RE: [Emc-users] trajectory planning

[Leslie M. W. <les...@al...>]

Hi Jonothan

Sounds like you might have experienced some slipped steps.

This could be related to the trajectory planner, because when
it plans incorrect large tangiential accelerations the stepper
may not be able to handle it.

Often mid band resonance is also a problem with steppers...
dampers or microstepping usually helps.

As far as original... The trajectory planner has not changed
much so you could substitute the word current.

The trajectory planning equations are outlined in a post I made yesterday.

And if the planner makes smoother motion profiles it is less
likely that the steppers will slip.

A trapezoidal plan basically has accel discontinuities that cause
the stepper pulse generator possibly to send unrealizable
commands.

But I am using servo only... I stopped using stepper in machine tools
15 years ago or so. So others might be more knowledgable as to the behavior
of stepper emc versions.


Leslie M.Watts
L M Watts Furniture
Tiger Georgia USA
(706) 212-0242
http://www.lmwatts.com
Engineering page:
http://www.lmwatts.com/shop.html
CNC surplus for sale:
http://www.lmwatts.com/forsale.html
CNC carved signs:
http://www.lmwatts.com/signwp.html




-----Original Message-----
From: emc...@li...
[mailto:emc...@li...]On Behalf Of Jonathan
Stark
Sent: Monday, December 29, 2003 7:48 PM
To: emc...@li...
Subject: Re: [Emc-users] trajectory planning


Quoting Leslie M. Watts (les...@al...):
> I work with servos, buyt I should note that the original trajectory
> planner probably wreaks havoc with steppers as well.

When you say "original", do you mean the code now compiled into
EMC by default?

I've noticed that even on simple commands with steppers,
there is some drift, or even instability with EMC that I
can't quite figure out.

At times with the table at it's end point, one or more
of the motors will jitter, or even the display will jitter,
fluttering between 2 values.

Sending the table 8 inches and then bring it back those 8 inches
will often result in 2 missed steps.  The interesting thing
is that it's not that the stepper motor has missed the step.
It's that EMC has apparently not sent the step.  Do others
have this problem?  I tried changing the edge of the pulse
I was clocking off of, thinking that step may have some how
happened when the direction was reversed, but the result
seems the same.

I'm running open loop, so there isn't any PID loop going as
there are no encoders.

Can someone outline briefly the trajectory planning concepts
used in EMC and how it is the same or different for servos
and steppers?  Is there some rounding that's happening that
could account for my step drift?

The discussion about trapezoidal vs. 4th order splines was
also interesting, but do splines make sense for stepper motors?
I can't quite grok how it'd be useful when you have to calculate each
step anyway.

Thanks,
Jonathan



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