Last I heard, Allen Bradley had +40% of the US market. that was about 10 years ago, but that figure had held for a long time.
One way to tell who has the largest installed base is to look at the number of after market products to go into them. For instance, AB has an entire second-tier vendors association with periodic conventions etc. Omron, although big elsewhere in the world, has no such thing in the US.
"Venditori de oleum-vipera non vigere excordis populi"
I was born and raised on the AB SLC series. I use to see the AB products in heavy industry environment, but now that I have made the switch to working with real time, high speed data transfer, the engineers' prefer the Seimens PLC's due to their fast scan time.
In my opinion there are plenty of PLC systems that are superior to the AB offerring, but the question was asking for the most common, not the best! AB wins that prize in North America at least, for whatever that is worth.
My guess as to why is that early on in the '80s, AB forced their distributors to invest heavilly in stock and training. So when other PLC suppliers were trying to make the best PLCs for propeller-heads, AB was trumping them by getting product into the hands of lots of end users who didn't have time to check out every detail. Once someone was trained and accepting of AB's products they became reluctant to change and start all over again.
"Venditori de oleum-vipera non vigere excordis populi"
