Its been a while since I did an update here. Our LinuxCNC Plasmac config is jumping ahead in leaps and bounds. I've had the parts laying around for years and never made my scribe mechanism but I think I better get onto it soon because scribes are now supported in Plasmac. This is done a little bit differently then in other systems becasue LinuxCNC supports multiple spindles so its just a matter of loading a scribe tool from the tool table that applies the tool offset from the torch by using standard tool change M6 commands and then specifying the spindle to use in the M3 command.
Plasmac has also gained inbuilt support for hole cutting where overburn and velocity slowdown is supported.
For guys cutting thick material, there is also a pierce only mode so you can do all pierces in the job and clean the dross off the plate before cutting out the parts. Stefan has used this to cut full sheets of 16mm and it all worked perfect! Here is a simulated demo of this in action
While this has been going on, I've been working with Peter Wallace from Mesa Electronics and today I finally got some real cutting done using a Mesa THCAD-5 card for Ohmic sensing. Peter wrote some new firmware for my Mesa 7i76e board so it can support up to three THCAD boards so I have one to sense the arc voltage and another for ohmic sensing. This is managed by a component I wrote that runs on LinuxCNC's servo thread 1000 times a second.
So here is the hardware:
Parts from the right of the unconnected blue relay in order:
24 volt power supply with 500 volt isolation
390 K scaling resistor to give a full scale of 24.5 volts (overkill mounted using a Mesa BUS strip that has 2 segments commoned together.)
THCAD-5 with 5 volt range (these can handle 500 volts overvoltage indefinitely)
Earth clamp for THCAD ground to table via enclosure bolted to it.
This circuit is powered up when the control box turns on it is powered all the time
So with this setup, everything in the ohmic circuit is 100% isolated so we don't even have to worry about protecting the circuit from arc voltages. I will say it took a long time for Peter to convince me of this!
So when I did some testing I found that the ohmic voltage rose from 0 volts to 24 volts over Z axis travel of 0.04mm and the 24V opto isolated relay I have been using turned on at 7.5 volts and goes off at 5 volts. This is likely too sensitive in some situations. So with the THCAD actually reading the voltage as the torch shield comes in contact with the material, I decided to enable a probe signal once the voltage exceeded an arbitrary 18 volts.
As an added bonus, I decided to add what might well prove to be a totally useless feature but I knew the THCAD can read a bit more than its full scale so by having a 24.5 volt full scale range, we can turn on a pin if the voltage exceeds 24.5 volts. Becasue we are using a 24 volt power supply, the only way that can happen is if the torch is actually cutting! So we have this cool and very wanky indicator we can use if we want to.
So there was only one way to see if this would work so I unleashed it on a job with 110 pierces and it worked flawlessly!
Even more amazing is I did not blow anything up as the circuit was powered for the full job!
The next stage of development for this is to modify the component so it turns on say somewhere between 18-22 volts and stays on until the voltage falls to say 1-5 volts so there is a big range of sensor hysteresis for nice stable sensing. I will add some GUI controls so we can vary the sensing sensitivity for different conditions. It will be interesting to see how this will work with a water table. That is the next step.
So becasue this circuit does not rely on any resistors and has such fine control over material sensing, I have decided to name this hyper sensing (which one wag suggested was ohmic sensing on steroids!)
One thing we heard was that with a water table, leaving the sense circuit powered all of the time could result in corroded consumables. Does anybody have any knowledge about that?