CNC Interface and Torch Voltage Capture

[EmptyNester]

One of the problems I've been having is that the LCD display blurs if values are changing quickly.

I haven't had a chance to put in opto-isolators for the serial port yet. But, I really wanted to get some data out of the THC based on what it was seeing. I realized that if I used a laptop running on a battery, I wouldn't have to worry about ground loops. So, I connected a TTL to USB adapter I got on ebay. I did a cut and captured the voltages. The graph below shows a plot of those values for a cut of of 1" circle.



The blue line is the raw voltage as red from the A/D. It is in counts (it's a 10 bit A/D, so it ranges from 0 to 1023). The red trace is the value after software filtering.

I started capturing voltages when the torch on signal went active. I put a 0 value in when the software had waited 500 ms. after the "arc good" signal went active. The 500 ms. delay doesn't seem like long enough to wait before starting height control.

Using the 500 ms period as a reference, it works out to an average of a sample about every 7 ms.

The software was running pretty slow (I think due to the slower than normal serial library I was using). I expect I can easly ramp the sample rate up higher, though it seems to be doing pretty good at the current rate.

You can tell that the software averaging is too slow. I was doing a sliding window average over 25 samples, so that ends up being a 175 ms average. The voltage is clean enough that I can do a much shorter running average (maybe 3 samples or about 20 ms?).

The op amp filter circuit is filtering well, but I ran a circuit simulation and found that some of the wierd behavior I'm seeing is actually how the circuit works (at 0 V in you get about 1.3 V out and the output won't go higher than 3.7 volts). So, I'm just going to need to play with the voltage divider more to get the cutting voltage in the middle of the op amp's range.

I had taken a class on LTSpice (analog circuit simulator) and it helped me figure out parts of the voltage circuit I didn't understand. It turns out that the 100K resistor on each side of the torch voltage coming out of the CNC adapter reduce a signal in the 110 V range down to the 60 V range. I had though I was cutting at a much lower voltage than is normal, I now know its due to the CNC port resistors.

I'm going to have to write a single post on all the details of the CNC port when I'm done. I've learned a lot of things that I've never seen documented in one place.