Hello everyone, it's been a long time since I wrote anything in this forum, today I have something to ask.
I have two plasma cutting tables, one of them 100% operational with Mach3 and an independent THC. I am still manufacturing the other table, I want to control it with Mach4, I am going to use the analog THC function "and I have a question about it.
Is it possible to use the method called "Arc voltage sampling" to set the correct plasma cutting height? Has anyone done it or knows how to do it?For those who do not know what I am talking about, in this link there is a video of Lincoln Electric with a very clear explanation of this
https://youtu.be/-OT8UWz69s0 and here is an article written by Jim Colt from Hypertherm in other forum
https://www.plasmaspider.com/viewtopic.php?t=20551, where he explains very clearly how this method works.
Arc voltage sampling is an accurate method of setting the torch height using a physical height input instead of dialing in a specific arc voltage.
Not many height control systems use this....though they all should! Arc voltage sampling was first used about 25 years ago on high end ($100k plus) industrial cnc plasma cutting machines as a way to make height control systems easier to use and more accurate...critical to good cut quality and long consumable life.
Here's how a conventional (non sampling) plasma height control works:
-Operator hits start button...z axis moves torch toward material to be cut.
-Initial Height Sensing method (torque sensing, ohmic sensing, stall force sensing, limit switch sensing (also known as "floating head"), inductive sensing and more) locates the surface of the material.....then the torch retracts to the suggested pierce height.
-Torch fires.....stays at the pierce height until pierce delay times out (suggested by torch manufacturer for material being cut)....then indexes down to an operator set "cut height" (from the torch cut charts).
-x and y motion starts....and when the machine accelerates to near programmed cut speed....the arc voltage feedback height control takes over to maintain a constant distance between torch and material. This feedback circuit reads the actual voltage between the torch electrode and the material.....and compares that voltage to a voltage set by the operator (from the torch cut charts).....if the read voltage is higher than the operator set voltage....then the z axis motor drives the torch closer to the plate until the voltages both match (actual voltage vs pre-set).
The problem with this method is that a lot of things affect the voltage reading.....if you change cut speed you need a different voltage to maintain the correct physical cut height, if the cut air pressure changes....you need a different voltage, as the consumables wear you have to set a different voltage, and more.
A system that uses "Voltage Sampling" is easier to set and use....as the operator does not have to set the arc voltage. Here's how it works with the differences in bold:
-Operator hits start button...z axis moves torch toward material to be cut.
-Initial Height Sensing method used is ohmic plate contact...because it more accurately locates the material surface as compared to any other method. It electrically locates the surface of the material.....then the torch retracts to the suggested pierce height.
-Torch fires.....stays at the pierce height until pierce delay times out (suggested by torch manufacturer for material being cut)....then indexes down to an operator set "cut height" (from the torch cut charts).
-x and y motion starts....and when the machine accelerates to near programmed cut speed....the actual arc voltage is read by the THC electronics.....which "lock onto this voltage (does not have to be set by operator) and the arc voltage feedback height control takes over (using the sample voltage) to maintain a constant distance between torch and material. This feedback circuit reads the actual voltage between the torch electrode and the material.....and compares that to the sample voltage. If the read voltage is higher than the sample voltage....then the z axis motor drives the torch closer to the plate until the voltages both match (actual voltage vs sampled).
The advantage of this....the operator only sets cut height (physical height) and the electronics locks onto the voltage sampled at that height. This calibrates the height control voltage / height relationship before every cut cycle.....and compensates for speed changes, air pressure changes and consumable wear...maintaining constant height. Ohmic contact is required with this based on its accuracy at setting the correct physical pierce height and cut height...as it uses a very light touch without deflecting the material as the other Initial Height Sensing methods (torque sensing, stall force sensing, limit switch sensing (also known as "floating head"), inductive sensing and more) do.
- Jim Colt Hypertherm
Guest
Show Posts
