Vertical Axis PID Tuning questions

Hi all, I've made some good progress on my Wotan Rapid 1 CNC HBM that I'm retrofitting to LinuxCNC. I actually have been very fortunate, the X Axis is dead on the money right out of the box. I didn't have to do any servo tuning. I've got the acceleration rate set at the stock 30in/sec/sec and machine maximum set at 200 ipm (that's what the old control was set at for max speed) and it will jog without error at 200 ipm. I know that sounds slow, but this is a large machine.. Table is 4ftx5ft and a rotary table at that. I have no doubt the table assembly is a couple of tons. The column that holds the Head slides in and out for Z axis and was almost too much for my 18k forklift to pick up.

The Y axis is another story.. Being a horizontal machine, the Y axis is the movement of the head up and down. I can jog slowly upward without issue, but going down is usually an immediate following error. What I noticed is that when I jog down briefly and then stop, the head will "coast" for a second... I'm guessing gravity is playing a part on this.

The machine has Allen Bradley 1388 analog servo amps. Is this something I should work on with the amp or work on in LinuxCNC? The amps have tachometer feedback, so I'd think that the amp would see the servo spinning with 0V input and try to bring it to a stop... But maybe not? For what it's worth, the Amps haven't been touched in this retrofit. They are set how they were with the old Dynapath control.

Thanks for any advice.
-Matt

If the drives have tachometer feedback they should be running in
velocity mode and should resist gravity. If it can be done safely, you might want
to make sure that the Y drive resists applied force even without feedback (P term set to 0)
It should at most drift slowly without feedback.

That is, without feedback and without commanding a move the drive will be given a 0
velocity command and should resist externally applied forces.

I had the same issue on my HBM It is a Mitsui Seiki HR5…also quite a beast. Fanuc DC servo motors and amps. 

In my case the drive on y tripped because of over voltage. This was caused by the regeneration when moving y axis down. 

I found that the piston inside the hydraulic counterbalance cylinder came off the rod and was permanently pushed up against the cylinder end, leaving the rod to travel up and down freely. So no counterbalance....

Fixed that and the problem went away.

Machine does use analog servo amps with tachometer feedback. I'll dig into that and see what's going on.

Machine has a weight & chain counterbalance that are in place. It also uses compressed air for the aerostatic ways, and that is turned on and working also.

I'll hook my oscilloscope up to the test points on the servo amp and see what's coming in there. Maybe be a simple bad connection.

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.