Gantry Limit switches & homing question.

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07 May 2019 21:01 #133094 by michaelrommel
Hi Rod and Andy,

thank you for your feedback, suggestions and the interesting concept, that you, Rod, implemented.

I have a XYYZ 4-joint machine and intend to use the auto-squaring feature, I have 10 proximity sensors at my disposal and enough free inputs on my mesa card. The sensors are of the NO type and I read in various threads, that it is a good idea to have a NC system, because any broken cable or sensor then immediately gets detected as error. That was the reason of my contemplating the "normally detecting metal" approach and have the limits / home detected as "end of metal".

I do not yet have any idea, how far my gantry / crossplate (X-Z) might coast - I wanted to have the limit switches installed first, before I experiment with powering up the motors and tuning them. They are JMC IHSV57 180W Servos.

Combining both of your approaches, I came up with the following concept, which would then use 2 sensors on each joint, making it 8 of the 10 sensors. With Andy's logic this might actually work.



Thank you for your support!

Michael.
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07 May 2019 21:21 #133096 by rodw
Looks good. The other way to do it would be to use a shared home/limit switch at one end and a limit switch at the other. Just that this way, you need to use a home offset big enough to move the sensor off the sensing plate at the end of the travel which reduces the machine working envelope.

Coasting is a function of homing velocity so it can be reduced by slowing the homing velocity down.

I know they say NC is better but in the end most of my sensors are NO as it comes down to what is affordable and available.

If you could get your hands on one more sensor, you add max limit sensors to all axes and just put the max limit sensor on one side of the gantry only and use it for both limits (Y and Y1).
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07 May 2019 21:24 #133097 by michaelrommel
Pretty good idea with the combined max limit for the two Y-joints! Thanks!!

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08 May 2019 01:14 #133106 by rodw
In fact, thinking about it, you probably don't need to combine them. Just rely on a sensor on one side/joint.

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