Vertical Axis PID Tuning questions

So the plot thickens here... I hooked a scope up to my amplifiers and was watching the command signal and the tachometer voltage for the x axis. When I jog, command goes right up and tach catches up very quickly. When I let off, command goes to zero almost immediately and drive shuts it down. Just what I expected. When I do the same on the Y axis, I'm seeing that behavior when I jog "up". But when I jog "down" and see the head coasting, I'm seeing that the command from the mesa board is actually slowly ramping down. So, the reason the unit "coasts" down is because the command voltage does not go immediately to 0 when I stop jogging. I looked and the PID config for Y is set to all the same values as the X (defaults) when I open the linuxcnc calibration box.

Any ideas what may be causing this?

What is the commanded Y velocity? (the PID output)
The analog output voltage should follow this.

How is the counterbalance setup on the Wotan, counterweight and chain or hydraulic?
If the head doesn't drift with the brake off and no power to the servo, you should be able to set the balance on the drives.

scotth wrote:

How is the counterbalance setup on the Wotan, counterweight and chain or hydraulic?
If the head doesn't drift with the brake off and no power to the servo, you should be able to set the balance on the drives.
 

Counterbalance is a weight and chain setup. Head doesn't move with the brakes released. 

The Wotan puts me at a disadvantage, but it should be close to a DeVlieg. The 1388's were rock solid and almost no trouble. With the mechanical balance ok and the drive balance zeroed, you should see near zero analog reference at axis stop. Make sure all drive train components are tight. Big machines like to hunt around zero position with analog drives and encoders at the far end of the screw from the motor or with scales. Screw windup or mechanical loss will drive you crazy on setup. I'm sorry I can't find my old 1388 manuals.

scotth wrote:

The Wotan puts me at a disadvantage, but it should be close to a DeVlieg. The 1388's were rock solid and almost no trouble. With the mechanical balance ok and the drive balance zeroed, you should see near zero analog reference at axis stop. Make sure all drive train components are tight. Big machines like to hunt around zero position with analog drives and encoders at the far end of the screw from the motor or with scales. Screw windup or mechanical loss will drive you crazy on setup. I'm sorry I can't find my old 1388 manuals.

Thanks for the insite. I do indeed see zero commanded reference with the head at rest. Verified with my o-scope. My X axis doesn't seem to hunt to bad for zero, I can't complain. 

For tuning in the drives themselves, did you just use a variable bench power supply for reference voltage and then switch it on and off with a switch to get the drive itself tuned in? I have a 1388 PDF from Allen Bradley and it shows how to tune the drive using an oscilloscope on the tach reference input. Maybe I need to tune that first, then go back to LinuxCNC.