Re: OK to switch and plug before paint?
Originally posted by mc5w:
One of the things that I have also later learned is that most machine builders will not allow any customer access to a programmable controller program, partly because of copyright, trade secrecy, and safety issues. What that means is that I have to debug the system using the garbage in garbage out principle. 99% of breakdowns associated with programmable controllers are caused by broken wires, misaligned limit switches, and jammed gearboxes. 0.9% of the time an I/O board for the programmable controller goes bad. It is a federal felony to modify somebody else's computer program and then use it for commercial purposes without their permission. Copyright holder NEVER give out permission for somebody else to modify their program because they want to pocket the labor dollars.
This is one of those things that is both true and not true. One of the reasons machine builders do not like to leave their programs accessable to the outside world is that people modify them and than deny they made any changes, claiming whatever the end result of the modifications are is the machine builder's fault.
I have been there a number of times where modifications made by someone after I left a working machine caused it to fail, and yet I was expected to fix it - generally the expectation was the fix would be free, even though the reason for the failure was a modification someone else made.
I don't like locking a program as it makes debugging difficult for the end user, assuming he has anyone competent to do the debugging. But I know a lot of OEMs that do lock the programs as a way of reducing trouble calls. Service departments cost money.
Often the people making the modifications have only a vague understanding of what they are doing and don't even realize what they have caused, even after the fact.
I prefer the guys having a problem call me and ask up front if they encounter a problem they can't easily fix themselves. Much easier that way.
I also try to build in good diagnostics so that it's fairly obvious just what is wrong and points the guy in the right direction. Although, some people just won't use the tools you give them.
Humorous anecdote time. OEM gets call from machine owner. Machine that had been running fine for almost a year now won't work. Machine starts to run but than just stops for "no reason".
OEM sends service tech out to look at machine. He makes similar report. I talk to him and ask if when this happens is there anything unusual going on, or are there any alarm messages being shown on the screen. Answer is no, but it only happens on Saturdays. Service tech is convinced the PLC program knows it is Saturday and is causing this problem. He tells OEM's company president he is 100% sure of this.
I finally end up going to site on a Saturday with the OEM's engineering manager. Maint guy and service tech are there. Operator pushes start button, machine starts to run, than stops. Big red banner comes up on CRT "LOW AIR PRESSURE". Operator reaches over and clears banner. "See, it just stops for no reason at all".
Turns out the plant shut off one of their compressors on Saturdays, and the pressure setting on the one they were running was right at the setting on the low air pressure switch on the machine, so on Saturdays sometimes you would get lucky and the switch would trip.
I asked if they always got the big red "LOW AIR PRESSURE" banner on the CRT when this happens and they indicated that was the case. The OEM's engineering manager and I just did not even know what to say for a while.
