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
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,
- myCNC measures Arc voltage
- Compares Arc voltage with Reference value
- 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.
PLC builder window, Cutting ON PLC procedure.