4.4.2 POSITIONER ELECTRONICS
The
Positioner Electronics consists
of
two major groups; the Position Control Logic and
the Positioner
Servo Analog Circuitry. The relationship of these subsystems to the Linear
Motor, Velocity Transducer, and Position Transducer is shown
in
Figure 4-2. The entire
arrangement comprises a servo mechanism whose purpose is to control the mechanical
position of the carriage and, hence, the position
of
the heads or a time derivative of
carriage position such
as
the carriage velocity.
The servo mechanism may
be
operated in one
of
two modes, a Position Mode where the
position of the carriage is controlled,
or
the Velocity Mode where the velocity of the
carriage is controlled. The servo is switched between Position Mode and Velocity Mode,
or
vice versa, by means of transistor switches which are controlled by the Position Control
Logic.
The
Position Control Logic determines the specific mode of operation on the basis
of commands input
to
it
from the interface and the status of the signals derived from the
Position Transducer.
Additionally,
as
shown in Figure 4-2, the Linear Motor
can
be
disconnected from the servo
and
operated
off
the Emergency Unload Capacitor. This emergency unload system
provides a means of independently supplying power to the Linear Motor during emergency
situations. This network is independent
of
the servo electronics for purposes
of
high-speed retraction of the heads from the storage surface during emergencies.
The
Emergency Unload Relay acts
as
a double-pole, double-throw switch to connect the
Positioner
Coil to either the output
of
the Power Amplifier and the Current Sensor, or to
the Emergency Unload system.
It should be noted that when the positioner is executing
an
emergency unload,
it
is not operated
as
a servo and, therefore, functions
in
an
open-loop
manner.
The Emergency Unload Relay Driver receives its commands from the Logic
PCBA via the
Emergency Unload Enable (LEUEG) line. When the Emergency Unload Relay is energized,
the positioner coil is connected to the servo; specifically,
it
is connected to the output of
the Power Amplifier
and
to the Current Sensor.
The
Power Amplifier, in conjunction with
this
Current Sensor, forms a vOltage-to-current converter providing a current
in
the
positioner coil that is proportional to the applied input voltage to the Power Amplifier. As
previously mentioned, the available force for moving the carriage is proportional to the
positioner coil current to a first approximation, and the Power Amplifier, being a
voltage-to-current converter, provides a current that is proportional to its input voltage.
Therefore, the output of the
Summing Amplifier, which is the input to the Power Amplifier,
determines the force applied to the carriage where the force is approximately proportional
to the output voltage
of
the Summing Amplifier.
The
Summing Amplifier input is the major summing junction of the servo. Applied to this
Summing Amplifier
are
the servo commands and the feedback which nulls these
commands. Therefore, the servo loop functions to reduce the voltage at the
Summing
Amplifier input
by
providing
an
output voltage which results in a feedback signal which
nulls the command signal. Additionally,
on
200
tpi models, a temperature compensation
signal is applied to this summing junction.
One of the input commands to the Summing Amplifier is the velocity reference.
The
velocity reference command is a request for a specific velocity when the loop is operating
in the fast velocity mode. This velocity reference is derived from the Velocity Function
Generator. The polarity of this reference is determined by the Polarity
Select Network
and
is applied to the input of the Summing Amplifier through a transistor switch.
The
Polarity
Select Network consists of
U9,
02,
and
tranSistor switch
03,
located
on
the Servo PCBA
(Schematic No. 102810).
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