HP 7925D Service Manual page 26

Theory of Operation
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
110
sector PCA-A2. The state of the CYL signal can be ob-
served at the test point on servo PCA-A3 labeled "CYL".
Clearing the seek check flip-flop clears the seek check
status bit (status bit 2
=
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
=
1), 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 ena-
bled (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
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 initialized by SKH. Since the new
cylinder address and the present cylinder address count
both match, the MATCH signal will become active
(MATCH
=
1). The state of the MATCH signal 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-A5 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
1-16
7925
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 of 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-A5 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-
nal (RET
=
0) and the active MATCH + SKI signal
(MATCH + SKI
=
1), activates the fine position FET
switch. With this switch closed, the current applied to the
linear motor coil will be determined by the POS signal.
The POS signal is used to provide radial (cylinder) posi-
tion information to the head positioning servo loop. This
signal is derived from the servo code which is magneti-
cally recorded on the servo surface (see figure 1-10). The
servo code consists of 6720 di-bits per revolution, although
three of these di-bits are not recorded in the index zone.
As the servo surface passes beneath the servo head, a
voltage is magnetically induced. The output from the
servo head is directly coupled to the input of the differen-
tial preamplifier stage on track follower PCA-A5. This
stage consists of two differential amplifiers coupled to-
gether by a filter network. The gain of the first differen-
tial amplifier is controlled by the output from the servo
AGC circuit. The differential output is filtered and coupled
to a second fixed-gain differential amplifier. The output
from the differential preamplifier stage can be observed
at the test point on track follower PCA-A5 labeled "PRE".
It will be approximately 1.4 volts peak-to-peak. This out-
put is then coupled to the input of the phase switchable
amplifier stage. Figure 1-10 illustrates the servo and data
track assignments, as well as the waveforms produced at
the "PRE"test point as the servo head moves across +odd
and -even servo tracks.
The phase switchable amplifier stage provides a low
source impedance servo code output which is either in
phase or ,180 degrees out of phase with the output of the