HP 7925D Service Manual page 195

7925
timeout fault (TOF
=
0) exists, the run/stop flip-flop is set
(RUN
=
1), and the run spindle flip-flop will be set to
generate the run spindle command (RS
=
0).
This command will reset the speed down latch and the
reverse direction detector, and cause an encoder pulse to
be generated. The encoder pulse will clock the initial phas-
ing information from the phase encoder into the phase A
and phase B flip-flops.
A-36. PHASE ENCODING AND DECODING. The
spindle motor is a brushless dc motor with two sets of
phase windings. Power is applied to each winding in a
prescribed sequence from the +36 and -36 volt supplies
through four current switches. Two switches are provided
for each phase winding because current is required to flow
through the winding in both a positive and negative direc-
tion. Each switch is activated three times during any
given revolution of the motor. It is the relative position of
the rotor with respect to the windings that determines
which switch to activate. Rotor position and motor speed
are derived by the phase encoder, from the encoder PCA.
The phase encoder circuitry consists of an encoder disc,
which is fastened to the bottom of the spindle motor shaft,
and encoder PCA-A10. The encoder disc is a thin metal
disc with three 60-degree slots spaced 60 degrees apart.
Encoder PCA-A10 consists of two identical circuits, one for
phase A and the other for phase B. Each circuit is com-
prised of a light-emitting diode (LED), a phototransistor,
and an amplifier/inverter stage. The PCA is attached to
the spindle motor housing so that the light from each LED
passes through the slotted area of the encoder disc and
strikes the associated phototransistor. When light strikes
the phototransistor, it conducts and the resultant output is
amplified and inverted. The LED/phototransistor pairs
are physically mounted on the PCA 30 degrees apart with
phase A arranged to conduct before phase B, therefore, the
output from phase A will lead that from phase B by 30
degrees.
The two signals from encoder PCA-A10 are routed to spin-
dle logic PCA-A8 where they are conditioned and in-
verted. They can be observed at the test points labeled
'~ENC.N'
and "ENCB". They are then coupled to the input
of the encoder pulse generator and two "exclusive-OR"
gates which act as programmable inverters. The encoder
pulse generator produces a pulse for each edge of both
spindle encoder sensors. 'lWeIve encoder pulses are pro-
duced per revolution. The frequency of the encoder pulses
at 2700 revolutions per minute is 540 Hz. The output from
the encoder pulse generator can be observed at the test
point labeled "ENCP". The ~'exclusive-OR's" invert the
encoder signals when the run spindle command is inactive
(RS
=
1)
to dynamically brake the motor.
When the run spindle command is active (RS
=
0), no
inversion takes place and the encoder signals are clocked
into the phase A and phase B flip-flops by the output from
the encoder pulse generator. The latched encoder signals
are then routed to the phase decoder network where they
are decoded to select the proper current switch. These
Appendix A
phase selection outputs can be observed at test points
labeled "PH1+", "PH1-", "PR2+", and "PH2-".
Figure A-7 illustrates the timing relationship of the two
input phase signals, the output from the encoder pulse
generator, and the four resultant phase selection output
signals.
If an overcurrent condition is detected -in a given phase,
that phase will be inhibited. Similarly, if an overvoltage
condition is sensed, power to that phase will momentarily
be interrupted. Both motor phases will be inhibited when
the run spindle command is inactive (RS = 1) and the
speed is detected to be down or at the moment the reverse
direction detector first detects that the motor has begun to
rotate clockwise (reverse).
The latched encoder signals are also applied to the reverse
direction detector which is used to detect a clockwise rota-
tion of the motor during speed down detection. In addition,
a 180 Hz signal is derived from the latched encoder sig-
nals. This signal is used to clock the timeout counter
during a seek, seek home, or normal head load or unload
operation.
A-37. SPEED CONTROL. As previously men-
tioned, motor speed is derived from the phase encoder
information. The two signals from encoder PCA-A10 are
conditioned, inverted, and applied to the input of the en-
coder pulse generator. The encoder pulse generator pro-
duces a pulse for every edge of the encoder signals. Twelve
encoder pulses are produced per revolution. The frequency
of the encoder pulses at 2700 revolutions per minute is
540 Hz. The output from the encoder pulse generator can
be observed at the test point labled "ENCP". This output is
routed to the phase and speed down detectors.
The phase detector is a 3-stage shift register. The output
from the encoder pulse generator is used to shift "O's" to
the right and the output from a 540 Hz reference clock is
used to shift "l's" to the left. The 540 Hz reference clock is
derived from a 2.25 MHz crystal-controlled oscillator and
a divide-by-4168 counter. The output from the .2.25 MHz
oscillator can be observed at the test point labeled
"3 MHz" and the output from the 540 Hz reference clock
can be observed at the test point labeled "720 Hz". Phase
detection is achieved by monitoring the center bit of the
shift register. This bit can be observed at the test point
labeled "PHASE".
When the disc pack is rotating slower than 2700 rpm, "l's"
will be shifted through the shift register because reference
clock pulses will occur more frequently than encoder
pulses. This will cause a "1" to remain in the center bit of
the shift register and maximum spindle current to be
commanded. As a result, the motor will begin to acceler-
ate. As the motor comes up to speed, encoder pulses will
begin to shift "O's" into the left-most bit. Eventually, this
will force the "1" out of the center bit. When this occurs, a
decrease in the center bit duty cycle will result which in
turn will decrease the spindle current command causing
less current to be delivered to the motor. At speed, the
center bit will toggle and the duty cycle will be nearly
symmetrical.
A-13