In addition to their ruggedness, another key in the Programmable Logic
Controllers is that they are designed to be user-programmable with a
high-level language.  The device's firmware (handles booting, communication,
and other low-level functionality) executes much higher-level application
programming.  This high level programming can be compiled or interpreted and
can be developed using ladder logic, function block diagrams, or a simple
text based language.

This high level logic can usually be updated across the network.  In this
way, application changes (for example, adding a chilled water flow sensor
requirement to your chiller example) would not require a firmware update,
but rather a very simple upload to the device.

Your application may or may not require this degree of
user-maintainability.  If not, a straightforward PIC solution would make a
lot of sense.  Otherwise a Programmable [high level] Logic Controller
solution may be the way to go (and as others have said, this could be
implemented with a PIC).

Regards,
Jeff


All a PLC really is is a small CPU configured in typically a ruggedized
> configuration, with additional Ruggedized I/O, and typically programmed
> using Ladder Logic.
>
> In recent years, the cost of a PLC have dropped rapidly.   The first PLC
> I worked with is now being sold as the DirectLogic 305 series by
> Automation Direct (at the time it was GE Fanuc Series 1, but TI was
> starting to make modules for it).    Back in the 80's when I was doing
> this work, the costs were significantly higher.   Now, they're really
> inexpensive.
>
> Take your application for example....   for a couple hundred USD
> (perhaps 300 w/ the user interface), you can have a complete "real" PLC
> capable of everything you describe.   The real strength of a PLC is that
> you can buy this off of the shelf hardware, hook up your control lines
> to it, and spend a couple hours programming it, and you're ready to go.
> And the "ruggedizing" needed in an industrial environment is pretty much
> already done.    It's also flexible (oh, you don't want it to run with
> the door open, well, just add a switch and a couple more lines of ladder
> logic, and off you go).      For industrial machinery where the actual
> programming in use is often unique to that specific machine, it's
> great.   If someone came to me with an application for an industrial
> machine or similar (like  your application), that only had a dozen or so
> total production runs, or if the requirements weren't stable from
> machine to machine, I would spec a PLC, and not even really consider a
> custom PIC solution.
>
> Now, back to your application.    Because your application needs are
> fixed, yes, you should be able to build for a lower component cost.
> The cost of a 18F, and some I/O conditioning, plus the interface, is
> going to be significantly lower than the cost of a PLC.   The question
> being is how many units are going to be needed, and how "fixed" is the
> needs for each machine.   As long as the quantity is high enough, and
> the needs from unit to unit aren't going to require a whole new PIC
> application, then I'd say go for it.
>
> -forrest
>
> Jinx wrote:
> > I'm assessing a job, which I think a low-end 18F can do fairly simply.
> The
> > logic is quite straight-forward. Inputs are two 4-20mA transducers, a
> mains
> > relay, and two temperature sensors, with a real-time clock. Outputs are
> > a couple of alarm lines, LCD, and a few pushbuttons to set parameters
> >
> > The scenario might be something like - If transducer1 is a certain value
> > and temperature sensor2 is a certain value, then turn on the mains and
> > measure temperature sensor1 after 1 minute to make sure the appliance
> > (a chiller) is running. And various IF....THEN combos like that
> >
> > I was talking over the pricing structure with a couple of people and they
> > both suggested that I base it at less than the cost of an 'industrial
> PLC'
> > which would be needed. Their thought is that the lower (but not too low)
> > price mentioned in the same breath as a PLC will make my PIC sound
> > more attractive. Can't say I'm 100% about that but they won't shut up
> > about PLCs !
> >
> > I'm a complete ignoramus in the PLC department
> >
> > http://en.wikipedia.org/wiki/Programmable_logic_controller
> >
> > To me, after reading that, it seems a PLC is way OTT for this job, and
> > especially if you know how to program a micro yourself. This assessment
> > may be for 300 units (optimistic "Everyone will want one" salesman !!)
> >
> > I guess the material costs, not including sensors, using a PIC may be
> around
> > NZ$70 (US$35-$40), and software perhaps a little less (@ Q = 300), then
> > there's markup on that
> >
> > Can anyone with practical experience of PLCs give me any advice as to
> > whether the cost of a PLC system is even worth considering ? And, if so,
> > how do prices compare ?
> >
> > TIA
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