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The output of the OIA converter is applied
to the position converter which used it to
develop one coarse position error signal.
This signal is directly proportional to the
O/A converter output and serves as the input
to the summing amplifier.
At the beginning of this. seek the coarse po-
sition siqnal is
maximum
so the input to the
sWllllling amplifier is a large siqnal.
Since
there is no velocity yet, the current
through
the voice coil is
max~,
causing maximum
acceleration.
As the positioner accelerates,
a velocity signal is generated by the
~ty
transducer.
This signal opposes the position
error signal.
Its amplitude, however, is
less and acceleration continues.
Coast Phase
Eventually, the amplitude of the position
error signal and the velocity feedback sig-
nal are equal.
The net error signal in the
loop drops to zero.
The summing amplifier
output follows, so current is cut off. Ve-
locit¥ is constant.
Friction losses will
tend to slow the positioner but, as it does,
the velocity signal decreases.
This allows
the position error signal to call for more
current.
Oec.l.rate Phase
B'raking action starts as the positioner ap-
proaches its selected cylinder.
The track servo circuit (refer to Track
Servo Circuit description) has been gener-
ating cylinder pulses as each cylinder is
passed.
These pulses are used to increment
the difference counter.
When T=64,
the position error signal, from the
desired velocity function generator
is reduced.
This causes the velocity sig-
nal to dominate and, since it is opposite
in phase to the position error, the summing
amplifier output switches polarity.
OppOSing
current passes through the voice coil. The
carriage decelerates.
Both the desired ve-
locity and velocity signals are decreasing
simultaneously.
Voice coil current decreases
proportionately.
The
1000
maintains speed along an ideal ve-
locity curve.
This curve is the analog ver-
sion of the number of tracks-to-go.
The
velocity curve is generated by the desired
velocity function generator.
Its output is
compared with velocity to achieve maximum
deceleration under all conditions without
overshoot.
The position signal is the sum
of the following:
3-48
1. The position error signal from the
poSition converter.
Its output is
a siqnal whose amplitude is propor-
tional to the number of tracks-to-go.
2. Integrated velocity from the velocity
integrator.
Integrating a velocity
siqnal provides a signal proportional
to distanceo
This signal is a sawtooth
wavefor..: it is pulled back to zero by
each cylinder pulse and increases in
proportion to velocity and time (dis-
tance).
The combination of the step-
ping-down output from the position
converter with the ramp integrated ve-
locity
signal results in .a smooth curve
of constantly-decreasing magnitude.
3. The function provided by the nonlinear
feedback around the desired velocity
function generator.
Stop Phase
Stop Phase begins when the difference coun-
ter indicates that there is one track-to-go.
When T=l, the velocity integrator signal is
pulled back to zero by the cylinder pulse.
Its output, indicating distance, increases.
When its amplitude indicates approximately
one-half track remains, Fine Enable sets the
Fine FF.
Desired velocity is disabled since
the coarse gate is opened by Fine being set.
The last half-track of motion is controlled
by the fine servo signal.
There is a slight
increase in position error gain in switching
from coarse to fine.
Fine servo and velocity
are applied to the summing amplifier through
the fine gate.
The summation of these two
signals controls the braking current.
At the start of the seek, the Slope FF is
set if the seek is to an odd-numbered cyl-
inder.
The slope Signal controls the phase
of the track servo signal applied to the
fine position amplifier.
This adjustment is
required since track servo signal phasing
is a function of the servo head position:
the Signal is negative when over negative
dibits and positive when over positive dibits.
Therefore, on forward seeks, the signal is
decreasing from a negative value toward zero
when approaching a data track with an odd
number; it it increasing from a positive
value toward zero when approaching a data
track with an even number.
The opposite is
true during a reverse seek.
Phasing of the track servo signal is selected
so that the fine servo signal opposes the
velocity signal during the last half-track of
the seek.
Both signals are decreasing.
If
83318200
A
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