What Limit Switches?

[A_Camera]

So looking for both homing and limit switches.

This is for Thor which will have movement speed of at least 10m/min. Mechanical or Proxy? Will possibly using Index homing once sorted but I need to be able to stop a servo run away, so looking for something that will work and reliably.

I definitely vote for mechanical NC limit switches. The reason is that if something goes wrong with them the machine stops, you look for the error and eventually will find it. If you use proximity switches this may not always be the case, regardless if you use NC or NO type. Of course, for limit switches, nothing else but normally closed (NC) should be used, they guarantee broken wire detection also, which you can't have if you use normally open connection.

For home switches... well, I think mechanical is best but accurate mechanical switches are very expensive. I don't use any home switches, but have heard that proximity switches are accurate too. I guess it depends on the accuracy you need and the method of homing. Some software are better than others.

[komatias]

Depends on the controller actually.

Mechanicals are fine for both limit and home but have you thought of maybe using the home on index function offered by some controllers? I have this on my GSK and I know that some of the CSMio's have this function too.

It would be the case of hitting a switch that initiates the slow movement and then does one full rotation of the screw until a flag is tripped or similar.

[Chaz]

So based on all of this, I am 'leaning' towards this type of limit switch. There is no 'end' sensor so if it runs part, it just pushes the arm.

Something like this -

[Chaz]

Any reason this will not be suitable?

This will be used to trigger into a CS Labs IP-A unit.

http://uk.rs-online.com/web/p/safety...tches/6211515/

[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?