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myCNC - advanced CNC control, software - THC - THC
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Frequently Asked Questions - THC


THC (Torch Height Control)

Warning: Complete THC configuration is necessary if new Plasma Cutting profile created. Easy way is to use some of default profile made for Plasma Cutting and edit it for your needs. This acticle is for advanced users.

To complete setup THC for Plasma Cutting you will need:

1. Connect Height sensor output to one of Ethernet controller ADC inputs

2. Go to THC configuration widget (Technology->THC tab).

3) Configure THC

a) check "THC enabled"
b) select ADC input that used to connect Height sensor.
c) "Arc voltage ref" is reference value that the THC system will try to keep by control Torch Height.
d) "THC PI-control P-ratio" and "THC PI control I-ratio" can be set separately for moving up and down. The ratios for "move down" are usually lower than "move up" to prevent torch diving.
e) Height sensing signal may be unstable just after piercing. It's possible to deactivate THC for given timeout after piercing by setting "THC start delay"s"
f) At finish of cutting a cut part may fall out  and height sensing may work incorrectly at the moment. To prevent Torch following falling part THC system can be deactivated just before cut is finished. Set "THC time pre-off" to enable this feature. Set "THC time pre-off,s" to 0 to disable it.

4) Enable THC in G-code program.

PLC controller should send a message to THC to start THC working. Normally a message to "START THC" is included in some of cutting ON commands (M02.plc, M20.plc, M71.plc C07.plc etc) and message to THC "STOP THC" is included in cutting OFF and program OFF commands (M05.plc, M21.plc, M74.plc, OFF.plc, M02.plc)

CNC register 0xa4 is used to start/stop THC control.
Write "1" to the register switch ON THC.
Write "0" to the register switch OFF THC.

Sample of code to start THC is shown below-

if (thc_enable!=0)
  //start THC control
  parameter=1; //Start Height sensing


Sample of code to stop THC is shown below-

if (thc_enable!=0)
  //stop THC control
  parameter=0; //Stop Height sensing
  texit=timer+2; do{timer++;}while(timer<texit);

5) Configure DRO for measured Arc voltage.

DRO definition as all screen configuration is situated in "cnc-screen.xml" configuration file of the profile. Arc voltage DRO definition is shown below.

<gitem K="0.075" where="thc-work" type="display" format="%3.1f"  address="adc-inputs" number="0" bgColor="black" fgColor="cyan"
height="30" displayWidth="180" labelWidth="120" fontStyle="bold" labelFont="14" orientation="horizontal"
<message>Arc Voltage, V</message>
<message_ru>Напряжение дуги, В</message_ru>

For given example we use ADC0 input as feedback channel for THC system. So we need to choose ADC0 current value as Arc voltage.

Attributes "address" and "number" define ADC input #0 to DRO.
Attribute "where" defines Widget name to insert the DRO.
Attribute "K" describes ratio to convert 12bit value (0...4095 range) to Volts. This ratio should be set according your plasma power source, voltage divider parameters, THC addon board input attenuation etc.
Attribute "format" defines C-like format to display Arc voltage.
Attributes "height", "displayWidth", "labelWidth" describe height and width for DRO and its label.
Attributes "bgColor" and "fgColor" define Background and Foreground Color for DRO.

Field "message" provides test message for Label.

6) Reference Arc voltage control.

To change Reference Arc voltage from main screen can be used spinbox-like item. The item configuration is shown below.

Attributes "type", "action" "name" define spinbox that show and change current state of CNC-variable #0xa3 (which is actually reference value for THC control). The rest attributes and fields are similar to DRO configuration.

<gitem K="0.075" where="thc-work" format="%3.1f"
type="kspinbox" action="cnc-variable-dec-0xa3;cnc-variable-inc-0xa3" name="display-cnc-variable-0xa3"
fgColor="cyan" bgColor="black"
labelWidth="120" displayWidth="60" height="60"
fontStyle="bold" labelFontSize="14"
<message>Arc Voltage Ref, V</message>
<message_ru>Опорное напряжение дуги, В</message_ru>

 7) Configured spinbox changes CNC variable 0xa3. The variable itself should be defined in cnc-variables.xml configuration file. While pressing "left"(decrement) and "right"(increment) buttons on the spinbox value of the CNC variable is changed according to increment/decrement value ("step" attribute) and range ("min", "max" attribute).

<value suffix="V" type="numpad" min="300" max="4090" step="2" name="cnc-variable-0xa3">1850</value>

If the variable defined inside of <item> section it will be show on Settings dialog as well. See screenshot below.

    <value type="numpad" min="300" max="4090" step="2" name="cnc-variable-0xa3">1850</value>
    <message>Arc Voltage Ref, ADC units</message>
    <message_ru>Установка напряжения</message_ru>
    <message_du>Boog Voltage Ref</message_du>
    <message_kr>ARC 볼트 기준</message_kr>
    <message_pl>Napięcie odniesienia łuku</message_pl>
THC Arc Volatge setup
THC Arc Volatge setup


Screenshot below shows both Arc Voltage DRO and Arc Votage reference spinbox.

THC Arc voltage DRO and Arc volateg Reference spinbox
THC Arc voltage DRO and Arc volateg Reference spinbox


8) Plasma Arc voltage is negative. Differential amplifier/attenuator can be used to convert Plasma Arc voltage to 0...5V rande. Ethernet control boards ET10, ET1(2016 revision) contains differential amplifier to accept signal up to 30V. If Plasma power source doesn't have built-in voltage diveder, some external divider 1:10 should be used.

ET5, ET1(old revision) ADC accept 0...5V signals, so external differential amplifier and voltage divider should be used. On a photo below is possible choice fro differential amplifier - THC add-on board.





THC (Torch Height Control) is sub-system that keeps constant distance between plasma torch and material sheet.

Most of Plasma Cutting THC use Arc voltage as feedback.

Plasma Arc voltage depends on

  • distance to material
  • material type
  • material thickness
  • plasma torch
  • torch consumables
  • others.

Considering stable plasma cutting process the only "Distance to material" is variable., other parameters are constants.

THC able to keep constant distance to material by control Arc voltage.

myCNC THC & IHC implementation.

MyCNC THC contains independent PI-controller. If THC is activated,

  1. myCNC measures Arc voltage
  2. Compares Arc voltage with Reference value
  3. Uses PI-control to drive Z axis and compensate the difference.

Driving Z axis implemented as variable "tool length" so no special Z axis programming need to use THC.

THC parameters can be set in "THC configuration dialog.

  • THC enable - enable/disable THC.
  • THC feedback channel - ADC channel number that used for THC control.
  • Arc->Voltage ratio - ratio to recalculate ADC value to real Arc Voltage
  • Alarm Move Up - Voltage difference value range that detected as system error.
  • THC Low Speed.
    Machine adds deceleration-acceleration on corners.If cutting speed is lower, cut width become wider and effective Plasma Arc Length is longer, Arc Voltage increased. Plasma Torch might go down if THC tries to keep Arc Voltage on corners while cutting speed is low. That's why temporarily disable THC while cutting spped is low is very important option for modern THC systems.
    If current speed is lower than "THC Low Speed", THC is temporarily disabled.
  • Hi/Lo event output port. Port number #63 is used to disable built-in THC on corners. In case 3rd party THC used, it's possible to assign any other output to disable external THC.
  • Arc Voltage Ref - Reference Arc Voltage value (in ADC units)
    There are 2 scenarios for Reference Arc Voltage
    1) Predefined Reference Arc Voltage value can be used (or loaded from Cutcharts)
    2) Machine starts cutting on "Cutting height" set by IHC procedure. After "THC start delay" THC measure current Arc Voltage and uses this value as Reference value. During electrode/nozzle lifetime Cutting parameters are changing and Cutting height might "float" on the same Arc Voltage. This method may have some advantages and helps to keep constant cutting height during full lconsumables lifetime cycle.

  • THC PI-control ratios - Separate PI-control ratios (proportional, integral) for motion up and down to fine tuning PI controller.
    In many cases there can be different behaviour of THC while lift up and down. If gun comes closer to material (arc voltage goes low), THC should act quite quickly to prevent torch damange.
    If arc voltage goes high, it may be because distance between torch and material goes up OR because torch comes along corner and cut become wider. PI ratios for moving down may be lower and slower (Low fast P-ratio, high slow I-ratio) to have lower down speed and extra prevent torch plunging on corners.
    First P/I value in the dialog is for positive arc voltage difference ( voltage high, move down), second value is for negative arc voltage difference ( voltage low, move up). It's recommended to set similar values to both ratios (up & down), then advanced users may play with ratios for fine tuning.

  • THC start delay. Right after piercing Plasma Arc and its voltage can be unstable. It's useful to start THC with 1-2 sec delay after pierce finished and cutting motionstarted.

  • THC pre-off. Small parts may fell down at the end of cut. If the falling happens while cut process is not finished, Torch may follow falling part and move down, leaded by THC. Switching OFF THC 0.3...2 seconds before cut finished might help with this error and save torch gun.


THC system should be activated/deactivated in "Cutting-ON" "Cuttning-OFF" PLC procedures. All PLC procedures are flexible and available for modification/customisation in PLC Builder. We offer "P1024" profile as a basic template for Plasma Cutting. Beside of THC ON/OFF handler PLC procedures contains a complete IHC (Initial Height Control) procedure handler. Cutting-ON procedure controls Initial probing, move to ignition height, pierce height, cutting height and all the delays.

IHC procedure described on a picture below.

A- Material probing (Voltage short circuit sensor or pneumatic valve activated probe sensor can be used)
B- Move up to Ignition height, Ignition
C- Move up to Pierce Height, Pierce Start
D- Move down to Cutting height
F- Start THC control
E- Ignition Height
F- Pierce Height
G- Cutting Height
H- Pierce Time
I- THC start delay

All the parameters can be set in Settings dialog or User dialog


 myCNC has Cutcharts for Hypertherm, Thermadyne, Kjellberg power sources. IHC and THC parameters can be loaded directly from the cutcharts.

myCNC cutcharts support for plasma cutting


CNC software, load cutchart process


PLC builder window, Cutting ON PLC procedure.

myCNC software, PLC builder, Cutting ON, IHC, THC start procedure



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