Seek Operation - HP 7925D Service Manual

Theory of Operation
linear motor coil. This current command can be observed
at the test point on servo PCA-A3 labeled "CC". This
current will cause the carriage assembly to slew in reverse
at 3.5 inches per second until it reaches its fully retracted
position (CRB
=
1). When this occurs, the RET and CRB
signals will both be active (RET and CRB
=
1). Together
these signals cancel the 1667 millisecond timeout cycle
and clear the servo enable flip-flop to disable the head
positioning servo loop.
In addition, the CRB and STOP signals will clear the run
spindle flip-flop to issue a stop spindle command (RS
=
1).
The door unlock solenoid will be energized to permit pack
access as soon as the spindle has been braked to a stop.
The heads will remain in their fully retracted position
until another head load operation is initiated.
1-39. SEEK OPERATION. A seek operation is used
to move the heads from their present cylinder position to
some other cylinder position. The disc drive can execute a
seek command whenever the heads are positioned and
settled over any legal cylinder (ACRY and SB
=
0). The
controller issues a seek command on the tag bus with a
cylinder address on the control bus. When the command is
decoded, the SK signal will become active (SK
=
1). This
will initiate a 120 millisecond timeout cycle, direct set the
first clock inhibit flip-flop, and clock the cylinder address
(DO thru D9) into the new cylinder address register pro-
vided it is legal (lCA
=
0).
The SK signal will also cause the CYL signal to become
active (CYL
=
1) to clear the seek check flip-flop on I/O
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.
As previously mentioned, the legal cylinder address
supplied by the controller was stored in the new cylinder
address register when the seek command was decoded.
This address provides destination information to the head
positioning servo loop. In addition, the least significant bit
of the new cylinder address (LSB) is routed to track fol-
lower PCA-A5 where it controls the phase switchable
amplifier and the programmable inverter at the input to
the peak detector circuitry. This bit will be active
(LSB
=
1) for odd cylinders and inactive (LSB
=
0) for
even cylinders. The use of this bit is discussed in detail in
paragraph 1-37. Further, the three most significant bits of
the new cylinder address are inverted and routed to RlW
preamplifier PCA-A6 as the DWA, DWB, and DWC sig-
nals. These signals are used to control the programmable
write current sink.
1-20
7925
The cylinder address comparator compares the cylinder
address stored in the new cylinder address register with
the count stored in the present cylinder address counter. It
produces a 10-bit digital difference from these two addres-
ses. It also produces a signal which indicates whether a
forward or reverse seek operation is required. If the pre-
sent cylinder address is less than the new cylinder ad-
dress, the forward FET switch will be activated and the
present cylinder address counter will count up (POSITIVE
=
1).
If the present cylinder address is greater than the
new cylinder address, the reverse FET switch will be acti-
vated and the present cylinder address counter will count
down (POSITIVE
=
0). Both commands (forward or re-
verse) assume that the addresses do not match
(MATCH
=
1),
the seek operation is not inhibited (SKI
=
1), a seek home operation is not commanded (SKH
=
0),
and a retract operation is not commanded (RET
=
0). If the
present cylinder address is equal to the new cylinder ad-
dress, the fine position FET switch will be activated and·
the current applied to the linear motor coil will be deter-
mined by the
pas
signal.
In the case of a forward or reverse seek operation, the
digital to analog converter converts the digital difference
from the cylinder address comparator into an analog cur-
rent which is applied to the input of the velocity curve
generator. The velocity curve generator produces a cur-
rent equal to a constant multiplied by the square root of
the analog current from the digital to analog converter.
The VC GAIN potentiometer on servo PCA-A3 provides
the means to adjust the seek time by varying the gain of
the velocity command. The velocity command can be ob-
served at the test point on servo PCA-A3 labeled "VC". If
the reverse FET switch is activated, the velocity com-
mand will be routed to the summing junction of the
summing amplifier. If the forward FET switch is acti-
vated, the velocity command will be inverted by a unity-
gain, inverting amplifier before it is applied to the sum-
ming junction. The summing junction also receives a
voltage which is proportional to the linear velocity of the
carriage. This voltage is developed by the velocity trans-
ducer and shaft and is fed back through the tachometer
buffer and FET switch.
The summing amplifier compares the buffered output
from the tachometer (measured velocity) with the output
from the velocity curve generator (velocity command) and
produces a current command which drives the difference
to zero. This current command can be observed at the test
point on servo PCA-A3 labeled "CC". The amount of cur-
rent available may be limited by the current command
limiter. This circuit is activated by the seek inhibit signal
(SKI
=
1).
The current command is coupled through the voltage gain
amplifier to the linear motor power amplifier via a closed
FET switch. Both of these amplifiers are located on PMR
PCA-A9. The FET switch and linear motor relay were
both activated when the head positioning servo loop was