Rods "Spaceship" Scratch built Plasma Cutter build

If you have a PID based on arc voltage moving the Z axis then I would be running than axis in position mode, I think.
And I would expect the arc-controlled PID to consist mainly of I-gain. I would start tuning it with P and D at zero and tune only I for the level of responsiveness I wanted.

(NB: I have never even seen a plasma cutter)

andypugh wrote: If you have a PID based on arc voltage moving the Z axis then I would be running than axis in position mode, I think.
And I would expect the arc-controlled PID to consist mainly of I-gain. I would start tuning it with P and D at zero and tune only I for the level of responsiveness I wanted.

(NB: I have never even seen a plasma cutter)

Thats a bit of a conundrum (not your knowledge of the subject) but Dewey's docs said

When used to regulate an axis offset, the offset_pid
component servo loop is an outer loop to the LinuxCNC
motion control. For external offsets, this loop is implemented by
a simple single-axis trajectory planner with constrained
maximum velocity and maximum acceleration that are fractions
of the limits available for the axis as set by the ini setting:
[AXIS_L]OFFSET_AV_RATIO. The inner loop effectively contains
an integrator and use of the pin is not expected
to be helpful; some amount of may be required
for stability.

We do cheat with plasma and give eoffsets 100% of the velocity and acceleration. If anything on today's tests more acceleration would help but we ran out!

I do value your input Andy. Fresh thinking is always appreciated.

I might add that my regression analysis of 9000 data points collected with halsampler showed a very close linear correlation between the voltage and torch height (with 99.5% confidence). I might look at the different stepgen modes.

Hi Rod,

At the time i wrote my component based on new version external offset branche and reverse feed branche, i could set the z axis eo speed value. This example lives in the opa config of the Eo branche i think. My current value for thc z axis correction speed is 200 mm/min. If you can not find it. I will show it.

Rod

It does look like your THC is still a bit "nervous" considering the the thickness (flatness) of the sheet you were cutting. This is pretty exciting as I want to see a working EO PID solution that does away with all the muxing and scaling feedback to keep the motion planner happy (ferror).

A simple way to limit acceleration may be to insert a limit2 component between your Z axis PID component and the step generator. The load pin can be used so that it is only applied when under THC. You may have to use a limit3 before the PID if your axis PID does as much hard limiting as mine. I will not have any time to try this before the end of next month because of travel.

John

John,

I see no problem using pid. All the thc work is normally done by one component.
It's up to the component level to make plasma cutting very easy for users. Maybe rod can add a simple timer to his used component to add a reaction delay in ms. This will solve his nervous floating cutting head to a much more stable floating head.
I dont know wich component he is using, but reacting z axis value after 5v plasma difference voltage is normal. Using this feature will eliminate nervous behaviour at the begin stage. My advice is 5v is 0.5mm reaction.