realtime independent motor control with linuxcnc

I see, yes I'm dealing with a variable surface, I'm planning to control the Z axis by one stepper motor, using a camera's feedback to adjust the cutting depth. Andy told me I can do this via a HAL pin called offset, and I'm trying to figure out how to do it starting from 0 knowledge. thanks for your suggestions btw, if you find my camera approach useful to you maybe we can help each other further.

btw do you know how to control the A axis? I need to keep my blade tangent to the cutting curves.

hey Andy, I begin to put everything together,
Before I feed the Offset HAL pin in real-time to adjust my Z axis, I need to check the motion mode to be in the mode right before LNC starts cutting things, there are （1: Traverse 2: Linear feed 3: Arc feed 4: Tool change 5: Probing 6: Rotary axis indexing） in total 6 modes. I suppose the mode I'm looking for is between the Traverse mode and a Feed mode. Can I pause the motion when the motion mode changes from Traverse mode to a Feed mode, then adjust the Z axis? How do I do that?

I think this topic is on two threads?

To keep the blade tangent research all things "tangent knife". I use "Inkscape" and the extension "gcodetools" and the tool "tangent knife" to generate the rotation of my mechanism to follow the trajectory of the XY moves. Works like a champ. All free opensource software and is supported by helpful folks on the "English Language" section of the forum of cnc-club.ru (Nick rocks). I don't need to be concerned about penetration angles, or sharp turns with the blade deep in the material so I don't know how well those features work or don't work.

There is a way to get LinuxCNC to automatically do the rotation using some "tangent" commands but I think I had problems with it reversing direction to get to the other side 0 degrees which made is less desirable for my project than having the moves in the gcode.

You may want to look at using a reciprocating tangent knife if cutting leather. They are commercially available but probably big bucks.

For height control you may want to look at other distant sensing technology than video. There are various optical and ultrasonic range sensors on sites such as Adafruit and Sparkfun that work with Audrino and such micro-controllers that may work for you.

Lastly what you probably should to do is use a laser. They cut leather like butter, are non-contact so they don't move the material and can cut fine detail adjacent to other cuts.

I think the THC Plasma and the Z-offset approach both can do well,
one thing I don't understand is that why not using real-time path tracking algorithms to do all the cuttings,
but to generate G-code for pre-positioning cutting mechanics. In my opinion, with good motor precision path tracking algorithms should produce very nice cut results as well ( and easier to implement/control in my case).
I'm doubting I can make my workstation work within two weeks especially if I need to buy a new physical board. (I have a tight time-line )

I generate my cutting paths ( a series of curves, mostly an arc) in the form of a series of coords.
Yes I'm not sure what the feed-rate should be.

but, I figured it out that if I use another driver (independent of linuxcnc) to drive my Z axis and C axis I will be able to achieve my goal very fast with linuxcnc controling the X-Y axis. I use linuxcnc to generate G-code (from depth image) and perform cutting actions; I use motion HAL logic pins to read and control the end-tool's motion mode( pause, Traverse, cutting etcs.) and coords etc. , and meanwhile I publish the motion info to another script ( a ROS node) that controls the Z and C axis accordingly. In this way I don't have to hack a lot HAL pins and configs ( save me a bunch of time).

Therefore, if i figure out how to pause the motion at the right time based on the motion state （1: Traverse 2: Linear feed 3: Arc feed 4: Tool change 5: Probing 6: Rotary axis indexing) , I will be able to run my cuttting station in a foreseeable time.

Could you give me another hint? ( I need to pause the X-Y motion just before and just after cutting a curve.)
If I import linuxcnc in a python scrip, I will be able to read linuxcnc.stat attributes that include (motion_mode (may be the 6 modes mentioned above), state（RCS_DONE, RCS_EXEC, RCS_ERROR.）,and coordinates and etcs.
code:
import linuxcnc
s = linuxcnc.stat()
If I then control the read rate of the linuxcnc.stats (running s.poll() at 50 Hz) , store the current motion mode in a variable, and the previous motion mode in another vairable, when I detect a switch between Traverse mode and linear/arc feed mode, I use
c = linuxcnc.command()
c.brake(linuxcnc.BRAKE_ENGAGE) ,c.brake(linuxcnc.BRAKE_RELEASE)
or
c.auto(linuxcnc.AUTO_PAUSE), c.auto(linuxcnc.AUTO_RESUME)

to pause and resume the motion.

If the above "if" all holds, I'm confident to coordinate my C and Z axis with linuxcnc by another script.
Do you think my plan will work well?

Thanks very much