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In order to smoothly close focus the lens must first be
within approx. 5um of the "just focused" position. This
position is achieved by a focus search sequence. The
lens is moved up and down
using a triangular wave
search voltage while the spindle motor is kicked and
kept rotating at an appropriate speed. The servo LSI
monitors the FE and RFO signals and, at an appropriate
point, automatically closes focus.
The conditions for focus close are :
1) The lens is moving from a far to a near position
relative to the disc,
2) FOK = HIGH (5V),
3) FZD (IC internal signal) was latched high and
4) FE = 0 (w.rt. REFO).
When the focus servo closes, the servo LSI's serial data
FIN
LENS POSITION
RELATIVE TO DISC
NEAR
RFO
FOK
output port, XSO, will show a high-low transition. This
is received by the microcomputer as an indication that
the servo loop was
closed and after about 25m§
it
begins monitoring the FOK output, via a LPF, to verify
that focus is still closed ; in the event of FOK becoming
low for an appreciable time, the microcomputer will
take appropriate action.
The various signal levels which contribute to focus
close are shown
in figure 16, which shows the case
where focus close has been inhibited.
In TEST
MODE,
using
FOCUS
CLOSE
MODE
1,
conditions 2 & 3 can be inhibited to allow the S-Curve,
focus search voltage and the actual lens movement to
be observed at ease.
REFO
'JUST FOCUSED" LEVEL
FOK
" THRESHOLD
LEVEL
!
THRESHO
HOLD
FZD
|
|||
|
UNTERNAL SIGNAL)
{
XSO
(IN THE EVENT
FOCUS !S CLOSED).
Fig.16 : FOCUS CLOSING SEQUENCE
1-22
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