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Rod's Spaceship Scratch Built Plasma Table

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Re: Rod's Spaceship Scratch Built Plasma Table

Postby Rodw » Sat Aug 05, 2017 4:54 pm

sphurley wrote:The divider needs some noise filtering in it or you will get false voltage readings. It should also include some surge protection for your low voltage equipment.

Steve thanks,the Mesa THCAD card used to measure the voltage contains surge protection and is specifically designed for plasma torch voltage reading. See Torch voltage is converted to a frequency that is sent to the Mesa 7i76e encoder input where it can be read by LinuxCNC in real time (1000 times a second). For testing, I have been logging the data with a FIFO buffer byte like this:

Code: Select all

X coord    Y coord     Z coord   Volts     THCAD freq
263.000000 380.000000 -55.234143 81.969943 10266.940452
263.000000 380.000000 -55.237488 84.435688 10460.251046
263.000000 380.000000 -55.240757 83.754051 10406.811731
263.000000 380.000000 -55.243952 82.781669 10330.578512
263.000000 380.000000 -55.247072 83.741494 10405.827263
263.000000 380.000000 -55.250117 83.004798 10348.071496
263.000000 380.000000 -55.253087 82.104535 10277.492292
263.000000 380.000000 -55.255982 81.969943 10266.940452
263.000000 380.000000 -55.258802 82.509978 10309.278351
263.000000 380.000000 -55.261547 82.917936 10341.261634
263.000000 380.000000 -55.264217 81.567819 10235.414534
263.000000 380.000000 -55.266812 81.835627 10256.410256
263.000000 380.000000 -55.269332 82.756924 10328.638498

Each THCAD card comes with its own calibration data which can be used to convert the frequency back to a voltage (shown in the above log)
Its a shame I can't log the multimeter output, but from observation, it reads higher than the software during a run once the torch gets to a constant cutting height. The THC is not enabled but you can see that the Z axis is moving in this excerpt.

Filtering will be done in software using LinuxCNC's lowpass filter. See

I've written a custom component to do the calculations that convert the recorded frequency back to voltage. I've embedded the lowpass filter code into this component but its disabled at the moment. One thing at a time!

I will be doing some more tests today. I've been lent a high quality Agilent multimeter as mine seems to be very slow reading values and updating the display.

The reality is that it does not matter what reading LinuxCNC receives as it will be used as an input to a PID control loop. Voltage conversion is just a convenience for the operator to display on screen. I may just use the raw frequency as the PID input.

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Re: Rod's Spaceship Scratch Built Plasma Table

Postby Rodw » Sat Aug 05, 2017 5:29 pm

tcaudle wrote:Your best method would be to have a frequency counter and measure the freq out put with an applied test voltage of exactly 100 VDC and adjust the pre-divider (scaler) with the 100K resistors so you get the correct frequency. Other voltages would work but the math gets more complex,

Looking back at this I am glad you are doing it and not me :o

Tom, thanks for your input, I really appreciate it. Currently I have the THCAD scaled with an external resistor to extend the range from 0-20 volts which is near perfect for a 16:1 divider as full scale should be 320 volts at the torch. The problem I have is that there are so many errors in the Everlast documentation I don't have much confidence that the 16:1 divider is correct. From our analysis, R1 can be either 197.5 k or 130 k based on the position of the link. so a 50:1 divider requires a 4k value for R2 and a 16:1 divider requires 8.6k for R2. (the link just shorts out a 67.5k resistor). Clearly something is not right.... 40 years ago I would have been able to solve the two simultaneous equations. Now I'm not so sure!

I'll go through what you say in more detail. We did think of adding a potentiometer in the circuit so we could calibrate it.

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Re: Rod's Spaceship Scratch Built Plasma Table

Postby Rodw » Sun Aug 06, 2017 5:13 am

Well, I took a punt and connected two 48V power supplies in series which gave me 96 volts and connected it to the CNC board inputs


it showed 96.4 volts. Pretty good I thought!


There was 1.95 voltage drop across the resistors on the raw outputs


Then read the divided voltage in the plasma control box


I remembered that I had inserted a resistor to scale the divided voltage in the interconnect cable as per the THCAD manual so this voltage is actually half the divided voltage which I confirmed with a multimeter on the CNC board.

So now I know why it has been said nobody has got an Everlast plasma cutter working other than on raw arc voltage!

Here are the voltage dividers I calculated:
50:1 Actually 76.145:1
16:1 Actually 24.529:1

So a quick tweak of the scaling settings in my Linuxcnc hal file, the voltage reported on screen is +- 0.7% of the faked torch voltage. The good thing about this is that this is right in the range I saw when I did some cuts yesterday. This pic from yesterday shows 83.6 volts after the raw voltage resistors (Add 1.95 volts).

During all of this I found another error in the Everlast manual. They say the circular connector pins are the same as this on the board. Whilst the circular connector is correct to the diagram, both sides of the raw voltage and divided voltage are swapped around at the board end. What a mess!

So another weekend behind but slowly making progress.

With the voltage reading nailed, I can move forward to set up some speeds and feeds and then get onto the THC side of things....

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Re: Rod's Spaceship Scratch Built Plasma Table

Postby Rodw » Sun Aug 20, 2017 12:34 am

Well, finally!

I have a PID based Torch Height Control. It worked on my very first cut :D :D :D

This is a screen dump from LinuxCNC's Halscope software Ocilloscope.
The green line is the torch voltage
The highest straight line is the setpoint of 85 volts
THe spikes are the cartesian velocity during touch off. You can it flattens out during the cut

But what really excites me is to see the arc volts start at about 95 volts, then the PID control kicks in and brings it straight down to the setpoint which it tracks pretty well. Being PID based there is no dead band in this configuration.
I will be having a beer to celebrate this later in the day :)

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Joined: Sun Apr 22, 2018 7:48 pm

Re: Rod's Spaceship Scratch Built Plasma Table

Postby greygizmo » Sun Apr 22, 2018 9:09 pm

This is an awesome thread. Any updates?

Posts: 115
Joined: Sun Aug 21, 2016 1:49 am

Re: Rod's Spaceship Scratch Built Plasma Table

Postby Rodw » Mon Apr 23, 2018 5:12 pm

greygizmo wrote:This is an awesome thread. Any updates?

I've had a couple of technical problems (read blew up some of the electronics hardware) so I've not done much with the build. Until that happened I was concentrating on finishing the physical build and designing a downdraft hopper

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