Here is pretty much an ideal sequence of events
At the start of the cut you need to find the top of the work. The usual way is to mount the torch on a small auxiliary slide with a limit switch. You move the torch down until it hits the work and trips the switch.
Next you lift to the commanded pierce height, pierce and start cutting. After a short delay to allow the arc to stabilize you turn THC on. If the actual speed drops below the commanded speed by a certain percentage (i.e on sharp corners) turn off THC. If the THC stays on when the machine slows down the torch will dive into the work.
At the end of the cut turn off THC and lift to rapid height.
Ideally it should also be possible to turn THC on and off via code. The machine should keep moving smoothly when this happens. It should not hesitate. Motion through corners and arcs should be as smooth as possible. Any hesitation will leave marks.
You can use SheetCam's cutting rules to generate THC on and off commands on corners if your controller cannot do this automatically based on the feed rate.
I believe most THC controllers have simple up and down outputs. When the up output fires, move the torch up at a fixed rate and vice versa. Alternatively you could sense the voltage directly through your ADC the do all of the THC logic inside your controller. Keep in mind the torch voltage is around 200VDC with possibly a few kV of high frequency AC superimposed so the voltage divider/filtering needs to be very robust. Opto isolation is also a good idea.
Les
On 18/02/2015 16:02,
jatinder.grewal9@yahoo.com (
jatinder.grewal9@yahoo.com) [sheetcam] wrote:
Argun thank you so much for your message.
Surely the 0-10v output can be used and we can reprogram that as per need. For your reference its a pure 10bit  0 to 10 volts through DAC and not a PWM type as used on Mach3 and this resolves coupling and inductive issues in the 0 to 10VDC signal.
Regarding the THC signals I would like to understand the basic concept so that we can program the unit accordingly. So to my understanding from the GCODE the controller will switch on the plasma for example at M3 and at this point what should be the reaction of Z axis?
We can control the Z axis separately from the GCODE and for example if we get a UP/DOWN signal from your THC then should the controller move the torch up or down as required by the your THC?
I have a lot of questions in this regards such as what should be the initial position of the torch before starting a cut, what should be the position of the torch at end of cut, what should the controller do when it gets a IN POSITION signal. Basically trying to understand how the sequence of things work for automatic THC.
Thanks again,
Jat
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Posted by: Les Newell <
les.newell@fastmail.co.uk>
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