On 4/25/2012 10:11 AM, Spehro Pefhany wrote:
On Mon, 23 Apr 2012 18:34:09 -0400, Tom Gardnermars@tacks wrote:

From flowchart to schematic to ladder

I can design machine control circuits using relay logic but it's a
grueling job for me. I don't do it often enough to be proficient.
Typically, my circuits have 8 or less 344PDT relays with about as many
switches and as many outputs like valve coils and such. I can do a
flowchart in minutes but can't easily translate to a schematic.

We only have a couple of PLC controlled machines. I did the flowcharts
and hired a company to do the programming. I would very much like to
become proficient at doing the whole job in house. I can think of a
number of applications that using PLCs to replace relay boxes would be
very beneficial.

I use a bunch of stuff from "Automation Direct" and I've been looking at
their "Click" line of PLCs. They seem inexpensive and powerful.

So, how do I get from flowchart to Ladder programming?

Start simple and build on it! Start by replacing a single timer or
something like that. Don't start with a whole machine unless you can
block out a whole bunch of uninterrupted time to bootstrap things.

PLCs are the right tool for your applications, IMHO, you just have to
play with them a bit more. You might want to start with a micro-PLC
that doesn't have zillions of features, and then move to something a
bit overqualified for your applications that you can standardize on.

The nice thing about being in full control of the programs is that you
will be able to tweak them without getting someone in.

BTW, I would be sure to implement any safety-related requirements
outside of the PLC. For example, don't route an e-stop switch or an
overtemperature alarm through the PLC logic, make it actually shut
things down positively if human safety could be compromised. That way
a PLC failure, glitch or programming error can't hurt anyone.


Good ideas, thanks!