Head Positioning - HP 7925D Service Manual

7925
voltage representing the spindle current command with
the average spindle motor current derived from the spin-
dle motor current sampling resistor and regulates the
motor current accordingly. This is achieved by applying
the desired spindle current command to the positive in-
puts of two differential amplifiers and the derived average
spindle motor current to the negative inputs. The unity-
gain inverting amplifier inverts negative current samples,
so that they may be processed as positive current samples.
The actual measured current sample can be observed at
the test point labeled "SMC".
The difference between the desired current and the actual
measured current is applied to the negative inputs of two
comparators. The output from a 22 kHz triangle wave
generator is applied to the positive inputs. This signal can
be observed at the test point labeled "22 kHz". A pulse
train is produced with a duty cycle determined by the
points at which the smooth dc voltage intersects the slopes
of the triangular wave. If there is a small difference be-
tween the desired current and the actual current, a low
duty cycle will be output from the comparators. Similarly,
a larger difference produces a higher duty cycle output. It
is the duty cycle that controls the pulse selection outputs
which in turn control the application of current to the
spindle motor windings. The output that regulates the
positive phases can be observed at the test point labeled
"P+", while the output that regulates the negative phases
can be observed at the test point labeled
"P-".
A-39. DYNAMIC BRAKING. When the RUN/
STOP switch is set to STOp, the spindle motor is dynami-
cally braked to a stop. Dynamic braking is achieved by
attempting to drive the motor in a clockwise (reverse)
direction while it is rotating in a counterclockwise (for-
ward) direction. This is accomplished by inverting the
information from the phase encoder circuitry. The
"exclusive-OR's" at the input to the phase A and phase B
flip-flops act as programmable inverters. When the run
spindle command is inactive (RS
=
1), the phase encoder
information is inverted. This will cause the opposite phase
to be driven which will brake the motor to a stop.
A-40. SPEED DOWN DETECTION. The speed
down detector monitors the encoder pulses, and when the
interval of time between transitions exceeds 0.7 of a sec-
ond, it direct-sets the speed down latch to declare the
motor stopped. With the run spindle command inactive
(RS
=
1)
and the speed down (SPD
=
0),
the spindle cur-
rent command to both motor phases will be inhibited. If
the speed down detector should fail to detect the proper
time interval between encoder pulses, the reverse direc-
tion detector will be clocked set at the moment the motor
first begins to rotate clockwise (phase B leads phase A).
When set, the reverse direction detector will inhibit the
spindle current command to both motor phases. In either
case, the yellow OFF LED will light when the spindle
current command to both motor p'hases has been inhibited
and the motor will remain stopped until another run spin-
dle command is issued (RS
=
0).
Appendix A
A-41. OVERCURRENT PROTECTION. The four
current switches, located on PMR PCA-A9, have over-
current sense networks associated with them. These net-
works sense the level of the current being applied to the
associated motor phase and if this current exceeds the
established upper limit, the appropriate current limit sig-
nal will become active (CLI or CL2
=
0). This will set the
associated current limit latch on spindle logic PCA-AS.
The state of the latch can be observed at the test point
labeled "CLl" or "CL2". When set, the latch will disable
the spindle current command to that motor phase. The
other phase, however, will remain operative to keep the
spindle motor rotating until the heads have been un-
loaded. In addition, the set output will cause the SPFLT
LED to light indicating that a spindle fault exists. It will
also signal the fault detection circuity through the inter-
lock chain to cause an emergency retract operation. The
current limit latches are reset by setting the POWER
switch to OFF, then to ON which causes SPS to momen-
tarily become active (SPS
=
0).
A-42. OVERVOLTAGE PROTECTION. During
spindle braking, the current switch circuits attempt to
drive the +36 and -36 volt supply lines to about 60 volts.
To protect against this condition, a pair of shunt regulator
circuits are employed to monitor the +36 and -36 volt
supply lines. If an overvoltage condition is sensed (voltage
greater than 42 volts), the active phase is turned off and a
bleeder resistor is switched in to lower the excessive volt-
age. The state of the disabling command can be observed
at the test point labeled "VL+" or "VL-". When the lower
threshold is reached (voltage less than 40 volts), the sys-
tem resumes normal operation.
If the spindle motor is jammed when the run spindle com-
mand is issued (RS
=
0), a stall condition will occur. Dur-
ing a stall condition an overvoltage is sensed by the shunt
regulator circuits, the active phase is turned off and
bleeder resistors are switched in to attempt to lower the
excessive voltage. If the bleeder resistors were allowed to
remain on, in the stall condition, the resistors would burn
out; therefore two regulator protection circuits are
employed to sample for excessive on time of the bleeder
resistors and to inhibit bleeder action.
A-43. HEAD POSITIONING SYSTEM
The head positioning system (see figure A-IS) consists of
circuits on microprocessor PCA-A2, servo PCA-A3, drive
control PCA-A4, track follower PCA-A5, and power and
motor regulator (PMR) PCA-A9. Further, it includes such
mechanical assemblies as the actuator assembly, carriage
latch solenoid, carriage back detector, and velocity trans-
ducer and shaft. With the exception of PMR PCA-A9, all
communication between PCA's occurs via motherboard
PCA-A 7 and interconnecting cables. PMR PCA-A9 com-
municates with the other PCA's through the main har-
ness. The purpose of the head positioning system is to
control the application of power to the coil in the actuator
assembly. This causes the heads to be accurately
A-15