Hacker News Reader

Comment by idiotsecant | Hacker News Reader

I am a controls engineer and saying something is CLOSED when it is actually only observed to be NOT OPEN is my personal windmill I tilt at regularly. People have deied before because what the screen said was that the valve is closed, but what the PLC actually observed is that the valve had left the open position, which was the only limit switch present. Maybe the valve is travelling slowly, jammed halfway open, etc. When operators make the complaint you just did I tell them that what I've told them is reality and if they want a true closed we need a closed limit switch.

View on HN

Ugh! I've had to deal with this in a situation where we needed two sensors to tell if an object was present or not, but there was only space for one. So we could tell if the payload was not present, but not if it was.

This meant that we would have to make assumptions like "you (other subsystem) say you had the payload and you gave it to me, and I believe you, but I can't prove that you did until much later in time."

And if it turns out that we weren't actually holding it, then recovering from that state required unwinding a whole series of decisions and actions made some time in the recent past.

Suffice to say, it made error handling for certain cases... challenging!

This was my first guess, although I do not have explicitly experience in life-or-death type of control systems. In such systems, does it make sense to have more than two states? Something like "open", "unknown but not open", "unknown but not closed", "unknown", "closed"? (Assuming that you have enough limit switches.)

quantum switch

That's exactly what you do. In all systems you should have two switches, and you annunciate on / off / travelling / error to the operator. Often you inherit something that doesn't have that.

In anything that really matters you have a continuous position transmitter instead of switches so your state is more obvious. In safety systems you might have 2 or 3 of those transmitters so you can have a backup or a vote

Thank you! Do you have any good references for voting systems? I've always wanted to learn more about those.

I thought this was an interesting and very related read that touches on voting controls: https://ntrs.nasa.gov/api/citations/20020039704/downloads/20...

Thank you!

Reply on news.ycombinator.com