Dynamic Braking; Speed Down Detection; Overcurrent Protection; Overvoltage Protection - HP 7925D Service Manual

7925
1-32. DYNAMIC BRAKING. When the RUN/STOP
switch is set to STOP, the spindle motor is dynamically
braked to a stop. Dynamic braking is achieved by attempt-
ing to drive the motor in a clockwise (reverse) direction
while it is rotating in a counterclockwise (forward)
direc~
tion. 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 stop spindle command
is active (RS
=
1), the phase encoder information is in-
verted. This will cause the opposite phase to be driven
which will brake the motor to a stop.
Theory of Operation
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.
With a disc pack installed (PIP
=
0);
the pack access door
locked (DL
=
0); the run/stop flip-flop set (STOP
=
0);
no
existing AGC fault (AGF
=
0), carriage back fault
(CBF
=
0), interlock fault (lLF
=
0), destructive write
fault (DWF
=
0), or timeout fault (TOF
=
0), the head
positioning system circuitry waits for the spindle to reach
operational speed (SPU
=
0). When this occurs, the RET
signal will become inactive (RET
=
0). This will cause the
SKH signal to become active (SKH
=
0) which will
1-37.
INITIAL HEAD LOAD OPERATION.
Once
the disc pack reaches its operational speed of 2700 revolu-
tions per minute, the heads will automatically be loaded.
The heads will fly above the surface of the discs supported
by a thin cushion of air. This cushion of air acts as an air
bearing to the heads. The air bearing functions as a very
stiff spring which is opposed by the leaf spring on each
head arm. These two opposing forces tend to cancel one
another at a flying height of 35 micro inches (0.89 microns)
at cylinder 0 to 27 microinches (0.69 microns) at cylinder
822. In order for the heads to fly properly several condi-
tions have to be satisfied. Among these are the cleanliness
of the air that surrounds the disc surfaces, the axial run-
out and flatness of the disc surfaces, and the flatness of the
head surface near the read/write gap.
The head positioning system (see figure 4-26) consists of
circuits on I/O sector PCA-A2, servo PCA-A3, drive con-
trol 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-A7. PMR PCA-A9 communicates with the other
PCA's through the main harness. 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 positioned over a specified cylinder during
an initial head load, forward or reverse seek, offset, or
recalibrate operation. In addition, it provides the means to
retract the heads under both normal and abnormal (fault)
conditions. Included in the following are discussions rela-
tive to an initial head load, normal head unload, forward
or reverse seek, offset, recalibrate, and emergency retract
operation.
1-33. SPEED DOWN DETECTION. The speed
down detector monitors the encoder pulses, and when the
interval of time between transitions exceed 0.84 of a sec-
ond, it direct-sets the speed down latch to declare the
motor stopped. With the stop spindle command active (RS
=
1)
and the speed down (SPD
=
0), the spindle current
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 direction de-
tector 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 phases has been inhibited and the
motor will remain stopped until another run spindle com-
mand is issued
(RS
=
0).
1-34. 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-A8.
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).
1-35. 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 vol-
tage. The state of the disabling command can be observed
at the test point labeled "VL+" or "VL-". When the lower
1-36.
HEAD POSITIONING SYSTEM
1-15