Problem- THC signal filtration

[margor]

Hello.
I am working on THC for my plasma ruter, based on PLC, but I have a lot of troubles with signal from plasma. So: does any body could lell me what kind of filter should be used between output (arc voltage) and analog input on PLC?

best regards

[lamicron]

Check this file I found, thanks to jarl_rugludallur that posted it in CandCNCSupport forum today.

Here are a couple of things I have learned over the last few years,
please add comments / tips from things you have learned.

00. HF/Plasma Grounding is all about inductance, not resistance. Make
sure you understand the difference.

01. The best conductors/wires for interference grounding are those
with the most surface area spread furthest apart(low inductance @
Radio Frequencies), flat copper strips, braided copper strips and
large diameter copper/aluminum pipes are typically the best.

02. Don't use aluminum conductors underground, above ground they are
almost equal to copper in inductance to higher frequencies and are
therefor very suitable.

03. Shielded cabling should only be tied to ground at one point,
either side or center. If possible try terminating all shielding at
the common ground point.

04. You should not be able to measure any significant conductivity
between the PC Case (Ground) and your Cutting Table/Plasma cutter
Chassis, resistance should be near infinite (a very high ohm value if any)

05. A cheap way to test your ground is to hook up a light bulb to the
ground and one phase (115v/230v), make sure you hook up to the phase
before a residual current circuit breaker, otherwise you will trip it
since this test will create an unbalance between phase/neutral.

06. Copper tubing and cladding is typically much cheaper than wiring
and is often much better at conducting high frequencies.

07. Don't put ferrite beads on grounding wires except if you don't
want them to conduct High Frequencies, make sure to check if your
power cables have molded beads on them and remember that they include
the ground wires.

08. I recommend adding "ferrite beads" to all leads from the plasma to
the sensor card except the ground (chassis), wrap the wires a couple
of times around the ferrite depending on size of ferrites and wires.

09. I recommend adding "ferrite beads" on all signal/power leads into
a computer/THC, this includes all home/limit switches, parallel,
serial, network and AC but don't put one on your ground connection
except if you are absolutely forced to use the same ground path for
both the plasma cutter and computer (try to avoid sharing ground for
them at all cost!)

[margor]

Thanks a lot!!! I hope it help.

[Torchhead]

The only true way is to have the analog signal from the Plasma be isolated (analog isolation) . Most commercial PLC's don;t offer that. Vaporizing metal at 30,000 degs with a high current arc creates LOTS of noise. Even WITH good grounding it can be a challenge to read the actual DC voltage and block the noise. Then you have the HF start and High voltage start systems that will burn up downsteam analog systems.

The post is correct in most aspects. Sometimes the best ground is no ground at all. Keeping galvanic separation keeps marginal grounding or potential ground loops to a minimum. even engineers argue among them selves about which is better: One common ground or isolated circuits with separate grounds. It is indeed all about inductance and impedence. Even short wires under heavy current become transmitters of noise.

TOM CAUDLE
www.CandCNC.com

[Big John T]

Originally Posted by margor Hello. I am working on THC for my plasma ruter, based on PLC, but I have a lot of troubles with signal from plasma. So: does any body could lell me what kind of filter should be used between output (arc voltage) and analog input on PLC? best regards

I would like to build my own THC too. Most of the answers on this board seem to be "you can't do that..." I'm guessing that you are scaling the voltage down to 0-10 or something like that for the analog input of your plc. What are you using to reduce the voltage? In your plc you should be able to use a PID circuit or other means to ignore the spikes.

John

[jimcolt]

Hypertherm Torch height controls utilize a voltage divider board that also incorporates filtering and analog type isolation....as well as a high voltage (15kv) isolation relay. The voltage divider divides the actual arc voltage as much as 50:1 (remember with high frequency start plasma torches there can be open circuit voltage before the torch fires...as high as 400 VDC), and filters out as much of the noise created by the arc attaching to the plate as possible. The isolation relay is open during arc starting (high frequency) to minimize damage to components that could be caused by the high frequency voltage.

Jim