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

Hi,

No, the error can only be between one step and the next - if it were
cumulative you would finish up with two steps crowded together at one point
and that just doesn't happen. Each step position may be a little away from
the ideal, but the next one will probably be bang on - or maybe a little the
other way. So, the maximum error you could anticipate would be double the
error of one step i.e. 2 step poles adjacent with the maximum error acting
in opposite directions. No one has actually specified what the 5% error
quoted actually refers to, but, if this is the maximum allowed in the motor,
then we would presumably be looking at a maximum for any one step of only
2.5%. In practice, of course, the real error will be much less as, if there
is an error in the poles at one side of the motor, this will be reduced by
the pull of the coils at the opposite side (taking the simplest case of two
sets of opposing coils and a simple single pole rotor. In the real world
however, the motor will have several coils on at once and the rotor has
several poles.) The only exception to this may be a simple, low power,
unipolar motor.

Ian
--
Ian W. Wright
Sheffield UK
www.iw63.freeserve.co.uk

----- Original Message -----
From: <cadcamcenter@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: 05 September 2001 15:55
Subject: [CAD_CAM_EDM_DRO] 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.
>
>
>
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