Re: machine inaccuracies fixed by software compensation

--- 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.

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.

What does the professionals say?

Peter

BTW, the last time I "shouted", someone contacted me offlist to say
that the 5% error is 5% of 360 degree if I remember correctly.