Mesa Cards and Inductive Proximity Switches

Guys,

I'm currently running mechanical limit and homing switches on a little CNC router using Mesa 5I24 and 7I76 cards. On a lathe I have just started rebuilding I want to use the same cards, but with inductive proximity switches instead. Probably something like the little Omron TL series.

I take it that the switches will play nicely with the cards. I guess that the switches will be fine for limit switches, but are they accurate/repeatable enough to be used as a homing sensor? I've had a look at the Omron data sheets, but can't work out if they are.

Cheers,

Steve

I´ve used inductive switches of 12mm diameter as homing swithches with 5i25 + 7i76 and I've achieved a repeatability of less than 0.1mm on linear axes and less than 0.05° degrees on circular axes (the sensor is 50mm away from the center of rotation of the axis). I'd guess that having the sensor farther away from the center of rotation should give you more repeatability.
I would recommend to double check if NPN or PNP output are more compatible with the card, so as to avoid having to use external components. I don´t remember exactly which one was better, I only remember having messed it up when I ordered the sensors and having to use pull up resistors to fix it.
Best luck,
Lisandro

In a lathe use encoders with index. No not think in anything else.
A inductive or mechanical switch is not enough for a lathe, especially in the X axis. A encoder with index is nowadays inexpensive (>40€) and will give you a good homing.

for 7I76 (and 7I77,7I70,7I66,7I84) inputs,
PNP type proximity switches are the best
(no external components)

I tried an Omron TL-Q5MC2 inductive sensor the other day. Long story short, very happy with the performance of it.

I mounted it in the tool holder on a manual lathe, connected it to 12vdc, monitored the NC output, and ran it back and forth past across the end face of a piece of 1/2" square mild steel. Had the sensor hanging as far out of the tool holder as I could, as the instructions said that if it was adjacent to, or shrouded by metal, it reduces the sensitivity. Set the gap between the sensor and the metal at the recommended range of 40 thou. Used the lathe DRO to measure the travel of the sensor.

When approaching the metal, the voltage slowly dropped from 12v to 5v over a distance of 10 thou', and finally 'triggered' down to 0.7v at 13 thou'. When retracting away from the metal, there was no gradual change in the voltage - at a specific point, which was repeatable to about 1 thou, the voltage jumped from 0.7v straight up to 12v.

A larger mass of metal as the target, (6" lathe chuck), only made minor improvements to the accuracy. A larger gap between the sensor and metal, (up to 150 thou), doubled the approach range, but made no difference to the accuracy of the retracting trigger point. Mounting the sensor with the body shrouded by the tool holder, (front face flush with the end of the holder), made the biggest change to the approach range, increasing it to 55 thou, but had little effect on the accuracy of the retracting trigger point.

I think that this would be ideal for a limit switch, and also a homing switch where it is set up to approach/trigger, and then back off/trigger to set the home position. Cheers, Steve

Thanks for the tip Andy - that's a top idea.

I purchased a pair of the Omron M5 threaded tube PNP shielded type, (E2E-X1B1) and tested them. They have a really crisp on/off point on the approach and retract, repeatable to about a thou'.

As suggested, I plan to bury them in the carriage cross-slide dovetail in drilled and tapped passages. Once I conform the travel limits, I'll drill corresponding 5mm divots in the mating part.

Cheers, Steve

PCW wrote:

for 7I76 (and 7I77,7I70,7I66,7I84) inputs,
PNP type proximity switches are the best
(no external components)

Damn, I have a 7i76e and I've been collecting NPN proximity switches. Can someone tell me what external components are required and where they should be connected on a 24 volt NPN proximity switch? I'm guessing a pullup resistor or something is required. What value resistor?