How does height adjustment on plasma table work?

[Pete.S.]

Hi guys!

How does automatic height adjustment on a plasma table work?

Does it change the height while cutting or just as the start of the cut?
How does it sense the distance from the torch to the material?

Thanks,
Pete

[Gamble]

Changes while cutting based on the voltage feedback it gets and what it's set to

[Pete.S.]

Changes while cutting based on the voltage feedback it gets and what it's set to

Aha, so it doesn't know anything about where the material is? It just tries to keep the voltage constant while cutting?

Also, before piercing it looks like the torch will go down and touch the material. Since there is no arc yet, I guess that is to make sure the pierce starts at the right height. Do all controllers do it this way?

[Gamble]

Ohmic sensor on the end of my torch. When it hits the plate the circuit is complete so it knows where the plate is. Then raises to your pierce height. Pierces the material then moves and adjusts.
Some use a floating head instead of the ohmic sensor.

[jimcolt]

I thought I'd post this as it is an often misunderstood part of the CNC plasma cutting process. There are a lot of things that go on in the background of THC (Torch Height Control) with an automated plasma system....and it is a very important part of the plasma cutting process. A cutting machine without automatic torch height control will exhibit poor plasma consumable life as well as poor cut quality. Unfortunately, even with a good THC, improper settings or lack of a basic understanding of how the system works can lead to the same results as having no THC! Here's a simplified explanation of what has to happen for every cut cycle with a proper THC:

There are 3 cut height settings on a good quality THC. Pierce height, cut height, and arc voltage control.

1. Pierce height is preset by the operator to the plasma torch manufacturers recomended pierce height for a given material and thickness. The THC indexes down to the plate before the torch fires, locates the plate (there are a few different methods for sensing the top surface of the plate such as ohmic sensing (usually the best method), torque sensing, stall sensing, capacitive or inductive sensing and limit switch (floating head) sensing.), then retracts to the pierce height. The torch then fires.....and should stay in place, no x y or z movement, until the pierce delay (recommended by the torch manufacturer for each thickness) times out.

2. Cut height is preset by the operator according to torch manufacturers recommended cut height. As soon as the pierce delay times out the torch rapidly indexes down to the cut height. At the same time x and y mottion begin driving the torch through the part program.

3. Arc voltage control uses feedback from the plasma system to measure the voltage between the electrode (negative) and the plate (positive). (some call this "tip volts", which is incorrect as the torch nozzle is known as the "tip", yet it is the electrode and plate that the voltage is referenced from) At a given cut speed and fixed torch to work distance this voltage remains constant. If the plate is warped so that it moves away from the torch during steady state cutting...the arc between the electrode and the plate gets longer, a longer arc means the voltage gets higher. The torch height control see's an increase in arc voltage and signals the z axis drive to move the torch closer to the plate. Arc voltage monitoring takes over torch height functions as soon as the machine is within a certain percentage of the programmed cut speed.....so as soon as acceleration gets the machine to a certain speed...the arc voltage control takes over. Consequently when the x and y motion decelerates below this threshold percentage...the avc is disabled so the the torch does not dive during corner or small feature slowdown events .

If your torch height noticably changes during the transition between cut height and avc, then you should adust your arc voltage setting so that there is no change during this transition. So, if the torch indexes to cut height, starts moving, then moves further away from the plate....reduce the arc voltage setting. A rule of thunb is that 5 volts will equal approximately .020" torch movement......so reducing arc voltage by 5 volts will move the torch closer to the plate by roughly .020", increasing voltage will do the opposite.

Things that affect torch height when operating in avc (arc voltage control)

1. Gas pressure fluctuation. Changing gas pressure changes the resistance of the plasma arc, which changes the arc voltage.

2. Speed change. Slower cut speed makes the kerf wider, which means the arc is longer, which means the voltage is higher...so the THC moves the torch closer to the plate.

3. Worn electrode. Plasma electrodes wear by forming a pit in the hafnium emitter. If the pit is .020" deep, the torch will run closer to the plate by .020"....as the arc is .020" longer and the height control is trying to maintain a constant voltage.

Bottom line....the proper physical height is more important than setting the exact arc voltage that is listed in the plasma torch manufacturers manual. Adjust the voltage so the physical height is correct...don't worry about the voltage reading!

I'd be happy to answer any questions! Jim Colt Hyppertherm

[Pete.S.]

Things that affect torch height when operating in avc (arc voltage control)

1. Gas pressure fluctuation. Changing gas pressure changes the resistance of the plasma arc, which changes the arc voltage.

2. Speed change. Slower cut speed makes the kerf wider, which means the arc is longer, which means the voltage is higher...so the THC moves the torch closer to the plate.

3. Worn electrode. Plasma electrodes wear by forming a pit in the hafnium emitter. If the pit is .020" deep, the torch will run closer to the plate by .020"....as the arc is .020" longer and the height control is trying to maintain a constant voltage.

Bottom line....the proper physical height is more important than setting the exact arc voltage that is listed in the plasma torch manufacturers manual. Adjust the voltage so the physical height is correct...don't worry about the voltage reading!

I'd be happy to answer any questions! Jim Colt Hyppertherm

Great post Jim!

A couple of questions though.

1. If the physical height is the most important thing to keep constant, why doesn't the CNC controller sample the arc voltage during the first say 1/2 second or so after it has reached full speed and then try to keep that voltage constant? Then the operator wouldn't set the voltage at all, it would be become whatever it was at the proper height.

2. Or even better, why not keep the physical height the same all the time? Meaning, why not use some other way of measuring the torch height other than arc voltage?

2. What kind of tolerance should the height control keep the height within without out it affecting the cut in a measurable way?

[jimcolt]

1. Better height controls have been using voltage sampling for over 20 years. Some of the entry level and light industrial tables use this to auto set the voltage and height.
2. Other methods (capacitive and inductive, sonar, laser) have been tested, used. Arc voltage is the most repeatable, reliable and electrical noise immune method to date.
3. With an air plasma we recommend staying within .004" of the recommended height. Most of the shielded Hypertherm process specs require a real time cut height of .060". When you look at Hypertherm high definition industrial plasma, height is recommended within .002" of recommended heights for best cut quality. Jim Colt

[Pete.S.]

Thanks Jim and Gamble!