Hypersensing with THCAD - better way to do ohmic sensing

Initially, I had a single threshold but then I did some testing of the opto isolated relays I was using originally to see the hysteresis. They turned on way below their specified minimum voltage (at about 8 volts) and went off at about 5 volts. I thought the low on threshold and narrow hysterisis might cause reliability issues. We figured that by adding more hysteresis would be desirable and maybe prevent false triggering. But the intent was always to provide the ability to adjust the sensitivity by changing the voltage threshold to suit different environments.

The original experiment was based on a premise by PCW that the THCAD might be able to measure the resistance in the circuit. Which it does becasue we can see the voltage change as contact is made.

For the record, using the high values to deal with the water table proved very reliable. There is well over 1500 pierces here I did last week.


The biggest issue I had was warpage forcing a probe fault as the plate was above probe height but Phill has shared a revised plasmac.comp with a changed algorithm to deal with this.

So now we have had some success, I will revise the component and the docs when I get a chance.

I wonder with the feedback from water table users that the algorithm is not changed to be more like:
1. Set a voltage threshold (say 23.8 volts) that must be exceeded to trigger
2. Set a number of servo cycles the voltage signal must remain a constant state before the ohmic-on signal (say 3 servo cycles)

So then when probing the voltage must exceed 23.8 volts and be trending upwards for 3 servo cycles before turning on.
And then when it fell below that threshold and was trending down for 3 consecutive servo cycles we turn it off.

By looking at the trend, it would be more responsive than just waiting for a number of servo cycles after the threshold was crossed.

Let me know your thoughts guys.

You do need to be aware that this will cause an increase in overrun.

Yeh I know. Accuracy is determined when we turn off the signal.hence the idea to track the trend. I still have to think it through a bit but if it is trending down, we might be more responsive than say a config that turns on at 22 volts and off at 1 volt which is what I had at one stage. (eg. The hysteresis consumes servo cycles anyway.)

I think another variable will be that as you do more cutting there may be more suspended metal particles in the water and that may change the resistance.

phillc54 wrote: I think another variable will be that as you do more cutting there may be more suspended metal particles in the water and that may change the resistance.

Possibly but the metal should sink to the bottom and the commercial additives should buffer pH etc to a fairly constant value. But it was always acknowledged that different environments would probably need different thresholds. I seem to remember that Islander261 got you to add the pulse function so he could get a preflow of air before his first pierce to combat this. I personally think I would have this problem with a relay config as the 24 volt relays I was using tested out as being 8 volts on and 5 volts off so the probe would stay on anyway. I did notice earlier before I understood the issue that the second probe would often fail unless the air was flowing...

At least now we can compensate for the issue in software... With the right algorithm...