D
Daniel Horn
Guest
Hello all!
I am trying to make use of a lens assembely from an older Sharp
Viewcam camcorder. I have figured out the magic secrets required to
drive the two stepping motors that move the focus and zoom lenses back
and forth, but I've become quite puzzled by the device that opens and
closes the aperture iris.
I think this device may be a third, but rather different, stepping
motor. Here are the details (from memory, I'm afraid... and
meausurements were made with rather antiquated equiment, so take with
a grain of salt):
The device is a small cyllinder, about 1.2 cm diameter by 1cm height.
Its shaft is connected to the iris mechanism, which limits its travel
to about a total of 45 degrees. When at rest, it sits at one extreme
of this range (with the iris closed).
It has four terminals, A B C and D. I measure about 80 Ohms between
A<->B and C<->D. All other combinations have infinite resistance.
When I apply 3 volts across AB, the motor instantly travels its entire
range, opening the iris, and stays in that position until I switch off
the power, at which point it snaps back to the rest position. If I
reverse the polarity across BA, the motor goes "click"... I suspect it
is trying to go the other direction.
The behavior is exactly the same (but opposite) for terminals CD.
With the measured 3 volts across the terminals, the motor is drawing
about 20 mA. (That doesn't seem to add up, but my circuit theory is a
bit rusty... inductance and all that?)
So... how do I get the motor to reach positions between fully off and
fully on? My first inclination was that this is a stepping motor, so
I've tried several combinations of what should have created a
"half-step" effect... but no matter what I do, it seems to be always
either completely on or completely off. I've tried applying a
constant voltage to AB while varying (in real time, via a simple
potentiometer) the voltage to CD. As CD increases, it eventually
reaches a point (around 2 volts) where the motor just snaps from open
to closed... with never a moment spent in between. Sorry, may have a
detail or two wrong there... in any case, I tried all the combinations
of current direction, etc. The four behaviors I could produce were:
1. Open until I reached the 2v threshold, then clicks closed
instantly.
2. Closed until I reached the 2v threshhold, then clicks open
instantly.
3. Closed throughout.
4. Open throughout.
I began to wonder... is this a stepping motor at all? It seems to
have an awfully large step size (at least 45 degrees, maybe more, but
limited by the iris mechanism itself), and the way AB and CD seem to
do identical things does not jive with what I understand a stepping
motor to be (although, to be fair, I've known about the existence of
stepping motors for no more than 3 days now). What else could it be?
Or could it be that *is* a bipolar stepping motor, but that
microstepping it will require circuitry more sophisticated than my
trusty potentiometer?
Daniel Horn
I am trying to make use of a lens assembely from an older Sharp
Viewcam camcorder. I have figured out the magic secrets required to
drive the two stepping motors that move the focus and zoom lenses back
and forth, but I've become quite puzzled by the device that opens and
closes the aperture iris.
I think this device may be a third, but rather different, stepping
motor. Here are the details (from memory, I'm afraid... and
meausurements were made with rather antiquated equiment, so take with
a grain of salt):
The device is a small cyllinder, about 1.2 cm diameter by 1cm height.
Its shaft is connected to the iris mechanism, which limits its travel
to about a total of 45 degrees. When at rest, it sits at one extreme
of this range (with the iris closed).
It has four terminals, A B C and D. I measure about 80 Ohms between
A<->B and C<->D. All other combinations have infinite resistance.
When I apply 3 volts across AB, the motor instantly travels its entire
range, opening the iris, and stays in that position until I switch off
the power, at which point it snaps back to the rest position. If I
reverse the polarity across BA, the motor goes "click"... I suspect it
is trying to go the other direction.
The behavior is exactly the same (but opposite) for terminals CD.
With the measured 3 volts across the terminals, the motor is drawing
about 20 mA. (That doesn't seem to add up, but my circuit theory is a
bit rusty... inductance and all that?)
So... how do I get the motor to reach positions between fully off and
fully on? My first inclination was that this is a stepping motor, so
I've tried several combinations of what should have created a
"half-step" effect... but no matter what I do, it seems to be always
either completely on or completely off. I've tried applying a
constant voltage to AB while varying (in real time, via a simple
potentiometer) the voltage to CD. As CD increases, it eventually
reaches a point (around 2 volts) where the motor just snaps from open
to closed... with never a moment spent in between. Sorry, may have a
detail or two wrong there... in any case, I tried all the combinations
of current direction, etc. The four behaviors I could produce were:
1. Open until I reached the 2v threshold, then clicks closed
instantly.
2. Closed until I reached the 2v threshhold, then clicks open
instantly.
3. Closed throughout.
4. Open throughout.
I began to wonder... is this a stepping motor at all? It seems to
have an awfully large step size (at least 45 degrees, maybe more, but
limited by the iris mechanism itself), and the way AB and CD seem to
do identical things does not jive with what I understand a stepping
motor to be (although, to be fair, I've known about the existence of
stepping motors for no more than 3 days now). What else could it be?
Or could it be that *is* a bipolar stepping motor, but that
microstepping it will require circuitry more sophisticated than my
trusty potentiometer?
Daniel Horn
