ladder and relay logic are NOT my strong suit! I don't do well with any
more than a few inputs and outputs. Is there a good basic book that
would help convert a process into a wiring diagram and parts list? A
lot of my stuff wouldn't be worth a PLC but a few relays and limit
switches are more in line.
I like some of the PLC simulators that you can run a simulation on and see
what's happening in the ladder logic.
I would recommend you pick a project that you want to convert to PLC.
List the Inputs and outputs
List the steps that the machine will go through, how it's started & stopped,
does it have automatic and manual operation?
Of the steps of the operation, define how each step is completed, a sensor,
photo eye, timer timeout, etc.
Also must consider how it will be started up after power loss or e-stop, if
it gets out of sequence, how do you want to reset....
Once you define what it needs to do, the ladder logic can go pretty smooth.
If you have any specific questions on how to get the PLC to do something,
ask, I'm sure someone will be able to help.
For an economical operator interface you can use a single push button and
light, human machine interface can communicate by Morse code! :-)
You may not want to require your operators to learn Morse code to operate
the machine but you can do simple flash sequences, i.e. alarm number 27
could be 2 flashes, pause, 7 flashes, long pause, repeat until cleared or
reset.
Come up with a project, once you get walked through a couple of problems
you'll figure out how to solve many other problems in a similar manner.
RogerN
I don't need anything on PLC. The stuff I have is two or three relays
and 3 or four limits. E-stop, start and reset will do for most things.
Any more complicated project, I call BOB...he does all my PLC stuff.
Not cheap but my time is worth more doing something else. I just want
to do small stuff with relays while watching TV.
Ok, relay ladder logic with relays!
In school, in Digital class, we learned Boolean Algebra and how to take a
more complicated piece of logic and reduce it down to a simpler form. At
work one time we had a project to come up with relay logic but they didn't
want to use a PLC. I wrote a Boolean expression and simplified it, ended up
taking 2 relays to do what they wanted. Anyway I thought you might want to
look up some info on Boolean Algebra, it can be applied to relay logic in
control panels.
By the way, some of the machines at work used sequencers, each sequence
would be active for a time and have sets of contacts that would operate at
the different stages of the sequence, that's another way to control a
machine sequence.
RogerN
------------------------------------------------------------------------
Hi Tom :)
Take a look at
www.cmhsoftware.com
Their 'Constructor' software does just what you are looking for.
About 10 years ago I requested and received a 'Demo' version which is
a 'cripple' version that doesn't save or print. But it sure proves a
wiring concept.
No connection with the company. Just like the software :)
HTH
Bob
rgentryatozdotnet
For a steel ladder, Rustoleum "Rusty Metal Primer" or "Clean Metal
Primer". Wooden ladders shouldn't be painted & aluminum ones don't need
to be.
Bob
<G>
learning ladder logic is worse than learning chinese. its just bass
ackward on all the thinking to me.
25 years ago, I would make the EEs explain every detail to me. I'd
take notes and get a minumal understanding. enough so i could ask for
a logic change here and point. Never really got on to it.
When I went into rebuild your own CNC controls hobby, I went with
Camsoft because it used computer programming logic instead of ladder.
Cost me a lot of $ at the time to stay away, and you know I'm a
cheapskate.
My brain doesn't take on new stuff as well as it used to, I wouldn't
stand a chance. prove you're smarter than me.
Karl
Nope! I have this machine with 2 hyd cylinders and six 3pdt relays and
two 4pdt relays that is so fubar that it needs a new control program. I
tried for a week and I'm stuck. Monday, I call Lakewood Automation and
write them a blank check. My brain is to mushy to get this job done by
myself. I need the brushes! The machine will make about 600/day of
solid-fill end brushes at $6 each. I'm an azzhole, the machine would
have paid for an upgrade by pros in a day. But NOOOO, I farted around
for a week.
If you can write down EXACTLY what you want in a logic flowchart, a EE
that knows this can whip out the program in jig time. make a guy that
don't know your process figure out the logic and you'll pay through
the nose and still be debugging it after he leaves.
I learned a very long time ago to be EXTREMELY specific about what I
needed the machine to do ahead of time.
Just my 2 cents
Karl
Karl,
If one gets down to that detail, ladder logic becomes as simple as any
other combinatorial logic.
All you have to remember is that it doesn't all get executed
simultaneously, as it might in a gate array, but rather each 'gate
network' (ladder statement) gets executed in succession.
LLoyd
On Sun, 21 Apr 2013 16:30:12 -0500, "Lloyd E. Sponenburgh"
I could always quickly write down what it needed to do, in english
with blocks and arrows. making the ladders just never clicked for me.
I did learn that most of of the EEs I worked with couldn't see the
overall picture of what the machine needed to do. two heads better
than one.
Karl
Karl,
You're old enough to remember the old "barrel sequencer switches" used on
machinery back in the day -- the type that had a bakelite cam stack with
about 10-100 thin cams, each with several lobes, and each engaging one or
more small leaf switches or microswitches. The cam stack was usually
turned by a solenoid and ratchet/pawl setup, so it had discrete
positions. Remember? yes? And remember the old Telco stepper switches
that had up to ten wiper arms and about 100 contacts per sweep of the
arms?
They were used to control everything from manufacturing machinery to
pinball machines, and constituted a very early form of ladder logic.
Each stop position in the rotation represented a line on the ladder
diagram. Each set of switches/lobes acting when the cam was in each
position constituted the logic in that ladder statement.
So, except for one-shots and rising-edge triggers, and other stuff that
can muddy up the picture... each line in a ladder diagram is nothing more
than:
-- a 'primary' input condition for that line
-- a set of logical operations (ANDS, ORS, XORS, etc) driven by other
input conditions. (can include inputs, other ladder statement outputs,
clocks, etc)
-- an output condition for the line.
That output condition may be used as an input condition for subsequent
lines (or even prior ones on the next loop through the ladder). The
lines are treated like the barrel switch -- each ladder statement is
interpreted and executed in sequential order, starting with the first,
and ending with the last, then looping back to the first. They're
scanned FAST, so from the human perspective, it all seems to be happening
at once, but they are sequential.
Most newer PLCs also permit the 'output' of a ladder statement to result
in the execution of a custom function -- usually a small BASIC-like
program that gets treated as if it were a hunk of physical logic.
I wish I could sit you down in my shop for a day. I'd have you
programming solo in extended ladder logic by the next morning.
LLoyd
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> fired this volley in
Crap... a statement like that would make it more confusing without saying
this... those logical operations are represented as RELAY CONTACTS in
ladder logic. I'll bet you can do simple relay logic; yes?
Now build 100 small relay logic machines, and set them up on a big rotary
switch so each one gets 'operated' one at a time.
That's ladder logic.
LLoyd
OH YES! I have parts list, an accurate and verbose description of the
process, flow chart, and my attempts at the program.
I've since called 4 companies and none can even look at it for 4 weeks
and $1,200/day 1 day min. They are crazy!
I've almost got it...the one cylinder gets power, extends to a switch,
retracts, extends, retracts, extends, retracts...... I can't figure out
how NOT to repeat that part of the program, it's like a skipping record.
What is supposed to happen?
what starts the cycle(s)?
what stops it?
Sounds to me that you are missing a "stop" switch for the retracted
position.
If you post what you have I may be able point to a solution.
As we discussed over the phone..sometimes you simply have to let the
experts do it. If it were a hobby...we can take our times, but when it
comes to "bottom line"...shrug
You know what I did six jobs ago as a Kid starting out - Central
Office Construction for GTE - 500 million relays looking for a phone
call to put through...
Put together all the crap you have so far, hopefully with diagrams of
how the machine was put together when you got it, and what wouldn't
work, and stick it in a Dropbox account and send me a link, or blow up
GMail - I can look at it tonight and might be able to spot the
critical error that was causing it to crash and burn. Add a jumper or
fix the limit switch, and get you running again.
Ho! Ha Ha!Guard! Turn! Parry! Dodge! Thrust! Spin.... (Sploosh!)
http://www.youtube.com/watch?v ŒuihrjLNAo (Worksafe, as long as
you're the Boss.)
It gets tricky but it's not that bad - you just have a relay start a
motion, then the next part can't start till it hits all (two, three)
limit switches that say 'part 1 done, start part 2'. Make that relay
lock on (or off) till it's part is past.
You have to resist the urge to over-simplify it, because that's when
power sneaks through a path you didn't intend and it goes GRUNCH! and
turns into a pretzel. (A very expensive one.) Limit switches to
detect and positively lock out potential crashes are a good thing.
The hard part is E-stops and resets. Especially when you have to
'reset' the machine in a certain sequence from an unknown state.
Unlock the ram, remove the injection feed tube, /then/ retract it.
Last resort, How's the weather in Cleveland in a few days, and how far
out do you have to book to get a decent airfare? (I know I'm going to
regret saying that, but you know how that goes.) Out of BUR Hollywood
Burbank if possible, NOT LAX.
* ** ** ** ** * *
But once you get the brushes built and out the door, get working on
the PLC conversion - No excuse for not being able to understand and
fix it. Even nicer if the machine has a display and can tell you
where it hurts.
Don't be like the guy with an almond-picking machine (I think...) who
had custom programming done on a early version of a SPLAT
micro-controller and they didn't give him the paperwork and program
printout - the machine died a flaming-glory style death, the old
company is gone, and now they have to reconstruct the programming
again from scratch. With the old papers and printouts it would have
been a simple rewrite to match the new hardware.
I'd send backup copies to several relatives in different states, too.
Ship them a 4-drawer Turtle fire file cabinet for in their garage.
That, or have "Iron Mountain" stick it in a Salt Mine.
Bruce Bergman, Lead Electrician and other Magiks, Westend Electric.
CA C-10 #726700
Pryy rvtug bar rvtug - guerr gjb fvk - fvk rvtug guerr guerr
--<< Bruce >>--
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