Thread: Inductive Limit Switch problem
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01-04-2020 #21
Interesting analysis. Re. the PCB headers, the only caution in a high noise environment is the protection of the '14 inputs to >5V spikes. The IC should have clamp diodes that should work to protect but worth bearing in mind.
I have to admit all my cheap boards have had opto isolated inputs which I think can only help for noise immunity for systems like this.
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01-04-2020 #22
If it were me I'd be doing the following:
1) Add a decently low (<= 2K2) pullup on the output of the switch to 24V
2) remove the diode
3) Increase R2 to 10K
4) add a 2K7 resistor across the PCB header pins
5) up C1 to at least 10nF
6) remove R3
This should give a decent voltage swing on the switch output and a 0.1....+4.5v ish swing on the input to the 'HC14 with a bit of filtering and a 12dB attenuation of noise on the switch output to boot.Last edited by Voicecoil; 01-04-2020 at 10:29 PM.
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02-04-2020 #23
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02-04-2020 #24
After I put up the last post I thought for a while and realised that using a diode was the wrong way to do this and that a simple potential divider with the BOB input seeing the open collector of the sensor directly was a far better idea as you suggest. I have a spare sensor and a spare BOB (that's one of the advantages of using a cheapo board instead of a $900 one) so I'll try those component values and check the volts. Thanks for working out the values.
It's tempting to try replacing the surface mount components on the BOB but I think an external bit of Veroboard with all the new bits soldered on will be the best option.
Doddy,
Come to think of it, I've probably got a quad opto-coupler or two in stock though I'll need to print up a surface mount PCB to use one of those. I feel a rummage through my drawers coming on!An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.
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02-04-2020 #25
Does Mach3 disable limits when homing, or is this an option?
I am not able to look at my machine as it is currently on the floor in my office, which I am refurbing.
Looking back through threads (to 2016), I sort-of answered this question, but left doubt. I use a NO inductive switch for homing, but a NC microswitch for limits and I just thought that a break in the homing circuit could cause a crash if the limit is deactivated.
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02-04-2020 #26
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02-04-2020 #27
OK, after serious consideration my priority has to be getting the #$%^*&^%$#@ing machine actually working so I can make some stuff with it. Playing about with circuit ideas is fine, but...
I'm not wasting a long weekend off work waiting for RS to send me some new cable and other components including 2mm pitch plugs for the PCB headers, so I've made a circuit board which connects into the existing screw terminals without the need to remove/replace surface mount components on the BOB itself. I expect it's going to work. If so, it'll do for now and I'll make a more reliable change to the wiring later. This machine is after all just a toy in my shed rather than the heart of a commercial operation.
Watch this space for details if it works!An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.
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02-04-2020 #28
Notice that Mach3 is already loaded
Thankfully, I had all the cabinets and worktop (25mm moisture proof MDF, 800mm deep) delivered before lockdown. All the carcases are built and in position. Next job is the worktops, and they are heavy sheets.
Anyway, My question is about homing and limits and I cannot find any reference to the query in the Mach3 reference manuals.Last edited by cropwell; 02-04-2020 at 02:03 PM.
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03-04-2020 #29
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03-04-2020 #30
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