Plasma CNC and THC (torch height control) with PlanetCNC

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Plasma CNC cutting machine is one of the most commonly used types of CNC machine.
It’s used mostly for cutting Steel, aluminium and other conductive materials.

Like any other type of CNC machine, plasma machine has it own specifics. Machine should be properly designed for such type of machining, considering slag, steel dust, high temperatures, moisture, grease etc..

Plasma cutter is a great source of electromagnetic interference. So special care should be taken into account when designing and wiring machine electronics.
Be sure to use shielded cables for motors, limit switches. External power supply for motion controller is very recommended. Plasma cutters itself sometimes do not satisfy standards of electromagnetic compatibility (EMC), so make sure that your plasma cutter is within EMC regulations.

Heat emitted from plasma could deform workpiece material, usually metal sheet. In such case plasma cutter would not cut at constant height which could result as a faulty workpiece or could damage plasma cutter in the process. This is usually solved with THC device. THC stands for Torch Height Control.

For your plasma CNC application you can use PlanetCNC motion control system together with PlanetCNC torch height controller.

Torch height controller consists of torch height controller and torch height sensor device.

Torch height sensor device measures arc voltage. Bigger the distance between the plasma torch and material higher the voltage and vice versa. So based on the measured voltage value we can maintain constant height of plasma torch above the material.

Torch height sensor device is connected with torch height controller via optic cable. Torch height controller then sends control signals (Up, Down, Arc OK) to motion controller which then dynamically compensates Z axis and thus maintains constant cutting height.

 

Features and specifications:

Torch height sensor device:
• Plasma arc voltage measurement up to 350 V
• Additional input for divided plasma voltage (1:50) up to 10 V
• Plasma voltage present signalization
• Full optical output isolation; transmitter and receiver connected over an optical fiber
cable
• Main AC power supply (110 VAC-230 VAC/50-60 Hz)

Torch height controller:
• Output signals: Arc Ok, Up and Down (normally-open solid-state relay outputs)
• Programmable reference and hysteresis voltage, delay time and anti-dive limit voltage
• Current plasma voltage presentation on LED display
• LED signalization of arc present, up and down signals
• Full optical input isolation; transmitter and receiver connected over an optical cable
• Test mode operation
• Wide power supply voltage range (6 – 36 V)

 
Torch height controller terminal description:

 
Torch height sensor terminal description:

 
 

Connection diagrams

High voltage source from plasma cutter:

 
 

Divided voltage output source from plasma cutter:

 

 

Torch height controller connection with PlanetCNC controller inputs:

Please note that Down, Up, Arc OK signals can be connected to any input of PlanetCNC controller since inputs function can be configured later in PlanetCNC TNG settings.

 

Controller input configuration in PlanetCNC TNG software

User can configure controller inputs for THC under File/Settings/THC.

Axis:
User selects machine axis from drop down menu. Selected axis will be height compensated.

Dec Pin:
Dec input pin number, located at INPUT header of controller. Select input pin number from drop down menu.
When this input pin is active, machine will move/compensate in negative direction.
Down signal from torch height controller is connected to this input.

Inc Pin:
Inc input pin number, located at INPUT header of controller. Select input pin number from drop down menu.
When this input pin is active, machine will move/compensate in positive direction.
Up signal from torch height controller is connected to this input.

OK Pin:
OK input pin number, located at INPUT header of controller. Select input pin number from drop down menu.
If set, software waits until this input is active and only then begins with program execution.
Arc OK signal from torch height controller is connected to this input.

Range Min/Max:
Low and high limit compensation travel.

Speed:
Speed of compensation moves. Units are mm/s or inch/s.

When torch height controller will activate its output, corresponding digital input of controller should activate.
You can observe status of controller inputs via IO tab of PlanetCNC TNG state panel.

 

THC related g-codes

THC compensation can be enabled or disabled, as also additional gcode parameters can be used so that machine behaviour will suit your plasma cutting requirements for better cutting results.

M54 – Enable/Disable THC
Usage: M54 P <Q> <R> – P = zero for Off, otherwise On
– Q = speed limit
– R = constant speed
If speed limit is set then THC will not move Z axis if machine speed is below set speed.
If constant speed is enabled then THC will not move Z axis if machine is accelerating or decelerating.

Example:
M54 P1 Q1000 R1 – enable THC for constant speeds (no acceleration) above 1000.