1394 Sercos with ControlLogix CPU Question

[BobLfoot 301]

Our brilliant engineering department recently upgraded our 1394 GML Commander and PLC 5 based line control servos to 1394 Sercos and ControlLogix CPU's. The change was more or less seamless, but we still have one or two lingering bugs that threaten to eat our lunch. I just spent over an hour on one of these and wonder if any guru's outhere have an idea why. Initial Problem Description - Line was emergency stopped by an operator to clear a material jam. On the restart the code is supposed to automatically home all axes. One axis did not home, but the line started anyway. It was at this point maintenance was called. Maintenance Response - Taking the Engineering issued "Cheat sheet" we opened RSlogix 5000 and went online. We then selected the axis in question and opened the motion direct commands window. As per our instructions we issued an MSO {this starts the axis?} which completed without error, We then issued an MAH {Which is to home the axis} this completed without error, but resulted in no motion by the axis. We then issued a series of MAM commands setup as Incremental, -10 position, Speed 10 Uinits/sec/sec with an accel and decel of 10 units/sec/sec. This did move the home tab to the home limit switch. When about 15 degress from home we again tried MAH and the axis homed. This allowed the machine to restart and run correctly. So far tonight we have had to do five technician assisted homings. Axis Configuration on Homing Tab - The axis configured for Active Homing Position 0.0 Offset 0.0 Limit Switch Normally open Reverse bi-directional homing speed 10 degrees / sec and return speed 1 degree per second. My real Questions - 1. How can the Motion Direct Command MAH complete without error and not result in axis motion and yet set the homed status flag? 2. We are consulting our AB Motion Specialist, but does anybody have any ideas where to go next? Edited 14 Jun 2006 by BobLfoot

[TechJunki 0]

I have recently seen this before, in my case the Axis State was Disabled and the Motion Direct Commands would say completed without error but then do not result in any movement. This seems somewhat decieving to me. From what you are describing that may not be the issue in your case if the MAM function worked and the MAH did not. Did you issue any other commands between issuing those two? When using the motion direct commands if you have the motion group highlighted in the tree view there should be a axis status box in the lower left of your screen, you may want to look at this and see what the Axis State is when this happens, you may also want to take a look at the Axis and Drive Faults, I believe the Axis State should say Ready when it is able to accept most Motion Direct Commands.

[BobLfoot 301]

There were no commands issued betwen the MAH and the MAM command. Occaisionally the MAH would produce a brief jump but no sustained motion.

[TimWilborne 108]

First you say one axis failed to home. So I'm guessing that there are more than one. Is it always the same axis that is getting you trouble? What flag are you using to say the axis is homed, let's execute the next command? Also look at MAS and maybe MSF instructions. It is a long shot but I can't remember what a MAS does to the homing status. I think the MSF causes an error I haven't used the homing commands in a couple of years. We have many M02AEs and 2098 drives so I still work with motion though

[Gerry 13]

Once an axis has been successfully homed the 'axishomed' status bit is set and stays set until the drive or PLC is powered off. If the program uses this status bit to determine if the axis is homed, then it may skip the MAH instruction. If the axis is sitting on or near the home sensor when the MAH instruction is initiated, then there may be very little/slow movement which could be mistaken for no movement.

[TimWilborne 108]

Another thing, under your axis properties go to the homing tab. What is your homing sequence set too?

[BobLfoot 301]

The axis is setting 120 to 150 degrees from home and is set for 10 degress per second. So homing should take 10 to 15 seconds by my calculations. Can the axishomed bit be toggled by logix 5000?

[TimWilborne 108]

Can it be? Unfortunately it probably can be. Should it be. It usually will cause you a lot of trouble. I know this is (or was) a big problem with message instructions What are you thinking is happening?

[Gerry 13]

I don't think so. However some instructions will. e.g. MASD The MAH resets the status bit on initiation, then sets it again on completion. Does the program condition the MAH instruction with an XIO of the 'axishomedstatus' bit? I've just recently had a strange, similar problem, where an axis was instructed to home, didn't move, but said it was properly homed. I traced it to the home proximity switch. On its own, the axis would home properly, no problems. But when two other servo drives were powered up, this particular home prox started oscillating. Tried replacing the prox - same result. Replaced with same brand, different size - OK. Replaced with same size, different brand - OK. Try monitoring the axis tag's "homeinputstatus" on a trend if you suspect any problem like that.

[BobLfoot 301]

Rockwell sent in their Motion Specialist yesterday to look at the system and he regrounded all the Drives and Home Limits and the problem dissappeared. Turns out we were getting false home signals from noise, so the drive thought it was done.

[TimWilborne 108]

Now those are the problems that will drive you crazy How did he find it or was it just a wild guess?

[Peter Nachtwey 15]

Why would it be a wild guess? Why would you assume that there is something wrong with the controller homing routines and not the wiring when the controller has been tested in 10000s of applications and the wiring in only one?

[TimWilborne 108]

No Peter I would not think it was something wrong with the Controllogix homing code and that is not what I asked. Programs will do whatever they are told to do right or wrong. I asked if there was any method to him finding that there was a noise problems or if it was a guess. If there was a method to him finding this then I would like to know about it for future reference. If he just went up and started messing with the wires then in my book that goes under the category of a wild guess. Nothing wrong with doing this, I do the same thing. I work in an extremely noisy environment and it is not a matter of is there noise on the line or not, it is a matter of if the noise on the line is affecting the operation of the machine. Anytime I think I can learn new I ask the question.

[OkiePC 14]

Home and registration inputs are very high speed inputs which makes them inherently very sensitive to noise. I have seen this same problem on 1394 and s-class controllers. Adding surge suppression and rerouting cables/grounding shields usually helps. I have one machine which still caused us trouble so I changed the program so that it does not home unless the controller is restarted or has a fault which could cause a loss of position. The operator can still command it to home, but it isn't part of recovery from E-stop anymore. Paul