CNC plasma is frying servo controllers after adding ohmic sensing

[clay1599]

I did post this to another forum but so far have not received any responses, so...

I have an old plasma table, Aviator XLT that came with a Burny 1250 controller, Thermal Dynamics PakMaster 150XL and PCM-120 torch for the plasma. The Burny broke again and it was not feasible to repair so I updated the machine with new controls and servos. The CNC controller is a Masso G3 Touch with Automation Direct SureServo2 motors and drives. The unit uses a Masso DTHC for torch height control. The Masso is mounted on the outside of a metal enclosure that rides atop the gantry with the remainder of parts inside.

I do not know how the Burny was detecting the work surface while probing (maybe monitoring the Z axis servo amperage?) so I converted the torch to a floating head style with a limit switch to signal the Masso that the surface was reached. Because of the gantry design, this probing was not reliably accurate. However, I did make many test cuts with this setup that worked OK. The plasma worked, the machine worked, the DTHC worked; the only issue was unreliable probing. I always intended to use an ohmic sensing setup anyway so I moved on to adding a CNC4PC PTS-1 Plasma Touch Sensor. The machine style shield cup sits above the nozzle height so I bought a drag shield cup and ground down the prongs so that they were slightly lower than the nozzle. I tapped the shield cup and used a ring terminal to attach a wire to the cup that would feed back to the ohmic sensor's input. The shield cup is electrically isolated from the torch. The PTS-1 Ohmic sensing unit has a dry contact output that is opto isolated.

When I first tested the machine with the Ohmic sensing, the Z axis servo drive fried immediately when the plasma arced. I reviewed my setup and realized that I had placed the PTS-1 unit inside the control panel, even though the output from the unit was opto isolated, the signal wire from the torch shield was still going inside the cabinet. I thought this was the likely culprit so I moved the PTS-1 to the outside of the cabinet and tried again with a new servo drive. Upon the torch firing, the X axis servo driver was immediately fried.

Looking at the setup, I do not understand why this is happening. I did also realize that I believe the original setup on the table used the external shielding of the servo cables to bond the different axes together. The new setup uses internally shielded cables so there is no extra bonding of the axes. Could this be the problem? Is it likely that bonding the axes to the star ground would fix this issue? I do not want to experiment and toast another servo drive if possible.

I am clearly missing something, either the problem of no bonding was already an issue and it was a coincidence that the drives are frying after adding the Ohmic sensing, or something about the Ohmic sensing setup is to blame.

Thank you

[weldguy]

Very strange, frustrating, and costly issue, I wish I had the answer but I don’t. I believe your plasma is HF start. If so what do you have for an earth ground? Is there any chance the screw for the ring terminal on the shield is contacting anything behind the shield?

[Joe Jones]

I hope you find the cause of your troubles.

Joe

[SegoMan DeSigns]

Couple of questions:

Do you have a common ground buss bar tied tied an earthing rod?
Is the control cable internal ground bonded at the controller end only?
Does the gantry / z axis slider have a ground lead to the common ground buss bar the same size as the work lead from the plasma unit?
Is the work lead to new metal or buffed rusty metal?
Limiting resistors on input wiring going into the G-3?
Have you posted this on the Masso forum? Give me the link and Ill help bump it up for you.

[BTA Plasma]

You cannot use a PAKMASTER or any High Freq plasma cutter with OHMIC without a total OHMIC Isolation circuit. The cheap ohmic sensors will backfeed voltage and even worse could be a loop into your controller/PC. Hypertherm uses a THC isolation module for OHMIC on Maxpro/HPR/XPR. High Frequency plasma cutters dont care much for diode/opto isolation circuits if the energy density it too high by the time it gets there. It will literally jump the traces. Kind of like a surge protector and a lightning hit...so much plasma power into the ground the energy jumps isolation.

[tcaudle]

The ohmic sensor uses voltage across the shield an d the plate (material) That voltage HAS to be separate from and floating so it does not let torch voltage get back to the control grounds., In short it needs to have a voltage source like a battery that is not connected back to any other ground on the system.
I know you have worked to bond all of the grounds together but work clamp on a plasma is not "Ground" . It is positive in relation to the electrode. what ever you clip the work clamp to better be a really good low impedance conduit to ground because the plasma tries its best to inject its noise onto that connection. For every foot you are away from the service ground the higher the impedance and the more local noise is at the connection.Grounds act differently at higher frequencies than at 60 HZ ,
HF start amplifies the issue the galvanic separation of the ohmic supply has to be able to withstand the HF high voltage source. Opto are great for isolating signals going into the I.O parts of the control but a shared ground on the ohmic side negates that separation