Hypersensing questions

phillc54 wrote:

The real issue is water entering the torch and bridging between the two electrodes used for torch ignition. I did find that a 120 amp nozzle has a much bigger orifice so is much easier for water to enter.

Is this during probing or between cuts?

To tell you the truth I don't know as htat was before I isolated the issue.

phillc54 wrote:

rodw wrote:

But in my environment I manages over 1500 probe event as last week and it all worked ort allowed me to recover from any fault. In one instance, over 500 pierces unattended...

So there is no issue.

Not for me

@snowgoer, I guess it wold be possible to sample the voltage when is_probing goes true but I'm not sure the approach from there. To make sense of this for your machine you need to play with halscope for a while.

if you change the circuit to put a resistor between the probe and table
you can lower the sensitivity to water contamination

Note that this resistor should be able to handle a short to the plasma voltage
as I mention before a 10K 10W resistor would work and raise the sensing current to 2.4
mA (vs 50 uA for the THCAD5 directly)

Note the the 24V supply should be turned off when not probing to prevent electrolytic corrosion

snowgoer540 wrote:

Does what I’m saying about the resistance even remotely make sense?

Kinda but no. Any additional resistance must be counted (eg raw arc voltage with 2 x 100 k Ohm resistors or the everlast divider board which converts a 16:1 divider to a 24:. BUt what does not make sense is the low resistance you are measuring compared with Myselfg and Ulethane as we had numbers in the MHz which are reported in this thread somewhere.

Anyway off to have a play myself..

snowgoer540 wrote:

Forgive me if I'm noticing something everyone else is already aware of. So I have my shield grounded to earth. The interesting thing is that when i dip the tip into water, I can make the false ohmic trip go away by removing the powermax ground from the table. The ohmic goes back to working like normal then (I can trip it with my fingers as it makes about 5 volts).

I also have PM45XP and can confirm, removing the work clamp from the table makes ohmicsense.ohmic-volts go to 0v

My thoughts were that the ohmic wire (on the shielded cap), which is now shorted to the nozzle+electrodes, is making its way back to the star ground point, or wherever the -24V "table" connection is tied to, via the plasma cutter. Just a guess, but I don't see where else it could be traveling through to make this connection. Obviously we can't detach the work clamp or interrupt its connection, because it just goes back to 23,24v whenever it is attached again.

I am not sure how to solve this problem electrically (also not an EE, but an ME) but I'm not seeing that there's much of a difference between the ohmic sense wire finding ground via the material (good) vs. shorted through water and the torch and wherever it ends up (bad) besides how clean the signal is. I think more filtering or averaging would help on the input, but it depends on how slow is it going to make the trigger, which is what Phil was alluding to about probe overrun. Perhaps it means we have to probe slower or just allow more float travel.

In a separate vein, I've been thinking of a possible plan B for ohmic sensing and water tables. Continue as normal, but if a clogged torched is detected through a threshold voltage or average voltage per time outside of a probe event, default to float switch trigger. Maybe it would kick in if ohmic-volts was floating around 22-23v after a cut for X number of threads, but not <21v since there is enough headroom for a clean signal to be processed. I know it doesn't solve the issue, but perhaps makes it fall back quicker.

Plasmac already throws an error if ohmic is triggered before moving to probe height, but this would be error handling that kicks in before it knows plasmac is going to have an issue with it and default to float switch.

rodw wrote:

snowgoer540 wrote:

Does what I’m saying about the resistance even remotely make sense?

Kinda but no. Any additional resistance must be counted (eg raw arc voltage with 2 x 100 k Ohm resistors or the everlast divider board which converts a 16:1 divider to a 24:. BUt what does not make sense is the low resistance you are measuring compared with Myselfg and Ulethane as we had numbers in the MHz which are reported in this thread somewhere.

Unfortunately that conversation was in the other thread (getting difficult to keep track).

snowgoer540, can you mimic these measurements? This is what I measured the other day.

Well, I went to work on a Sunday to have a play for you guys. I have a Thermal Dynamics A120 120 amp machine.

So at the start, everything was nice and normal as everything was dry. Then I used my finger to splash some water into the nozzle and sure enough the volts went up to about 21.6 volts. This is the result of a probe event. I am really interested to see the behaviour when the contact is broken.




So what I'm seeing is a definite downward slope as the connection breaks. This is what I was expecting to see.
And when we zoom in



So it would be good if you guys could post similar screen dumps.

I'm still thinking about how best to proceed from here but clearly we can measure the ramp down as I expected.

I am wondering if the resting voltage could be sampled when is-probing goes true but I don't quite know what the next step would be.

I am assuming you guys are just seeing the 23.5 volts after the torch is wet and not on a dry machine. That is what I thought Uthayne said earlier. Can you confirm? All I hav eto do is blow some air through the torchand its clean again. But I have had to open the torch to dry it before.

snowgoer540 wrote:

PCW wrote:

if you change the circuit to put a resistor between the probe and table
you can lower the sensitivity to water contamination

Note that this resistor should be able to handle a short to the plasma voltage
as I mention before a 10K 10W resistor would work and raise the sensing current to 2.4
mA (vs 50 uA for the THCAD5 directly)

Note the the 24V supply should be turned off when not probing to prevent electrolytic corrosion

This resistor is in addition to the 390K 1/4 watt already there?

I'm not going there yet. Try this instead.Works here for me.:
Note the component is now named ohmic2.comp
you need to halcompile --install ohmic2.comp (non source code guys will need ot use sudo and you may need to install the linux_dev stuff. I'm not really up with deb installs.

Install as per my attached spaceship_ohmic.hal
There is a new mode pin to set the mode 0 = existing, 1 = new algorithm)
Also there is a pin called num_readings that determines how many readings it should average. I chose 5 to start with.

So this only uses the ohmic-threshold (high voltage)

After it triggers, It will wait until the average of the last 5 readings < ohmic_ threshold. Set this pin to suit your setup
If this works, its a cool bit of code

I'm keen to get some feedback here becasue I had another idea which I will look at once we have some feedback..

That is to add a mode 3 being auto mode. In this mode, every time we commence probing, we sample the voltage as we start probing. Then during probing we monitor the voltage and remember the maximum voltage value seen.

So then we could set the probing threshold to be say half way between the two values and use that on the next probe. The worst case if probing fails because water got in the nozzle, it would right itself on Plasmac's re-probe because we would have re-calibrated on the failed probe.

Let me know your thoughts...