I have a macro focusing rail system for my SLR that I use to take "stacked" macros, where you take a picture, move the camera a tiny bit closer, take another picture, move it a tiny bit more, take another picture, etc.
There is a device called a StopShot that does exactly what I want, but it costs over $500 US, and I already have a precision set of macro rails. Plus, this is an excuse to build something with my Arduino, and use stepper motors as well!
To get good results you need dozens of images, and it would be best if I could adjust the rails using a motor instead of turning the knobs by hand. There are at least 2 reasons:
It's best not to touch the camera rig, since you tend to cause small movements that throw off the alignment between images
The rail system has fine control of the focus, and very tight adjustment knobs, but there are no markings or detents. You can't measure or control the amount that you turn the knob between shots, aside from putting a paint mark on the knob and mentally dividing a full turn into 1/4 turns, and then each 1/4 turn into 4, for a total of 16 very rough steps.
I need to turn the knobs slowly and precisely, by very small increments (64 steps per rotation would probably be good, maybe more.) In normal use the system would turn a knob by a tiny fraction, wait a few seconds for any vibration to settle down , trigger the camera, then repeat. 3-5 seconds between adjustments would probably be about right.
For moving the rails to one or the other limit of travel, 1 or 2 RPMs would be fine as a max speed. Getting accurate rotation without any missed steps is more important than fast rotation.
The control knobs are very stiff, to keep the rig from slipping. The motion is smooth, but there is quite a bit of resistance. It takes a firm turn to get any rotation. It almost feels like viscus fluid damping.
How can I measure/estimate the amount of torque I need, and thus the size of stepper I should get? I have a torque wrench for working on my camera that measures torque down to 2 NM, but no lower. I don't have a good sense for estimating torque, and don't have any tools to measure torque less than 2 NM. From what I've read, even fairly heavy duty steppers seem to have torque in the range of single-digit newton/CM.
Since my speed needs are very low I could probably get away with a geared stepper. That would give me more steps/rotation plus higher torque, but I don't know how to figure out what I need.