Hello Guest it is November 20, 2019, 02:40:45 AM

Author Topic: Soft stop at limit switches  (Read 13870 times)

0 Members and 1 Guest are viewing this topic.

Offline Jeff_Birt

*
  •  1,107 1,107
    • View Profile
    • Soigeneris
Re: Soft stop at limit switches
« Reply #30 on: September 13, 2010, 08:33:55 AM »
I agree with you Hood  :)

If you hit a limit then something went very wrong. It is only safe to assume that the machine has no idea where it is at. The only way to always know your starting back off with the machine in the right place is to re-home it.
Happy machining , Jeff Birt
 

Offline jve

*
  •  128 128
    • View Profile
Re: Soft stop at limit switches
« Reply #31 on: September 14, 2010, 02:32:42 PM »
hi guys
i was just reading this topic and i always try to use the softlimits but i am retrofitting a big cnc lathe with mach3 turn and some how i canot config the soft limits settings
i am working with the machine and i am trying to understand the soft limits settings
do you guys use softlimits in mach3 turn?

Offline Hood

*
  •  25,856 25,856
  • Carnoustie, Scotland
    • View Profile
Re: Soft stop at limit switches
« Reply #32 on: September 14, 2010, 02:50:03 PM »
They dont work in Turn, supposed to be getting fixed but will be Rev 4 before they are done.
Rev4 of Mill will be first then maybe the poor relation (Turn) will get looked at after that.

Hood

Offline jve

*
  •  128 128
    • View Profile
Re: Soft stop at limit switches
« Reply #33 on: September 14, 2010, 02:56:06 PM »
do you know when rev4 is coming out ?

Offline Hood

*
  •  25,856 25,856
  • Carnoustie, Scotland
    • View Profile
Re: Soft stop at limit switches
« Reply #34 on: September 14, 2010, 02:57:55 PM »
No one does, not even Brian.
Hood
Re: Soft stop at limit switches
« Reply #35 on: September 16, 2010, 05:49:04 AM »
If you hit a limit then something went very wrong.
Hello,
That's one point of view. Other points of view I wanted to explain in this thread. Anyway, the first question was a "how"-question. So I can read between the lines that there is no chance to achieve this behaviour in the current version (e.g. by a script or something) and this would be a totally new feature to be requested to the programmer.

So there is a two step solution:
1st On my own machine I removed all limit switches.
2n Now it's on to  convince the programmer that this feature is absolutely necessary for a future release.

Thank you for this discussion.
Nicolas


Offline Hood

*
  •  25,856 25,856
  • Carnoustie, Scotland
    • View Profile
Re: Soft stop at limit switches
« Reply #36 on: September 16, 2010, 06:35:26 AM »
Can you remind me why there is a need for this other  than forgetting that you have not enabled softlimits?
I looked back and I see this as your reasons.
Quote
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.

1. That may well be true but surely that would be up to the machine builder to provide a remedy for this? Especially when you remember that a Hardware Limit is an emergency situation so hardware means should be used rather than relying on software.
2. Mach is open loop so there will always be the high possibility that you have lost position when stopping in an emergency situation (Hardware Limits or E-Stop) It is no big deal to re-reference again, in my opinion, in the rare instance that soft limits have not protected you.
3a. Operator error and could be overcome with some VB in the cycle start button or hiding or disabling or requesting a password on the softlimits button to stop an operator disabling the soft limits.
3b. Dont understand this, Hard Limits are placed at the extent of the axis so how would the working area come into play. Hard  Limits are a per axis thing and turning them into a software limit operated by physical switches is not going to help as far as I can see.
3c. That is the job of the soft limits or the operators programming responsibility. High End CAM and very expensive controllers have work envelope, fixture and tool awareness features but are complicated to set up and really only worth it if doing long production runs I would think.
3d. Not sure what you mean by that, softlimits are there to do exactly what you are wanting and why they would be useless when working is not understood by me.
4. Again use soft limits and only rely on your Hardware Limits for emergency situations when, in the unlikely event, Soft Limits fail.

Just my opinions of course ;)


Hood


Re: Soft stop at limit switches
« Reply #37 on: September 16, 2010, 07:41:53 AM »
Can you remind me why there is a need for this other  than forgetting that you have not enabled softlimits?
I looked back and I see this as your reasons.
Quote
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.
1. That may well be true but surely that would be up to the machine builder to provide a remedy for this? Especially when you remember that a Hardware Limit is an emergency situation so hardware means should be used rather than relying on software.
-> Of course, it's possible to add a hardware that will provide a "soft stop" when clock is interrupted. This would be a special hardware for dealing with a missing software feature.
2. Mach is open loop so there will always be the high possibility that you have lost position when stopping in an emergency situation (Hardware Limits or E-Stop) It is no big deal to re-reference again, in my opinion, in the rare instance that soft limits have not protected you.
-> No, it's no big deal to re-reference for many machines and situations. For other situations, it cost's you some minutes to re-reference. So operators ponder whether to hit "stop". Instead of "first hit stop, then think about if it whether it was necessary". This costs you some split-seconds in every maybe dangerous situation. So it's a security gain to make effort for a restart after "stop" as comfortable as possible.
3a. Operator error and could be overcome with some VB in the cycle start button or hiding or disabling or requesting a password on the softlimits button to stop an operator disabling the soft limits.
-> Better not. Constraining operators should have a very good reason. So better find another solution.
3b. Dont understand this, Hard Limits are placed at the extent of the axis so how would the working area come into play. Hard  Limits are a per axis thing and turning them into a software limit operated by physical switches is not going to help as far as I can see.
It's about soft limits. They are only one pair (min/max) per axis - so working area by soft limits is always a rectangle or a cuboid or some hypercuboid in higher dimensions for four to six axis systems. But if the mechanics do not allow the same extrema on each axis for every position or all other axes, soft limits will not work.
3c. That is the job of the soft limits or the operators programming responsibility. High End CAM and very expensive controllers have work envelope, fixture and tool awareness features but are complicated to set up and really only worth it if doing long production runs I would think.
Yes, of course. And on the other hand, moving limit switches is very fast and simple on some machines.
3d. Not sure what you mean by that, softlimits are there to do exactly what you are wanting and why they would be useless when working is not understood by me.
If soft limits always did that thing they should, you would not need any limit switches. Machines have limit switches, so there's need of them. (Friedrich Nietzsche would have good arguments against this opinion, but I'm quite sure he will not visit this board.)
4. Again use soft limits and only rely on your Hardware Limits for emergency situations when, in the unlikely event, Soft Limits fail.
As I said before: On many machines, limit switches told to Mach are no emergency situation. Limit switches for emergency situation are behind them and stop machine by hardware when Mach failed to stop machine.
Just my opinions of course ;)
Hood

Many greetings
Nicolas

[Edit: Removed some typos]
« Last Edit: September 16, 2010, 07:46:35 AM by Nicolas S. »

Offline Jeff_Birt

*
  •  1,107 1,107
    • View Profile
    • Soigeneris
Re: Soft stop at limit switches
« Reply #38 on: September 16, 2010, 08:21:57 AM »
Quote
Yes, of course. And on the other hand, moving limit switches is very fast and simple on some machines.

And moving limits around is a VERY silly thing to do. Limit switches are a last chance safety system. Moving them around allows for the possibility of driving past one which is an open invitation for disaster as now there is no last line of defense between the moving bits and a very nasty hard stop.

Limit switches are there to protect an axis from being driven beyond its possible travel. Soft limits are there to provide an early warning system. If you have need to provide some safety around a given work envelope then you talking about a different subject entirely.

In complex robotic systems each axis has its limits of rotation (1), there are fixed physical limits and you don't screw with them. You most often also describe a safe position (2) by moving the robot to a position considered safe where other moving fixtures will not contact it. You also define a work envelope (3) by move the robot to various positions within its work cell that describe the limit of how far it should move whilst in operation.

1) is a physical limitation of each axis, the same as what a limit switch protects on a typical CNC machine. It has absolutely nothing to do with #2, or #3
2) is a simple 'move each axis to this position' type of thing.
3) is much more complex because it involves complex kinematics calculations to make sure no part of the robot protrudes from the work envelope. As Hood mentioned before, this work envelope stuff is something typically taken care of by the CNC programming software as it can take things like fixtures and other limitations in considerations where most CNC controllers do not have the capabilities to do so.
Happy machining , Jeff Birt
 
Re: Soft stop at limit switches
« Reply #39 on: September 16, 2010, 08:28:29 AM »
Limit switches are a last chance safety system.
Why must every limit switch be a last chance safety system?