About homing and switches...

[routercnc]

To use one sensor and two targets... Arent the sensor pointing towards one of them, how can you make it work in two directions?

Imagine the sensor is mounted to the gantry and pointing down. It moves sideways over the target, not directly towards it. At some point when it is directly overhead it will trigger and stop the axis. If there is a fault it will just glide on past without damage until the gantry hits the hard stop further along.
When it is moving the other way, again it is looking down and will eventually glide over the target at the other end which will trigger the axis to stop. So one sensor, two targets.

It has been shown to be just as effective using a parallel approach as a direct approach. As long the target gets within 4mm (or whatever the rating is) it will trigger no matter how it got there.

[routercnc]

As you are using CSMIO, as Paulus mentioned, this will take the prox sensor 24V directly. It has taken me a while to find my original drawing but for anyone with a cheap 5V BoB this is what I had to do to get it work. Without the diodes it was very unreliable at triggering. With the diodes it is 100%. Maybe these should be fast acting types, but mine were regular types and the homing is very repeatable. Sorry for the side track on the OP.

[JW1977]

Imagine the sensor is mounted to the gantry and pointing down. It moves sideways over the target, not directly towards it. At some point when it is directly overhead it will trigger and stop the axis. If there is a fault it will just glide on past without damage until the gantry hits the hard stop further along.
When it is moving the other way, again it is looking down and will eventually glide over the target at the other end which will trigger the axis to stop. So one sensor, two targets.

It has been shown to be just as effective using a parallel approach as a direct approach. As long the target gets within 4mm (or whatever the rating is) it will trigger no matter how it got there.

Offcourse, thats how its done, thanks!
Well then i have a pretty good idea how to do this for X and for Y.
Im not really sure about Z yet. How do you put the "down" limit for the Z axis?

[routercnc]

Offcourse, thats how its done, thanks!
Well then i have a pretty good idea how to do this for X and for Y.
Im not really sure about Z yet. How do you put the "down" limit for the Z axis?

Lots of people don’t have one since the height of the workpiece and stick out of the tool are variable so there is no obvious place to put it unless you set it up in a variable position to suit each job

[AndyUK]

Lots of people don’t have one since the height of the workpiece and stick out of the tool are variable so there is no obvious place to put it unless you set it up in a variable position to suit each job

I'm planning on putting mine so that the ballnut doesn't impact the BK/BF mount, and so that the carriages don't fall off the linear rails.

[john swift]

As usual
the devil is going to be in the detail
when looking for the solution to problems connecting limit switches to breakout boards
for instance the LJ12A3_4_Z_BX proximity switch does not have the simple open collector NPN transistor output you may expect going by some adverts and data sheets
It has a hidden 10K pull up resistor connecting the output to the positive supply !


with ref to the diagram in post 24

adding the diodes works with the usual TTL inputs found on most BOB's
that requires the switch to either ground the input or go open circuit

some Chinese BOB's have opto isolated inputs with a PC817 input LED and current limiting resistor connected to +10V

the Geckodrive G540 has its opto isolator connected to +12V and has a 2K2 current limiting resistor


so you don't always see 5V across the BOB's open circuit input and ground terminals !

BOB's like CNC4PC's C31 need a positive 24V input
so if you are using proximity switches you need one with a PNP output
to pull the BOB's input up to the positive supply

many Chinese BOB inputs connect to either a 74HC14 or 72HC245 logic
IC input
(see the upper left hand corner and centre of this diagram for the Chinese BOB input
Lower left hand corner for the C31 )



( I prefer N/C switches because a broken wire will be trip the axis limit
with N/O switches a broken wire will not be detected )


John

[Clive S]

Well written John 👍

[cropwell]

As per reply #17 to this thread:

John, this topic was discussed (with the internal circuit you reprint) in 2016, till I got sick of it. The LJ12A3etc is designed for industrial 24v control sensing, but it works equally well switching amateur 5v (without diodes) if your BoB pulls the pin to logic 1 at 5v. The supply voltage to the device electronics has to be min 10v max 36, it will work below 10v, but not reliably, presumably because the oscillator duzzn't buzz. It has an internal 10k, but even so, the thing operates for practical purposes, just as if it is a NO relay on the blue and black wires. I happily use them with a 12v supply with the black wire connected directly to the BoB input.

Peace and Love

Rob-T