Centroid Community CNC Support Forum

Can anyone explain what the Drag Plot graph represents and how to interpret it?
This is what I have learned so far.

The red horizontal lines indicate the bounds acceptable limits for the machine as it is currently configured.
The graph displays a graph of the motor pid out/current used over the distance of the axis.
A "U" shape graph shows that the ballscrew is not aligned with the axis movement. stuff like that is hard to tell without it.

Most this information came from cnckeith. Thank you cncKeith!

It looks like the graphs X axis is the distance moved.
I think the graphs Y axis shows the amps used to move the axis.
Why are there 2 different graphed lines on the X axis? I am guessing one is for movement in one direction and the other is for movement in the the opposite direction.

What is the yellow line? I am guessing it is actual amps used to move the machine axis. But on my Z axis (Which comes up with no excess drag) The yellow lines are above the red lines for mach. movement in one direction and below the red line in other direction. ??? The seems like the power to move is greater than the red line which indicates power limits. ??

What I am asking is a explanation of what this all means.??

My information could well be out of date, but:

1) The horizontal axis of the graph is axis travel, in Machine Coordinates. Zero is at the machine home position for that axis.
2) The vertical axis of the graph is servo load, against an arbitrary scale. Most commonly, 32 units on the vertical axis would equal 100% of available torque, but that can vary depending on your servo tuning values (Ki in particular).
3) The red horizontal lines represent plus and minus 25% of available torque. When full scale is 32 units, you will see the horizontal lines at plus and minus 8 units.

The 25% guidelines are an arbitrary, but reasonable, suggested limit on torque to move the axis with no cutter load. It is not at all based on the machine design or installation. For example, if you change your drive belt ratio from 1:1 to 2:1, the torque required of the motors will be cut in half, but the red horizontal lines will not change. The axis will be just as hard to move as it was before, but the motor will have an easier time moving it because of the improved mechanical advantage.

4) The yellow traces are the results of the most recent drag test: one trace going from minus to plus, and one trace going from plus to minus.
5) Red traces show the history of past drag tests, so you can see changes over time.
6) A "U" shaped graph means that the motor requires more current to move the axis near the ends of travel. This may indeed mean that the ballscrew support bearings are out of alignment with the ballnut. It can also show a combination of heavily-worn ways and overtightened gibs.

Again, the vertical scale is in arbitrary units. It is scaled from the output of the PID algorithm. In torque-control mode, the PID output corresponds fairly directly with motor current (Amps), which in turn corresponds fairly directly with shaft torque. However, the control software has no knowledge of how many Amps you get for 100% PID output, nor of how many in-lbs torque that would be at the motor shaft.

Thank you! I could not ask for more.
It's still a little confusing but still clear enough to figure out.

Where did you glean this information? I went through everything I could find all docs including all bulletins.
I am going on 69 so I probably overlooked something

I deleted
x_drag.out
y_drag.out
z_drag.out
And the red horizontal drag lines were gone just like you said, they represent the previous drag test.