DOKK / manpages / debian 12 / linuxcnc-uspace / ohmic.9.en
OHMIC(9) HAL Component OHMIC(9)

ohmic - LinuxCNC HAL component that uses a Mesa THCAD for Ohmic sensing

loadrt ohmic [count=N|names=name1[,name2...]]

Mesa THCAD Card component to scale input and outputs from the Mesa THCAD5, THCAD10 and THCAD300 cards. Which is designed to allow user configurable voltage threshold for ohmic sensing. Scaling of the Plasma arc voltage by a voltage divider is supported.
Output pins are provided for:
ohmic-volts (the voltage sensed on ohmic sensing)
ohmic-on (true if ohmic-volts >= ohmic-threshold)
arc-on (true if arc voltage is received eg. full scale reached or exceeded)
Actual voltage as read from the THCAD card (0-300V, 0-10V or 0-5V depending on the THCAD version used.

Normally, we would use a THCAD-5 for ohmic sensing in conjunction with a 24 volt isolated power supply and a 390K resistor. (voltage divider = 4.9) This would result in a full scale reading of 24.5 volts which is above the power supply output voltage.
So if full scale is reached, it can be assumed that the THCAD-5 is sensing an arc voltage. In this case, the circuit will remain protected by the THCAD's ability to tolerate a 500V overvoltage indefinitely.
It is optional that power to the Ohmic sensing circuit be disconnected unless probing is in progress ut this adds additional complexity.

0 volt calibration data for THCAD card in Hz
Full scale calibration data for THCAD Card in Hz
THCAD Divider set by links on THCAD board (1,32,64 or 128
THCAD Fullscale (5, 10 or 300)
The velocity returned from the THCAD and read by the Mesa encoder input
The divide ratio (default 1:1)
The threshold volts above which Ohmic sensing is set to be true
The threshold volts below which Ohmic sensing is set to be false
True if full scale (eg arc is on)
Measured thcad voltage
Calculated Ohmic voltage
Threshold plasma torch voltage
True if probing

THCAD5 card using a 1/32 frequency setting and a voltage divider internal to the plasma cutter with range extended to 24.5 volts with a 390K external resistor as per the manual. Additional information and wiring diagram is contained in the Plasma Primer in the main Linuxcnc documents.

loadrt ohmic names=ohmicsense
addf ohmicsense servo-thread
setp ohmicsense.thcad-0-volt-freq 122900
setp ohmicsense.thcad-max-volt-freq 925700
setp ohmicsense.thcad-divide 32
setp ohmicsense.thcad-fullscale 5
setp ohmicsense.volt-divider 32
setp ohmicsense.threshold 22
setp ohmicsense.ohmic-low 1
net ohmic-vel ohmicsense.velocity-in <= hm2_7i76e.0.encoder.00.velocity
net ohmic-true ohmicsense.ohmic-on => plasmac.ohmic-probe

Rod Webster

GPL

2023-02-10 LinuxCNC Documentation