Deceleration and soft limits

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28 May 2023 10:41 - 28 May 2023 10:43 #272328 by beefy
Before I came to Linuxcnc the system I used said that the axis would decelerate to the soft limit position. Therefore I had to allow for deceleration travel distance. So lets say my deceleration distance at max speed was 10mm, I would keep any work pieces at least 10mm away from the soft limit positions, so there was no chance of the feedrate being slowed.

Looking at the Linuxcnc docs it states the follow for the soft limit values in the .INI file:

When this limit is exceeded, the controller aborts axis motion.

That is not very explanatory.
Does it mean the axis will decelerate only upon reaching a soft limit, and thus will travel further due to deceleration.
Or like my previous system, will it decelerate so that it stops AT the soft limit position
Last edit: 28 May 2023 10:43 by beefy.

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28 May 2023 18:15 #272355 by rodw
Replied by rodw on topic Deceleration and soft limits
I don't think it's anything you need to worry about. If properly set up and homed, the system will know if it's going to hit a limit when you run a gcode command so it will refuse to run. ie if a move is going to go out of bounds it won't run. This seems to be checked when a gcode file is opened.
Soft limits should be inside limit switches and limit switches whould be inside hard stops.

I rrecommend you at least  install home switches if you haven't already.​​​​ I gave plenty of room (90mm) after hitting a home switch to decelerate to a stop and moved close to the hard stop with a HOME_OFFSET. This let me home at a fairly fast speed.

Alsoo one thing I got wrong for a long time was not using G54 for running programs. QTplasmac sort of enforces these as when you touch off to x0,y0 it's normally in the G54 space. The system will still know where the limits are even if you set x0,y0 to 10mm from the soft limits.

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28 May 2023 20:11 - 28 May 2023 20:12 #272365 by beefy
Replied by beefy on topic Deceleration and soft limits
Thanks Rod.

It is important though, especially on my table (need all the distance I can get). My deceleration distance at rapid speed is 9.26 mm.

My limit switch is just 3mm behind the soft limit position, so if deceleration starts at the soft limit position, it will trip the limit switch, which I don't want.

However, if the system decelerates to the soft limit position, so that the controlled point goes no further than the soft limit position, there's still a few mm to go before hitting the limits. That's what I do want, and thus I need to know how the system works in order to put the correct values in the .INI file.

I'm guessing the system does decelerate so that it stops at the soft limit position, but the docs just don't explain that.

Might just have to experiment and see which one applies. With 9.26mm of deceleration distance I should see the difference quite clearly.
Last edit: 28 May 2023 20:12 by beefy.

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28 May 2023 20:40 #272368 by tommylight
If set properly, the machine should never ever hit limits.

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29 May 2023 03:17 #272392 by rodw
Replied by rodw on topic Deceleration and soft limits

If set properly, the machine should never ever hit limits.

Listen to the Oracle!

Forget your old CNC environment. Linuxcnc won't let you  go outside your soft limits. If it does happen, something is very wrong.  it will hit your limit switch and stop very fast!
I'm not sure if it respects the dcelleration or not. It was so long ago I set my machine up I can't remember. I went years without having a limit error once configured.

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29 May 2023 06:41 #272404 by beefy
Replied by beefy on topic Deceleration and soft limits
Guys, I appreciate the replies but I think you are on the wrong track with me a little.

I understand the setting up of limits, home switches, etc, and I understand that soft limits should prevent the possibility of running into the limit switches when the system is set up correctly. That is the way my old system worked too.

I simply wanted to know how Linuxcnc works with regards to deceleration and soft limits, so that I can set up my system correctly AND determine the closest any part to be cut can be to the soft limits.
In your last post Rod, it seems you have actually answered my question when you said:

Linuxcnc won't let you go outside your soft limits

Therefore, the system MUST take into account the current speed and the deceleration rate, and start decelerating BEFORE reaching the soft limit position, so that it is possible for it to stop at the soft limit position, and not go any further.
With my system at rapid speed, it would therefore start decelerating 9.26mm before the soft limit position, so that it could come to a stop at the soft limit position.
For my plasma cutting that means for the fastest feedrate I would ever expect during cutting, I need to calculate the deceleration distance at that speed, and ensure any cut part is at least that distance plus kerf width, away from the soft limits positions.
That way the feedrate will never be slowed by deceleration because the controlled point is too near a soft limit.

It also means I'll be safe with my limit switches 3mm further away from the soft limit positions.

Thanks for the help and feedback.

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29 May 2023 09:34 #272412 by dm17ry
Replied by dm17ry on topic Deceleration and soft limits

So lets say my deceleration distance at max speed was 10mm, I would keep any work pieces at least 10mm away from the soft limit positions, so there was no chance of the feedrate being slowed.


there's no reason in doing so. if the trajectory is inside soft limits (it has to be) then there won't be any difference in speed no matter how close it approaches them. the machine has to slow down to stay on the trajectory anyways, soft limits proximity doesn't affect speed planning in any way.

it only makes sense when jogging - then yes, linuxcnc will slow down before reaching the limit with defined acceleration to be able to stop at the limit.

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29 May 2023 11:10 #272421 by beefy
Replied by beefy on topic Deceleration and soft limits
dm17ry,

of course, thank you so much for that explanation. I couldn't see the forest for the trees. My brain can have a habit of getting locked on a train of thought.

This is great news because it means I've "gained" nearly 20mm extra work length on my X and Y axis over what I thought I had.

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29 May 2023 11:29 #272423 by tommylight
Ok, lets try a different way, we do cut often right up to the machine limits, never had issues.
You are right, it will decelerate near soft limits, but cutting is done at relatively slow speeds compared to machine abilities, so i never noticed.
Talking abot plasma here.

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