Re: [CAD_CAM_EDM_DRO] Re: Re: step pulse timing resolution

----- Original Message -----
From: <ballendo@...>
To: <CAD_CAM_EDM_DRO@egroups.com>
Sent: Friday, October 13, 2000 8:27 PM
Subject: [CAD_CAM_EDM_DRO] Re: Re: step pulse timing resolution


> Jon Elson Wrote:
> >Well, using a timer, it is easy to do arbitrary granularity for ONE
> >axis, but it gets into big trouble with 2 or more! What if you have
> >two axes that need pulses that will come out very close together?
> >Of course, you can have some code that figures out when this is
> >going to happen, fudges the pulses at the same
> >time, and then compensates later. The more axes, the more fudging
> >will go on.
>
> Jon,
>
> You are exactly right.
>
> Unless you are running co-ordinated and independant timing for each
> axis you WILL have these sorts of issues. And if perfection of motion
> is the goal, you will be limited TO such a setup.
>
> Everything we do here is about defining, determining and controlling
> the error(s).
>
> Its always:
> 1.Where are we? (current position)
> 2.Where SHOULD we be? (commanded position)
> 3.Are we close enough to where we SHOULD be? (error)
> 4.If not,What do we NEED to do to get there? (correction)
>
> With one timer, we must "slave" the axes to some "master".
> Traditionally this "master" is the axis making the longest move. All
> other axes are run at some sub-multiple of this.
>
> As long as these sub-axes(distance) are factors of the "master", time
> and position correspond and there is no problem. For example:
>
> With x=12,y=4,z=3 (numbers are in steps/units to move) we can set any
> arbitrary "rate" of pulse output, and the position at any point is
> exactly where it should be.
>
> But if x=12,y=5,z=7, we've got problems! This is where traditional
> stepper systems and servo systems diverge.
>
> As a blanket statement(please don't beat me up re: this) Stepper
> systems control position directly; servo systems control position as
> a function of time.
>
> So the stepper system says, How the heck am I gonna get 5 and 7 steps
> divided evenly into 12??? and it does the best it can, which means
> the position is NOT going to be where it SHOULD BE.
>
> The servo system says, No problem, I'll speed up(increase current) to
> the sub-motors such that we all arrive together. The analog component
> of the servo system comes in and saves the day. BUT, the servo GETS
> its position information from a DIGITAL encoder(typically). Leading
> to the same sort of problem, but on "the other side, so to speak" of
> the problem.
>
> So how is all this resolved?
> By accepting that we're not always gonna be where we think we are(#1
> above) and; we may not BE ABLE to do anything about it(#4 above).
> So we modify our assumptions re: error(#3).
>
> The net point of all this is to say that "good enough" is a LONG way
> from mathematical perfection. And that's OK. If our parts meet our
> needs.
>
> Ballendo
>
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