Re: [CAD_CAM_EDM_DRO] Re: machine inaccuracies fixed by software compensation

cadcamcenter@... wrote:

> --- In CAD_CAM_EDM_DRO@y..., "Carol & Jerry Jankura"
> <jerry.jankura@s...> wrote:
> > Doug:
> >
> > Isn't the 5% error rating on the motor the positional accuracy of
> the last
> > step, and not cumulative?
>
> Hi,
>
> I am still learning about steppers and servos, so do correct me if I
> am wrong.
>
> Regarding inaccuracies in a stepper motor, can I suggest that a 5%
> error in a 1.8 degree stepper can be cumalative over 199 steps, but
> beyond 1 revolution (200 steps) it is not cummaltive. Let me explain
> further, taking the extreme example of the first 199 steps all off
> by -5%, and try to see what should happen when it rotate a complete
> revolution.
>
> A 5% error in a single step (1.8 degree) is 0.09 degree. Let's say
> each of the first 199 steps has an error of -5% (-0.09 degree), and
> thus would have accumalated an error of 0.09x199 = 17.91 degree. The
> 199th step should be at 358.2 degree, but is now incorrectly
> positioned at 358.2-17.91 = 340.29 degree.
>
> It is however impossible for the error to be negative over the whole
> revolution, which means that the last step must have an error of +
> 17.91 degree. That is, the next step will take the motor through
> 1.8+17.91 = 19.71 degree. So the last step will then take the motor
> to 340.19+19.71 =360.00 degree, back to its original position.

Well, it is most likely that the type of errors in these motors will be
mostly symmetrical, ie. if the first half rev is largely negative, then the
second half will be largely positive. So, probably, the worst case error
will be 1/2 of your 17.91 degree. Actually, I think the worst case error
will
end up being less than this amount, as such a motor will have no torque
at certain positions, or be guaranteed to lose steps even with no load.

>
> This should mean that the maximum possible error will be 17.91 degree
> which is just a little short of 18 degree (5% of 360 degree)
>
> I think it will be simpler for us to assume that 5% error in a
> stepper means a possible error of 5% of 360 degree over 1 cycle but
> the error is not carried over into the next cycle.

I did some calculations of what a worst case error could be that would not
cause a motor to lose whole steps, and it comes out to about 1% of
a full revolution, I think.

Jon