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.

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 hte 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


ohmic.N (requires a floating-point thread)


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

0 volt calibration data for THCAD card in Hz

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

Full scale 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 Fullscale (5, 10 or 300)

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 (default 1:1)

ohmic.N.ohmic-threshold float in (default: 18)

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

ohmic.N.ohmic-low float in (default: 5)

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

ohmic.N.arc-on bit out

True if full scale (eg arc is on)

ohmic.N.thcad-volts float out

Measured thcad voltage

ohmic.N.ohmic-volts float out

Calculated Ohmic voltage

ohmic.N.ohmic-on bit out

Threshold plasma torch voltage

ohmic.N.is-probing bit in

True if probing


Rod Webster