Tom Jenkins
Lifetime Supporting Member
Thanks Peter - you expressed what I was thinking.
Tom
Tom
A major problem of inexperienced and many expert programmers is that they usually do not know what they need to know. This is why specific industry experience is important when designing a process.
I believe we need to be teaching risk analysis, as in "how to figure out what bad things will happen if things don't work right". For example, if you have a crane using an electro-magnet to transport steel, should an E-stop really disconnect all of the power sources?
A major problem of inexperienced and many expert programmers is that they usually do not know what they need to know. This is why specific industry experience is important when designing a process.
I believe we need to be teaching risk analysis, as in "how to figure out what bad things will happen if things don't work right". For example, if you have a crane using an electro-magnet to transport steel, should an E-stop really disconnect all of the power sources?
I just worked at a foundry that had 2 overhead cranes that carry over 10,000 lbs of steel over workers routinely.
While I was working on one of the cranes rebuilding the rectifier, I asked about the machine to determine exactly how it worked. I did not understand how it could be expected to hold steel when the rectifier failed.
I had never worked on an overhead crane and I suggested we call a crane specialist and have the crane certified OK by a reputible crane service company. Such companies are certified and licensed by the state.
No dice,
Then I discovered that niether crane had a UPS to hold the magnet in case of a rectifier failure. I brought this up and was told the cranes were not required to have a UPS because of their age!
All new cranes are required to have a UPS.
And I suspect if you call the crane service for repair they would require a retrofit to add a UPS to give the crane pass on safety.
The metal cabinet and guard rails looked like play doh after the bar fell off the magnet when the rectifier failed.
I have run into this at every job I have ever worked.
I lost an arguement with a engineer about the MCR and Estop issue.
He used the Estop as an input to the PLC and then used the PLC to stop the machine.
I argued that the Estop should control the stop condition directly and ALSO be an input to the PLC so the control knows why thing have stopped and can do a reset.
I lost this arguement.
They contacted OSHA and got the OK to do it their way. Or thats what I was told.
Thats not good.
I don't believe they contacted OSHA.
Surely you can just quote the regulations to them.
In Europe we have the EN written standards to work to.
I am not convinced that either of you actually dissagree with me.
No, they do not allow the PLC to handle estop functions -- unless it's a specifically rated/designated "safety PLC". An EStop needs to put the machine into a "neutral energy" state.Does OSHA really allow the PLC to shut down the outputs on an estop? I thought they required estops to be hardwired and to remove power from outputs as needed (ie, on furnaces, we only shut down things in motion (belts, fans, etc)). Thanks
matt
The problem is what would you test?
Question 1) Explain the difference between Unitary, Modular and Rack PLC sytstems, include diagrams to aid your explanations.