HP 7925D Service Manual page 198

Appendix A
positioned over a specified cylinder during an initial head
load, forward or reverse seek, offset, or recalibrate opera-
tion. In addition, it provides the means to retract the
heads under both normal and abnormal (fault) conditions.
Included in the following are discussions relative to an
initial head load, normal head unload, forward or reverse
seek, offset, recalibrate, and emergency retract operation.
A-44. 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 microinches (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.
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 (ILF
=
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
initiate a 1667 millisecond timeout cycle, set the servo
enable flip-flop, and direct set the seek home flip-flop. The
state of the SKH signal can be observed at the test point
on drive control PCA-A4 labeled "SKH".
The SKH signal will also cause the CYL signal to become
active (CYL
=
1) to clear the,seek check flip-flop on mi-
croprocessor PCA-A2. The state of the CXYL signal can be
observed at the test point on servo PCA-A3 labeled "CYL".
Clearing the seek check flip-flop clears the seek
check status bit (status bit 3 = 0).
In addition, the COF signal will become active (COF = 0)
to clear the offset magnitude and sign registers on track
follower PCA-A5. This will ensure that any offset infor-
mation stored during a previous offset operation will be
cleared out so that it will not affect the positioning of the
heads.
With the servo enable flip-flop set (SEN = 1 and SEN = 0)
and the DRDY and RET signals active (DRDY and
RET = 0), the ECS signal will become active (ECS = 1).
This causes the CSOL signal to become active (CSOL = 0)
to energize the carriage latch solenoid permitting carriage
movement. Also with the head positioning servo loop en-
abled (SEN =0) and no existing power supply fault (PSF =
1), the linear motor relay on PMR PCA-A9 will be ener-
gized to permit current to be applied to the linear motor
A-16
7925
coil. These conditions can be observed at the test points on
PMR PCA-A9 labeled "SEN" and "PSF". The SEN signal
also enables the linear motor power amplifier (LMAE = 1)
after a 60 millisecond delay to ensure closure of the linear
motor relay contacts.
With the seek home flip-flop set (SKH = 1 and SKH = 0),
the new cylinder address register and present cylinder
address counter will be cleared by SKH. Since the new
cylinder address and the present cylinder address count
both match (both are zero), the MATCH signal will be-
come active (MATCH = 1). The state of the MATCH sig-
nal can be observed at the test point on servo PCA-A3
labeled "M".
Since the heads are not yet positioned over the servo zone,
the AGC signal from track follower PCA-AS will be inac-
tive (AGC = 0). The set output from the seek home flip-
flop and the absence of the AGC signal (AGC = 0) will
activate the +slew FET switch on the servo PCA. With
this switch closed, a constant velocity will be developed
and an appropriate current will be applied to the linear
motor coil. This current command can be observed at the
test point on servo PCA-A3 labeled "CC". The coil will be
repelled from the linear motor magnet to push the car-
riage assembly supporting the heads along the rails at
approximately 3.5 inches per second.
A voltage which is proportional to the linear velocity of
the carriage is fed back through the tachometer buffer and
FET switch to the summing junction of the summing
amplifier. The tachometer buffer is a unity-gain amplifier
used to eliminate the effects of temperature on the velocity
transducer signal. The voltage developed is used to pre-
cisely control the head positioning servo loop during the
initial head load operation. This voltage can be observed
at the test point on servo PCA-A3 labeled "TAC".
The velocity transducer and shaft are used to develop this
linear velocity voltage. The velocity transducer is a cylin-
drical coil assembly mounted in the center of the linear
motor magnet assembly. A magnet is attached to the car-
riage assembly by a supporting shaft. The motion of this
magnet as it passes through the coil generates the linear
velocity voltage. The magnitude Df the voltage is propor-
tional to the linear velocity and the polarity indicates the
direction of motion.
As the heads approach the head loading area of the disc
pack, they are forced away from the disc surfaces by the
air pressure developed by the rotating disc pack and the
air distribution system. The heads will actually fly above
the surfaces of the discs supported by a thin cushion of air.
When the outside edge of the outer guard band is first
detected by the servo head, the AGC signal will become
active (AGC = 1) to disable the forward slew operation.
The state of the AGC signal can be observed at the test
point on track follower PCA-AS labeled "AGC". The seek
home flip-flop will be clocked clear by the leading edge of
the AGC signal. The set output from the seek home flip-
flop (SKH
=
0) together with the absence of the RET sig-