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.


ohmic.N (requires a floating-point thread)


ohmic.N.is-probing bit in

True if probing

ohmic.N.ohmic-low float in (default: 21)

The threshold volts below which ohmic sensing is set to be false

ohmic.N.ohmic-threshold float in (default: 22)

The threshold volts above which ohmic sensing is set to be true

ohmic.N.thcad-0-volt-freq float in

0 volt calibration data for THCAD card in Hz

ohmic.N.thcad-divide float in (default: 32)

THCAD divider set by links on THCAD board (1, 32, 64, or 128)

ohmic.N.thcad-fullscale float in (default: 5)

THCAD full scale in Volt (5, 10, or 300 Volt)

ohmic.N.thcad-max-volt-freq float in

Full scale calibration data for THCAD Card in Hz

ohmic.N.velocity-in float in

The velocity returned from the THCAD and read by the Mesa encoder input

ohmic.N.volt-divider float in (default: 4.9)

The divide ratio

ohmic.N.ohmic-on bit out

True if ohmic circuit is closed (material is sensed)

ohmic.N.ohmic-volts float out

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