Re: [CAD_CAM_EDM_DRO] Re: AMAZING STUFF!

My intention to hookup DRO and use it as positional information came about because I have a warn lead-screw on my Bridgeport type milling machine. The cost of replacing the past it parts would be far inexcess $600 greater than the machines worth. I decided to start at the end and look around for software that could use this type of input, none was found. It seems that the whole thing is turning into a nightmare with every part of the project being developed from scratch. With hind-sight it would have been far cheaper to buy a new machine but I thought that there must be people out there that would have a use for such a system and have carried on. It will be done. I will keep you posted as to the results, good or bad.

----- Original Message -----
From: Ray Henry
To: CAD_CAM_EDM_DRO@onelist.com
Sent: Sunday, March 12, 2000 2:53 PM
Subject: [CAD_CAM_EDM_DRO] Re: AMAZING STUFF!


From: Ray Henry <rehenry@...>


>Message: 16 [CAD_CAM_EDM_DRO] Digest Number 399
> Date: Sat, 11 Mar 2000 19:18:39 -0500
> From: "Matt Shaver" <mshaver@...>
>Subject: Re: AMAZING STUFF!
<big snip>
>What I wonder is what will the control algorithm do when it senses that the
>axis is lagging? In a servo system the amplifiers are commanded to increase
>the motor speed. In a stepper system this would imply increasing the step
>rate. If the motor has stalled, increasing the step rate won't do any good.
>Steve Stallings stopped by today and we discussed this very issue. He felt
>that lost steps were most often due to resonance problems and if you've just
>lost a few steps because you went over a "resonance bump", then it may indeed
>be possible to "catch up".

I'd think that EMC can be taught to catch up, but we will soon know from
the folk that are putting together the hardware for their emc stepper mills.

But this thread started out with the implied use of a linear encoder to
overcome large backlash from worn or inexpensive leadscrews and I want to
insert a caution with this. Since the computer is dealing with a
difference between actual position and commanded position and is computing
that difference over time, it will attempt to drive the axis motor faster
the longer the difference exists. If the axis has a large backlash by the
time the motor drives the screw to the end of it you will likely get
overshoot and the process will begin again.

You may be able to damp out some of this oscillation with a combination of
deadband and backlash compensation as well as PID values but my guess is
that there will always be times when an unexpected axis leap occurs. Such
things happen already with commercial mills with axis slop and backlash
compensation.

This effect along with occasional axis jumps the size of the backlash when
milling or turning forces change direction and the result may be an
unusable machine. We have the control tools to try this now but won't
really know until someone hooks up steppers and linear strips to an old
mill. If you do, count me in on the tuning of the thing.

Ray




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