8.2.1
HALL EFFECT TRANSDUCER
This magnetic transducer
is
located within the spindle hub. It senses spindle po-
sition and provides dynamic feedback proportional to motor speed.
Two
com-
plete square
waves
are generated for each revolution. This positional information
is supplied to
both
the speed reference circuits and motor driver control transis-
tors.
To
prevent
both
phases
of
the motor from being selected simultaneously,
transistor 2A inverts the Hall input to coil A.
8.2.2
SPEED REFERENCE CIRCUIT
Using the charge time
of
C5
as a standard, comparator lA, pin
2,
will
output
a low pulse indicative
of
speed error. The charge time
of
CS
is
set
at
the factory
by adjusting R2 to 3,600 RPM nominal.
8.2.3
PULSE SHAPER
This circuit smooths out the incoming error pulses for ease
of
integration.
8.2.4
INTEGRATOR
Speed error information
is
integrated and transformed into usable levels by IC
lA
(pin
7)
whose output determines the current flow per revolution through each
motor' phase.
8.2.5
CURRENT LIMIT
Transistor Q6 senses the current
level
drawn during motor start-up and limits
it to
3.S
Amps by controlling the
"on
time"
of
2A (pins
3,
12).
8.2.6
MICROPROCESSOR: MOTOR ON
This signal is maintained low by the microprocessor as long as the index sensor
indicates motor spin-up within a specified periode
If
the motor does
not
rotate
the signal
is
set high and motor current
is
removed. This circuit prevents damage
to the PCB and motor if, for sorne reason, rotation cannot occur.
8.2.7
MOTOR DRIVERS
Current is supplied
to
both
motor phases by IC 3A. Hall information
is
used
to
supply current to the correct phase, while the integrator error information
controls the current level.
8.2.8
DYNAMIC BRAKE
At power
on
C12
is
charged up. When power
is
interrupted,
C12
becomes the
source keeping QI, Q2 and
Q3
on. At this time, transistor
Q3
shorts
both
motor
phases
to
ground allowing motor generated EMF
to
bring the spindle to a stop.
32
ST212
PRODUCT
MANUAL/Rev.
B
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