ballscrew cumulative error was Re: E.M.C. PID Tuning - HELP !!!!

Jeff,

This is not really that unusual. Very often ballscrews have a
cumulative error. More so with less-expensive ballscrews.

In your case the lead is slightly less than "advertised". So you
compensate by taking more steps per unit, and it all works out...

Hope this helps,

Ballendo

--- In CAD_CAM_EDM_DRO@y..., "ja_erickson" <ja_erickson@y...> wrote:
> good day ray,
>
> i first have to say thanks alot for your insights and time in
> assisting me. this machine has the original steppers from
bridgeport
> and they do not have any feedback capabilities. i just returned
from
> the garage with a real good feeling as i have had a good tuning
> session. i have arrived at a setting of 100 for the 'p' variable
and
> a 5 for the 'i' variable. by adjusting these two variables in unison
> i was able to use a lower 'p' value which seemed to smooth the
> physical stepping characteristics of my motors at slower feedrates
> while allowing them to attain greater rapid traverse velocities. in
> comparison i needed a higher 'p' value to achieve the same rapid
> feedrates without the 'i' value. i determined that my machine has a
> backlash of .0015" and added that to the .ini file but the machine
> began to wander before and after its commanded moves which i think
is
> a really strange situation. in order to rectify this problem i
began
> to experiment with the 'deadtime' variable and found that by using
a
> very small value '.0004' it made the wandering go away but
at '.0003'
> the wandering only happened once in a half hours worth of testing.
> i wrote a small g-code program and had a dial indicator set up on
the
> test axis and began to see if the machine was missing or adding
steps
> as it processed the file.to my surprise there was a cumulative
error
> happening as the program ran i ended up having to increase the
input
> and output steps to a wierd value of 2008 instead of the calculated
> 2000, but this machine now runs and repeats flawlessly. i have read
> somewhere that if the motor driver that your using expects to step
at
> 1 and the software starts a step at 0 then this anomily may happen
> but i dont know how e.m.c. operates. could you give any
explainations
> or suggestions on what might really be going on with this value of
> 2008? i must complement you and your work with e.m.c. as ive seen
> your name associated with it all over the place. if it matters i am
> using a microstep "gecko" drive for this retrofit project.
>
> thanks again
>
> jeff
>
>
>
>
>
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., Ray Henry <rehenry@u...> wrote:
> >
> > Hi Jeff
> >
> > Welcome to the wonderful world of tuning the EMC to a retrofit
> machine.
> > Perhaps I missed them but I need the answers to a few questions.
> Is this a
> > servo machine? Are these the stock servo drives and motors?
What
> are you
> > using to get position feedback to the PC?
> >
> > Now a few thoughts. I write down the values for all of the
> relevant
> > variables before I make any changes. Old values get a line
through
> them
> > and the new value below. Then make a comment on better or worse
> motion as
> > a result of the change. This way you can see your progress and
not
> worry
> > much about over correction. Make only one change at a time. If
> the change
> > improves motion make a change in the same direction again. If
the
> change
> > makes motion worse, step back half way.
> >
> > There is a tuning widget under the calibration menu but some have
> reported
> > that it doesn't always change the running values and save them to
> the ini
> > file the way that they expected. Check back to the ini file
fairly
> often
> > with an editor so that you can make sure the values you want to
> change are
> > really doing it and the values that you want to remain constant
are
> also
> > correct. Don't save from the editor while the EMC is running.
> When you
> > shut down the EMC, it will write final values to the ini and
report
> a diff
> > from the old ini.
> >
> > I also tend to listen to motors while they start, run, and stop.
A
> > grinding or whining sound means to much gain. An oscillating
> frequency
> > while moving at a constant commanded velocity often means the
gain
> is to
> > low.
> >
> > You are doing good to lower the accel rate. Do it the same for
> both max
> > and default. Remove the I and backlash values, double the P
> value. Now
> > gradually increase the acceleration until you get response near
> where you
> > expect them. If you get following errors, double those values as
> well.
> > Don't worry just yet about jerkiness as long as the machine
doesn't
> rock
> > from foot to foot <g> or constantly trip out with following
error.
> >
> > When you have the speed near where you want it, begin to back off
> the gain
> > (P) until you see some overshoot when you accel or stop and some
> hunting
> > during constant commanded velocity. Increase the gain a bit to
> reduce this
> > problem.
> >
> > There is nothing in the EMC quite like the older notion of gain
> break but
> > you can use Feed Forward to do something similar. Jon Elson
> mentioned that
> > he uses a FF1 value of 8.00 with his bridgeport. You could try
> adding in a
> > bit of this and see if it improves the motion that you have
> achieved above.
> >
> > Be careful with (I) and (D). (D) can easily cause your machine
to
> run wild
> > but will kick up the command when the difference between actual
> velocity
> > and commanded is great. (I) on the other hand will cause an axis
> to lag
> > behind changes in velocity because it integrates over time. (I)
is
> reset
> > when you change direction but can be changed by compiling with a
> flag.
> >
> > FF0 is an offset from actual position. I don't use it at all and
> can't
> > comment on where one might. FF2 is also a mystery to me in it's
> > application.
> >
> > You might also remember that the motion modules and maths in the
> EMC are
> > intended to cover the whole range of motion including rapidly
> varying loads
> > and other conditions that might be encountered by vehicles. They
> are not
> > specifically intended for machine tool use.
> >
> > HTH.
> >
> > RayH -- U.P. Michigan
> >
> >
> > >    From: "ja_erickson" <ja_erickson@y...>
> > >
> > > evening all,
> > >
> > > i have a few questions and oddities that have arisen during a
> tuning
> > > attempt of e.m.c. for my bridgeport series 2 retrofit project.
> > <S>