Tuning an open-loop system

I have an AvidCNC-based gantry router table with ClearPath servos, driven by LinuxCNC via a Mesa 7i76e card. I'm noticing what seems like a bit of overshoot, and also some ringing, in the resulting toolpath. Let me try to explain:

Overshoot

I use a Haimer to reference the workpiece (in this case, I'm referencing some faces of the 4th-axis assembly that I've measured from the rotation axis). In Y, the reference position is -57.434 mm. If I zero the Haimer on this position, then issue g1f2000y-67, g1f20005-57.434, it routinely overshoots by 0.03 mm. If I feed in at f100, the Haimer lands on 0.0.

Ringing

Here I'm talking about ringing in the sense of a step input into a system, the resulting output oscillates around the desired value. This is most visible when I'm cutting, say, a sheet of plywood, and take a 90° turn. For the first couple centimeters after the turn, you can see some waviness in what should be a straight edge.

Now, this is likely due in part to the lack of stiffness in the machine, and it being on these footmaster casters (I'm gonna go back to the fixed Avid feet). I'm usually cutting pretty fast when I see this.

But the overshoot above, f2000 (mm/min) isn't particularly fast, and F1100 is painfully slow.

I wonder if my accelerations are too high? Are there more ways to tune such an open-loop system?

You can find my config here: github.com/JetForMe/router-table/tree/ma...uxcnc/config/router3

I would suspect that you would need to tune the drives themselves

You would expect ringing if you are exceeded the drives acceleration
capabilities. That would be an easy thing to test (say reduce the accelerations
in 1/2) This will also reduce induced ringing on the mechanics

I maintain some large industrial routers with Fanuc controls, where I battle with this problem anytime something in the drivetrain wears out making that axis a little loose. Bad bearings on the pinon, or transmission shaft even a bad loosening belt. But barring a mechanical issue I'd look at the drive tuning.

Here is a good example of acelleration induced machine that is not stiff enough. In this case 0.5g acceleration with a lead into a notch to denote a fold line. The rapid reversal in direction from near 0 degrees to near 180 degrees to the x axis caused a lot of wobble.


If you are seeing this in a straight cut away from any change in direction and you are cutting along an axis, I would say it has to be the clearpath tuning.

Hi JetForMe: I’m an engineer at Teknic, and I realize this thread is a few weeks old at this point, but I came across your questions and wanted to offer some advice. As some of the other members have noted, there are many factors that can cause the problems you are seeing. The commanded accelerations (and/or velocities) could be too high, the servo tuning could be sub-optimal, the mechanical system could have issues (worn, slipping, sticking, the casters you mentioned could be contributing, etc.).

Given all the possible contributing issues, the best way to troubleshoot is to use ClearPath’s built-in diagnostic tools. Please feel free to call us for support. One of our engineers can “dial into” your system and help diagnose what’s going on (usually pretty quickly). We can look at the torque utilization, visualize the command signals, compare how close the motor is tracking compared to the cut quality, and much more. If the tuning is sub-optimal, we can optimize it for you.

If you would like some assistance, please give us a call at 585-784-7454, or use our "Contact Us" form online at www.teknic.com/contact/.

Best,
Bridgette G. - Teknic Servo Systems Engineer

PS: ClearPath is a fully closed-loop servo, so if the mechanics are good, and you don’t exceed the motors’ torque limits, you should get excellent cut quality. Your system is only open loop in the sense that you may not be adjusting your feed rates when you are exceeding the motors’ torque limits. But you can close this supervisory loop as well by getting high level feedback (HLFB) from ClearPath. HLFB can warn you of any situation where the performance is compromised, and most CNC controllers will be able to adjust accordingly. We can help you set that up as well, if you’re interested.