How to estimate/measure the size of stepper I need

The background:

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:

1. It's best not to touch the camera rig, since you tend to cause small movements that throw off the alignment between images

2. 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.

Suggestions?

I think the first decision is what drive mechanism - leadscrew or toothed-belt.

This affects the steps/revolution, torque requirements.

You also need an estimate of the load mass and friction in the
guide rails - for instance is the camera always moving on the level or
does gravity have to be overcome too?

What resolution do you need for the camera movement? - that affects the
belt v. leadscrew decision. Most hybrid steppers are 200 steps / revolution
these days, so that's probably an assumption you can make. Remember
microstepping gives you a little more resolution, but not dramatically more
(microstepping is mainly about reducing noise and resonance).

[ And I should add once you have the drive system you can estimate or measure
the friction (which may be dominant). A NEMA17 size is likely to be enough
as they go upto 0.4Nm pull-out torque. Smaller steppers like NEMA11 are
actually harder to find anyway. ]

And one last thing, a stepper-driven rig doesn't need to be stiff, the motor itself
holds position very strongly. You do need to keep the motor powered up all the
time though.

A simple (if crude) way to get a reasonable estimate of torque is to attach an arm (perhaps a light stiff piece of wood) to the rotating element and hang weights (perhaps coins in a small tray) from it.

Beware that the movement of a stepper motor from one position to the next is not smooth - as its name suggests, it moves in steps.

...R

I am working with an existing rail system. I think it uses rack and pinion or similar drive inside.

It is designed to hold the camera very strongly so that it doesn't move on it's own.

I want to turn the existing adjustment knobs on the rail system with steppers. My plan is to glue nuts or gears onto the knobs and attach the steppers to those.

We can assume that the camera will be level.

200 steps/revolution is plenty - more than enough, even. Like I said in my previous post, 64 steps/revolution would be sufficient.

There is a lot of friction/damping in the rail system so there is quite a bit of resistance to turning the knobs. That's what I want help measuring/estimating.

MarkT:
I think the first decision is what drive mechanism - leadscrew or toothed-belt.

This affects the steps/revolution, torque requirements.

You also need an estimate of the load mass and friction in the
guide rails - for instance is the camera always moving on the level or
does gravity have to be overcome too?

What resolution do you need for the camera movement? - that affects the
belt v. leadscrew decision. Most hybrid steppers are 200 steps / revolution
these days, so that's probably an assumption you can make. Remember
microstepping gives you a little more resolution, but not dramatically more
(microstepping is mainly about reducing noise and resonance).

[ And I should add once you have the drive system you can estimate or measure
the friction (which may be dominant). A NEMA17 size is likely to be enough
as they go upto 0.4Nm pull-out torque. Smaller steppers like NEMA11 are
actually harder to find anyway. ]

Robin2:
A simple (if crude) way to get a reasonable estimate of torque is to attach an arm (perhaps a light stiff piece of wood) to the rotating element and hang weights (perhaps coins in a small tray) from it.

Beware that the movement of a stepper motor from one position to the next is not smooth - as its name suggests, it moves in steps.

...R

Having the motor move in steps as fine. In use, the rig will turn the adjustment knob by a small amount, wait several seconds for vibrations to settle down, take a picture, then repeat. I'm not doing video, so stepwise motion is fine.

DuncanC:
There is a lot of friction/damping in the rail system so there is quite a bit of resistance to turning the knobs. That's what I want help measuring/estimating.

See the first part of Reply #3.

...R