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Author Topic: Clearpath motors coasting beyond limit switch and crashing into bumpers ?  (Read 521 times)

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I am using Clearpath motors on my DIY router.  Mach 4 on a win10 current model PC and a PMDX-126 BOB and ESS. I have been fighting the limit switches. It seems that the motor coasts past the point where the limit switch makes contact. This makes it crash into the bumper and fault the Clearpath motor.

I wish I could make the CP motor stop faulting but it seems that is not an option.

I know the CP motors don't actually coast and this happens on every axis so I expect teh motor is ramping down to zero. So where would there be a setpoint to shorten the delay between the limit switch activation and the motor stopping?

What is a reasonable distance (or time) between the limit switch activation and the motor stopping moving? I only have about a 1/8 of an inch travel between them.

I know people use prox sensors and their range is not huge so certainly I have something set wrong or more people would be discussing this issue.
« Last Edit: December 30, 2019, 12:33:53 PM by OldePhart »
when a limit switch triggers Mach does an Estop and it commands the machine stop at what amounts to infinite acceleration,
and NO servos can do that, as infinite acceleration requires infinite current. Therefore your servos fault.

The simple expedient is never to let your machine trigger the limits. In industrial practice it is considered a failure if a
machine 'limits' out. It should have been stopped by soft limits prior to the physical limits being triggered. When a machine
Estops due to a limit event the machine must be re-referenced, time consuming and therefore expensive in a production

You should do similar, set up your soft limits and set them 2-3mm 'inside' your physical limits and then you'll never touch
the limits. Note this absolutely requires that you reference (home) your machine at the beginning of each Mach session.

A second alternative is to set the limits a little inboard of the bumpers and have a script to decelerate and stop, ie
<FeedHold> and <Stop>, any axis that triggers its 'limit' but WITHOUT causing an Estop.

My wife left with my best friend...
     and I miss him!


After typing out a reply questioning why a limit should trigger an E-Stop,  I was able to see where I created a misunderstanding - I should have said HOME switches. When I'm speaking to people who know about these things I need to be specific, my apologies for that.  I am of course referring to the HOME position limit switches which are used to zero the DRO.

I can't move the existing switches I chose as they are plunger style and if I move them outward there is then not enough plunger travel and the motors crash into those instead. I realize that if there is no other solution to stopping the motor more quickly I have no other choice but to go to lever rollers and move them inward and so be it. But a prox sensor has very short range and people use those . . .?

And the tech in me wants to see if there is a setting in mach or ESS to make the motor stop faster. I may find out that is a bad idea further down the line when actually cutting...
I use roller plunger snap action microswitches as home switches. They are set up several mm 'inside' the machine boundaries.
They are operated by a little ramp screwed to the axis bed. This means that should the axis overrun the switch is not damaged.

I would suggest roller plunger switches a little inboard of machine limits. I would use prox sensors as limit switches.
I know most people try to combine limit and home switches into just one switch but then have exactly the problem you describe.
I prefer one roller plunger home switch per axis and two prox sensors at the limits per axis.

My wife left with my best friend...
     and I miss him!
I assumed there was only one switch at that position from the start. Never occurred to me to have a limit and home near each other. I'll change switches.

what motion controller do you have?

I use an ESS and that means that I can have the second port as inputs so I can have one input per switch, 3 homes and
6 limits and still have inputs left over for probes, Estops etc.

If you are using a UC300 an ESS or a 57CNC then you have plenty of inputs, there is no need to be stingy with them.

If you have separate homes and limits Mach is never confused about which event has occurred. If for instance you haxe the X home and X- limit
combined on one input then when you are referencing your machine you wish Mach to interpret a signal on that input as a home switch. Thereafter
it is to be treated as a limit and cause an immediate Estop. Mach can and does treat them differently but if it gets confused can you blame it?
Whereas if you have separate switches if ANY of the limits operates causes an immediate Estop at all times and conditions likewise the
home switch always gets treated as a home event, never any other interpretation required.

My wife left with my best friend...
     and I miss him!
I have an ESS connected to a PMDX-126/107.

I do have the available inputs and I am using them for home switches, I have no limits at all, well soft limits.  The single input question was for confirmation, I didn't think it was a good idea to combine them either, but I have seen schematics that do just that so it was worth asking.

I was looking at the MB3 for ESS, that BOB looks so much simpler to set up than the PMDX-126 !

the MB3 has a good reputation. It has a good balance of inputs and outputs with both single ended and differential

My only concern is that you are then locked into that manufacturers interpretation of a 'good balance'.

I use MB2's from Homan Designs in Australia. The model code is that close to CNCRoom's MB03 to cause confusion but
I believe Peter Homan was first in by many years with the model designation. The MB2 is a bi-directional one port BoB.
It does not have any opto-isolated inputs, relay outputs or a PWM to analogue circuit.  What that means is that the
flexibility is mine, I make and add the additional circuitry as I require. This suits me as electronics is my thing.
If electronics is not your forte then the MB03 is a good alternative.

My wife left with my best friend...
     and I miss him!
Wait... the only MB2 (and MB3) I know about is from CNCroom (Thailand) . There is another vendor from Australia with a board called the MB2 ? Tough for us neophytes to get up to speed with that kind of name duplication thing going on.


Peter Homann has been making these for years and well and truy predates the CNCRoom boards.

My wife left with my best friend...
     and I miss him!