It should
be
noted that the polarity of the reference, and hence the direction
of
motion, is
determined
by
the power supply voltage in the particular circuit. Likewise,
08,
in
conjunction with
R49,
CR25,
and
R50,
R46,
determine the Reverse Slow Mode velocity
reference.
Additionally, command signals Track
Offset Plus (NLTOPG) and Track Offset Minus
(NL
TOMG) from the Position Control Logic
can
be
used to determine position mode offset
through use
of
position reference voltages derived in the Mode Control Circuits. The track
offset function is employed in the position mode for diagnostic purposes. The two logic
signals, Track
Offset Plus (NLTOPG) and Track Offset Minus (NLTOMG) operate transistor
switches in the Mode Control Circuits to generate a position mode reference, thereby
offsetting the heads from the nominal track cenerline.
Referring to the Servo Board Schematic
No.1
0281
0,
it can
be
seen
that NL
TOPG
controls
transistor switch
05
which, in conjunction with
R33,
CR15,
R34,
and
R35,
determines the
magnitude of the Track
Offset Plus position reference. Likewise, Track Offset Minus
controls transistor switch
06
which, in conjunction with
R38,
CR19,
R39
and
R35,
determines the Track Offset Minus position reference.
In
addition to the commands controlled by logic signals that may be input to the main
summing junction,
an
offset correction may
be
introduced by the Servo Offset Adjust,
R22,
on the Servo PCBA. There is also
an
external input test point provided to enable the
introduction
of
external test signals into the summing amplifier.
TP19
and
R40
on
the
Servo PCBA perform this function.
Two separate and distinct feedback paths to the main summing junction
are
provided. One
of these feedback loops is closed at all times when the servo is energized. This path,
as
shown in Figure 4-2, is from the Velocity Transducer, through the Velocity Transducer
Amplifier, to the Summing Amplifier input. The Velocity Transducer Amplifier merely
amplifies the
low level signal from the Velocity Transducer to a high level signal that is
proportional
to
the velocity
of
the carriage. Since this feedback path is closed at all times,
a damping in the Position mode of operation is provided.
In the Velocity mode, this
feedback
nulls velocity commands.
The other feedback path is used only in the Position mode. This path takes the X
+ 0
signal from the Position Transducer, conditions
it
to a voltage signal in the Position
Transducer Amplifier, and then switches
it
to the Summing Amplifier input. The transistor
switch
013
on
the Servo PCBA performs the switching function and is controlled by the
Position Mode signal (LPMXG) from the Position Control Logic.
As previously discussed, four signals are provided to the Position Control Logic which are
derived from Position Transducer signals. These four signals are the X
+
0,
X +
90,
Heads
Retracted, and Position Transducer
Index. They are converted from current signals into
voltage signals
by
current-ta-voltage converters that are contained in the Position
Transducer Amplifiers. These current-ta-voltage converters
are
U1
,
U6,
U2,
and
U7
located
on
the Servo PCBA.
Since the output
of
the Position Transducer Amplifiers is
an
analog voltage and therefore
unsuitable for direct
application to logic, they are converted into digital signals by
analog-to-digital converters. These converters are, in essence, a special type of Schmitt
trigger
and
are
comprised
of
U11
and
U10
on
the Servo PCBA.
The
signals which are fed
back to the Position Control Logic
are:
Position Reference Clock (SPRCG), Position
Ouadrature Clock
(SPOCG), Heads Retracted (SHRXG), and Position Transducer Index
(SPTIG).
These signals
are
utilized by the position control logic to determine the operation
of the positioner servo
in
conjunction with commands from other parts of the logic
and
the
I/O
interface.
6300
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