The mysterious world of PLC's

I had to laugh at an incident this week.

I was asked through a third party if I could give a price and go to a company to take out a program from a plc and put the program in a new plc.

When I say 'third party' it transpires it was more like a 20th party and chinese whispers were abounding all down the line.

The plc type changed it's model and manufacturer at least three times. No one in this chain had a clue what a plc was, never mind what was wrong with it.

The 'third party' would not give me the name of the company, just the area - for my pricing details. Nor them my name. It must have been so we didn't collude and cut them out of the loop. (which I wouldn't do anyway.)

After many attempts at trying to find out the brand, model and problem. I gave a price for taking the program out and changing the unit if need be. (all parts to be supplied by others)

So, I arrive on site. I am passed from pillar to post gleaning a small amount of information at each station 'it's the brain that has gone' - 'the computer is faulty' - if I hit it with a screwdriver, it works for a bit'

I finally get to the hallowed land where this computer/brain/plc lives. It is a Mitsi FX2n 64MR with a relay output blown that controls directly - a DC valve.

I get an interface relay from my car, install it and reprogram a spare output to this relay. All is working perfectly again.

The manager, looking at my new relay comments that it is amazing how small they make things these days. I agreed, interface relays are quite small. He says how do they fit all the brains into that little thing.
Brains? - that is just a relay to stop the dc valve taking out another output.

Where's our new plc he asks. You don't need one I tell him. But we have paid for one, I want a new one.

No, you have paid for a possible program transfer or the problem to be solved.
We were told it would cost at least £400 and for that, you would take the program out, supply a new plc and disconnect and reconnect the new one.

'The plc alone costs over a thousand pounds' I don't care if you have underpriced the job, want a new one.

He is off in a storm and the next news I am called by the third party 'he is not happy with you - you have not supplied a new plc'
'So you thought for £200 I was going to supply a plc - make and model unknown, program and fit it and you would just double my price and let me spend thousands doing it!'

In the end I went to see the top man at this company, I explained how he had actually got the job done very cheaply and it was properly fixed and they would have no more trouble.

I explained to the 'third party' that next time the word 'plc' comes up ask me to deal directly and I would not cut them out of the loop.

All that for a blown output - I'm glad it's Friday, going for a beer

Job well done.

I love these tribulation stories. It makes me feel good that the public is not picking on just me. Thanks for sharing.

But this leads to the ultimate question, how do you answer your kid's question, "Daddy, what do you do at work?"

My job has changed now, but I used to reply with:
"I fix the mistakes made by the mechanical Engineers."

I used to use " Electrically compensate for Mechanical deficiencies using automation "

I like to think of myself as a professional monkey-proofer.

-Dave

SLC500 said:

I used to use " Electrically compensate for Mechanical deficiencies using automation "

Click to expand...

I believe that is the definition of a controls engineer.

"No, I can't program the machine to break the laws of physics."

Not just mechanical engineers but civil engineers as well...

"I don't care if the controller output is 100% and the inlet valve is fully opened. There's something wrong with your control system because the flow won't get up to setpoint." Does head pressure on a gravity water feed mean anything to you?

Mike Lamond said:

Not just mechanical engineers but civil engineers as well...

Click to expand...

One day at about 4:30, (I got off at 3), a big time flurd-toater came rolling through and dumped about 3 inches of rain in about 40 minutes.

My phones from the plant 13 miles west of me. "Paul you gotta come back out here, all the beamshears are down, toeguard too, and al lthe plycutters except number four". I queried, "Lightning get a sub? Main substation upstairs?"

He said, "No, water." All the panels are flooded. All these machines' servo drives and PLC 5 panels were no longer on the north wall, the north wall got moved, and they sloped the new roof toward the place where the old roof ended. Corporate plant engineers built a huge exapnsion along the length of a mile long plant, and sloped the roof toward the plant along the whole zone 2 area. They said, "Yeah that's how we expand, every other expansion gets sloped the opposite way as you go across...Sorry, Mr. Engineer dude but you don't need a 30' ceiling above beamshears and plycutters, and the expense of dealing with all that runoff. "That's how we've always done it..." If I had a nickle.

Now, I am no construction expert, but I built a few caroports and roofs and patio covers, and not once did I ever slope them toward the house!

So, the drain pipes were flooded, these panels had big rectangular holes in the tops with dynamic brakes in expanded metal cages. And some roofer must have fudged up really bad because the operators described a water fall from the corner along that whole seam.

So, when I got there they the bus locked out, and they were sucking up the wter with the big machines, and all the panels were opened except one. When we opened it, there sat the PLC-5 16 slot rack at the bottom like all the rest of these machines, full of ananlog and AC I/O running a panel next to it full of servo amps...obsolete ones...I think they were 1392-DS for digital servo.

The panel was filled like a bathtub halfway up the 1771 chassis. half of it was underwater when I opened the door, which I had probably ensured was sealed to keep dust out...I'd never had to deal more than a bad drip from roof leaks over panels before that day. Usually we controlled those professionally with blue and green curtains of plastic web material, and funnels and bucket. Not this day, though.

All the PLC-5 survived, but we used about 6 discrete i/o cards and only had enough spare servo amps to get half the machines runing product by ten pm.

I think we used two cases of lectro-solv.