GML (1394/IMC-S Class) Overtravel Faults

[OkiePC 14]

This is not really a PLC question, but I'm sure that some of you have dealt with this before. The problem is with A/B 1394 and S-Class motion controllers' overtravel inputs and trapping faults. When an overtravel fault occurs, the global fault code changes to a 3 and the axis status changes to 9, but as soon as the limit switch is Okay, the fault codes go back to normal. I've had two different situations in which an intermittent OT limit caused troubleshooting confusion. The fault handler was unable to identify the cause because it cleared up before any of the fault reporting logic could occur. On one machine it was a loose connection. On the other, it was a drive belt that was whipping the limit switch lever very rapidly (barely visible to the trained eye). The only way I was able to diagnose the first problem was to put a print block in the beginning of the fault routine and watch the terminal window. I've tried copying the status words to user variables at the beginning of the fault routine, and this works occasionally, but most of the time (with the second problem especially) it's just not happening fast enough. The motion controller always detects the fault and jumps to the fault routine, but usually reports no fault. The maintenance techs scratch their heads for a while and then call me. Is there a way to programatically trap hard overtravel faults that is more reliable? Thanks Paul C.

[OkiePC 14]

I copied my post in an e-mail to Rockwell Software. I got a call this morning and had a lengthy discussion about changing some firmware to solve my issue. The technician from RS said that I had already done (in GML) what he would recommend and that there is no retentive evidence of hard OT conditions anywhere else in the controller. Also, this is the only fault condition that is not retentive. I suggested to him that the controller firmware be changed to either retain the axis status or the global status to allow us programmers to make more bullet proof fault routines and annunciation. He said he'd pass that along to the marketing/development teams, but could not guarantee that anything would be done. We have a huge investment in 1394/S-Class controllers (probably a hundred of them) in this plant, and have had Rockwell's involvement in several other major issues. They know we are generally unhappy with the 1394 system module reliability, so maybe they'll do something to help. Anyway, I complained and I feel better. PC

[wildswing 0]

Hi Paul, I'm not sure how much help I can be. Sounds like you have overtravel fault set to report status only, right? Our's are set to stop motion. Are you using the "when end or fault" block? I don't. I found it too restrictive so I wrote a task that simply watches for Global fault > 0 then immediately copies all fault info to some user variables without interupting any tasks. Want more info? Just ask. I've done all of our programming in-house. I'm kinda isolated (geographically) so this is the first time, aside from tech support, that I've been able to talk to anyone who uses these things. I knew I wasn't alone!

[OkiePC 14]

The axis config for Hard OT is set for stop motion. I am using the "when end or fault" block. The first blocks in the fault routine copy all of the axis statuses, global status, and runtime errors to user variables. It's just not fast enough for my particular situation (75% of the time). In my case the drive belt was whipping during axis reversal and hitting the OT limit switch momentarily (just a few milliseconds). The "fix" was to bend the lever about 45 degrees to get it clear of the drive belt. It's strange that the hard ot faults are the only ones that aren't retained. I like GML commander, and have had very little trouble with S-Class controllers, but I think that AB should have left some things alone when they bought Creonics. AB screwed up the autotune feature as well. The creonics autotune feature was very thorough and accurate, but with AB motion controllers, it's virtually useless. Anyway, yes we use a lot of their motion controllers (s-class and 1394). So, for now, we're stuck with these quirks... Paul C.