What Limit Switches?

[Neale]

I was talking to a friend recently who passed on some of his experience of using large numbers of mechanical switches in machine applications. He told me that the most common problem leading to unreliable triggering, lack of repeatability, etc, tended to be due to oxide build-up on the contacts. Two things you can do to help - use a higher signalling voltage (one reason why industrial machines use 24V rather than 5V) as this helps create microscopic arcing at the contacts which helps keep them clean, and also don't use them with high impedance input circuits. Drawing, say, 10mA or more through the contacts similarly helps them self-clean.

Personally, my machine is for home rather than commercial use so I'm only thinking of my own safety rather than meeting H&S legal requirements, and I've chosen to use proximity switches for combined home/limit and limit purposes. NC, of course, and mounted as pass-by rather than crash-into with the gap reduced to a minimum to help repeatability. Targets are long enough to ensure that the switch will remain triggered even if the moving part overruns. There is a failure mode in which they will fail in a non-safe mode (open collector output transistor goes short-circuit) but as they have built-in indicator LEDs, it is very easy to test them individually by just waving a small bit of metal near them and seeing if they trip.

I bought a box of 10 from eBay. So far, one seems to be faulty on arrival. One has a very slight but visible transition between on and off (judging by the LED brightness) but I've yet to test this with a real motion controller connection. The others seem to give a very sharp on/off transition and just turning the ballscrew by hand and watching the LED, I'm getting better than 1 div repeatability on a 0.01mm DTI. I expect that using the real motion controller will give even better results. This afternoon's job is completing the control box limit switch wiring...

[Chaz]

I was talking to a friend recently who passed on some of his experience of using large numbers of mechanical switches in machine applications. He told me that the most common problem leading to unreliable triggering, lack of repeatability, etc, tended to be due to oxide build-up on the contacts. Two things you can do to help - use a higher signalling voltage (one reason why industrial machines use 24V rather than 5V) as this helps create microscopic arcing at the contacts which helps keep them clean, and also don't use them with high impedance input circuits. Drawing, say, 10mA or more through the contacts similarly helps them self-clean.

Personally, my machine is for home rather than commercial use so I'm only thinking of my own safety rather than meeting H&S legal requirements, and I've chosen to use proximity switches for combined home/limit and limit purposes. NC, of course, and mounted as pass-by rather than crash-into with the gap reduced to a minimum to help repeatability. Targets are long enough to ensure that the switch will remain triggered even if the moving part overruns. There is a failure mode in which they will fail in a non-safe mode (open collector output transistor goes short-circuit) but as they have built-in indicator LEDs, it is very easy to test them individually by just waving a small bit of metal near them and seeing if they trip.

I bought a box of 10 from eBay. So far, one seems to be faulty on arrival. One has a very slight but visible transition between on and off (judging by the LED brightness) but I've yet to test this with a real motion controller connection. The others seem to give a very sharp on/off transition and just turning the ballscrew by hand and watching the LED, I'm getting better than 1 div repeatability on a 0.01mm DTI. I expect that using the real motion controller will give even better results. This afternoon's job is completing the control box limit switch wiring...

Thanks. Good info. I'm using 24V, so this will help.

Can you link what you bought please?

[Neale]

Thanks. Good info. I'm using 24V, so this will help.

Can you link what you bought please?

Sorry - it was sufficiently long ago that that seller doesn't seem to have them any more. They were "LJ12A3-4-Z/AX NPN NC 4mm Inductive Proximity Sensor Switch DC 3-wire 200mA", which are available from quite a few other suppliers. I paid £23 for a box of 10 but again, prices vary and that was a couple of years ago.

[Chaz]

Sorry - it was sufficiently long ago that that seller doesn't seem to have them any more. They were "LJ12A3-4-Z/AX NPN NC 4mm Inductive Proximity Sensor Switch DC 3-wire 200mA", which are available from quite a few other suppliers. I paid £23 for a box of 10 but again, prices vary and that was a couple of years ago.

Thanks.

[Chaz]

https://www.amazon.co.uk/LJ12A3-4-Z-.../dp/B0053FA8B8 Seems to be the same?

[Neale]

Not absolutely sure. These specifically say "iron sensing" or somesuch. Because I don't have the original listing for mine, I can't say what they are. Proximity switches are sometimes iron only, sometimes anything metallic (although with reduced sensitivity, as I understand it). Mine certainly trigger on aluminium. Of course, if your trigger component is iron or steel, then that's not important.

[Chaz]

Not absolutely sure. These specifically say "iron sensing" or somesuch. Because I don't have the original listing for mine, I can't say what they are. Proximity switches are sometimes iron only, sometimes anything metallic (although with reduced sensitivity, as I understand it). Mine certainly trigger on aluminium. Of course, if your trigger component is iron or steel, then that's not important.

Ive ordered 3 of them to test and see. They should be fine.

Thanks

[Kardacian]

Ive ordered 3 of them to test and see. They should be fine.

Thanks

Would you consider using Hall Effect sensors? No moving parts extremely accurate and will outlast mechanical switches. Take a look at these, they run on 5 Volts but I can modify to use higher voltage if needed. They operate in the same manner as a NO switch. However a NC can also be created and will not be expensive.

https://www.tindie.com/products/Kard...ll_prod_search

[Boyan Silyavski]

Would you consider using Hall Effect sensors? No moving parts extremely accurate and will outlast mechanical switches. Take a look at these, they run on 5 Volts but I can modify to use higher voltage if needed. They operate in the same manner as a NO switch. However a NC can also be created and will not be expensive.

https://www.tindie.com/products/Kard...ll_prod_search

Great but 5v and not shielded = Problems

[Kardacian]

Great but 5v and not shielded = Problems

The voltage problem can be solved easily, what voltage do you need to support. I run these without shielding and have had no issues. What would be an ideal shielding for your environment?