LinuxCNC, Plasma, THC questions

Long time Mach3 user on Mill, Router and Laser. Sorry about that but I am very accustomed to it.

However I am in the final stages of building a plasma table and the particular requirements are a little different. So I am researching LinuxCNC and in particular how it incorporates THC.

With Mach3 when the controller is given up/down signals by the THC (eg Proma) the LPT driver takes care of these movements.

The disadvantage of that is that those Z axis moves are bang bang. That is, there is no acceleration ramp. Full go or stop.

This has the problem that the Z axis performance, and hence the THC performance, is limited by that lack of a proper acceleration ramp.

For example my Z axis can happily run at 1800 mm/min with acceleration ramp but only 600 mm/min without it.

So to my questions,

Does LinuxCNC, when responding to up/down signals from the THC (eg Proma), does it incorporate acceleration ramp into those moves?

Is there an advantage in respect of the above question, to using Mesa THCAD?

Another Mach3 disadvantage seems to be latency in the response to up/down signals. Is this better with LinuxCNC? Again is this better or not with THCAD? Or Proma?

I hope I am asking this in the right place.

Thanks

Greolt

This is the thc component I use with the Mesa THCAD
linuxcnc.org/docs/html/man/man9/thc.9.html

store.mesanet.com/index.php?route=produc...oduct&product_id=128
You can set the velocity of the thc correction with

thc.correction-vel float rw
The amount of change in user units per period to move Z to correct

Rick G

Thanks Rick.

But does that velocity involve an acceleration ramp.

Maybe that is a Mach term. Dou you know what I mean?

andypugh wrote:

Yes, because the LinuxCNC step-generator function has built-in acceleration and velocity limiting. (I suspect that this might also be true of Mach, if you look in detail).

Yes Mach has the same of course.
BUT moves of the Z axis in response to THC up/down signal do not. They are what I describe as bang bang.

tommylight wrote:

........................ from what i remember, there is a setting in mach for setting the reaction speed in regards to the maximum speed and acceleration set for Z axis, and it is in percent. I do not recall the exact naming, but it is there.

Thanks for the response Tom

In Mach3 there is a setting for THC Z axis speed. It is expressed as a percentage of max velocity (or rapid speed) as set in motor tuning.

However the THC up/down move does most certainly NOT include an acceleration ramp. Let me be clear, this is via the LPT driver.

I have no certain knowledge of how external pulse generators like SoomthStepper , for example, handle THC Z axis moves. Although I believe it is the same.

So my question remains, does LinuxCNC make THC Z axis moves the same as Mach3 or does it include an acceleration ramp?

andypugh wrote:

The LinuxCNC step-generator has a built-in acceleration limit that you can configure.

So does Mach3. Exactly that.

I really don't wish to sound rude or arrogant. Please forgive me if I do. But that does not answer my question.

Sure seems like it answers the question

Thats is if you set the acceleration limits of the stepgen
those limits will be obeyed regardless of where the stepgen
position or velocity command comes from, be it axis position, THC position, THC velocity, phase of the moon, or whatever