Wiring 24v NPN Proximity Switch to 5v BOB - Why does'nt it work

[Clive S]

Now I'll have to draw out the combined circuit of four of those (NC versions) strung in series, so that I can use a single input for combined limit switches, just to reassure myself that it should work - even though I've wired it up on the bench and it does seem to.

Not sure but I recon they will be ok for limits but if used as home combined.
I think there will be an increasing delay put in to the circuit the more you connect.

[Neale]

Couple of points there. Personally (and without looking up switch data sheets to see if they quote switching times) I suspect that compared with typical machine speeds, any delay will be insignificant (*). That's based on gut feel rather than hard data, though. Second point is that I would be more concerned about repeatability than actual switching time. I don't really care if the response time is, say, 100uS as long as it is always 100uS, so that the machine always stops in the same place. Doing a bit of my usual back-of-the-envelope sums, I reckon that at full tilt my ballscrews will be turning at 1000rpm. Say, 17rps. 800u steps per rev, so 13600 steps/sec. That's about 75uS per step. In practice, final homing is done at, say, 10% of that, so as long as the switch responds repeatedly to the nearest 3-400uS, you should be able to stop at the same ustep. Dean's demo of homing repeatability using proximity switches a while back supports this.

(*) what probably matters more is that you always home at the same speed as I suspect that exact switching points will depend on speed of approach to target but that is something that's under our control. It only matters for homing anyway; don't really care exactly where the machine stops if it hits a limit as long as it stops before something gets broken!

[cropwell]

I think there will be an increasing delay put in to the circuit the more you connect.

Apart from the inductive sensing circuit, I see no capacitative or inductive components that will introduce any time elements into the switching circuit. Sorry for the delay in replying

[Clive S]

Apart from the inductive sensing circuit, I see no capacitative or inductive components that will introduce any time elements into the switching circuit. Sorry for the delay in replying

I was looking through this- http://www.ab.com/en/epub/catalogs/1...23/Wiring.html

It states "response time is equal to the response time of the first sensor plus the sum of the turn on times of the others. "

But all this has been killed to death many times. Can't beat having each sensor on its own pin.

[cropwell]

I was looking through this- http://www.ab.com/en/epub/catalogs/1...23/Wiring.html

It states "response time is equal to the response time of the first sensor plus the sum of the turn on times of the others. "

But all this has been killed to death many times. Can't beat having each sensor on its own pin.

This only seems to apply for two wire sensors in series. Three wire sensors in parallel won't be affected. Phew: our 'homes' are secure, we can all sleep safely in our machine beds this afternoon.



Rob

[Boyan Silyavski]

Ok. Now that io have you here :-) , i have another quick question . If BOB has transformer that powers limit switches and at the same time spindle speed output, and you connect a couple of limit switches in series and find that spindle speed output does not work right. Then could you add additional transformer positive and negative to the BOB outputs, same 12V vdc, but from another supply , so you kind of "help " it?????????

[cropwell]

Ok. Now that io have you here :-) , i have another quick question . If BOB has transformer that powers limit switches and at the same time spindle speed output, and you connect a couple of limit switches in series and find that spindle speed output does not work right. Then could you add additional transformer positive and negative to the BOB outputs, same 12V vdc, but from another supply , so you kind of "help " it?????????

In theory you could, but it is not good practice and I wouldn't recommend going anywhere near that solution.

How about a diagram of what you are doing with some component info ?

There is probably another issue here. I would guess that your 12v power supply might be under-rated for the job, but having said that I don't know what load you are pulling. You could try adding a reservoir capacitor across the power line and also some decoupling capacitors near the prox switches, just in case you have switching transients going back on the power line.

If you are talking about another 12v supply, why not use it for the sensors only and leave your working spindle speed controller alone ?

Cheers,

Rob

[Neale]

You say that you put two limit switches in series and it "does not work right". Does it work with just one limit switch connected? Does it work with no limit switches connected? I've been looking at a couple of examples recently where the BOB or motion controller has an analogue output for spindle speed, and it looks as if these often have a separate ground connection that should not be connected to the same ground that the limit switch inputs use on the BOB. I agree with Rob, though - it would be a lot easier to understand if you could provide a diagram and some details of the devices you are using. It's too easy to guess and make the wrong assumptions otherwise. If you are talking about spindle speed control here (not power to the spindle motor) it would be surprising if the problem was a lack of power because the limit switches and speed control inputs take quite low amounts of power. Interference is possible but shortage of power - probably not the problem.