Ohmic sensing with IPTM-60

 

Here is what a sensing cycle looks like under good conditions.
Green: Sensing output signal: 1 if contact, 0 of no contact
Cyan: Voltage between plasma tip and work piece
Purple: Enable ohmic sensing signal
Red: Z-axle position.

First there is a 15 ms reaction time for the relay to switch on, I call it setup time.
After the setup time has expired, the contact evaluation starts.
There are several conditions.
1. Voltage drop below a set voltage limit like 12 V
2. Voltage drop from voltage at end of setup, like 5V drop
3. Overall voltage drop over one cycle, like 5V
4. Overall voltage drop over three cycles, again like 5V.
For good conditions like then one in the picture all conditions triggers at the same time in principle.

With water on the table and too low short circuit current configured,
we get something like the orange curve. A damped curve with reduced "swing".
No sharp transitions between open and short circuit.
 

Of course I tried to fix this in software, therefor the four conditions to trigger a sensing contact
The right thing to do was to lower the R1 value and increase the short-circuit current.