THC problem

Hello Everyone,

We made a plasma cutting machine and now change the controll software to LinuxCNC.
It works good but I have some problem about THC.

I use Mesa 5i20 with Mesa THCAD 10V.
The plasma source is a Hypertherm Pmax 1650.

For the thc control I add the following parameters:
vel-tolerance: 5
volt-setting: 82 (from the user manual, finecut 40A, 3mm thickness steel, we measure it during cutting and it is good value)
volt-tolerance: 1

If I enable the THC the head is move only down and crashed into the plate.
I changed the presage of the thc.vel-scale to negative to look what happen and the head is rise up more than 100 mm from the plate.

I look in the show hall config - thc.zpos.in and it is very high value.

If I had disabled the THC the head was moved down. And after manuall moving the Z it is also make a correction move.

Have You got any idea what is wrong?

Thank You!

Regards,
Csaba

I would start by using hal watch to make sure the THC voltage is correct when the torch is working.
That is if the torch is cutting at 82 volts is the THC voltage in Hal also 82 volts?
This will confirm the voltage is read and the scale is correct.

Rick G

Ricks advice is good, check that the voltage is scaled correctly first.

One other thing missing that may or may not be important is that the encoder scale is never set.

It looks like you are assuming that encoder scale defaults to 1 and it may in fact have this default
but I dont see that specified anywhere in the hostmot2 documentation so I would set the encoder
position scale in the hal file.

Does the Hypertherm Pmax 1650 have different torch voltage divider settings, or are you using raw torch voltage and an external voltage divider?
The Mesa THCAD 10V expects a 0-10 volt input.

If all else is well and the voltage is incorrect the offset and scale of the voltage are set here in your plasma-5i20.hal

Rick G

Thank You for the answers!

I watched the encoder scale in the hal config. It is 1, it seems that the default scale parameter is 1.

The HT plasma source has an own arc voltage output with max 300VDC. We use resistances to reduce it max 10V for the Mesa THCAD 10V.

We have made some test cutting to check the thc.volts.
Material 5mm thick steel plate , 60 A - according to the HT manula the arc voltage is approx 135V.
During cutting I see the halconfig/ pin / THC / thc.volts and the thc.zpos-out.

Test 1: volt setting is 135V, Velocity tolerance 5, Volt tolerance 1
Result: head is crashed into the plate, thc.volts is changed betwen 100-115V, thc.zpos-out is below -30 after a 100mm cutting.

Test 2: volt setting is 100V, Velocity tolerance 5, Volt tolerance 1
Result: head is rised, thc.volts is changed betwen 120-136V,

Test 3: volt setting is 124V, Velocity tolerance 5, Volt tolerance 1
Result: head is crashed into the plate, thc.volts is changed betwen 124-135V, thc.zpos-out is below -30.


One more remark, if I uncheck the Enable THC function after the test 3, the Z is rized up with the value of the thc.zpos-out.



Regards,

Csaba

You probably have the THC scale/offset wrong

Note that if you use a negative THC scale the THC offset must also be negative
(or set the encoder scale to -1 and set thc scale and thc offset to positive numbers)

You can verify that you have the scaling right by monitoring
the thc components thc.volts pin while varying the input voltage.

The next thing is to verify that the correction motion direction is correct for
too high and too low thc voltages relative to the thc.volts-requested pin

As PCW said it looks like a problem with scale/offset and compensation direction.

Can you disconnect the voltage input to the THCAD from your plasma power supply and apply a know voltage such as a battery to the THCAD and confirm that the thc voltage in hal is correct?

Rick G

That is a good idea!
In this case we can check it independently of other parameters and influencing factors.

We will try it!

Thank You!

Csaba

Hi,

We have tried to give dc voltage form an adapter to the THCAD.
We set for volt setting 20V and also gave 20V form the adapter.

In hal pin the thv.volts is 0,65..V in the case of 0V input, and ~21V in the case of 20V input. So we have 0,2V differece as each 10V.
I think it is quite good, but have to adjusted.

The problem was that the z.pos.out was made only negative direction movement despite of we have changed the input voltage below or above 20V.

Csaba