Re: experiments with EMC smoothness

["Dave Engvall" <dengvall-at-charter.net>]

List:
Les's comments on "jerkiness" caused me to do some playing with my bench
experimental system.

System:
SEM MT30H4-44 servo motor
SD 1525BR servo amp with a 70 v supply.
a 2.5" dia flywheel 2.5" long mounted directly on the shaft with a helical
.25 x 6 mm coupler
to a heidenhain ROD 426.000-2500 encoder
controller: ppmc with encoder and dac cards running ppmc_dionull in place of
ppmc_dio

This system is not very loaded so the settings are fairly agressive.
Emc:

P = 1020
I = 0.11
D = .1
FF0 = 0.0
FF1 = 0.0
FF2= 0.0
Input scale = 40000 0 simulating a .25" pitch ball screw
max and default accel = 10.16 m/sec**2


With a short program that makes successive 1" moves...from x = 0 to x = 10
and then a single return to zero move.

G1 X 1 F n00
G1 X 2
.........
G1 X 0
M2

If one logs this move using drive voltage vs. position (since there is no
acceleration vs position) then
at low velocities the voltage curve swings in a dampened oscillation from
+10  V to -7.5 V on the first swing and then dampens out to about 4 V +/- 1
V all the way to 10" where one sees the same thing for the reversing of the
axis and then again as it decelerates approaching zero. Following error
maxes at about 0.005" and decays about the same as the drive signal.

With a single move to 10" or n = 1 to 3 there is curve that looks just like
the slow speed moves. Only when I tried n = 4; therefore 400 ipm which is
max speed for the system do I start to see small dampened oscillations in
the drive voltage at the start of each new block of code. The oscillations
are about twice the voltage swing for a move in equilibrium.

I was expecting much larger transitions but at least for my system they
don't happen.

Results from high speed systems with real loads to push should be
interesting.

OK guys it's open season!

Good luck!

Dave Engvall









----- Original Message -----
From: "Les Watts" <leswatts-at-alltel.net>
To: "Multiple recipients of list" <emc-at-nist.gov>
Sent: Tuesday, May 20, 2003 7:13 AM
Subject: experiments with EMC smoothness


>
> As some of you know I have been investigating observed jerky motion
> when EMC is used at high speeds and accelerations as some of us typically
do
> on cnc routers and the like.
>
> I ran some simple little tests where one axis is commanded to move
> linearly with a long series of short G1 moves at various speeds.
>
> I found that at virtually all programmed speeds short adjacent moves
> run much much slower than the same total motion programmed as a single
move.
> A machine-gun like jerky behavior is also noted.
>
> I found this surprising since EMC is supposed to have blended trapezoidal
> motion planning with a cubic subinterpolator.
>
> Let me briefly describe what these do.
>
> Trapezoidal motion planning refers to the velocity profile in a move.
> >From the start point to the destination point the motion is divided into
> three areas.
>
> The first is accelerating up to speed. This is done at constant
> acceleration,
> so velocity increases linearly.
>
> The second area is the cruise region, where the motion is at constant
> velocity and therefore zero acceleration.
>
> Finally there is a negative acceleration (deceleration) region where
> velocity is linearly reduced with time until motion ideally ceases at
> the desired destination point.
>
> Now it’s easy to see that a series of commanded moves would be in
> exact stop mode, always decelerating to a stop at each waypoint.
> Inertia and PID compensation would prevent complete stops, but it
> would still be arbitrarily choppy.
>
> That’s where the blending comes in. Generally if another move is
> seen coming by the planner it will skip the deceleration phase
> and call the next waypoint. In that way velocity only has to be
> changed if it is commanded.
>
> So a series of identical short moves could be as smooth and seamless
> as an equivalent long commanded single move because the blending
> suppresses those deceleration/acceleration regions between segments.
>
> But I am observing that EMC is not fully doing this in G64 contouring
> or any other mode at any speed.
>
> Puzzled, I looked through archives. I found an old letter from Fred
> on the subject which I will quote some excerpts.
>
>
>
> >The EMC doesn't do S-curve velocity profiling, one of its many
> >deficiencies. It just does trapezoidal profiling. The code for this is
> >in emc/src/emcmot/tc.c, and Will's working on adding support for 6-axis
> >motion (orientation control). If you need to limit acceleration spikes,
> >there's no real satisfactory way at this point. I think the best that
> >can be done as a workaround is to reduce the acceleration value, but
> >this will increase corner rounding.
>
> >The way the motion controller works is by looking at each motion segment
> >(straight line or circular/helical arc), computing its total length in
> >Cartesian XYZ space, and planning a constant accel to the specified
> >speed, cruising, and then decelerating to the end point. Adjacent motions
> are
> >blended together, the second being started when the
> >first begins to decelerate. This causes rounding, which can be made
> >crisper by increasing [TRAJ] DEFAULT_ACCELERATION. The blending is also
> >broken, in cases where three or more segments need to be blended
> >together.
>
>
>
> I think the last sentence explains the observed jerky motion-
> apparently for paths of more than 3 segments THERE IS NO MOTION BLENDING.
> The only control is to reduce DEFAULT_ACCELERATION to
> a low value. Perhaps it is only  cubic interpolated intermediate
> points that are blended - I’m not sure.
>
> This was realized by a further quote of Fred’s letter:
>
>
> >The motion controller obviously needs to be improved, in at least these
> >areas:
>
> >1. Blending. It should be possible to blend any number of adjacent
> >segments. A guest researcher got this to work, but it wasn't quite
> >finished and it's not compiled in the distribution (it's the
> >"segmentqueue" code in emc/src/emcmot if you're interested).
>
> I took a look at segmentqueue.c, segmentqueue.h, and the aparently
> currently used blended trapezoidal routine tc.c and it header file.
> I wonder what wasn’t quite finished? Was it just some housekeeping stuff?
>
> I would also ask that folks have a look at these files as well.
> I am familiar with the math routines for these functions but it is
> of course very hard to follow exactly what’s going on in that code.
>
> Perhaps segmentqueue is 99% finished and could be instituted with
> only a little work. I of course have no idea what is not working
> yet with it.
>
> This would be a powerful performance increase for EMC if I understand
> things correctly. As I have studied this I have
> concluded that it would be more powerful than things like
> quintic interpolators and the like.
>
> For very slow speeds and accelerations like 10ipm and 3 in/sec^2
> EMC can be fairly smooth. But those of us using cnc routers or plasma
> are forced to go much much slower that many of our servo machines can
> handle,
> because we get a pulse of current at maximum allowed on EVERY SINGLE
> PROGRAMMED WAYPOINT. That is what I am seeing anyway. Intermediate cubic
> interpolated
> waypoints may be blended, but that in itself would seem to be of little
use.
> And my tests show that it is not a computer speed issue. Or at least I
don't
> think so, because it happens at any programmed speed.
>
> Let’s work on this. We could be talking about a tenfold increase in
> trajectory
> smoothness!
>
> Please excuse the double post if you are on both lists.
>
>
> Leslie Watts
> L M Watts Furniture
> Tiger, Georgia USA
> http://www.alltel.net/~leswatts/wattsfurniturewp.html
> engineering page:
> http://www.alltel.net/~leswatts/shop.html
> Surplus cnc for sale:
> http://www.alltel.net/~leswatts/forsale.html
>
>
>