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?

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?


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.


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


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

Scott

--- In CAD_CAM_EDM_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
>