Hi BFoelsch,
Depending on what you are trying to do a PLC can make life very easy. Most
people who go the PC route on automation designs do it because they are
comfortable with a programming language or technique, but they totally
underestimate the difficulty of managing real world I/O, which is very easy
with a PLC. Secondly, very few, if any, PC- based solutions that I have seen
permit on-line editing. There is a great deal to be said ( in the industrial
environment) for the ability to make program edits while the program is
running, and again, this is pretty much standard for PLCs and pretty much
unheard of with PCs. And of course, the PLC operating systems are rock-solid
compared to the Microsoft offering.
That is also what I have observed. There is another reason for a PC:
Everyone is familar with its file system and you can also write your
report right on it. No need to schlepp the laptop into a clean room
environment. Usually you can't take anything in there, or at least
shouldn't.
Online edit is possible though. We have done it when debugging
auto-bonders, laser cutters and so on. Single step, stop, manual head
movements to try something out, slow advance to see why a linear motor
didn't run smooth, it is all no big deal if the software can do that.
The OS was often non-Windows. There are some really good ones such as
QNX where latency times are very predictable. DOS, by the way, is also
rock solid and very frequently used in automation.
PLC would be even better but it does have to compete price wise. I am
pretty puzzled when I look at the distribution system. When I asked at
some local electricians places some didn't even know what PLC is. Others
said it can only be purchased through a 'few special outlets', whatever
they meant by that. One guy got so frustrated by the VAR runaround that
he bought his Moeller PLC unit through EBay.
Yes, the AB product is not the price leader, but it is the most popular,
most understood and most supported.
I assume they could do a lot more in sales if they were more available.
An example is our pellet stove. These have to run an auger, several
motors and must monitor 4-5 sensors. Low quantity products, the perfect
spot for a PLC unit. Yet they all go through the trouble of having their
own micro controller boards developed, something that just can't make
much sense in an industry where a few thousand units a year is a lot.
Regards, Joerg