I have a Hypertherm 1250. What is the useful range for the arc voltage signal? The manual states that it is 0 - 300. I pretty sure that I won't be cutting at 0 volts so what is a real low and high limit for cutting? I'm building a THC from scratch and the wider the range the more pins I need to input the number to the pc.
If you have an operators manual for a Hypertherm Powermax 1250....simply look on the cutting parameter charts....these charts are in every Hypertherm manual and provide info regarding cut speed, correct consumable parts combination, amperage, physical torch to work distance, pierce height and arc voltage for each different material thickness. All of the Hypertherm manuals are downloadable in .pdf format on the www.hypertherm.com web site......just click on the library tab in the upper right corner of the home page.
I just looked for you....the lowest listed voltage is for cutting 10 ga stainless steel at 40 Amps with the Fine Cut consumables is 63 Arc Volts.......and the highest listed voltage is 156 Arc Volts for cutting 1" carbon steel at 80 Amps. I would suggest making your cutting voltage range a little wider than that in case you ever want to cut thinner and thicker.....maybe go 50 VDC to 175 VDC. The open circuit voltage of the 1250 power supply can be close to 240 VDC.....so be sure your circuit can handle that level.....other plasma systems go as high as 400 VDC open circuit voltage.
Other must have features on an arc voltage style height control:
AVC freeze......whenever the cnc motion slows (such as in cornering and fine feature contours) to 80% or less of the suggested cutting speed at a given arc voltage....expect the torch to get closer to the plate to compensate and maintain arc voltage. Most height control systems communicate with the cnc...and freeze at the currrent height in slowdown situations. When the speed is back up...the AVC (arc voltage control resumes.
Kerf crossing....a similar dive will occur when the kerf is crossed at the end of a cut. The height control must be able to recognize the rapid rise in arc voltage when the arc crosses the cut kerf....and must ignore this to avoid a dive and collision with the plate.
Initial Height sensing....A method of finding the surface of the plate....then retracting to approximately 1.5 to 2 times the recommended cutting height is necessary to allow piercing without damaging the torch consumables. Most height control systems use the torch (before the arc is initiated) to touch sense the plate , and use either a limit switch or z axis motor current to indicate where the plate is....then retract an operator set distance to the proper pierce height.
Cut height....as soon as the pierce is complete....but before the torch starts moving, it is important for the torch to quickly index down to the recommended cut height. The arc voltage height will not operate at this time as the voltage is not correct until the cut speed is achieved...so this is usually a known distance that gets the torch to the physical height recomended by the torch manufacturer. The torch remains at this height until cut speed is reached...and AVC is activated.
End of Cut retract....this is required to avoid collision with tipped up parts when traversing to the next cut out.
These area few things that are necessary to make an automated arc voltage torch height control work well....all modern industrial plasma height control systems have these and many other features that are designed to provide the best cut quality, cycle times and especially consumable life.
I have no real experience with this, but I am working on building my own THC as well, and I think I can answer your question.
The plasma cutter can operate at 0-300 volts, but I would think that you would want to use a voltage divider (resistors) to bring it down around 0-5 volt range. The THC control circuit then uses that proportional 0-5 volt signal to determine the arc length.
Zero volts would be zero inches, the cutting tip touching the sheet. I'm going to guess that it will never actually be zero because there is always some resistance between the tip and the sheet, but it could get close to zero. The maximum voltage will be determined by the resistance values you use for the voltage divider, so it can be whatever you want it to be.
Thanks for the very informative post. I have a Bridgeport style CNC milling machine that I just retro'd and have started doing my homework to put togther a plasma/ router table. You mentioned subjects I haven't run across elsewhere and hadn't even considered such as height freeze in slow turns etc. You probably just saved me hours of head scratching and posting my problems to beg for help.
Wow Jim thanks for such an informative answer. I feel dumb as many times as I have looked at the cut charts in my manual the pages are somewhat dog eared. The only thing I didn't have handy was the fine cut chart...
You bring up some interesting points that I did not consider. The plan is to use the controller (EMC2) to control the torch height. If I can input the arc voltage as a digital number into EMC2 the rest would be pretty straight forward. I didn't consider the arc voltage changing as I crossed a kerf so I'll have to see what I can do to ignore large swings in arc voltage in the program.
To maintain proper torch height (remember that you are skimming about .060; .040 for finecut, off the top of the material) at 100 to 400 IPM you have to be able to hold the gap volts to within about +- 1 but the voltage will spike when you pierce, and change as the table slows for corners and tight cuts. It's hard to set a voltage that won't result in a head crash from the spike caused by cutting air. It's not a good idea to cut crossing kerfs anyway. Most CAM programs can't deal with crossed toolpaths and apply any form of offset.
All of the moves Jim lists are essential, and effective THC is not just reading the Torch volts. What you are building is a closed loop "servo" and things like response times and dynamic range are part of the equation. Doing all of that in a hostile atmosphere of noise, poor grounding, etc is all part of the challenge. 5 volts of change can mean enough head movement to have a serious crash or loss of arc.
As always, the devil is in the details.
You THC has to be able to be preset from about 70 volts to 165 volts and have a resolution of 1/4 volt or less across the range. 10 bits of A/D should do it nicely.
All of the other parameters can be handled by either the CAM program or the control program.
BTW the TH300 analog THC and MP1000 digital THC have been ported to EMC. You should be able to find the information.
EMC2 can "know" when the velocity is changing so that part is easy to block out torch movement in corners. Being able to set the voltage in EMC2 also makes sense to me. I'm just trying to dumb down the analog to digital part and keep the "brains" in EMC2...
What is a good response time? What can you "get by" with?
Why does the preset need a resolution of 1/4 volt?
The electronics...and sometimes the mechanics of the torch height control can probably achieve 1/4 volt resolution (rule of thumb, 1 volt change in arc voltage is approx equal to .004"....although that varies with different processes)....but if you can hold arc voltage to plus or minus 1 volt....you will have a very good system.
Arc voltage THC from 10 years ago had a plus or minus 5 volt deadband....so the height could vary as much as .040"....todays plasma systems need accuracy in the plus or minus .010" range or plus or minus a little over a volt for best performance. Since minute variations in cut speed, surface condition, material content, plasma gas flow/pressure, and when a cloud passes under the moon all affect the arc voltage/torch height relationship......there really is not a good reason to try to get much better than 1 volt.
Many really nice arc voltage THC's end up getting "detuned" a bit to minimize torch z axis oscilation!
Jim, Thanks again for the information. You have been a big help in understanding the big picture. This will be a nice learning experience I bet. I'm by no means an electronic engineer but I'm pretty good at figuring things out given some hints and facts...
Given that this is a portable CNC plasma cutter and I don't plan on cutting big parts out (48" x 48" table) but I might be making a 48" cut across a sheet. What does the torch height affect the cut shape and/or dross amount? If your too high or too low but not too low that you run into the material what does each do?
When the torch to work is too high (voltage too high) expect the cut edge to have a positive bevel (bottom of the part bigger than the top) and expect top edge rounding and top dross. When the voltage is high....the plasma is producing more power (killowatts) because the actual cutting power is Amperage (constant current with most plasma systems) X Voltage. When more power is applied...more heat is also applied to the plate causing a deeper HAZ (heat affected zone) and a higher chance of warpage.
When the torch is too low...the cut edge can have a negative bevel.....bottom dross is common (not enough power) and the chance of collision with the plate increases.
Further.....one pierce too close to the plate can ruin a set of consumables that can last for 100's or 1000's of starts.....
When manufacturers design torches and consumables....this balancing act of height, energy and collision avoidance is carefully accounted for! That is why the highest quality plasma systems have a lot of choices for consumables....to allow you to fine tune the cutting process to your satisfaction!
I notice that many of the low cost Chinese import systems have a "one set fits all" mentality ....probably because a minimal amount of R&D and engineering time was spent developing the processes. Less engineering means the units can be sold for less. Hypertherms high end industrial plasmas....such as the HPR130 offers over 20 different consumable combinations to cut all combinations of materials between 30 and 130 Amps!
Arc Voltage Range
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