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Author Topic: Soft stop at limit switches  (Read 13899 times)

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Offline Hood

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Re: Soft stop at limit switches
« Reply #10 on: September 10, 2010, 08:12:09 AM »
The word "some" is there again and afraid that is the problem, one group of users want it one way and another want it another way.
One thing though that is for sure is Soft Limits when properly configured do exactly what you want (and actually more). It is the operators responsibility to ensure that the softlimits are active and if that cant be guaranteed because of forgetfulness then I suggest you set up additional measures such as the adapted first screen page as I mentioned earlier. Additional things that could be done are you could make the Cycle Start into a VB button which looks to see if the machine is Referenced and Soft Limits enabled before it will start the code. You could have a hidden Soft Limit button so an operator can not disable Soft Limits and I am quite sure it would not be beyond most people who know VB to have a password protected Soft Limit button.

If on a machine, say for example that has linear slides and very little friction, it may well travel further if pulses are removed. It should be noted however that Hard limits are an emergency feature so your machine should have proper means of stopping as fast as possible, motor brakes and braking resistors come to mind.

Hood

Hood

Offline Hood

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Re: Soft stop at limit switches
« Reply #11 on: September 10, 2010, 08:16:09 AM »
I will also add that it is a common misconception that to reference the machine you need to first disable the soft limits. That is not true and if it is needed it is because soft limits have not been configured correctly in the first place.
Hood
Re: Soft stop at limit switches
« Reply #12 on: September 10, 2010, 08:25:04 AM »
I better should have titled this thread with "softer stop at emergency condition or limit switches".
When soft limits are applied and work, limit switches are not needed, anyway. When limit switches are not needed, the whole discussion is futile. i think we all agree that limit switches are useful- if we could omit their use totally by using soft limits we could save a some money and construction effort. So why is the response to the question: "How can I make my stopping process more effective?" - "Don't come to a situation where it's needed!" ?

Many greetings
Nicolas

Offline Hood

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Re: Soft stop at limit switches
« Reply #13 on: September 10, 2010, 08:41:24 AM »
As said Limit Switches are exactly that, and are a last form of defence and it is up to the system builder to use these in a way which best suits their machine.
 If it was an option to have them set the way you want then I have no objection but if it was to be the way they had to be set then I would have a huge objection.
 The way you want them  is relying on software doing something, i.e. decelerating the axis when an input is seen, if the software fails to do that where are your limits now?
 So afraid it is up to you to petition Brian to add that feature for you and as said I have no objection as long as it is an option and not the only way.
Hood
Re: Soft stop at limit switches
« Reply #14 on: September 10, 2010, 08:43:40 AM »
Here is my personal list of arguments why it makes sense to care about the possibility of a controlled slowdown at a limit switch on in an emergency condition (some of them are not my own ideas):

1. Some machines stop faster when there is a controlled stop instead of killing the drive clock.
2. On stepper drives, hitting "stop" usually results in lost steps, causing the need to re-reference position. So operators usually ponder whether it's really necessary to hit "stop" but maybe "line feed" does as well. That's not the sense of a "stop" button to ponder before hitting it!
3. Sometimes, soft limits simply don't work.
  3a) Sometimes soft limits just are switched off by operator
  3b) Some machines do not have a rectangular working area!
  3c) Some machines have changing dimensions of working area.
  3d) If soft limits always worked, they would be useless.
4. At some machines (usually smaller ones), the impact on the machine by the abrupt stop when triggering the limit switch is even harder than that one caused by crushing into the elastic dead stop.

I think every single reason itself is enough to motivate for having an adjustable ramp for "emergency slowdown".

Many greetings
Nicolas

Offline Hood

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Re: Soft stop at limit switches
« Reply #15 on: September 10, 2010, 08:48:41 AM »
Having the machine ramp to a stop when a limit is seen is purely a software feature and if that software fails then you dont have any limits.
 Limits should not only be in software but should be mechanical and it is something you should think about.

Hood
Re: Soft stop at limit switches
« Reply #16 on: September 10, 2010, 08:51:31 AM »
So afraid it is up to you to petition Brian to add that feature for you and as said I have no objection as long as it is an option and not the only way.

Of course, every "adjustable ramp" implies the possibility of a sudden stop. In fact, it's exactly the point that either I want to know how to implement this in existing Mach or want to pose a feature request for future release. But before the second, I'd like to discuss its usefulness, otherwise I don't see any chance of implementation if its reason is not as clear as day.

Many greetings
Nicolas


Re: Soft stop at limit switches
« Reply #17 on: September 10, 2010, 08:54:18 AM »
Limits should not only be in software but should be mechanical and it is something you should think about.

100% Ack. But the same is for those limits already implemented.

Offline Hood

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Re: Soft stop at limit switches
« Reply #18 on: September 10, 2010, 09:06:44 AM »
Of course, every "adjustable ramp" implies the possibility of a sudden stop. In fact, it's exactly the point that either I want to know how to implement this in existing Mach or want to pose a feature request for future release. But before the second, I'd like to discuss its usefulness, otherwise I don't see any chance of implementation if its reason is not as clear as day.

Many greetings
Nicolas
In effect you are wanting two sets of Soft Limits and so will need additional switches to act as Hard Limits in case both SoftLimits fail or are you just going to rely on software?

I think it would be possible to do what you want by not having the switches set as limits but rather set as normal Inputs. You would then have a Brain or macropump monitor them and if seen to issue a feedhold.




100% Ack. But the same is for those limits already implemented.

I do not understand what you are meaning here.

Hood
Re: Soft stop at limit switches
« Reply #19 on: September 10, 2010, 09:19:51 AM »
OK, perhaps my vocabulary maybe insufficient:
I'd like to say:
Hard limits which are security issues, of course have to be casted in hardware. Something having nothing to do with Mach at all. Mach only gets a notice that an emergency stop has occured. And, maybe if it makes sense, that a limit switch has been triggered. Anyway, it's not Mach to decide to stop the machine because this has already been done by hardware.

Hard limits I talk about in this discussion are those limit switches connected to a breakout board and to Mach which decides to stop motion by cutting clock when they're triggered. However, these limits stop motion by software anyway - so they're as reliable as every software thing, regardless if they're implemented with a ramp or not.

Soft limits I do not want to talk about at all. They're fine.

Many greetings
Nicolas