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Author Topic: Soft stop at limit switches  (Read 13900 times)

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Offline Hood

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Re: Soft stop at limit switches
« Reply #20 on: September 10, 2010, 10:47:31 AM »
What you are calling Hard Limits are really hardware activated Soft Limits, to be fair it is likely the way more than 90% of Mach users use and think of them. It is also the way two of my machines are set up.
So to answer your question, it is not possible to configure Mach to do as you wish through normal means but may be possible via macro pump or Brains but probably only to issue a feedhold.

Should it be an option in Mach, in my opinion only if its an additional option using separate inputs from the limit switches. Reason is I want my limit switches to act as true limits, ie my E-Stop circuitry is invoked via hardware but in addition Mach is told to cut pulses. That way  I have both hardware and software activated Limits from the same switch.
Then if wished additional switches could be input to Mach to act as ramp down hardware activated soft limits via  another input, called maybe something along the lines of Ramping Limit Inputs.

Hood

Offline stirling

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Re: Soft stop at limit switches
« Reply #21 on: September 10, 2010, 11:57:37 AM »
Nicolas - If I understand you correctly, on the one hand you have concern over what I'll call the "bang" that a system suffers when the step pulses are simply "cut off" and you suggest that a ramp would be kinder. BUT on the other hand you suggest that a ramp could actually slow the machine down (deccelerate) faster than simply cutting off the step pulses.

But there surely is the contradiction. The "bang" is the result of rapid decceleration and you're wanting to INCREASE that deceleration.

Or have I missed something?

Ian
Re: Soft stop at limit switches
« Reply #22 on: September 10, 2010, 03:34:22 PM »
@stirling:
No, Ian, you did not miss the point- as I told in the first post, we have been discussing this topic in a german board for a while. I (for my own machine) want a ramp for slower deceleration, because at my machine, the "crash" to the dead stop is much softer than the stop made by Mach. Others want a ramp because for their drive deceleration works faster with a ramp.

@bob888:
OEM code for Reset is "1021", have a look here:
http://www.artsoftcontrols.com/MachCustomizeWiki/index.php?title=OEM_Buttons

Greetings
Nicolas


Re: Soft stop at limit switches
« Reply #23 on: September 10, 2010, 03:54:16 PM »
Should it be an option in Mach, in my opinion only if its an additional option using separate inputs from the limit switches. Reason is I want my limit switches to act as true limits, ie my E-Stop circuitry is invoked via hardware but in addition Mach is told to cut pulses. That way  I have both hardware and software activated Limits from the same switch.
Then if wished additional switches could be input to Mach to act as ramp down hardware activated soft limits via  another input, called maybe something along the lines of Ramping Limit Inputs.

Having the possibility to separate between "ramp stop inputs" and "cut-off inputs", maybe even with two separate stop conditions, would be the Rolls Royce solution. Having the choice between an adjustable ramp and a cut-off for the one existing stop condition would be the Toyota Yaris solution. When having none at all, implementing the Toyota would be enough.

Greetings
Nicolas

[Edit: correcting some stupid false friends in vocabulary]
« Last Edit: September 10, 2010, 03:59:25 PM by Nicolas S. »

Offline Hood

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Re: Soft stop at limit switches
« Reply #24 on: September 10, 2010, 04:04:21 PM »
Then you really need to eMail Brian and request that this is added to the list of things to do. There is a chance that it may be easy enough to do in Rev4 so the sooner you request the greater the chance of it happening :)
Hood
Re: Soft stop at limit switches
« Reply #25 on: September 11, 2010, 02:35:40 AM »
Hi, when the axes touch the limit they should stop mechanically opening the motor contactor and brake with a low value resistor (resistive brake) while sending an alarm to the driver and the match for them to stop immediately. The limits of limit switch should stop the machine mechanically after out and then back to the reference.

I also think that the stop through the match should not have infinite deceleration but not a slowdown as low as 200 mm / s ยจ 2. Maybe 800-1200 would be a good option.

regards
Renato

Offline stirling

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Re: Soft stop at limit switches
« Reply #26 on: September 11, 2010, 04:34:58 AM »
Nicolas
I (for my own machine) want a ramp for slower deceleration, because at my machine, the "crash" to the dead stop is much softer than the stop made by Mach.
OK - by replacing the word "crash" with deceleration. what you've just said is:
"I (for my own machine) want a ramp for slower deceleration, because at my machine, the deceleration to the dead stop is much softer than the stop made by Mach."

Therefore: why don't you just arrange your limit switches to be triggered towards the end of the deceleration caused by your "dead stop". So "at limit" your machine hits your dead stop - slows down with your "softer" deceleration and then hits the limit when it's almost stopped.


Others want a ramp because for their drive deceleration works faster with a ramp.
Then they're going to get a harder "crash" than at present. Do they understand this?

Ian

Offline BOB88

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Re: Soft stop at limit switches
« Reply #27 on: September 12, 2010, 01:41:19 PM »
Very interesting discussion.  One thing that comes out is that Mach3 should have a way, in CONFIGURE---General settings, to set soft limits to default to ON at all times.  Then no one would have to worry about forgetting them.  

Regarding the "controlled stop" at E-Stop or hitting Limit Switches, many good points were made.
For some of us, I'm sure the main thing we wish Mach3 could do is NOT to lose knowlege of its location, even after those errors.  With steppers (without encoders) , this can't be done if the step pulses stop, because it is the step pulses that update the DROS position.  That's why we like soft limits, feedhold instead of STOP, etc.  With servos, there are encoders.  If Mach3 tracked the encoders and updated the position DROS at all times, even after stop or E-stop, it would be easy ( in absolute mode) to reset, and continue without having to rehome at all.  Unfortunately, it doesn't, so a "controlled" stop could retain the position in some cases, namely hitting the STOP button on screen.  As for hitting limits,  the soft limits already  somewhat do the "soft stop at limits" being asked for.  The slow zone could be reduced making the axis decelerate faster.  The minimum stop time is limited by the step motor capability.  Early CNC's did in fact have two sets of limit switches.  First one stopped all movement but retained position.  Second one killed power, applied brakes (especially on Z-Axis where gravity would bring a thousand pound head crashing down, etc.)   More later..if you don't drum me out of the corps.  

Offline BR549

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Re: Soft stop at limit switches
« Reply #28 on: September 12, 2010, 02:21:24 PM »
You CAN have mach turn on Softlimits through a script I believe BUT softlimits CANNOT work UNTIL the machine has been referenced(;-). Even a modern High $$$ cnc has to follow the same process. Yes I know some have linier encoders that retain the position values(;-) Mucho Bucks compared to mach.

LOOK at Rogers encoder board I think the latest version can recover the lost position or at least has the potential to do so AS LONG as the power has not been lost. You can also do it with MACH brains and shaft/linier encoders.  I know in the past we could using Macros and the brains.

Just a thought, (;-) TP

Offline Hood

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Re: Soft stop at limit switches
« Reply #29 on: September 12, 2010, 05:46:29 PM »
I just dont get why so many people crave Mach not losing position if a limit is hit, it only takes a few xeconds to reference and you are guaranteed to be back to a point you and Mach know.
Everyone to their own I suppose :)

Hood