Thanks to all for your comments.
In fact, the application is a motorized focuser (for telescopes). Depending on the aperture of the “telescope” (refractor or reflector, doesn’t matter), the required precision is not the same. A scope opened at f/15 does not need much precision, while a f/4 one will require very high precision.
But… depending on the position of the focal point, the focuser may have to do a long travel to reach this point. If I do this travel at 1/8 stepsize, it will take very long time, hence the “high speed” option with full step (for example).
@ Architect : this could be an option but there’s a small problem with such presets. You will have to use the lowest stepsize as a unit, so that precision is available, but in turn you won’t be able to go to some positions while in full-step mode, since (in this case) it will have to be a multiple of 8 (starting at the current position). e.g. : if I’m at position 100, in full-step mode I won’t be able to go to position 105. 108 will be ok, as will also be 116, but not 111.
You see the problem ?
@ Nicolas : I (also) need torque : But this may not be really mandatory if I use some kind of gearbox.
But you’re right on one point : precision is less in lower stepsize modes… By design.
It’s not the first driver I’m writing for steppers (or brushed CC motors) : http://www.lsp-fr.com/astro/wp/archives/144 for example
But this time, I wanted to fully use all the features of the EasyDriver, even though it eats up a lot of IO pins.
Btw, I could also use high-step sizes to do some pre-positionning, then use 1/8 or 1/4 for precise positionning. Again, I have to be careful with “high-speed” because the load can go as high as 3-4 kg on a focuser and moving this load too fast could be risky, mainly because of the price of the “load” : big CCD camera + filterwheel + interferential filters + optical correctors ???
I will still need some “ramping”, anyway.
@ Gus : maybe cost and ease of use, at first glance. Brushed motors driven via PWM and gearbox are very easy to deal with and moves are still precise enough to reach good focus (in my case). But here, accuracy is not good enough, though.
@ Rajesh : yes, you may need encoders on steppers. Since they can miss some steps because of not enough torque (because of too much load or because of a stall), for example, your driver can believe you’re at some position while you’re not !
And believe me, in astronomy, it can happens often if you do not monitor your telescope mount correctly.