Precise homing?

[Bubba]

michaelthomas,
I can't really respond to the smoothstepper as I do not use one and am not familiar with it.

[hub]

Ok, so as I don't have an index pulse from the encoders (plus, I'm thinking of letting the Z-axis be driven by a stepper without an encoder).
So I have to make the index pulse myself. I'm thinking of a disc with a mark and a sensor. Optical would be the better way in this case I think?

[Al_The_Man]

One other potential problem to look at to get the home position precision you are looking for, is Mach does not know exactly the position of the servo's because the position is not confirmed back to the controller, when a marker is received.
When Mach registers the marker as home and stop the servo's, if the servo's are not suitably responsive to the stop command there could be a an overshoot error however slight?
In a closed loop system, when the marker is found and the zero position captured, any slight overshoot of the servo's is known and hence the axis position is known relative to home zero.
It may not be an issue, but one to keep in mind.
Al.

[hub]

One other potential problem to look at to get the home position precision you are looking for, is Mach does not know exactly the position of the servo's because the position is not confirmed back to the controller, when a marker is received.
When Mach registers the marker as home and stop the servo's, if the servo's are not suitably responsive to the stop command there could be a an overshoot error however slight?
In a closed loop system, when the marker is found and the zero position captured, any slight overshoot of the servo's is known and hence the axis position is known relative to home zero.
It may not be an issue, but one to keep in mind.
Al.

Very true. My drive has an output that tells if it's in the commanded position or still trying to get there. I could use that as well.
But could it be done with simple switches only..?

* Linear mark sensor on -> AND rotary index sensor on -> AND 'On Position' on.

Just all of them in series would work?
Would Mach3 "get it"? It would be an extremely short pulse and only happen if all 3 are ON.. And it would probably need to go back & forth a couple of times until settled..

[michaelthomas]

I see.......Mach tries to back off the sensor to register home, doesn't it? I was wondering how that would react to just a pulse. Maybe it wouldn't matter.

[hub]

I see.......Mach tries to back off the sensor to register home, doesn't it? I was wondering how that would react to just a pulse. Maybe it wouldn't matter.

I believe Mach just assumes a simple sensor is attached (microswitch for example), when it switches to 'ON', Mach reverses and goes slow until it goes off and stops.

[michaelthomas]

Ya.....but if it can't stop directly on the pulse......it would overun it and re-trigger the pulse on its way back, which would create a fault if it was being used as a limit switch as well.

I don't think you could count on any hysteresis in the index pulse.

I don't have any practical experience in this......I'm just trying to think it through, so i may be way off.

[hub]

As I have understood it, hysteresis is taken into account. As far as Mach 3 is implemented.

[michaelthomas]

I guess what I'm trying to understand is how wide the pulse may be. If you could stop directly on it, would it be continuous like a switch?

Rather than stress myself with things I won't fully understand without a lot of study.....I would like someone to chime in and say, " This is how I did it, and it works great".....lol

May not happen, though.

I can follow directions very well......but designing a circuit is more of a stretch.

[hub]

I guess what I'm trying to understand is how wide the pulse may be. If you could stop directly on it, would it be continuous like a switch?

Rather than stress myself with things I won't fully understand without a lot of study.....I would like someone to chime in and say, " This is how I did it, and it works great".....lol

May not happen, though.

I can follow directions very well......but designing a circuit is more of a stretch.

I don't know. It could be ON for 180deg and OFF 180deg or anything in between. And the length of the pulse in time would depend on rpm.
But, in my opinion, the length of the pulse doesn't matter as long as it can be detected. What you are looking for is the rising edge (or falling edge) of the pulse.

[hub]

And for the " This is how I did it, and it works great" maybe take a look it this:
Electronic home switches made easy!

[michaelthomas]

OK.....that makes sense.

I have only a rudimentary understanding of electronics.....its just not enough in this case....lol. It never occurred to me that the pulse could occupy more space or time than just an instant.