Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions thread
Bob Watt Contributing Member • Posts: 532
Look at the detailed schematic, focus control line missing


Look at the detailed schematic. All the lines with one
exception (going to a box called the "display") from the AF calculation
module near the bottom, are shown as inputs. To achieve control of the
lens focus there MUST be a control line going from
the AF calculator to the lens uprocessor. This missing
control line (or maybe the arrow is on the wrong end) is in
fact by necessity an error signal telling the lens AF motor
to drive + or - from the present position. It does look like
this feedback loop is a virtual one, coming from a uprocessor
after a comparison, and thus subject, for instance, to a mismatch
between the step size of the "drive quanity detector" and the
control signal, which could cause discrete jumps. The triangle
however is NOT a comparator, it is the "drive actuator", according
to the image in the writeup.

So while there may be discrete mismatch issues wrt size of
the error signal bits and the drive quantity detector bits, that the
lens uprocessor can then miss by + - one discrete unit (the bigger one),
the system is indeed a feedback loop. It must be. A one directional
system like you discribe ("step 20 units, ok all done, nothing more")
would be prone to the problem you are talking about, but can not
be the system really used. If you doubt that, then explain how the
lens hunts back and forth before settling down. A unidirectional
system would move and stop until you once again pulled the
trigger. Only a real time feedback loop (hardwire or virtual thru
a uprocessor) has the inherent lag that results in hunting. A
servo loop of this type will have a damping constant that may
be adjustable to allow over, under or critical damping during the
feedback process. Critical damping will cause it to slew to the point where
the error signal is 0 and stop. Under damping will overshoot and
hunt (one of the dread 28-135IS focus issues). If the damping constant
is out of adjustment, particularly if it is overdamped so it never quite gets
where it needs to be before the error signal becomes too small
to act on, the focus will never be quite right.

I do not think your conjecture holds up under scrutiny. I suggest
everybody visit this page and watch the replay and mouse around
in the image on the page to find what the boxes are
called (like the triangle which is called the drive actuator, i.e.
the USM motor).

Bob Watt

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