Mesa Board with high tolerance for high frequency with 24v Step-gen?

Hello everyone,

I've been employing the 7i96S on various machines for some time now, but recently I've encountered a persistent problem on a particular machine. The step generators for X, Y, and Z abruptly cease functioning, and in some instances, have even burned out on the board. The issues didn't occur all at once but rather sequentially. During this process, I've inadvertently damaged two boards specific to a machine that runs on a MAX200 Hypertherm. I'm suspecting that interference from the pilot arc is still permeating the system in some way.

To clarify, I've established complete isolation of the Mesa board system from other electronics. Likewise, the I/O circuit is entirely isolated (my circuit triggers a separate circuit). Additionally, I've separated the power supplies and avoided grounding (I did attempt grounding initially, but the outcome was worse due to the 'dirty' interference-riddled ground).

Interestingly, I've never encountered this sort of issue with other plasma systems, only with this specific machine. Consequently, I'm considering shifting from the 7i96S to a more resilient model that features 24V-compatible step generators. These are likely to be more resistant to burnouts.

Given the hardware at my disposal, I could potentially utilize a 7i92TM with a parallel port China BoB, or a 7i92TM+7i77 configuration. However, I suspect this might result in a similar situation to using a 7i96S, as I'm unsure about the stepgen max voltages (which are 5V for 7i96S).

Are there any specific strategies or modifications that you all think I could implement? The machine functions fine for a few days, then the pilot arc subsequently terminates, effectively killing one of my stepgens. 

I'd truly appreciate any insights or suggestions you could provide. Thank you in advance!
 

I agree with tommylight. I think this is a grounding/shielding issue.

When the arc starts you can have transients in the 100s of volts
in the table ground. If this couples into the step/dir signals, it _will_ cause damage.

You can improve the EMI resistance of the step/dir outputs somewhat by

1. Using single ended drive (+5V and STEP-,DIR-)
2. Adding a say 0.01 uf capacitor from STEP-/DIR- to ground

This guide may be of interest:
www.hypertherm.com/Download?fileId=HYP103900&zip=False

Aciera wrote:

This guide may be of interest:
www.hypertherm.com/Download?fileId=HYP103900&zip=False

Be sure to implement everything in this guide. No shortcuts. If you use ohmic probing, use a decoupling capacitor where the probe wire enters the enclosure as its a very effective RFI antenna.

These machines are RFI beasts. I know of at least one commercial table with issues so don't feel too bad.

the machine's control boxes are full metal. every box is grounded.

The probe im using is a floating head with a NO/COM limit switch. I'll be changing that to a PNP proximity sensor soon.

I'm feeding the servo drives with +pulse +dir -pulse -dir, but i'll try to change to +5v , pul-, dir-.
Also, I'm not sure i understand the condenser strategy. I should place the condensers directly on the board, right? I tried to recreate PCW's idea with the picture below (on msPaint).

I will also be changing the cable that goes to the motor drivers, to something properly shielded all the way.

The connectors at my servo drives are 27 d-sub cables, which also have a ground ending. 


I have had situations where everything was grounded, the 5v psu was grounded, the 24v circuits were grounded, the board was sitting on a metal plate that was grounded, and i still had step-generators frying. 
Physically modifying the board with something that will protect it (like 0.01f capacitors?) is what i need to make sure this stops happening. Is the schematic correct?

Also, if my servo drives don't work with single ended step signals, how can i wire the capacitors?

I suspect the limit switches might also be acting as RFI antennas, but i'm not sure. it could also be the probe switch.

Thank you.


EDIT:::

I've done some research. My idea is to put a ceramic 0.01uF 50V capacitor at every step generator, at 5v and (gnd). 3 capacitors in the board.
Then, a 100uF 25v condenser at the 5v power supply (+) and (-), and another at the 24v input/output power (+) and (-), and another in mesa field power (+) and (-).

I am also thinking about placing a 100 ohm resistor in every pulse+, pulse-, dir+, dir- wire, creating a RC filter for each wire, with:
100 ohm, 10^-8 F.   the cutoff frequency is fc= 1/(2*pi*R*C) = 1/(2*pi*100*10^-8) , fc=160kHz
My steps are at a bit less than 2000, and a maxvel of 3000 mm/min... resulting in 100khz max frequency, so below the cutoff frequency of my RC filter.
Is this a good idea?!

For differential signals you would need a capacitor from each step/dir pin to ground
Are the servo drives mounted in the same box as the 7I96S?
 

which approach seems to be best? i added the picture now.
I also plan to add 100uf capacitors at the +- of each power supply.
also planning to add ferrite rings around every step generator cable

initially, the drives were in the same box. metal, over the gantry, grounded.
after some of these issues happening i moved it to another box, where the monitor and PC are.  also grounded.

 

the machine's control boxes are full metal. every box is grounded.

What about ground wires from the the motors back to the star ground point?
What about copper ground stake attached to the star ground point?
Read that Hypertherm document and ensure everything they say to do is done. You did not acknowledge this.