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thc:thc

Torch Height Control (THC)

THC checkup

Check ADC channel Number

Arc Voltage measured value is displayed on the myCNC main screen. Arc voltage signal should be connected to the proper ADC input number. The number should be set up in THC configuration dialog.

Connect the test signal (for example 24V from test power source) to THC ADC input and check the voltage value displayed. Test signal should be connected through Arc Voltage Divider you use (not directly to the board ADC input).

If you don't see any voltage on the display, then

  • goto “Diagnose” widget,
  • find the ADC channel number your test signal connected to



    (it is ADC0 - channel#0 on the screenshot example)

  • setup this channel# as THC feedback in THC settings widget (THC feedback channel was changed to “ADC0”, Save button on the top-right of the screen clicked)



  • go back to the main screen and check the Arc Voltage display.



    Now it shows Arc Voltage. However, the value is not correct ( 36.8V instead of 24V) and the Voltage ratio should be adjusted.
  • go to the THC settings widget again and fix the “ADC/Voltage” ratio. The value should adjusted to
    K1=24/36.8=0.6522


    Current ratio is “0.1” (on the screen above), so total ratio would be

    K=0.1*24/36.8=0.06522


  • Change new ratio, press Save



  • and the result on the main screen is



Check THC/PID settings

Now it's a good time to check default THC/PID settings

This is just default values you can start playing with on the screenshot above. “Recommended” values depend on machine speed, sheet thickness, plasma power source and torch type.

"Alarm Arc Voltage difference..." and 
"Alarm Arc Voltage Rise" settings help to prevent the torch diving while
 moving through the cut, on low speed corners etc. 

However, if the reference voltage is very different from the real measured voltage 
(more than these values), Height control may not start at all. You may want 
to temporarily set these values to "1000" to disable this feature and test 
the THC.
THC PI-control ratios may vary depends on a sheet thickness 
and plasma power source type. 

The higher P-ratios absolute values, the higher Height Control Speed. 
According to our experience, I-ratios should be set to "-2...-6". 
Higher I-ratio values may lead to oscillation during height control. 

Both P- and I-ratios should be NEGATIVE for Arc Voltage Height Control. 
This means - 
The higher Arc Voltage (if compare with reference position), 
The Lower should be Position.

Check THC is ON

The next step is to check if THC is ON in user settings.

It can be a button in User settings of an item in PLC settings. Both settings actually are the same and represent the same variables.

Find a good Reference Voltage value

If you don't have a recommended Reference Arc Volatge value tables from power source manufacturer (Cur Charts), you may need to find a good reference arc voltage value. To find it

  1. Disable THC & IHC
  2. Set desired height from the sheet
  3. Make a test straight cut and check Arc Voltage during the cut (it is “96V” for the screen below).

Set the reference voltage value

Check THC working

Turn On back the THC in User settings and make test cutting.

  1. Check the “Arc Ref” voltage is about the same as the measured “Arc Voltage”



  2. Check “THC Low Speed” led is OFF



  3. “THC Speed” display should show the current Height Control Speed. The speed is in internal units, so absolute “THC Speed” value might not make sense, but you should see that -
    1. “THC Speed = 0” means THC is off



    2. “THC Speed” is positive - Height Control moves torch UP, usually Arc Voltage (and the current height) is lower than reference value



    3. “THC Speed” is negative - Height Control moves torch DOWN, usually Arc Voltage (and the current height) is higher than reference value



That's all about THC Height control troubleshooting. The article might be updated in case we get some customers feedback.

Registers to control THC

Variable Name Value Comments
THC #0
GVAR_THC0_ARC_VOLTAGE_REF 7011 Arc Voltage Reference value. This register is written while Load Cutcharts. It can be updated automatically if THC is activated in “Measure” mode. In this case, after THC activated and given “THC delay” controller measures real Arc Voltage, save this value as “Arc Reference” and keeps this value during the cut. Rotary encoder can be connected to this register to implement on-the-fly Arc Voltage adjustment
GVAR_THC0_ARC_VOLTAGE_ADJ 7012 Adjustment Value for Reference Arc. Either potentiometer or rotary encoder can be connected to this register to be able to adjust Arc Voltage reference on the fly VolategArc Voltage Reference value.
GVAR_THC0_ARC_VOLTAGE_REF2 7013 Register contains sum of Arc Voltage Reference register (#7011) and Adjustment register (#7012). The software automatically updates register value if any of register 7011,7012 is changed. The register is available for reading to display adjusted Arc Voltage reference value
thc/thc.txt · Last modified: 2018/08/16 13:52 by es