630D
which would indicate a gross malfunction of the speed control.
The
Increase Motor Speed
(NLlMS1 )'obtained
from the
Speed
Status Flip-Flops is
fed
to the Servo PCBA.
A portion of the Spindle Speed control circuitry is located on the Servo PCBA. Basically,
there
are
two functions performed by
this
circuitry; the generation
of
a signal to the Motor
Control PCBA which is synchronized to the line frequency and causes the trigger to occur
at a specific time during the power line cycle.
Generation
of
a signal which is synchronized to the line frequency is necessary because
the basic power line is the power applied to the drive motor
by
the Motor Control PCBA.
This line synchronization is accomplished by a Zero Crossing type of synchronizing
network in which a pulse is developed by the Line
Synchronizer when the line voltage
passes through the zero volt condition.
The
trigger to the Motor Control PCBA must
be
supplied at a specific time during the
power line cycle. The time
of
occurrence
of
this
trigger must be proportional to the integral
of the binary error signal. This is done
to
provide a proportional power control to the drive
motor main winding that is the time average
of
the binary error signal (NLlMS1).
Phase angle control is obtained by the use of a
Ramp
Generator which is synchronized to
the line voltage
by
the line synchronizer. The output of the integrator is compared with the
voltage developed
by
the Ramp Generator in a Voltage Comparator. The results
of
this
comparison are used to generate a trigger signal which is
fed
to the Motor Control PCBA.
The
Motor Control PCBA is, in essence, a bi-directional power switch isolated from the
normal machine ground. The
SWitch
is turned on by the trigger signal from the Servo PCBA
and then turns itself
off
as
the line current passes through zero. Also contained
on
the
Motor
Control PCBA is the necessary interconnection wiring for configuring the drive
motor for the two basic types
of
power line operation,
110v
and
220v
(refer to Paragraph
4.8).
The trigger
Signal obtained from the Servo PCBA is applied to Current Amplifier Transistor
Switches via
an
Opto-Isolator device.
The
Current Amplifier Transistor Switches apply
gate current to the Triac
Switch which selects the power line onto the main winding
of
the
drive motor. During normal speed control, when power is to
be
applied to the drive motor,
the specific time for switching in the power is determined by the voltage comparator
on
the Servo PCBA. The Triac Switch allows current to pass through the drive motor winding
for that portion
of
the line cycle. Since the power line voltage has both a positive and a
negative excursion in a given cycle, power may be applied twice during a cycle.
The
phase
angle of that power application
can
be
determined and controlled by the comparison of the
Ramp Generator with the integrated increase motor speed
Signal.
The control circuitry contained
on
the Servo
PCBA
is enabled by the Not Drive Motor
Enable (NLDMEG) signal. Thus, when
it
is desired to
have
no power applied to the drive
motor (e.g., when the machine is stopped) the Circuitry is correspondingly commanded by
the state
on
the drive motor enable line. When
it
is desired to stop the disk from rotating
and
bring it to a halt,
it
is necessary to develop a braking torque to slow the disk down to
the stop condition in a reasonable amount of time. This is necessary because the rotating
assemblies
have
a considerable inertia and the time for the spindle to coast down to a stop
without
an
additional braking force would
be
excessive.
In
order
to
develop this braking
torque, the drive motor is operated in a special mode during that portion
of
the stop cycle.
This is referred to
as
a brake cycle and is defined by the Brake Cycle Enable (NLBCEG)
signal derived from the
Start/Stop Control Logic
on
the Logic PCBA. When the Brake
4-24
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