Height Controller

Overview

As of 7.13.13 - we are considering capacitive height control, and we are moving to Oxyacetylene as the first implementation (electronic noise-free environment) and then to plasma cutter (RF noise) after basic operation is developed with Oxyacetylene and 1/16" accuracy cuts are attained at Factor e Farm. At this point, we have on our hands a $2000 CNC torching system with 3 hose machine torch and height control for a 5'x10' system, rivaling commercial systems at over 10x the cost.

Discussion

Kliment says: I don't really see capacitive sensing working when the torch is on, since the electrical noise from the arc will be louder than the charge difference we can expect. I've been looking into measuring either the arc voltage or arc current and adjusting to that.

Arc current would be easiest to measure but I don't know how meaningfully that varies given the plasma torch is a constant current driver. We can measure current without modifying the torch at all, by simply clamping an inductive current sensor to the cable. Measuring voltage is a bit more involved, we would need to attach a very low resistance resistor network inline between the ground return path and the workpiece. That would allow us to measure the arc voltage by measuring across one of the resistors in the network. This also gives us data on arc current, and would be cheaper to implement. The torch itself does measure the voltage internally but it's not exposed anywhere so we can't use it directly without modifying the device.

I've also done some work on the toolpath generator. The latest version (QT based) doesn't seem to work on anything but Windows unfortunately but I grabbed an earlier one (tkinter based) and it had so many configuration parameters that it was easy to configure to be reprap-compatible. I got this to work by ad-hoc playing around, and am working on figuring out which of the changes are actually necessary.

Sorry for being incommunicado for so long, I was on a trip where I expected to have useable internet but turned out not to.