Advice for multiple PLC practices

Just for a little backround.... I have been in the electrical field for 6yrs and have been involved with PLCs for the past year or so. I am now the only electrician in our plant and am solely responisble for our automation system.

My question comes from the fact that this is the only PLC system I have ever seen / worked with and do not know the "standards" for doing things.

Our plant consists of 7 major machine centers and the connecting conveyers. Some machines have thier own dedicated PLC for the machine and conveyers. Others have thier own PLC, but rely on another for the conveyer system. Still other systems still have hardwired start / stop operation.

The PLCs are on a serial over Ethernet network. This is a monitoring system only with no control elements. All PLCs are Idec, most FA3S, some FC5A.

This approach makes it difficult to modify the system when changes are made.

I have thought about adding more controllers so that each one runns its dedicated system and no more. Each machine would have its own PLC. The conveyer systems would be seperate, and the consoles would have thier own to with a serial connection to the "main" PLC for the machine.


Real question:

1. Would this be considerd poor practice?

2. Is there a general rule on thumb for how complex a system needs to be to justify its own PLC?

3. Can I run the outputs from one PLC to the in of another for communication. I know it works, but is this "bad practice".

4. Any other advice?

Thanks for any help. Our PLC system was quite the mess when I got here. No I/O labels, no rung labels, no descriptive anything. So I have come a long way on my own, but it's time to ask for help.
--Alex

A lot of this would have to do with your requirements justification for using a PLC usually depends on its function vs cost. ie would be say using a PLC to replace hardwire components such as timers and relays. Normally with conveyer systems you have several considerations
Do the conveyers require interlink controls ?, Do the PLC's set the rate of speed that the conveyers run at? My own preference in this circumstance is to have one PLC per main line of conveyers then pass the status to the monitoring PLC. Having one PLC per machine is a good idea as it can reduce the amount of equipment downtime due to a single PLC failure. Again you need to consider interlinks for the conveyers. I've seen signals run from one PLC to another I honestly don't see it as a bad practice and its one thats been commonly used

Could you expalin what you mean by interlinks?
--Alex

Passing the running status from the front conveyer to the contact of the second conveyer. to put simply if one stops then you want to stop the conveyers supplying that conveyer

Got it. Your interlinks are what I would call interlocks.

Yes, there are a few outside interlinks in the conveyer system, but for the most part they are all internal.

No. In this instance our PLCs are not controlling conveyer speed. They are all either contactors or VSDs with one preset speed.

So what you are saying is that you prefer on main PLC for the entire system, with seperate subsystem PLCs that report back to the main unit?
--Alex

Correct course this will depend on your company requirements for a plant the size I work in its unreasonable so we generally have a PLC running its own machine then use hardwire interlocks for other conveyer systems however we don't monitor 90% of our machinery from a central location. Those that do has the PLC's report to a master which reports to the monitoring station or direct to the reporting station depending on how big the machine is

Normal practice is for machines to have their own PLC and the conveyors their own.

Whether you have multiple conveyor PLC's depends on the layout, if you have parallel lines it makes sense to have PLC's per conveyor line. If you have one serial line, it makes sense to have a single PLC for the conveyors interlinking between the machines.

Serial comms is not recommended for interlocking conveyor control, OK for passing messages where that is required but not running signals.

Easiest and most reliable is inteposing relays, have used Profibus and Ethernet though.