Choosing a plc and overall control strategy

Hello everyone, I have been reading this forum for a while and would first like to thank everyone for the helpful information and links to resources. I will be replacing a plc on a custom manufacturing machine within a year. The original control vendor no longer makes or supports plc's, and my employer would rather be proactive and schedule a replacement. The machine is relatively simple, about 260 discrete i/o, and a couple analog i/o. About 160 of the i/o are on three pneumatic valve/sensor manifolds, an average of 50 feet from the control panel. The current controls use a proprietary fieldbus for the distributed i/o. The manifold vendor has DeviceNet and Profibus manifold slave nodes available, as well as multi-pin nodes, to run all the wiring back to the control panel. As always, cost is a concern, and it seems that replacing the valves and manifolds could easily exceed the cost of a new control system, so it would be preferable to use the existing valves and manifolds. If I "home-run" all the wiring back to the control panel, I have many options to choose from, at the cost of extra wiring. Using a plc with a Profibus or DeviceNet master will simplify wiring, and provide easier alteration/expansion down the road, but increases the plc cost a bit, and reduces my choices. Any advice or suggestions would be appreciated.

If cost is an issue and you have distributed I/O on the machine have a look at Omron and CompoNet. It is about half the price of Device Net and does not require any special cables - just 1 pair screened. I normally only use 0.75 sq. mm twisted pair screened - nothing special at all.
The only thing you have to watch, as with any network, is resistance over long distances as the terminating resistor is 120 ohm. The master has to be able to see the terminator.
I normally just use a switch mode power supply for the 24VDC supply as well.
There are a variety of I/O modules available - digital and analogue.
Omron also have slice I/O that may be worth a look as these are not all that expensive either and work as they should without fuss.

Brian,
Something to consider- do you need high speed IO for these remote points? Will it need to be scanned in step with your ladder logic? Some (most?) field IO will be scanned at a different rate or out of sequence with your CPU scan.
This may be moot if things are running relatively slowly. What other features and requirements do you have? Specifying what you need to do will help us answer your questions better.

It looks like the fastest event on the distributed i/o would be a proximity sensor that is on about 3-4 ms and would need to be reliably detected.

If there are only a few IO's that need high speed, you can always home run them to the PLC, and use a fieldbus setup for everything else.

CompoNet will update 1000 I/O to the PLC over 30 metres on a twisted pair in 1ms. You could then program interrupts in the PLC.

If you are thinking about a field-bus you may want to consider Allen Bradley's Ethernet/IP.

Cheap cabling and you can now connect it in a ring. The type of field I/O would depend on the density at each I/O location. You have a choice of point I/O, Flex I/O, or ControlLogix. You can also add a 1788-en2dn to interface with your DeviceNet equipment. CompactLogix controllers are alot less costly when compared to ControlLogix and the CPU's are just as powerful.

What are your main design concerns, i.e. reliability, cost, etc..?

In the end, you may pay less for something in the short run, but you need to think about support. Choose a platform that you can either support internally in your company, or you have contractors in the area that can support you.

A couple of opinions:

I prefer to use micro PLCs at a number of locations, networked back to a master. That let's you keep equipment protection and safety logic and anything that needs high speed at the local node, with the master just coordinating the various sections of the machine.

In the overall cost of a project, the PLC and I/O cost are almost insignificant. The cost of engineering and programming are usually more significant. That means as a rookie you should pick a supplier with good local support and be willing to pay a little more for that. I've bought a lot of stuff from internet only folks like AutomationDirect and they are a great value, but I didn't use them for my very first PLC venture.

I use a lot of communications networks to things like valves and VFDs and it works well. I do NOT rely on control over a communications network to I/O outside my PLC panel for anything that counts on high speed or involves personell saftey. Use your judgement when setting up the architecture.

Thats really good advice Tom, I totally agree.

I second what Tom said. very good , sound advice

I prefer to use micro PLCs at a number of locations, networked back to a master. That let's you keep equipment protection and safety logic and anything that needs high speed at the local node, with the master just coordinating the various sections of the machine.

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Quite often yes but it also depends on the speed of the remote I/O - I generally find CompoNet is really fast and as there is no Ethernet network IT do not want to own it.
Spend half my time onsite arguing with IT at a customers premises - use something that is NOT Ethernet and they do not want to know. CompNet is a token ring - mention that and they run for cover.

BobB said:

mention that and they run for cover.

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Or, tell them that if the network goes down, they should expect to be on call 24/7 to provide support. IT guys like their sleep

I have installed Ethernet in multiple plants always islanded from the IT networks. As of yet, I have not run across any issues with IT trying to take over.
I guess I am lucky so far.

Thanks for the replies everyone, definitely some great food for thought. I have not worked with Omron controllers in a very long time, going to do some research on their current offerings. The machine in question does lend itself well to using multiple controllers, being divided into three essentially separate operations. The higher speed operations could be isolated to two points of about 20 I/O. The company I work for manufactures HVAC equipment, so the "in house" controllers are unsuitable for this application. On a previous controller replacement we went with a Beckhoff system, their support was good and their North American headquarters are near to us.

If you are going to have a look at Omron the CJ2M is the go. It also uses CJ1M I/O cards.
For remote I/O have a look at CompoNet. As I mentioned it is pretty darn quick.
The CJ2M is available with or without Ethernet IP and uses USB ports for programming.
If you use multi PLCs then Ethernet IP auto transfer words on the network is really easy to set up - easiest I have seen so far.