New to PLC: How to choose a controller?

[kilolima 0]

I am new to PLC, and I have a project that requires some temperature sensing/feedback and valve controls. It is a part of a bigger automation system, and it will expand further to integrate with other parts of the system in the future. The bigger system has optical isolated RS-232, GPIB and CANBus in place for device communications. Most equipment in the facility has been around for more than 20 years. This new addition will likely to be there for a long long time. So, I am concerned with compatibility and how easy to buy replacement or additional parts in the future. I saw many different companies that make PLC products, Rockwell (Allen Bradley), Omron, Automation Direct, etc. The price difference is quite significant between, for example, Rockwell and Automation Direct. Are they all compatible with each other, in terms of both hardware and software? Is one better than the other for a given application? In general, I would like to get some pointers about how to choose the right controller. I appreciate any of your help. Thanks in advance.

[TimWilborne 108]

The physical hardware and software is not interchangable between brands. There are serveral "Open" networks that allow them to work together. Devicenet and Ethernet/ip to name a couple. As for which on is better, that is a matter of opinion. Maybe the first thing you should look at is what brands do you have currently and what brands are locally supported in your area

[kilolima 0]

Thanks for your reply, TW Controls. Currently, we don't have any PLCs in the whole facility and no one in the facility has any knowledge or experience in PLC. So, I am particularly interested in picking something easy to start/use and flexible in terms of compatibility and expandability. So far, I found Automation Direct (DirectLogic) has the lowest cost PLCs. Is there any disadvantage of using that brand of controllers? I am located in Toronto, Canada. I took your advice and checked Automation Direct's support page. They do have an affiliate in Toronto.

[Steve Bailey 118]

If you don't already have a good understanding of PLCs, then Automation Direct may not be your best choice. That's not because there's anything wrong with AD's product line. It's because you're not the ideal customer for AD's business model. One reason that PLCs from Allen Bradley, Siemens, GE, et al are more expensive than those from AD is that the former have a large staff of field sales engineers, applications specialists, and distributors to help support the OEMS and end users of their products. Automation Direct realized that there was a large group of users who didn't need that much support and hand-holding. By eliminating the field sales and support personell they minimized their overhead. They put some of the money that saved into their excellent help desk and some of it into maintaining a larger inventory than most of their competitors. They created a company that has excellent telephone support, but it's not very easy to get someone to visit your facility and assess your needs. The other major PLC makers depend on their distributors to maintain close relationships with their customers and stock the components most likely to get sold. For someone in your situation, I would recommend that you develop a relationship with a distributor that you feel comfortable with; someone with enough technical expertise to be able to understand your processes and make sensible recommendations about automating them. In my opinion, that's more important than the cost of the components and certainly more important than which brand of PLC you settle on. At the end of the day, any of the major brands of PLC will do the job. They wouldn't be able to stay in business if they didn't.

[TimWilborne 108]

I second everything Steve Bailey said. Also when bringing one of these guys in, you can explain to them what you are wanting to to now and what your future expansion plans are. They will recommend the most economical solution at will still allow you to upgrade in the future. The future expansion was one of the things that would worry me about Automation Direct. Not that it can't be done but many of the others are easier. But another thing to look at is who is going to build you controls. If you are going to do it, then your local Plc rep that you decide on would be the place to start. But if you also need to locate someone to build your controls, you may want to look into that first. You may find that on particular Plc is more prevalent in your area.

[kilolima 0]

Thank you very much for the recommendations. Really appreciated. I will phone up some local distributors and see what they can do for me. Thanks again. And Happy Holiday.