and provides a nearly
smooth curve.
As the position error de-
creases, the summing amplifier control signal
decelerates the positioner to keep the
velocity signal/position error signal dif-
ference to zero.
When the counter indicates one track to go
to the desired destination counter
=
1023,
the integrated velocity signal is reset by
the regular cylinder pulse.
The integrated
3-58
velocity, which indicates distance, brings
up fine enable when about one-half track of
travel remains.
'rhis sets the Fine FF which,
in
turn, clears the Coarse
1'1'.
Desired veloc.ity no longer has an effect:
the position error is supplied by the fine
servo signal.
'rhis siemal is the track
servo
signal from the track servo circuit.
'l'he
amplitude of the signal is proportional to
the distance" between the present head posi-
tion and the desired cylinder.
Since the
desiJ:a:i
destination is track 160,
bit 2 0 of the Cylinder Address register is
·0·.
'rhis caused the Slope
FF
to be cleared
at the start of the seek.
As
a result, the
track servo signal is inverted to form the
fine servo Signal.
In
all seeks, the fine
servo signal is phased to be opposite to the
velocity siqnal. Since, for forward seeks,
the velocity signal'is positive-qoing from a
negative value toward zero, fine servo must
be
negative-qoing toward zero so that these
two signals can oppose each other.
'rhe dibit pattern causes a track servo siq-
nal to have a positive slope While approach-
ing an even-numbered cylinder.
Therefore,
the track servo siqnal must be inverted to
serve as a usable fine servo (position error)
signal.
If the seek had been to an odd cyl-
inder, the Slope FF would have been set and
the track servo signal would not have been
inverted.
As the positioner approaches
track
160, the fine servo signal approaches Ov.
'rhe summing amplifier responds to this de-
crease in amplitude by decelerating the posi-
tioner so that the sum of the velocity and
poSition error equal zero and all motion
stops with the servo circuit at null.
With the Fine FF set, the On Cylinder detec-
tion circuit is enabled.
It receives the
analog Signal from the fine servo amplifier.
When fine servo is less than about 0.3v, the
read/write heads are about 0.0003-inch from
nominal data track centerline and On Cylin-
der Enable comes up.
If the 4.75 ms delay
initiated at the start of the seek has timed
out, the 1.75 ms On Cylinder delay is trig-
gered.
When it times out, the On Cylinder
FF sets and the heads are selected again to
permit Read/Write operations.
This causes
Positioner Busy to drop, which in turn, for-
ces
a DEN
to the controller.
Since the positioner is not locked by a me-
chanical mechanism, the servo circuit con-
tinues to be enabled following the seek.
If
the positioner should drift slightly, the
track servo signal decreases.
This Signal
(fine servo), becomes
a
pOSition error input
to the summing amplifier.
This drives the
positioner back into place.
83318200
A
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