skip process detection problem

I have a product that must run through 3 different but consective stations on the same machine. Each station has it's own plc.Some times an operator might forget and skip one one of the stations(1st or 2nd stations). I am trying at this time to modify all three separate programes so the 2nd station will expect some kind of signal from the 1st station, and the 3rd station expects a signal from the 2nd station in order to proceed. Is there any other methods I can use(proramming and or hard wirring). The other thing I will be concern with is;if any solution I will try to implement should not affect production cycle time. Thanks

If you hard wire signals between the stations, you may need interposing relays if the different PLC's are not all fed from the same power supply or transformer.

What PLC's are you using? Perhaps they have some communications capabilities?

Sounds like a general hand shaking scenario. Station 1 signals to Sta. 2 that Sta. 1 Complete. Sta. 2 should acknolodge this signal and then Sta. 1 can take the signal away... Ditto for Sta 2 to Sta 3. Not sure how your operators can skip stations so don't know if that will be fool proof enough. Then again, when you try to make things more fool proof, someone will just invent a better fool

Norm's description is definitely the norm-al way you'd go about it, but I got to thinking about your application...

What you're trying to do is more of an interlock scenario, so this might be a better solution...

• 1.) #1 completes cycle and tells #2 that it's done (permitting #2 to start)
  
  2.) #2 completes cycle and tells #3 that it's done (permitting #3 to start)
  
  3.) #3 completes cycle and tells #1 that it's done. This allows #1 to re-start, and also cancels the "permit" signal to #2, which in turn, cancels the "permit" signal to #3

If you use the handshaking method, there's nothing stopping the operator from running #1 again (or #2, after #3 starts). My method locks them out until #3 completes.

Each machine will know the status of the process, and less I/O needed on #2 as well!...

beerchug

-Eric

The PLC I am using is Omron. Please is this built-in function"HAND-SHAKING".Although this is a human error on the operator behalf, Managment always blame it on the machine setup and demand machine change and not further operator accountability. In any solution to this problem , managment want to reduce the wating time between the stations. Th three plcs are not connected together nor have any communication cables between them. I can't help thinking ; maybe alot of us work for fools that demand foolish stuff.Thanks

Is there some unique feature added to the product at stations 1 and 2 that could be detected at subsequent stations?

For example, if station 1 drills a hole in the piece, perhaps a sensor at stations 2 and 3 could detect the presence or absence of the hole and prevent further operations if it is not present.

"Handshaking" is not a built-in PLC function. It is a term that refers to the technique of one device sending a signal to another, and then the receiving device sending an acknowledgement back to the sender.

nmk,

While it is certainly possible that one is working for a fool, that does not necessarily mean his request is unreasonable.

It sounds to me like the request is reasonable... at least, to the extent that you have described the situation.

You said...
I have a product that must run through 3 different but consective stations on the same machine. Each station has it's own plc.

Is there any reason that all three stations can not be controlled by one PLC?


You said...
Some times an operator might forget and skip one one of the stations(1st or 2nd stations).

Does the operator(s) insert the product (by hand) into one station, then do the operation, then remove the product (by hand) and insert it into the next station (again, by hand)?

Or, does the product move automatically from one station to the other?

In either case, can two or three stations be operating at the same time?


HAND-SHAKING:

Hand-Shaking can be accomplished through a complicated communication link (requiring all of the necessary cables and software), or it can be as simple as...

(This depends on the operational timings of the three stations. This can be done in other ways, but you have yet to describe the timing... so I can't provide the "correct" answer. This is just one way to do it...)


If it is the case that the product goes through all three stations before the next product is processed, then...

On Start-Up, an Output from Station-3 to Station-1 is turned ON.

At Station-1:
If the signal from Station-3 is present AND the Output from Station-1 to Station-2 and -3 is OFF, then it is "OK" for Station-1 to operate.

At this point, neither, Station-2 nor Station-3, can operate.

When Station-1 finishes a cycle, it turns ON one Output. That Output goes to an Input on Station-2 and Station-3. The signal from Station-1 to Station-3 causes Station-3 to turn OFF its' signal to Station-1.

At this point, neither, Station-1 nor Station-3, can operate.

At Station-2:
If the signal from Station-1 is present, AND Station-2 is NOT sending a signal to Station-3, then Station-2 can operate. When Station-2 finishes a cycle, it turns ON an Output to Station-3. While the Output to Station-3 is ON, Station-2 can not perform another operation.

At this point, neither, Station-1 nor Station-2, can operate.

At Station-3:
If the signals from Station-1 AND Station-2 are present, then Station-3 can operate. When Station-3 finishes a cycle, it turns ON an Output to Station-1. While the Output to Station-1 is ON, Station-3 can not perform another operation.

When the signal is sent from Station-3 to Station-1, Station-1 turns OFF the signals to Station-2 and Station-3. When the signal to Station-2 goes OFF, it turns OFF its' signal to Station-3.

At this point, only Station-1 can operate.

You should probably include a reset button to allow restarting the sequence without doing a shutdown.

Again, there are variations on this theme... the method that you choose to use depends on the timing.

I learnt to program a long time ago, however I remember a few golden rules.

1. Will it pass the idiot test. (Can an operator do something that will cause the system to collapse and die.)

2. If it can be broken it will get broken.

3. Avoid manumatic system where you can. (The automatics is easier to blame, it does not normally shout back.)

Looks like the system employed failed on all three accounts.

I'm unsure of the exact process being carried out which would allow an operator can miss out a station. However follow the interlock idea from above and embarrass offenders by sounding a klaxon if they try to go out of step. Surprising how quickly they learn.

Can you give us more information as to the actual function of the system.

Interlocking

As the others have indicated, if you supply more detailed information on your process, you will receive a number of fixes for your problem.
Having the PLC blamed for holding up production, when it is only doing its job, seems to be a common practice. Don't sweat it , just fix the most recent complaint and move on. Sometimes automation can replace the lack of operator training and sometimes it can't.

Roger