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either is greater, the summing amplifier
makes minor braking current adjustments.
When the heads are on cylinder, both signals
are zero and current is zero.
When the fine position signal is less than
about 0.3 volt, the positioner is, for all
practical purposes, positioned over the data
track.
This initiates the On
Cylinder~.
After 1.75 milliseconds, On·Cylinder is
generated.
The fine servo siqnal remains active even
though On Cylinder is up.
This is the track
following or position error operation. Since
the positioner is not mechanically locked in
place, it can drift off cylinder.
As
long
as it is precisely positioned, the dibits
read from the adjacent dibit tracks are
~
and opposite.
Should the carriage move, one
dibit signal will increase in amplitude.
This results in a slight track servo signal
which is translated into the fine servo sig-
nal.
The summing amplifier, in turn, senses
this off-null condition and drives the
po-
sitioner back on cylinder.
If the positioner goes off cylinder suffi-
ciently to cause the fine position signal
greater than 0.7 volt for more than 800
~
seconds, the On
~ylinder
signal is lost.
This sets Seek Incomplete, Fault, and de-
selects the heads.
If the unit is reading
or writing at the time, Pack Unsafe is also
set.
The loop also permits positioner offset if
the
program requires it for error recovery.
Shift Position In (Tag 19) command will pro-
vide a positive bias input to the fine posi-
tion amplifier.
This is now an error signal
to the summing amplifier to cause a motion
forward of 400 microinches.
The motion stops
when the bias voltage and track servo vol-
tage cancel.
Shift Position Out (Tag 18)
causes a 400-microinch reverse offset.
Ei-
ther offset may also be provided by switches
on the maintenance panel.
Positioner Busy
status is up for 100 milliseconds following
receipt of Shift Position Out or Shift Po-
sition In.
DEN is then generated.
Short Seeks
The preceding explanation of the basic seek
operation presumed long seeks that permitted
the positioner to attain maximum velocity.
Maximum
velocity·
of about 66 ips requires 70
tracks acceleration time.
During short
seeks, gating is identical although relative
phasing of the error signals will vary.
During seeks less than 128 tracks certain
signals are available immediately:
integrated velocity, non-linear
83318200
E
feedback to the desired velocity function
generator, and a position converter output
not clamped at its maximum value.
These
signals generate a position error voltage
to accelerate the positioner.
The voice coil saturates for a shorter time
but the primary function remains unchanged:
acceleration occurs when the position error
signal exceeds the velocity signal; braking
occurs when the velocity signal exceeds the
position error signal.
TYPES OF SeEKS
Introduction
The drive performs 3 basic types of seeks:
•
First Seek - Initiated by pressing the
START switch, it causes the heads to
load and position themselves at cylin-
der 000.
•
Direct (Forward/Reverse) Seek - Per-
formed when the controller commands
the drive to move the heads from one
location to another.
•
Return to Zero Seek - Initiated by the
controller it causes the drive to posi-
tion the heads at cylinder 000.
First Seek (load)
This function, also known as the Load se-
quence, involves the activities that a unit
must perform before it can effectively res-
pond to a Read, Write, or Seek command from
the controller.
This function consists main-
ly of power supply relay sequencing and
~~
checking by the unit's logic.
As a result,
no actual selection of the unit is required
and very little drive/controller signal ex-
change occurs.
Successful progression of the
function assumes that all circuit breakers
are on, disk pack is installed on spindle of
unit, and the interlocks are closed. Success-
ful completion of a First Seek is signified
by the occurrence of DEN and the lighting of
the Unit Number indicator.
Standby status
(SSR bit 2 5 ) drops.
Initiation of the function occurs when the'
START switch is pressed (Figures 3-21 and
3-22).
Refer to the power supply discussion
in this section for additional sequencing
information.
Positioner Busy status is
available.
The logic enables motor relay
K3.
This causes release of the hysteresis
brake and starts the spindle motor.
At the
same time, the First Seek Interlock is ener-
gized to initiate a First Seek Interlock
Delay.
~n
SIC
33 and above,
~e
first .seek
I
in~erlock
is replaced by circuitry located on
the _SAV card.
3-49
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