BobLfoot

PLC Law

158 posts in this topic

If you ask my automation tech, they say it can....blah....

Share this post


Link to post
Share on other sites
I'm sensing theres a story behind this one!?

Share this post


Link to post
Share on other sites
I would definitely like to hear the story behind this one!!!! Pictures for Crash of the Week??? Edited by cmoore73

Share this post


Link to post
Share on other sites
I did a service call at a place on Wednesday that I had to field integrate a Cognex vision system. I didn't ask if the index table was on station, so I set the camera up and setup the vision system. Customer told me they didn't have any parts to run, so I couldn't run the machine. So I had to leave. Next day they called me and said they got parts and the vision system doesn't work. They looked on the vision screen and saw their part was half off the screen. I thought someone bumped camera. So I went out to look at it AGAIN, figured out the index table wasn't in position when I set up the camera...silly me....I didn't even think to ask, I just assumed it was on station. No crash picture, thank goodness!

Share this post


Link to post
Share on other sites
That is a Robot law.

Share this post


Link to post
Share on other sites
You don't know how many times I've heard "Something in the program changed..." I was really, really green when I first heard this one. I could barely spell PLC yet... and someone was using this. I couldn't convince them that the program couldn't change itself. Nobody bothered to check the hardware. Sensors had gotten hit by a forktruck and were pointing at the floor. It took about 3 years to teach them that the program doesn't change unless externally changed... If the program that used to work isn't working, look for hardware problems first. This is a great thread!!

Share this post


Link to post
Share on other sites
Yeah, this one probably differs by plant. 90% of the machines around here max out at 100 rungs or less. This "solution" came from having several electricians laptops die without being backed up to the network. They lost the original ladder for a SLC that had been OEM locked (S:1/14) by a company that's now out of business. I'll have to reverse engineer it someday when they give me time... Bwahahaha! Let's amend this to... PLC Law #16: Keep only the latest copy of the program accessible to maintenance. The meaning of "accessible" may differ by plant, but shouldn't include waking the engineer up at 3am!

Share this post


Link to post
Share on other sites
Perspectives certainly differ. My latest project had 425 rungs (just checked the number), containing 4104 instructions in total (many large rungs that wouldn't fit on a single sheet of paper). And it was a small program! It's more normal that we're working on programs at about the double size... Our view on this may also be influenced by being an machine builder. Our customers basically don't have any business messing up the program, that privilege belongs to me and the other two programmers...

Share this post


Link to post
Share on other sites
Woah -- big differences here. I just finished a system that was maybe 150 rungs. The system I did before that was a little bigger... The program printed out over 3000 pages of code!! One of the ones I'm working on right now is at 600 rungs, probably on its way to 1000. The other PLC that will be in that system will probably be 2500-3000 rungs. On all of these, I've done a lot of simulation. This has drastically reduced startup time. Yes, it's cost me some time during development, but it has reduced startup to basically IO check and setting timers and preset values.

Share this post


Link to post
Share on other sites
I just started using simulators for my Mitsubishi software, and they are extremely helpful. The very day I installed them, I was going over an updated program for a customer. He called me an hour later to complain that thus-and-such didn't work on the update, and I had already found and fixed the issue! Too bad I didn't get the software earlier... It was especially nice, because I was using GX Simulator (PLC) and GT Simulator (HMI) in conjunction, and could basically simulate the entire control system.

Share this post


Link to post
Share on other sites
If I count right then I offer the following: PLC Law #26 - The Controls People can always count on being the last to leave and the first to return to solve problems on any PLC job. Even when the problem is not PLC related. From a suggestion of P Daniil in this post.
1 person likes this

Share this post


Link to post
Share on other sites
Never was a truer word said well done Boblfoot

Share this post


Link to post
Share on other sites
Law #27 Share Your Updated Work.. It always happens that when changes/updates happen no copy is left by the people who done the changes. Law #27.1 AFTER CHANGES MAKE SURE THEY WORK So many times after changes machine no work and no programmer who done work is in the building

Share this post


Link to post
Share on other sites
#28. Machines with intermittent faults always seem to work okay for hours when the programmer is at the location. When the programmer leaves, they may start failing within five minutes...

Share this post


Link to post
Share on other sites
#29 Problems with controls and automated systems at plants happen every day. But it is only the day after you visited all problems must have a connection with your visit. #29.1 Some specific faults in controls and automated systems are there for years already, and the operators know exactly how to deal with it. Until the plant supervisor notices.... then all of a sudden it is a matter of life and death. I even know an operator who would power cycle the PLC every time he got stuck in the HMI. Nobody knew about the problem for about 6 years until the plant supervisor's eyes popped out when the operator did his daily routine

Share this post


Link to post
Share on other sites
PDL that "Japanese restart" works real well with Omron and Toshiba PLCs, don't try that with an AB all it's outputs function like HR relays in omron. Edited by BobLfoot

Share this post


Link to post
Share on other sites
Law #30. - Any problem which goes away by itself will return by itself (but at the least convenient time.) Never imagine you have fixed a problem unless you are sure what the problem was in the first place. Andybr

Share this post


Link to post
Share on other sites
I knew there was a well thought reason why this plant was put on Omron

Share this post


Link to post
Share on other sites
Law #31. - Check for Proper equipment grounding before power up Edited by rpraveenkum

Share this post


Link to post
Share on other sites
Law #32. - Don't Have to many backups it will lead to confusion, Have your Backup in organized way in Proper place.

Share this post


Link to post
Share on other sites
Actually that is one of the things I don't like on some PLCs. It is a personal thing but I am trying to make it simple and obvious for those who will end up supporting it, many have little experience with PLCs and even less with diagnosing things which are not seen in the program code. I prefer to have all memory retentive and code/instruction to clear what need to be cleared on powerup. This makes it obvious for everyone, regardless of experience level. I choose not to rely on brand spcecific features that might require 'insider knoweledge' or getting used to. This includes configuring latched areas, fancy features (side effects) of exotic instructions etc. Sure it works and it might save a cycle or two but if it's not something that every PLC will have, I try to avoid it. It doesn't have to be PLC, same goes for HMIs which don't share much even with most basic features (alarms, message displays, buttons).

Share this post


Link to post
Share on other sites
I am suprise no one posted this one yet. Law #33. - The program NEVER fails.

Share this post


Link to post
Share on other sites
Pay attention Chris

Share this post


Link to post
Share on other sites
You mean you don't have our t-shirts coming yet Chako?

Share this post


Link to post
Share on other sites
Law #33. - The program NEVER fails. So does that mean we dont have to listen when the supervisor/mechanic/operator and every one else says there is some thing wrong in the P.L.C. ALLELUIA Rodney

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now