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Thread: DIY Torch Height Control

  1. #1
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    DIY Torch Height Control

    I am working on making my own torch height control for my plasma table. I can get the signal fairly easily from the Powermax45 because it outputs a 1/50 voltage out the back. I am going to use an Arduino microcontroller to sample this every half second or something similar and compare the analog input to a set parameter and adjust the height accordingly.

    The problem I am running into(in my head) is how to interface the Arduino with the Gecko G540 I am using.

    Example...

    I dont remember the voltage range the Powermax outputs but I know it is higher than what the Arduino can handle so I will be using a voltage divider to be decided at a later time.

    Arduino gets a signal...signal will be 0-1023

    Arduino compares signal...Input signal is 700, set range is 500-550

    Arduino outputs signal...Lower Torch(output 1) <---How would I make this actually control the stepper motor? Intercept the signal from the DB25 connection and use pin 6 and 7 to control it?

    Like if I need to lower it, set pin 7(step direction) high or low and then pulse or power pin 6 to get motor to move?

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  2. #2
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    Anyone?



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    hi tjb1,
    My impression is that users of the machine project log are more mechanicaly oriented, like myself. I have some electronics background, so I can understand your question but not answer it. I suggest posting your question in the Electronics section of cnczone.
    Arthur



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    I'm just looking at DIY torch height control, how are you getting on with it?

    I did similar with a spark eroder and stepper. Basically it fired the arc then looked at the residual voltage on the capacitors. Fully charged was no spark, move down. Zero volts was touching, move up. In between was do nothing.

    No microprocessor required.

    I used two comparators, one to say volts too high, one to say volts too low and latched both outputs with one 74LS74 flop.

    A 555 timer generated the step pulses.

    If either comparator indicated movement I allowed the pulses on to the stepper.

    The DIRection was either of the comparator lines.

    Worked fine and dandy, when cutting it usually oscillated one step.



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    I'm thinking about torch height control also. I also have the G540 and Powermax 45 and am using EMC2 (now LinuxCNC). Am wondering if one can use the torch height control configuration PLASMA-THC using the G540 VFD output (0 - 10V) as the voltage to compare (now that I've found out how to get the 50:1 divided arc voltage and arc ok signal while using the hand torch) so its settable in LinuxCNC. Then just use two comparators to tell LinuxCNC to move Z up or Down. May be missing some of the fancy features but is that to simple and why can't I find anything on the web about people trying this already?



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    I won't know if it is simple until I get my oscilloscope on it

    I think divide the volts down, low pass RC filter, buffer the volts with a unity gain op amp then two comparators and two pots, one pot to set the threshold, one to set the no step hysteresis.

    If nobody is posting DIY there are two possible explanations...

    a: It is horrendously difficult due to unforseen complications.

    b: Anyone who can do it wants to sell us expensive THC



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    I havent gone any further into this, been busy actually designing the table and school work but what I think I am going to do is...

    Run 50:1 voltage output through a voltage divider into analog input on arduino.

    Run "Motion Start" into a digital input.

    Run both Z-step and Z-direction into digital inputs.

    Take Z-step and Z-direction from outputs and go to G540

    Arduino will be sampling and if "Motion Start" is low, I will pass the Z-step and Z-direction right through to the G540 with no modification. When arc starts and the cutter sends the motion start, that will go to the Arduino also. When Arduino gets this it will take the plasma voltage and compare it to my set range and then output its own steps-direction pulses while ignoring any information coming from the computer for Z-step and Z-direction.

    Then hopefully when computer shuts off arc, motion start will shut off and the Arduino will pass the Z commands from computer right to the G540 with no modifications.

    I have also decided just to use an "ohmic" shield and get the start height using that in the program instead of the Arduino.

    So basically Arduino takes over Z when the cutter sends the motion start signal, anything other than that and the Arduino is just passing the information through. Im just using an Arduino because I have it and they are cheap and easy to use(for me at least). I may be able to get all of this to run on an ATtiny85 if I can drop an IO or modify it to use the RESET as an IO and if I cant I will just use an ATmega328p. The tiny is like $3 the mega is like $5 so its really not that bad.

    Problem is I do not have a G540, steppers or a computer running Mach3 yet.



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    I'm not sure what thickness you plan to cut but on the thin stuff you will be cruising along pretty fast according to the plasma cutter manual. At high speeds, you are gonna need THC at something substantially faster than "every half second or so".

    Other than that, your approach seems logical.

    I think a simple approach like Robin has mentioned could work here also.

    Use the "motion start" output to switch the step and direction signals from mach to your "controller". That will put your controller in control during the cut and mach in charge every other time.

    The controller could output a steady step pulse stream, maybe like 100 hz or something to start. The controller could be a simple comparator between the commanded and actual arc voltage. The output of the comparator could toggle the direction pin. Due to the constant step pulse stream, the torch would oscillate up and down around the setpoint. I am not sure how much. In theory anyway, this could get you a working solution with only a couple dollars in solid state components and no programming required.

    Matt



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    It all sounds pretty simple...make sure you cover the following issues as well....like most of the available plasma THC systems do:

    -Find the plate surface then retract to manufacturers suggested pierce height. (different for different power levels and thicknesses...must be operator setable)
    -After pierce starts...activate a timer that freezes x, y and z motion until the pierce is complete. (different for different power levels and thicknesses...must be operator setable)

    - Once pierce delay timer completes, index rapidly to a physical cut height as recommended by torch manufacturer. (different for different power levels and thicknesses...must be operator setable)

    - Once x and y have achieved at least 80% of cut speed....activate arc voltage feedback circuitry to control height. (some THC systems sample the arc voltage at the beginning of every cut, after motion is near programmed speed and lock on that voltage, others use an operator set voltage that will maintain the physical cut height)
    -when machine decelerates for corners and fine features....freeze the z axis to eliminate diving.
    -when crossing the start kerf at the end of the cut....use kerf crossing voltage anomaly recognition to minimize torch diving.
    -be sure to filter divided arc voltage to minimize hash and noise that is the result of the plasma arc interacting with the molten metal movement....voltage output varies quite a bit.
    - As the torch electrode wears...the voltage reading will increase....causing the torch to be closer to the plate. Allow for automatic compensation if possible.

    These are a few features that make plasma arc voltage height controls work well. I'll be happy to offer advice as I have been working with these devices for over 35 years. I would love to see a functional, stand alone THC system that would work on any cnc / plasma for a low price. Industrial THC's cost between $5000 and $12,000.....including torch lifter/drive.


    Jim Colt



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    Been thinking. Rarely a good idea at my age...

    A stepper motor is the wrong tool for the job, it wants a little DC gear motor driving a screw.

    A power op-amp to drive it so it can receive plus or minus volts to go up and down with a two wire connection.

    It needs to be able to rise to the top and drop down to touch. I can sense those with two microswitches. If I split the motor power line with two diodes I can route the plus through the "top stop" switch and the minus through the "bottom hit" switch. That way it will stop automatically at top or bottom.

    To do torch height I send the error to the motor. If it's way out it goes quick, if it's barely out it hardly moves at all.

    That only leaves rise to the pierce height, still thinking on that, want to keep it simple.



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    Doing my daily reading I found this post by Jim...

    "The diving in the corners can be stopped with a setting in your Mach3 that freezes the height control whenever the cutting speed gets below 80% of the program speed. I know the setting is in there...but I am no expert on Mach3 systems....so you will have to consulat with someone that is!"

    Wow, thank you so much. That just made me rethink this. I was about to give up on it after reading your last post about the corners and everything and trying to decide how to monitor that but at least to get started I can use that Mach3 output to stop the THC. Now to figure out the pre cut area crossover...this would be so much easier if I had Mach3 and a plasma table. Fall cant come any faster.



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    Im gonna tackle the corner and pre cut crossover by just setting it not to move after dropping the voltage below a certain level. I am going to do some testing on some different materials and thicknesses to see what voltage it maintains. The max I was able to get out of it was 6.62 and that was 45amps pulling the torch up until it lost the arc.

    So far I have just been playing with an old printer using an arduino and adafuit motor shield. I have not hooked it up to the cutter yet, waiting on the cpc plug to arrive but im *adjusting* the voltage with a potentiometer and its working quite well now but no idea how it will work with the actual nema steppers. I will have to sniff out the signals and make the arduino output them since I wont be using the motor shield anymore.

    If anyone is interested this is the code. This is running on a Arduino UNO using a Potentiometer on the 5v, Ground, Analog Input 0, and an Adafruit Motor Shield with a Bi-Polar 4-wire stepper motor hooked to M1 on that shield. None of the serial code is required, I was just using it to verify what was happening.

    Code:
    #include <AFMotor.h>
    
    
    AF_Stepper motor(48, 1);
    
    void setup() {  
      
     Serial.begin(9600);
     Serial.println("Serial Started");
     
    }
    
    void loop() {
      
      analogRead(0);
      Serial.println(analogRead(0));
      if (analogRead(0) < 550 && analogRead(0) > 450)
      {
        Serial.println("Torch In Range");
      }
      if (analogRead(0) > 550)
      {motor.step(1, FORWARD, DOUBLE); 
      }
      if (analogRead(0) < 450 && analogRead(0) > 75)
      {motor.step(1, BACKWARD, DOUBLE); 
      }
    }




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