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 THCAD2, THCAD5, THCAD10, and THCAD300 cards.
Allows user configurable voltage thresholds for ohmic sensing.

Output pins are provided for:
ohmic-volts -the voltage sensed on ohmic sensing.
thcad-volts -the actual voltage measured by the THCAD.
ohmic-on -true if ohmic-volts >= ohmic-threshold, false if ohmic-volts <= ohmic-low.

A THCAD-5 would often be used for ohmic sensing in conjunction with a 24 Volt isolated power supply and a 390 kΩ series resistor resulting in a voltage divider of 4.9.
This would result in a full scale reading of 24.5 Volts which is above the power supply output voltage.
The circuit will remain protected by the THCAD's ability to tolerate a 500 Volt over-voltage indefinitely.
It is optional that power to the ohmic sensing circuit be disconnected unless probing is in progress.

True if probing
The threshold volts below which ohmic sensing is set to be false
The threshold volts above which ohmic sensing is set to be true
0 volt calibration data for THCAD card in Hz
THCAD divider set by links on THCAD board (1, 32, 64, or 128)
THCAD full scale in Volt (5, 10, or 300 Volt)
Full scale calibration data for THCAD Card in Hz
The velocity returned from the THCAD and read by the Mesa encoder input
The divide ratio
True if ohmic circuit is closed (material is sensed)
Calculated ohmic voltage

The below HAL example assumes a THCAD5 card using a 1/32 frequency setting and a voltage divider internal to the plasma cutter with range extended to 24.5 volts by a series 390K external resistor as per the manual. Additional information and wiring diagram is contained in the Plasma Primer in the LinuxCNC documentation.
Example Calibration Data: 0V = 122.9 kHz, 10V = 925.7 kHz should be entered as 122900 and 925700.

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 4.9
setp ohmicsense.threshold 22
setp ohmicsense.ohmic-low 21
net ohmic-vel ohmicsense.velocity-in <= hm2_7i76e.0.encoder.00.velocity
net ohmic-enable ohmicsense.is_probing <= plasmac.ohmic-enable
net ohmic-true ohmicsense.ohmic-on => plasmac.ohmic-probe

Rod Webster

GPL

2024-07-13 LinuxCNC Documentation