NAME

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

SYNOPSIS

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

DESCRIPTION

       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.

       EXAMPLE:
       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

FUNCTIONS

       ohmic.N (requires a floating-point thread)

PINS

       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

AUTHOR

       Rod Webster

LICENSE

       GPL