Re: Basic stepper control question

Hi Tom,
Just tossing out an idea:
If this is a non-cost sensitive project, a "brake" mechanism might be
a workaround. The brake could be devised to be clamped when a
solenoid is inactive (free movement when energized.) if current draw
is an issue over time.

That's interesting about the 50 verses 200 clicks that I would expect
(in my inexperience). This proves by itself the motor will settle to
a natural detent when inactive, in this case a one in four chance of
being what you want. I'll count one of my moters too. Mariss will
know I'll bet.

Thanks,
-Lee

--- In CAD_CAM_EDM_DRO@y..., Tom Kulaga <tkulaga@m...> wrote:
> I've got a problem, and I know you guys can help me out. I'm
involved in a project at work that uses a stepper motor to drive a
mechanism. The motor only runs to get the machine to an optimal
position, then stays there indefinately. Nothing fancy, full step
mode, low current, little bitty NEMA 17 motors, etc, but once we get
it into position, it absolutely cannot go moving around. I've done a
ton of mechanical design to get all the backlash out of the system,
and to thermally compensate the whole fixture so the critical point
stays right where we want it even when the air temp goes WAY up.
>
> Here's the problem: in order to keep the controller and motor
temps down, the electrical engineer on the team instructed the
software guy to kill all current to the motor once the move is
complete. Now, our test engineer is seeing random jumps right about
when the power is cut. None of us knew that the current was being
shut off till just recently, and when I heard about it, I theorized
that shutting off the current causes the motor to fall off the
intended position, but only some of the time.
>
> I came up with this theory while playing with a 200 step per rev
motor, and noting that you only feel 50 little "clicks" per rev when
you spin it by hand. If you shut off the current, wouldn't you
expect the motor to drop back to the nearest "click" position, moving
you 1-2 steps away from where you really want to be?
>
> The easiest thing to do would be to test it by leaving the current
on, but the engineer I'm disagreeing with is also the project leader
and a really opinionated "I'm never wrong" kind of guy. If I, a
mechanical engineer, question the electrical aspects of the design, I
damn well better have something to back me up. That's where you come
in. Am I wrong or right? If I'm right, is there some reference
material, white papers, or other fact-based evidence I can use to
support my conclusion? Frankly, I don't think this guy knows
steppers from a hole in the ground, but I'm pretty sure he'll argue
with me anyway. Having proof that I'm right would simultaneously fix
a serious problem as well as shut him up. That would be, well,
enjoyabl e to say the least. :)
>
> Thanks for anything you can offer, guys.
>
> Tom Kulaga