RE: [CAD_CAM_EDM_DRO] Servo Error, - Was backlash.

Sorry Scott.

I try to make descriptions easy, but often don’t succeed.

Jon is pretty switched on as well, and way more experienced at some of this
than I.

See comments below, hope these help



Regs Mark.



Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Polaraligned
Sent: 25 January 2007 12:46
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Servo Error, - Was backlash.



Interesting Jon. I guess I did not understand what
Mark was talking about when I first read his post.
Thanks for making it clear to me.
I am currently set up with my servos for 16000 counts
per inch of travel. If you figure it out, 128 counts
is a whopping .008"!!! That is a lot of error and still
have have the drive say everything is OK. But that
is following error. When the motor stops, the drive
should "hunt" for the actual position. Is there a case
when this is not true and the drive just stays where it
is?



Yes. Whilst this isn’t quite correct, think of it that the drive doesn’t put
enough power in to overcome friction for that last little bit of follow up.

The P parameter sort of looks for the error and applies power proportional
to that error. The more the error, the more power and faster it goes towards
position. But if the error is small, there isn’t enough power. Think of
pushing a chair across a floor, you can apply a little pressure and nothing
happens, so if you wanted it to move a short distance, you wouldn’t apply a
lot of force, you’d probably give it a short sharp nudge, it it still didn’t
move, you’d wack it a bit harder until it does. If you wack it too hard it
will overshoot. Think of the I as correcting this. Some cheap servo drives
are just P and D they have no I parameter.




Why would the servo controller not move the motor
to the command postion within say several counts?,even
if it is over or underdamped. I do not understand how
tuning would change this. I thought tuning just changed
how smoothly the servo went to command postion. I never
thought it would prevent the motor from getting there.
When I am single stepping the motor,
my dial test indicator shows clean solid .001" steps.
Is it possible that my motors are not powerful enough
and the drag on the table just prevents the servo
from getting to it's command position?

The easiest method to know whether there is a following error is to take the
encoder signal out to another counter, perhaps connect it to the mpg input
for mach. Then if mach sends say 100 pulses, you would expect to see 100
pulses worth of movement on the encoder. Some servos actually have an output
that shows the difference between the correct position and the actual
position, sometimes this is a digital output, sometimes analogue.

I have the G340's with the jumper for step multiplying.
I am currently in the X5 position with 1000L encoders.
If I move the jumper to the X2 position I will get
40000 counts per inch of travel. This will reduce my
possible error to .0032" of travel before an encoder trip.
Would this reduce any backlash induced by motor tuning?
I would guess not.

I would tend to try to adjust the servo’s better. I am using 10,000 lines
per inch with my Rutex drives, which gives 40,000 counts. It took a day of
playing to get the first drive right, but the error now is always zero

Can anyone tell me what I realisticly can expect for
backlash measured at the table???? I understand that
it can never be truly zero.

I thought I would look at the backlash on mine today, When I picked it up
the back lash wouldn’t allow a 1/10 though per division DTI to move, it has
been run now and new fresh oil is everywhere. My Y and Z axes are just under
0.5 though backlash.

I found the gib strips needed tightening on my X axis, lifting the table I
had 3 though of movement. ¼ turn of screw and it is tight and runs over full
travel nice an level, running DTI mounted on head down onto table, so no
curvature. My X axis has a little more backlash 0.9 thou at the ends and 1.1
in the middle. I put DTI on the end of the lead screw and I have about 0.7
though of end play, so it looks like I need to tighten that.

Hope this at least gives you something as a reference. The BP half though
figure sounds about right.

Thanks for pointing this out Mark, and thanks Jon for
explaining what Mark was saying so I understand it.

Scott

--- In CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com, Jon Elson <elson@...> wrote:

>
> Mark Vaughan wrote:
>
> >2 Are the servo drives set up correct. When you set a servo up, it's
> >response will look nice with P and D parameters, but it will stop

before it

> >gets to position or just after, there will always or near always be an
> >error, it needs a little more oomph on the last bit.
> >
> This is an incredibly good point. Your system is open-loop back to the
> computer (I am
> assuming, with Mach 3), so you have no idea what the actual position
> is. You only know
> where you commanded it to go, and that it is within +/- 128 counts so
> that the Gecko
> didn't fault. (It really has to be much closer, but depending on a
> range of parameters
> such as motor resistance, Ilimit setting and supply voltage it can be
> tens of counts.
> Mariss at Gecko swears this is impossible due to the I term, but a
> friend of mine
> has a machine in town with G320 drives, and you can turn his handles
> 1/10th of
> a turn and they will just stay there! This is about 50 counts on his
> motors. He really
> needs to increase his supply voltage, but it still shouldn't do this.)
>
> There is a test point inside the Gecko drive where you can monitor
> instantaneous
> position error with a scope or DVM. I think the manual tells how to
> convert voltage
> into position error, in encoder count units.
>
> It might be a good idea to read the position error at the ends of

travel

> when doing
> this test. Just the overhanging weight of the table will increase
> friction at the ends
> of travel. It may be that all the error you are seeing is following
> error, and maybe
> electronic tuning on the servo drive would reduce it substantially.
>
> Jon
>

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