Amount of PLCs

Hey everybody,

I got an offers from integrators for production line : conveyor, 8-10 robot - some scara some 6 axis + few processes. But mostly vision and pick and place on conveyor.

One integrator offered everything on 1 Beckhoff IPC EtherCAT, other integrator offered 4 Siemens PLCs 1517F on Profinet.

I was working as integrator before in automotive and on line with this amount of robots (or more) we were using 1 Siemens 1516F.

I do not know yet the answer from them but can you wise people see why would they offer so many PLC devices? Are there any advantages?
Is beckhoff that superior compared to siemens?

My rule of thumb on adding a PLC was associated with independence. If sections can be operated independently of each other then I would use a separate controller. In this way the section can be taken out of service and powered down without affecting other areas. Many times when everything is placed on one controller, sections then become interdependent and taking down one section will affect others when it is unnecessary. If you have some sort of sequential process train in which taking down one section will not allow the others to operate then putting it all on one processor makes sense. This is all assuming that the one processor has the resources available to control everything.

If you have a buffer point between processes you can easily run them on separate PLCs. Anything that dumps into a bin/box/loading deck/etc and then trashes its tracking data as its now a randomized mess and the next system will generate its own new tracking data. Go for a separate PLC. a PLC losing a part will not cause full plant shutdown. Just that section and the rest can feed into or off the bins on either end.

PLCMentor said:

My rule of thumb on adding a PLC was associated with independence. If sections can be operated independently of each other then I would use a separate controller. In this way the section can be taken out of service and powered down without affecting other areas. Many times when everything is placed on one controller, sections then become interdependent and taking down one section will affect others when it is unnecessary. If you have some sort of sequential process train in which taking down one section will not allow the others to operate then putting it all on one processor makes sense. This is all assuming that the one processor has the resources available to control everything.

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I don't see why would you need a separate plc for that purpose. You can design software so that different parts can be operational or not.

Don’t put all your eggs in one basket.

TurpoUrpo said:

I don't see why would you need a separate plc for that purpose. You can design software so that different parts can be operational or not.

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And when you need to change some part and switch off power supply?

goghie said:

TurpoUrpo said:

I don't see why would you need a separate plc for that purpose. You can design software so that different parts can be operational or not.

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And when you need to change some part and switch off power supply?

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Also a smaller spares inventory, if there is any duplication.

The need to do maintenance, repair, upgrade, modifications to the control system itself makes it sensible to split it into functional smaller parts that can operate independently.

PLCMentor said:

My rule of thumb on adding a PLC was associated with independence. If sections can be operated independently of each other then I would use a separate controller. In this way the section can be taken out of service and powered down without affecting other areas. Many times when everything is placed on one controller, sections then become interdependent and taking down one section will affect others when it is unnecessary. If you have some sort of sequential process train in which taking down one section will not allow the others to operate then putting it all on one processor makes sense. This is all assuming that the one processor has the resources available to control everything.

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This has always been my philosophy as well.

Usually the reason for using less PLCs is to keep the cost down.

Segmenting the design makes it more resilient but if they are all on one line and you can't run with one part down then that argument breaks down too.

If the entire line depends on all sections running for production, then I see no compelling benefit to have multiple PLC's if you can do it all with one PLC without losing performance/speed/synchronization.

I would be really interested in understanding the integrator's reasoning for 4 Siemens PLC's?? That is a significant cost increase compared to the other integrator's Beckhoff solution. Moreover, you'd be getting an inferior communications bus with Profinet vs EtherCAT, not to mention it (Profinet) is more expensive.

If it were my choice, it wouldn't even be much of a discussion - I'd go with the Beckhoff solution. However, I'd probably consider a CX over an IPC. More rugged and you can order with more cores (up to 12). Those additional cores come in handy when you have multiple processes, sections, etc.... With the Beckhoff solution, you'll also get EtherCAT as the comms bus. A superior protocol compared to Profinet.

I currently ask integrator why 4 PLCs, I can see the reasoning for 2 for sure, maybe can make it 3 but that would be overkill for me.

Other integrator is offering some software completely developed by them as they say, it is running on windows IPC and is using EtherCAT as a protocol. So I am not even sure how does it work completely but it sounds really interesting.

The more stuff that one PLC controls, the longer its scan time. SCADA/HMIs can also affect scan time with lots of data requests.

If very fast reactions are needed (<10ms), then it's best to have several small, quick processors rather than a juggernaut.

Roju said:

I currently ask integrator why 4 PLCs, I can see the reasoning for 2 for sure, maybe can make it 3 but that would be overkill for me.

Other integrator is offering some software completely developed by them as they say, it is running on windows IPC and is using EtherCAT as a protocol. So I am not even sure how does it work completely but it sounds really interesting.

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I would never opted for IPC with proprietary software unless I'm planning to close production in 3-4 years.

goghie said:

I would never opted for IPC with proprietary software unless I'm planning to close production in 3-4 years.

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Can you argument that please?
Thanks