Plasma THC



The plasma arc signal is used to control the height of the Z axis.

The signal is a high voltage (approx. 300V and during ignition even several 1000 volts), it is brought back to a safe voltage using a 1:50 plasma voltage divider. This divider is already built into the plasma current source or can be obtained separately from the plasma current source supplier. The signal from the 1:50 voltage divider is connected to analog input 5, see the drawing above.

The plasma is started using M3, which switches the tool output ON, this can be connected with a relay to the plasma current source. The plasma is on if an output from the plasma source to an input of the CPU indicates that the plasma is ON. This input is optional. The corner output from the CPU is a signal that becomes active when the CNC is reducing velocity in corners. This signal can be used by the plasma source if supported to reduce the plasma current.

With 1:50 the voltage is brought back from 300V to approx. 6 volt. The interesting work area for the THC lies between 60 and 200 volts. The input of the CPU in handle max 10V. The processor can read values for 0-1000 increments (10-bit).

To make full use of the 10-bit resolution the range can be adjusted to the working area for the THC. The 60-200 volts are reduced to 1.2 – 4 volts. So we want that the processor reads 0 when the input is 1.2 Volt and read approx. 1000 when the input is 4 Volts. This can be achieved by adjusting the 3 potentiometers on the iCNV600 board.

GAIN Analog in 5

This potentiometer defines the range of the input voltage, it can be set such that the max processor readout is approximately 1000 from about 4V to 10V. For the plasma, we want a range of approx. 200V

Lower Limit analog in 5

This sets the low voltage where the processor reads 0.

Upper limit analog in 5

This sets the voltage where the processor reads the maximum value, approximately 1000.

For the adjustment, a variable voltage supply is needed. The Eding CNC software is started and the IO page is selected to view the analog value.

Step1

• Apply 4 volts to the analog input 5.

Step 2

• Turn the lower (marked with L) limit potentiometer counterclockwise until the value read no longer changes. Now the lower limit is set to 0Volt.

Step3

• Turn the upper limit potentiometer (marked with U) counterclockwise until the analog value read no longer changes. Now the upper limit is disabled.

Step 4

• Adjust the gain potentiometer until the analog 5 value read is just at its max (approx. 1000) and does not change anymore.
• When varying the input now between 0-4V you should see the value in the screen vary between 0 and approx. 1000. So now input=0 ==> analog value 5 on IO screen = 0. And when input = 2V ==> analog value 5 is about 500 With 4V input ==> analog value 5 shows approx. 1000

Step 5

• Apply 1.2 volts after the 1:50 divider is our lower arc voltage working value and resembles 60V plasma voltage.
• Turn the lower limit potentiometer clockwise until the value reads 0 volts.

Step 6

• Apply 4 V to the input, this is our max working voltage and resembles 200V plasma voltage.
• Turn the upper limit potentiometer such that the value reaches near 1000. This is all about the calibration of the analog input with the potentiometers on the board. We have now mapped a plasma voltage from 60-200V after the 1:50 divider 1.2-4V to the full scale of the processor analog input (0-1023). This will give the optimum THC performance.

The software also must know the mapping of the 0-1023 input to arc-voltage. The software uses an offset and multiplication factor to transform the analog 5 value read to a representation of the plasma arc voltage. VoltPlasmaOnDisplay = voltInput * adcMulFactor + adcOffset The adcOffset = 60, because at 60V the ADC input value is zero. The adcMulfactor = VoltPlasmaOnDisplayRange / InputRange = (200 - 60) /1000 = 0.14 If your requirement of working arc voltage is different from 60-200V, apply the steps above with your values.

Step 7: Final check

• Apply the adcOffset 60 and AdcMulFactor 0.14 in cnc.ini under [PLASMA]
• Set the machine setting to isPlasma in the setup.
• Turn the input voltage between 0 and 4V and see that the Arc Voltage on the Operate Page displays approx. 60-200 Volt. If this is the case, the 3 potentiometers on the board are calibrated correctly.

Step 8

Read the plasma manual for setting up the plasma THC-related parameters in the software.