RE: Follow-Error

[Ray <rehenry-at-up.net>]

Tim

This would sound like it might need to be 1.414 times the fastest axis then.

Thanks.  I'll try that.

Ray


On Mon, 18 Jun 2001, you wrote:
> Ray,
> 
> This brings back a conversation I had with Fred Proctor about this long ago.
> If I remember correctly the MAX_VELOCITY in the [TRAJ] section does need to
> be larger than the one in the individual axis sections. The reason for this
> is if you were doing a 45 deg move and had both axis's moving at max rate
> the velocity of the tip will be greater than the individual axis velocity.
> The parameter in the [TRAJ] section needs to be sufficiently high so the
> resulting component does not exceed it.
> 
> Hope I remembered this properly.
> 
> Tim
> [Denver, CO]
> 
> > -----Original Message-----
> >
> >
> > Malte
> >
> > After a little more work here, I have come to the tentative conclusion
> > that you should try setting
> >
> > [TRAJ]
> > ..
> > MAX_VELOCITY       =  1.5
> >
> > to a value larger than the value that you set for any axis
> >
> > [AXIS 0]
> > ..
> > MAX_VELOCITY  = 1.2
> >
> > IMHO this allows some overhead that steppermod can use to feed in
> > pulses that otherwise build up as following error during acceleration and
> > during running at max velocity.
> >
> > (You would not need to translate the experimental details below)
> >
> >
> > DETAILS OF MY EXPERIMENT.
> >
> > 1 - Percent of Max Velocity and Following Error.
> >
> > On my first run, I set all Max Velocity parameters to 15 which
> > gave a rapid
> > feedrate of 900 mm/min.  I also had several processes running which may
> > account for some of the differences between this experiment and the others
> > below.  All of these values are averaged from five runs at each feedrate.
> >
> > feedrate 900 ->  34 mm
> > feedrate 890 ->  46 mm
> > feedrate 880 ->  83 mm
> > feedrate 877 -> 108 mm
> > feedrate 875 -> 150 mm
> > feedrate 874 -> 183 mm
> > feedrate 873 -> 230 mm
> > feedrate 872 -> 307 mm
> > feedrate 871 -> 480 mm
> > feedrate 870 -> 500 mm without trip
> >
> > First reading of this suggests that I may have exceeded the
> > ability of my PC
> > to spit out pulses when I try to travel faster than 870 mm/min using 1600
> > pulses for each mm.  But keep reading.
> >
> >
> > 2 - Max Velocity problems with steppermod
> >
> > When I change the max velocity to 10 rather than 15 and I am moving at
> > 600 mm/min I again quickly run into following error at 600 ipm.
> > However at
> > 592 mm/min the control is able to complete the move as programmed.
> > Although I didn't do as rigorous a test of this as at 900 mm/min,
> > I suspect
> > that the curve would look about the same.
> >
> > I changed the max velocity to 5 rather than 10 and I am moving at 300
> > mm/min I again run into following error but it is nearly at the 500 mm
> > distance.
> >
> > So the easiest conclusion, that there is a max that I'd exceeded at 900
> > mm/min doesn't prove to be a good answer.  It would seem that the correct
> > answer has more to do with a percent of the traj max velocity and that
> > the following error problem is compounded at higher max velocity values.
> >
> >
> > 3 - Acceleration problems with steppermod.
> >
> > it would seem that the ini values
> >
> > DEFAULT_ACCELERATION       =  20.0
> > MAX_ACCELERATION       =  20.0
> >
> > affect how soon a following error will occur when an axis is started at
> > or near maximum velocity. I'll illustrate by giving some average distance
> > traveled before the following error message stops the control.  I am using
> > your ini with max velocity = 15.  This gives a max travel rate of 900
> > mm/min. (35 ipm)  I used the K6II-400 and executed the mdi command
> >
> > g1 f880 x500
> >
> > I repeated the procedure several times and averaged the distance traveled
> > before following error.
> >
> > acceleration 01.0 -> 233mm
> > acceleration 02.0 -> 170mm
> > acceleration 03.0 -> 161mm
> > acceleration 05.0 -> 159mm
> > acceleration 10.0 -> 134mm
> > acceleration 15.0 -> 130mm
> > acceleration 20.0 -> 126mm
> > acceleration 25.0 -> 124mm
> >
> > I can complete the entire 500 mm at a feedrate of 870/900 with an
> > acceleration of 20 and 875 with an acceleration of 1.  Below are some
> > values of feedrate and the distance traveled before following error stops
> > the axis.
> >
> >
> > 4 -  Discussion
> >
> > It would seem from the above test that at max velocity, steppermod has no
> > way to feed in steps that accumulate as following error during
> > acceleration
> > or that build up as left-over pulses during rapid travel.
> >
> > Quote from Fred Proctor regarding steppermod and binning.
> >
> > "A version of emcmot.c that uses the "binning" algorithm for allocating
> > pulses that causes resonance in some systems. This was the
> > original stepper
> > motor algorithm added to the servo-only emcmot.c, using the STEPPER_MOTORS
> > compile time flag. This creates a second task that runs faster than the
> > motion controller, typically 10 cycles for every motion cycle.
> > During these
> > cycles, the pulses resulting from the motion cycle's calculated position
> > increment are spread out as best as possible. "
> >
> > If the variable TRAJ MAX_VELOCITY sets the number of spaces within each
> > bin to exactly match the number of steps needed to achieve max velocity
> > then there is nothing left over for steppermod to use to catch up and
> > following error gradually accumulates until it reaches the trip setting.
> >
> > 4 - Possible solution
> >
> > If Traj Max Velocity sets the number of spaces in a bin and max velocity
> > for any single axis never quite reaches the number of steps set, then
> > steppermod should be able to feed in steps that have accumulated as
> > following error.  To test this using much of the same procedure used
> > during the acceleration tests above, I set
> >
> > [TRAJ]
> > MAX_VELOCITY = 16
> >
> > and each axis
> >
> > MAX_VELOCITY 15
> >
> > and ran in mdi using g1 f900 x500.  It completed the full move every time.
> >
> > 5 - Possible problems with this solution
> >
> > Some have observed times when axes do not seem to move according to
> > a straightforward interpretation of the values set for Max Velocity.  It
> > was suggested that setting all Max Velocities the same might solve that
> > problem.  If you decide to implement the solution suggested above, would
> > you watch for times when you might get the faster rate rather than the one
> > set for each individual axis.
> >
> > Hope this helps
> >
> > Ray - (ultracrepidarian: (n., adj.) a person who gives opinions
> > beyond his scope
> > of knowledge.)
> >
> >
> >  On Sun, 17 Jun 2001, you wrote:
> > >
> > > Hello,
> > > when I dirive the axes in manual mode I get a Follow-error
> > after some time.
> > > Here is my conf.
> > > Thanks a lot
> > > Malte