Why use a Cell Controller in Manufacturing plant vs Robot Controller?

Good morning all,

I have many robot cell controllers here where a fanuc robot loads and unloads a part through various machines, then drops them out on an exit conveyor. All of the machines in the cell already have its own PLC, but they are all tied to a Main PLC / Cell controller PLC, which controls when each machine will cycle, making sure the robot is out of way, part present, fixture unclamped, etc etc.

This seems common in my facility, unsure about yours. Why have a separate cell controller, when this can be tied to the fanuc robot controller (for example R30iA or R30iB?). So instead of machines receiving signals from cell controller plc, why not just receive directly from robot?

I feel the master plc is unnecessary? What's the reason behind having a master plc controller in these cells?

Thanks,

Separation of concerns?
Often the PLC guy doesn't have robot experience so doesn't want to / shouldn't access the robot code.
Some robot controllers have limitations like only 8 inputs, only three outputs. It is natural in this case to use a plc for all the io etc. They also might not have online changes. Then when the system designer is used to doing it that way, that's the way it is forever more, regardless of increased features of the particular robot in use.

75% of my projects are robot cells. Robots are very good at motion. But they are terrible for logic. The master PLC is much better at handling the logic for all of the auxiliary operations (conveyor stops, cell gate entry, etc) AND for instructing the robot when its okay to perform operations.

What a robot can't do is perform safety logic. The additional cost for safety functions in a PLC is getting so low thst it can't be ignored for the added benefit.

As we speak, I'm commissioning (3) robot cells, each tending 3-4 CNC lathes. Its so much easier to first program the robot to do the motion tasks, interlock it to the cell PLC, an let the PLC tell the robot to perfrom tasks as they come up. The PLC is interlocked with the CNC equipment, incoming and outgoing conveyors, etc so it becomes real easy to program all that coordination in the PLC with ladder.

Yes, it is cheaper if you have just one CNC and simple conveyance with simple safety relays but I still prefer the cell PLC.

@All Ah okay, that makes more sense, I did not think of the safety aspect and the cost of additional i/o. Thanks for both of your inputs!

Any more information is welcome.

25 years ago when I worked around robots in cells, having individual machines with there own PLC's allowed us to move and change cells around very easily.
Think of an OEM, he has 10 machines individually controlled, but by mixing combinations he can create multiple cell variations.