HOMEAXIS failure

[Garyhlucas]

I have a program in the BRX to home the X axis and two Z axis on the machine I'm building.  I have twice had the HOMEAXIS fail to stop moving the axis towards the limit switch after seeing the switch.  I can see the switch is on but the servo is still moving towards the switch.  The switches are AD high precision switches mounted so they can slide by without damage but each axis has actually hit the physical end stops.  I can see the position counter still running while the switch is active.  It is set up to reverse direction and back off the switch for final position.  It is possible it isn't reversing and is still looking for the switch to open while moving forward.  One axis homes in the minus direction while the other two home in the plus direction.  Restarting the PLC seemed to clear the problem each time.

While on the subject.  It would be nice that on reversal off the switch you could set it to go some number of steps further.   Generally speaking you don't often want to set the home position exactly on the switch because you can't also use the switch as a safety end limit during operation.  So I try to make the counter zero with enough room for deceleration so that I can go there at speed without triggering the limit switch.

[Garyhlucas]

More data.  So I tried homing this morning and the two Z axis went fine.  However the X was parked on the limit switch.  So the switch was close when home axis was called and the axis moved further in the direction of the limit switch until it hit the stop and faulted the servo.  If you are already on the limit switch you should skip the motion towards the limit and move in the direction off the limit.  Otherwise you need to write a bunch of code moving you away from the limits before you have homes the machine.

[BobO]

More data.  So I tried homing this morning and the two Z axis went fine.  However the X was parked on the limit switch.  So the switch was close when home axis was called and the axis moved further in the direction of the limit switch until it hit the stop and faulted the servo.  If you are already on the limit switch you should skip the motion towards the limit and move in the direction off the limit.  Otherwise you need to write a bunch of code moving you away from the limits before you have homes the machine.

That's the difference between "Rising Edge" and "High Level". Rising Edge wants to see the actual edge. High Level checks for high before moving.

If the built in home search doesn't do what you need, you can use AXSCRIPT to build your own.

[Garyhlucas]

I get the difference.  Shouldn't high level or low level be used for homing?  If you re homing in one direct then edge triggering is clearly preferred.  However when you select finding the home switch at speed, ramping down and backing off I would think that that high or low level should be used going onto the switch and edge triggering for final position.  I really like the way the BRX provides so much high level functionality with so little programming time.

By the way, I am starting a new job on the 18th of April.  I have already brought them into the BRX world and I will be doing lots of motion projects going forward.  We have 4 on the table right now.  I am leaving behind long drives to and from work, and can work from home lots of the time.  No more automating worn out junk machines.  No more new Chinese machines that you first tear completely apart to correct all the problems, and redesign stuff that can't work before you even get to controls.

[BobO]

Shouldn't high level or low level be used for homing?

Maybe...that why both level and edge are options. You best know your machine.

I just mentioned it because it sounded like your failure was due to specifying edge instead of level. Maybe I misunderstood.

[Controls Guy]

Shouldn't high level or low level be used for homing?

Not necessarily.   Imagine an axis with a short enough travel and short overtravel such that it homes on the initial approach rather than reversing or double-reversing.   In that case, if you used level as opposed to edge and you were already in the overtravel when you started, home wouldn't be repeatable.

[Garyhlucas]

I would like to apologize for bringing this 'problem' up.  The problem was me, I completely missed the setting for High Level triggering vs Edge.  Works fine now.

[Greg]

I would like to apologize for bringing this 'problem' up.  The problem was me, I completely missed the setting for High Level triggering vs Edge.  Works fine now.

Garyhlucas (et al), if you ever chose to do a homing routine that is not covered in the AXHOME instruction, you can utilize the AXSCRIPT and write your own. But if you'll notice, when you pull up the AXSCRIPT instruction, there is an <Import> button near the bottom right. If you press that button and browse to:

c:\Users\Public\Documents\Do-more\Designer2_9\Projects\Examples\AXSCRIPT

...then you'll see some examples using AXSCRIPT you can import. You can use the homing ones as templates to craft your own (there are a few others that are not homing). If you click on one of them there is a quick view that comes up allowing you to look at their contents. There is even a text version of velocity/position graphs showing you how they work. There are:

Blended Move.txt
Common Home One Switch.txt
Common Home Two Switches.txt
Flying Saw.txt
Registration Min Distance Move.txt
Simple Home Position.txt
Simple Registration Move.txt
While End.txt

[Garyhlucas]

Greg,
I appreciate you bringing this to my attention. I changed jobs two weeks ago and now have three motion control projects in the works.