[B]HOW DTHC (THC/AVC/DTHC) WORKS[/B]
Automatic Torch Height Control (often called just THC) works by reading the Arc Gap voltage while cutting. Plasma uses constant current cutting.
If the Current stays constant and you vary the gap (either by moving the torch or moving the material UP or DOWN) then the voltage will change in
proportion to the change in arc gap. Much like the altimeter on a plane (that measures barometric pressure to determine altitude) the Arc voltage
indicates the RELATIVE distance from the end (tip) of the nozzle to the top of the material. The change in voltage for a change in height is a
small percentage of the overall cutting voltage. A 1% change in voltage (100 to 101 volts) is equal to several thousands (typically .025 or more)
of arc gap change so the THC must be able to see and act on a small change in a large number. The THC control must take the actual cut voltage
and compare to to a preset “target” and move the Torch Up or Down to try and correct the height based on it’s arc voltage. The process forms a
“servo loop” where an “error” voltage from a preset is used to physically move the torch Up or Down to “correct” the error. Under normal cutting
conditions the voltage stays constant but certain conditions that effect the arc gap voltage can skew the gap volts and case the THC circuit to
overreact. The feedrate (how fast the tip is moving across the material) determines the current density and the Gap Volts. A slower feedrate will
cause an increase in Gap volts (if no THC servo is there to correct). With THC engaged the circuit will sense the higher voltage and based on the
error created lower the torch to try and correct. This condition appears when a CNC machine has to slow it’s feedrate to make a sharp turn or to
cut small detail. The DTHC has added features that sense a non- standard change in arc volts and locks downward movement to prevent “head bounce”
[LIST]
[*] Arc Gap = Arc volts
[*] 1volt (change) = approx .025”(change)
[*] >Arc volts = > Arc Gap. (greater the Arc Gap the greater the Arc Volts)
[*] Z moves opposite Arc Volts based on Preset Volts. Torch Volts Above Preset LOWERS torch; Torch Volts below Preset RAISES torch.
[*] Control has “window” (Span Volts) where no UP or DOWN occurs. (prefect cut height) Anything inside the Span (+ or -) from the Preset generates NO change. SPAN VOLTS is set in 1/4V increments in the Cut Profile and stored in the DTHC memory.
[/LIST]