Re: [CAD_CAM_EDM_DRO] Re: EMC stg2mod.o and problem with homing

Asim Khan wrote:

>!! !! AT HOME POSITION AXIS POSITION COUNTERS DONOT TURN EXACT 0.0000
>EVEN WHEN THE POSITION DISPLAY IS SET TO MACHINE !! !! !!
>
>

Of course. The actual position is never what you ask for, just within some
acceptable error bound.

>Hi group!
>Due to problem in emc developer installation i was unable to install
>it on a P-I system. The developer install was causing the
>installation to process hang when it tried to install the kernel
>source files. I had changed cd drives but no better results!
>
>Since i was unable to install developer from the CD so ,To experiment
>a shortcut method to fix the homing
>and index pulse detection problem i copied the newstgmod.o from
>a fixed system to this P-I machine. I ran the system, the system
>is able to detect index pulse and can stop and turn green but the
>only trouble now i am having is that at home positions the axis
>counters donot turn an exact 0.0000 but instead they give some offset
>like X 0.0206 Y0.0345 Z 0.430.
>

Oh, my! Those are quite large differences, especially the Z.
You should investigate axis stiffness. Apply manual force to the motors
in such a way that if they took off it could not pull your hand in and
break fingers. Possibly using the palm of the hand on a belt sprocket
or something like that. There should be a VIGOROUS resistance
from the motor to any perceptible movement. (Don't be worried if
you can move the motors with STRONG effort from your hand, all
servo systems have some peak torque which thay cannot exceed.)
But, if you can move the motor more than a few thousandths of an inch
(few hundredths of a mm) with a small effort, then the stiffness of the
axis is far too low. This is primarily a problem with proportional
gain (the P term in the PID settings). But, gain affects everything,
including stability. If you increase P, you may have to adjust other
settings to keep the loop stable. Your P of 60 is VERY LOW! I
run my servos around P=600 - 1000! For stability reasons, you may not be
able to go that high. But, with 25000 counts/inch, you should be
able to settle to 1"/12500 ~= 0.00008" of the commanded position.
But, there's a lot more to this tuning business. First, the servo amplifier
should be set so that a velocity command of about 9 V = the maximum
speed you want the machine to move at. Then, MAX_VELOCITY
should be set to that maximum. Your MAX_VELOCITY is set
to 5. Do you really plan to go to 300 IPM? If so, then 9 V into
the servo amp should deliver 300 IPM. If not, then set MAX_VELOCITY
(both in the TRAJ section and in the AXIS sections)

Note that my machine has encoder resolution of .00005" (50 uIn)
and settles at rest to about .0001", or within about +/- one encoder
count of the commanded position. I do not use ANY deadband, but
this is a true analog servo system with DC tach, and so it is
enormously self-stable at rest. Without a DC tach, there is no
micro-velocity feedback between encoder counts to keep the loop
closed, and so deadband may be needed to prevent annoying hunting.

Jon