Michael Rigby-Jones wrote:
>>More current = more light. The drive coil will spin the rotor magnet 
>>against a (weak) spring. It works exactly like an analog meter 
>>except it 
>>is moving magnet not moving coil, and it has a feedback winding.


>Bearing in mind that these coils are normaly driven in a 
>closed loop from the video output amplitude, I have to 
>wonder how repeatable the mechanism would be from just 
>applying fixed currents.  Any stiction will give some hysteresis 
>in the transfer function, which could make getting a repeatable 
>aperture setting a little tricky.

They are very repeatable. The way stickion is tackled is to
have a 'buzz' on the control current, about 60H at 10% amplitude
should do.

And if he wants specific apertures (never done in reality since
all we really care about is getting a nominal video output level),
he should approach the desired setting from the same direction each 
time. e.g. always open the lens toward the f stop you want.

Robert

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