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. The main challenges are installing a height control system - which we decided to do as a separate, modular unit - and integrating control software into the system.
See also: Sensing_Distance_from_Work_Piece
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.
Chuck says: A few comments on torch height control:
1. Plasma and oxyfuel are different environments. While it is conceivable to develop a single sensor system that handles both, this is a hard problem and I suggest we first approach them as different tasks. Sensor system can be swapped out at the same time we swap torch heads.
2. For plasma, initial height sensing (IHS) and maintaining torch height during cut are two different tasks.
3. For plasma torch height control *during the cut*, arc voltage sensing seems far and away the most proven method. Many plasma power supplies have voltage sense outputs, but if we need to use a power supply without this feature we should simply plan on installing voltage sense terminals. The electrical components/issues are a. voltage divider resistors b. heavy noise filtering c. common-mode rejection (e.g. opto-isolation) in connection to control electronics d. stable and sensitive transfer function (must see ~0.5V change on ~150V signal)
4. A perfectly functioning arc voltage height control still does not address the IHS function at all; this is separate hardware. I think there are several possible approaches we can consider.
5. Start of cut operations need to be properly sequenced. These operations are different between oxyfuel and plasma. These operations are also different between edge- and piercing-start cuts. a. for oxyfuel there is, at minimum, a cutting oxygen valve; preferably there are proportioning controls for preheat fuel and oxygen as well. b. for plasma there is a signal to the torch telling it to start the arc, and a signal from the torch indicating the arc has successfully transferred to the workpiece (could be a current sensor on the workpiece ground wire). Preferably there is proportioning control of the plasma current. c. synchronization requriements between xyz toolpath and torch controls/sensors need to be evaluated: what communications are required between a separate z-control arduino and the main CoolRAMPS? ChuckH (talk) 20:47, 14 July 2013 (CEST)
"PSoc4 Pioneer Kit" is a cool little board using a Cypress processor that can do capacitance sensing.
Cap proximity demo: http://www.element14.com/community/message/76985