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Author Topic: Issues with using inductive proximity sensors as limits on Shopbot  (Read 3693 times)

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I am currently in the process of retrofitting a Shopbot PrsAlpha to work with Mach 4 and the PMDX-422, and I'm having trouble getting the limits to work right. The problem is that the Shopbot uses a single inductive sensor on each axis (which responds to a small metal assembly at each end of the axes). This means that there is not a distinct signal for X++ and X--, and the same goes for Y. I was hoping that Mach 4 would be able to discriminate between the positive and negative limits based on the current direction of movement, but this doesn't seem to be the case. Instead, when I set up the inputs for X++ and X-- as the same input pin on the PMDX, the software triggers both the positive and negative limits, and does not allow me to jog back within bounds. My current solution is to simply enable the X-- limit, and just use the limit override when it hits the positive limit (since it will read it as the negative limit and will not otherwise allow me to put it back within bounds). Of course this is not ideal, and I worry especially that others using the machine may be confused or override and move the wrong way. Any suggestions would be much appreciated!

Offline BR549

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Re: Issues with using inductive proximity sensors as limits on Shopbot
« Reply #1 on: July 22, 2015, 08:01:07 PM »
YOU only really need one limit per axis as you already have. It will trip at either end of travel.  To be able to  jog OFF the limit select THAT option in Gen Config. Then you can jog off of a limit just remember where you are going(;-). 

(;-) TP

Offline dude1

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Re: Issues with using inductive proximity sensors as limits on Shopbot
« Reply #2 on: July 22, 2015, 10:00:47 PM »
you can work around it there is a small problem with M4 does not look like it`s fixed yet

you can use custom name / input name and call it X +- and have it set to the inputs in Mach have a look here it may help its different controller yes but it may work it`s the setting what`s important hopefully bob will see this and say if it a go or not

  http://warp9td.com/index.php/kunena/7-general-discussion/5321-homing-in-mach4?start=6
Re: Issues with using inductive proximity sensors as limits on Shopbot
« Reply #3 on: July 22, 2015, 11:07:56 PM »
I have just tested ganging the home, neg limit, and pos limit switches all on the same input and it works as it should in Build 2522 and 2568. If the signal is active you cannot jog or otherwise move the axis if Limits Override is not turned on. When you turn on Limits Override jogging works fine in both directions. It is true that Mach4 does not try to figure out which way the axis was moving when the limit condition occurred. This is not practical because the machine could be powered up with the limit switch already active, or a limit could trip due to vibration even with no axis motion if it was very near the trip point.

The previous poster's description of a "custom name" is a requirement imposed by the ESS plug-in. This is not a "problem" in Mach4. It does not apply to the PMDX SmartBOB because all signal names are already custom names and they will appear in the Mach input configuration dialogs where they can be used for multiple functions without further effort on the user's part.

Steve Stallings
www.PMDX.com
« Last Edit: July 22, 2015, 11:18:36 PM by Steve Stallings »
Steve Stallings
www.PMDX.com

Offline dude1

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Re: Issues with using inductive proximity sensors as limits on Shopbot
« Reply #4 on: July 23, 2015, 12:18:41 AM »
thanks for the correction Steve 
Re: Issues with using inductive proximity sensors as limits on Shopbot
« Reply #5 on: July 24, 2015, 11:17:48 AM »
Thanks for the information and suggestions, guys! Based on this, I think that I may end up investing in secondary limit switches to discriminate between positive and negative switches. Because I am mainly retrofitting this machine to be usable by engineering students who have probably never done CNC work before, I am always worried about students accidentally jogging it the wrong way using an override, especially if they don't realize the reason it stopped was because of a limit switch.

Offline BR549

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Re: Issues with using inductive proximity sensors as limits on Shopbot
« Reply #6 on: July 24, 2015, 12:47:56 PM »
That reall y does little good as you can JOG in any direction.  you COULD create a button that ONLY allows you to jog in the opposite direction.  My opinion if they cannot look at teh machine and SEE where they are at and determene which direction they need to jog  they need to stay away from it in the first place. In reality IF they are hitting the limits they have already messed up the part.

Rule #36  MAKE SURE the part will cut in the space alloted BEFORE you push the CUT button. A simple dry run around the perimeter does very well to see this AS does looking at the  part MIN MAX values and use a tape measure. A miin max of 6x6 will NOT cut on a 4x4 table.

Common sense goes a LONG way with using CNC and keeping ALL your fingers .  I would be teaching that part FIRST.

Just a thought (;-) TP