CTRIO2 Homing Issue

[Mike Nash]

Do-more, H2-CTRIO2 (qty 3) and two axes giving the same unexpected behavior on home seek.

I have a typical industrial roller arm limit switch for home input on CH1-C input. The axis moves reverse to the limit (rising edge), forward again to the falling edge and then resets it's output to zero. But the limit switch is not off at this point. That severely messes me up if another home sequence gets triggered because there is no rising edge so it moves to the overtravel limit.

I have trended the input but do not see the falling edge there either. Both motors are moving as expected, but both end with the home limit made. I know I may have some issues with the belts being too loose and a limit switch may not be ideal, but that is what the original system was using. AC servo (step, direction), toothed belt to ballscrew.

I will probably add a move to the home routine to get off the limit and also check for it being made already, but was wondering if there is anything I am missing.

[Controls Guy]

Is there a double-reverse make-break-make option?  I think the Control Logix has that, I assume for long axes so you can home relatively fast because the first make of the limit switch isn't accuracy critical.  That way, even if it starts homing while on the switch it should still right itself.

I've also done homing by using an on-the-fly coordinate redefinition with a registration input.  IOW, sail past the home switch (typically a photo looking through a very narrow slot, like 0.010-0.020), and just redefine the axis position on the fly, then stop.  You end up on the other side of home, but you're synched to the machine axis coordinates.  Have no idea if CTRIO has that capability.  I think the new Do-More's are going to (lots of motion stuff built-in, iirc).

[BobO]

Do-more, H2-CTRIO2 (qty 3) and two axes giving the same unexpected behavior on home seek.

I have a typical industrial roller arm limit switch for home input on CH1-C input. The axis moves reverse to the limit (rising edge), forward again to the falling edge and then resets it's output to zero. But the limit switch is not off at this point. That severely messes me up if another home sequence gets triggered because there is no rising edge so it moves to the overtravel limit.

I have trended the input but do not see the falling edge there either. Both motors are moving as expected, but both end with the home limit made. I know I may have some issues with the belts being too loose and a limit switch may not be ideal, but that is what the original system was using. AC servo (step, direction), toothed belt to ballscrew.

I will probably add a move to the home routine to get off the limit and also check for it being made already, but was wondering if there is anything I am missing.

Not fully understanding the issue. Are you saying the switch is already made prior to running the profile?

And yes, the new hardware home search has the option of edges or levels, which is probably the solution to the problem.

[Mike Nash]

While not on the home switch, initiate home seek. Axis moves in home direction until home switch goes true (rising edge). Axis reverses and moves back away from home and should reset the position to count as the home switch input goes low (falling edge).

While the motor movements look right and the home position appears repeatable (if started off home). However, the home switch is not low at the conclusion of the sequence, but it should be. Same issue with two axes.

IF, another home sequence gets initiated while the home switch is true, the motor just drives off the back side because the CTRIO2 is looking for a rising edge that it is not going to get.

I can't select "level" for the limit 1 type, just rising, falling, or either edge.

And I can't swear the home switches aren't going low then bouncing back on somehow. I did not have a lot of chance to look into this today.

[BobO]

It's a mechanical recoil issue. Try adding some filter time to the input.

[Mike Nash]

Thanks BobO, I'll look into that tomorrow. I forgot the filters were even there. It looks like I have it at the default 1000nS.

If that doesn't work, I think I'll enable a different CTAXLIMT if the home is already on and just reverse to the falling edge. That would be easy enough.

I need to nail these positions as close to the thousandths of an inch as I can and home is critical.

[Mike Nash]

IOW, sail past the home switch (typically a photo looking through a very narrow slot, like 0.010-0.020), and just redefine the axis position on the fly, then stop.  You end up on the other side of home, but you're synched to the machine axis coordinates.

The sailing past is what I need to prevent! 

On and back off looks to be best for this app as there is an overtravel just past home (which needs to be adjusted I noticed after stalling into the hard stops.)

I think the new Do-More's are going to (lots of motion stuff built-in, iirc).

I've several smaller projects just begging, but I don't know that they can wait.

[Mike Nash]

Well I just realized I completely forgot about the filter on the home switch today. I wound up selecting a home sequence based on home switch status at the start and that seems to work fine, though involved. It also prevents unintended behavior.

Then I found I needed to pre-load a non-zero current output for the CTRIO and encoder feedback after home is found. That also was more involved than I cared for.

None of it is terrible, just a little harder than pleasant for what I need to accomplish, mostly due to the handshaking/handoff between steps.

It will be cool when these are all in one neat instruction (hint, hint).

[BobO]

Well I just realized I completely forgot about the filter on the home switch today. I wound up selecting a home sequence based on home switch status at the start and that seems to work fine, though involved. It also prevents unintended behavior.

Then I found I needed to pre-load a non-zero current output for the CTRIO and encoder feedback after home is found. That also was more involved than I cared for.

None of it is terrible, just a little harder than pleasant for what I need to accomplish, mostly due to the handshaking/handoff between steps.

It will be cool when these are all in one neat instruction (hint, hint).

I believe AXHOME will do everything you need. Now you just need a controller that supports it.

[Mike Nash]

Oh man... you had me hooked by the fifth word and then dashed my hopes. I am very aware it is easy to miss instructions and functionality - I was hoping!

Then again I go to pop out a quick rung in a D0-06 only to finally have it dawn on me that I'm trying to use a Do-more instruction.

It is still interesting to find things sometimes harder in another PLC brand than Do-more. But the opposite is true at times too.

(I think the hardest thing is to type "Do-more" rather than "D0-more".