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.
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.
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.
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.
Karl Townsend fired this volley in news: email@example.com:
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 E. Sponenburgh" fired this volley in news:XnsA1A9E2CA2B27Elloydspmindspringcom@126.96.36.199:
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.
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.
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!)
(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.
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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
I debugged an EE's failing relay circuit to a relay that took too long to open, because the clamp diode didn't dissipate the energy stored in the coil fast enough. The schematic appeared correct; it was a quirk of the component that caused the problem. jsw