Seriously?

[gar]

081228-1935 EST

KentAT:

I differ with your definition of "ladder logic" being synonymous with "PLC Logic". Ladder logic or ladder diagrams existed long before the PLC to provide an easy means to describe relay logic for a complex machine control.

The PLC design using ladder diagrams was modeled after relay logic diagrams to provide an easy transition for plant electricians.

See
http://en.wikipedia.org/wiki/Ladder_logic

Ladder logic is a philosophy of drawing electrical logic schematics. It is now a graphical language very popular for programming Programmable Logic Controllers (PLCs). It was originally invented to describe logic made from relays. The name is based on the observation that programs in this language resemble ladders, with two vertical "rails" and a series of horizontal "rungs" between them.

The PLC started in 1968 with a request for an electronic system to replace relay logic at General Motors Hydramatic Division.

A quick search with Google brought up some of the early history of the PLC, but not a real clear discussion of the early participants and their contributions.

.

 

[tom baker]

Learning PLCS is a great benefit and they are used in nearly all industrial processes. Its a different way of thinking from being an electrician, less hands on and more abstract.
Your benefit is you understand the electrical side of the wiring.
Hop over to http://www.plcs.net/ take a look around and see what you think.
There is another forum I hang out, better than this one but I don't have the address at home.

go for it, you will not regret it

Tom
Certified Control System Technician III (12 years experience)

 

[KentAT]

I differ with your definition of "ladder logic" being synonymous with "PLC Logic". Ladder logic or ladder diagrams existed long before the PLC to provide an easy means to describe relay logic for a complex machine control.

The PLC design using ladder diagrams was modeled after relay logic diagrams to provide an easy transition for plant electricians.

You are correct. My intention is to point out what I consider the most fundamental difference of ladder logic as used in programming a PLC versus ladder logic used in relay based system schematics.

Kent

 

[DesignerMan]

You may say relay logic and PLC's are close, yeah, in operation. You ever try to design a PLC system to match relay logic?

I have designed PLC systems to match relay logic controls and visa versa on occasion. Designing industrial control systems is what I do for a living and I can tell you that it isn't as complicated as it seems. Getting some hands-on experience will make everything become clearer. Relax- you'll get it.

 

[gar]

090118-0951 EST

nemo76:

The early design philosophy for PLCs was to create an input language similar to relay ladder logic rather than use a boolean algebra format. This was to make it easy for electricians familar with relay ladder logic to work on PLCs. However, early Modicon units were a real problem because of limitations of memory and four contacts maximum per rung.

It is much harder to go from many PLC logic designs to a relay circuit than relay to PLC. Today there are probably many times when you can not go from PLC to relay without adding something other than relays. Also very much harder to wire and debug relay logic of any complexity than a PLC.

.

 

[weressl]

090118-0951 EST

nemo76:

The early design philosophy for PLCs was to create an input language similar to relay ladder logic rather than use a boolean algebra format. This was to make it easy for electricians familar with relay ladder logic to work on PLCs. However, early Modicon units were a real problem because of limitations of memory and four contacts maximum per rung.

It is much harder to go from many PLC logic designs to a relay circuit than relay to PLC. Today there are probably many times when you can not go from PLC to relay without adding something other than relays. Also very much harder to wire and debug relay logic of any complexity than a PLC.

.

Couple of things:

1. Ladder logic IS a graphical representation of Boolean logic.
2. PLC's were conceptualized initially as replacement for relay controls.
3. As soon as you got inside of the microprocessor; the door opened to allow much more functionality to be implemented in PLC's than in classic relay control.
4. There is seldom, if ever, any need to go from PLC to realy.

 

[DesignerMan]

Couple of things:

1. There is seldom, if ever, any need to go from PLC to realy.

I agree- the core control system our company designs is PLC based. However, on occassion we do get customers that want relay logic control instead of PLC. Since every system we design is custom you basically have to take a PLC control concept and reverse it to relay logic.
Kinda fun really! :grin:

 

[WastefulMiser]

I once had a customer that wanted to use a relay-based transfer instead of a PLC because they said "it was easier to troubleshoot".

Hahahaha.

I wish I could have seen their face when they saw the final product with all the components required and wires going every.

 

[iaov]

My eperience with PLC's showed that some guys are realy good and understand right away. Some electricians never realy understand them or get comfortable with them. Trouble shooting with a PLC is wonderfull provided you realy understand the machine and/or process it is operating. With complicated industrial machinery sometimes the hardest thing is to figure out what it is supposed to do to start with.

 

[Vod-Vil]

Here is the easiest way to learn.

Buy a Micrologix 1000 on ebay for 100$
Download a free version of rslogix500 from allen bradley's website.
Buy a book called Programmable Logic Controller Tutorial by Stephen Tubbs

This gets you started.

The book tells you how to wire the PLC and gives a good introduction to basic programming.You will start with making the lights blink in certain sequences and work up to programming an assembly line at a bottling plant.

After you get through all the exercises you will know how crazy these PLC's can get.

Good Luck and have fun with it.If all else fails you can program your christmas lights and be the envy of the neighborhood.

 

[IMM_Doctor]

Nothing complicated about Industrial Machinery

Nothing complicated about Industrial Machinery

My eperience with PLC's showed that some guys are realy good and understand right away. Some electricians never realy understand them or get comfortable with them. Trouble shooting with a PLC is wonderfull provided you realy understand the machine and/or process it is operating. With complicated industrial machinery sometimes the hardest thing is to figure out what it is supposed to do to start with.

Working on Large Industrial systems is as easy as working in a large commerical building. As an electrician entering a 20-story office building, do you have to know every electrical device in the buliding? NO, you may be there to fix only what is broke, such as the lights in Room 1202. So as an electrican you don't start at the transformer outside, the switchgear, the feeders, the distribution system, but you start directly at the lights in Room 1202, Easy.

As a industrial controls electrican, you approach large systems the same way. You get the 100 sheet electrical drawings, and merely zero in on the ONE device that is not functioning, and work your way back to the lack of continuity. Most machines consist of nothing more than On/Off devices, similar to a single-pole wall switch, or an incandecent load. There are also Analog devices, but these are easy to comprehend, as they are merely some percentage of On/Off.

If you run into a component that you don't understand... Google it, and get a PDF manual instantly.

 

[dconrad]

Man oh man, what an opportunity. Paid to learn, Not sure what kind of plc they have but most A/B- modicon etc are pretty straight forward. Depends allot on who's writing the program. There's lots of books out, just learn the basics first and the rest will come. Most all industries are run by them in one form or another and its fun to see how other peoples mind work when writing logic. GO FOR IT, YOU WONT BE SORRY.

 

[wptski]

081213-1415 EST
On a separate subject I have a major problem with someone using a sequencer mode instead of using combinatorial logic in an assembly machine. A halt in the middle of a cycle is major problem because of the need to go thru a manual operation to get home. It may be easy to write the code, but a mess out on the assembly line.

gar:

Sounds like Dept 78 Line #3 at Ford Sterling! To do a simple pallet stop replacement on the "C" section, all was needed is to lock out the main conveyor and the air. Because of the way it was wired the jobsetter had to restart every operation in that loop if the panel box was turned off! We had a problem pallet stop that was 15' from panel box. I've done that job many times with the jobsetter standing guard at the panel box. There wasn't much physical danger in doing the job without observing proper lockout or I wouldn't have done it but if I got caught, it meant the remainder of the shift plus one day off!

 

[gar]

090318-1137 EST

Bill:

My reference was to a machine at Dana where the center section was lifted up, flipped over to put the pinion flange up, we measured drag torque, the part was shifted 10 feet, and put down on a fixture to apply RTV inside the tube bores. From here it went to the tube press.

The designer of the logic for the machine would only work in AB sequencer mode. Thus, you had to manually step thru each operation from wherever it was stopped to get to home.

My design philosophy on any machine where I could provide input on the machine control logic was that there should be a return to home button. In manual or more correctly a semi-automatic mode this button could be pressed, obviously safety precautions were included in the startup of this operation, and the machine would automatically return to its home position. This did not override any states that would be prohibited.

Thus, in a gaging operation where there was some problem part way into the cycle you could stop and return home. This was especially useful on the case shim stations where there was a complex sequence of getting to the gaging position. You may have used this function on some of the case shim stations. This was a technique I got Trio to use about the mid 1970s, and predated the machines that went into Van Dyke and Sterling.

I wonder if you many have been assigned to work with me at some time over the years. Maybe you remember Bob Farmer. He was pretty much the Trio Ford mechanical service man. I had him with me at Van Dyke for almost 24 hrs one day when the lines were first setup there. I was stuck with doing some major debugging of the relay logic on the pinion shim machine.

76 was when the first machines were built for Van Dyke. 77 and 78 were the first Sterling machines.

.

 

[don_resqcapt19]

The only thing you have to remember in troubleshooting either PLC or relay logic is that it was working before it broke, (unless it is a brand new system) and there should be no need to make any changes with the logic...just find the input or output that is not working. I don't know how many times I have found troubleshooters moving a wire or changing the program to make a system work. Often that results in more problems down the road. Find the real problem and fix it.

 

[wptski]

090318-1137 EST

Bill:

My reference was to a machine at Dana where the center section was lifted up, flipped over to put the pinion flange up, we measured drag torque, the part was shifted 10 feet, and put down on a fixture to apply RTV inside the tube bores. From here it went to the tube press.

The designer of the logic for the machine would only work in AB sequencer mode. Thus, you had to manually step thru each operation from wherever it was stopped to get to home.

My design philosophy on any machine where I could provide input on the machine control logic was that there should be a return to home button. In manual or more correctly a semi-automatic mode this button could be pressed, obviously safety precautions were included in the startup of this operation, and the machine would automatically return to its home position. This did not override any states that would be prohibited.

Thus, in a gaging operation where there was some problem part way into the cycle you could stop and return home. This was especially useful on the case shim stations where there was a complex sequence of getting to the gaging position. You may have used this function on some of the case shim stations. This was a technique I got Trio to use about the mid 1970s, and predated the machines that went into Van Dyke and Sterling.

I wonder if you many have been assigned to work with me at some time over the years. Maybe you remember Bob Farmer. He was pretty much the Trio Ford mechanical service man. I had him with me at Van Dyke for almost 24 hrs one day when the lines were first setup there. I was stuck with doing some major debugging of the relay logic on the pinion shim machine.

76 was when the first machines were built for Van Dyke. 77 and 78 were the first Sterling machines.

.

gar:

Bob Farmer was one of the names you mentioned in a PM but I don't know him.

The case shim selector is a complex lifting operation and always was a problem on "certain" lines only. Management never did want to take the time to figure out Line 3 rarely had a problem with the three stations and Line 1 never had all three running while I was in that area! They'd burn up bearings on the drive shaft, spin the center in the shaft or break it right off! It had a overly tremendous thrust load on the center and shaft bearings but they could never understand that it's only supposed to lift and turn the differential. At one time, they'd do a bearing job on one station every weelend just for PM.

 

[gar]

090318-1359 EST

don:

The pinion shim I was debugging was not completed before it had to be shipped. In many cases this is typical of what is required to meet production schedules. In the auto industry when a new model comes out machines have to be in a plant by a certain date to run the first tryout parts. If the machine can accomplish building a part that is all that is really required at that time. It may not meet cycle time, and may not even produce the best part. Then you work to improve the machine before the next sample parts, and finally when you get close to real production you try to be within the cycle time requirement, and building good parts. On this pinion shim station one of the debugging goals related to improving repeatability.

Also there are limitations of what can be done in a tool builders shop. You have few parts, in this case not even production parts, only prototypes, with which to work. This means you reuse the same parts over and over. Not until you can get a full production running with production parts can you really do final debugging. There is a big difference between having 4 or 5 parts and having a supply of one new part every 13 seconds.

In that time frame repeatability was on the order +/- 0.001" . Over the years I have been able to get this down to the range of +/- 0.000,15" on some machines. Little of this improvement had to do with the electronics other than some algorithm changes. We always resolved 0.000,1". Much of the improvement was in mechanical and hydraulic changes.

The line and time frame was 1976 and the decision by someone, probably Ford, was to continue using relay logic rather programmable controllers. The machines were for a new type axle for Ford main production. A very major change from the banjo type. This became the standard type for all cars and light trucks. If you expect to meet production deadlines you do not make more changes than necessary. Design cycle time was I believe 13 seconds part to part. About 3 seconds in pallet transfer. Thus, relay logic was probably a wise choice at that time. PLCs were quite limited, but could have done the job. Several years earlier on a Buick line the logic was with PLCs. Quite a difficult process getting that line running.

.

 

[gar]

090318-1416 EST

Bill:

Just by mentioning Farmer's name again I was trying to maybe jog some memory.

I do not know why the case shim bottom driver had to have too much force. This might have been cycle time related in order to move the bottom rapidly and an unwillingness to reduce pressure after the driver was up. There were always major problems with balancing the side ram forces. There were ideas I had on ways to reduce the side ram forces, but I could not get anyone to put up the money to work on anything different. There are lots of small changes that can improve repeatability from part to part but it is very difficult to get people to experiment.

.

 

[wptski]

090318-1416 EST

Bill:

Just by mentioning Farmer's name again I was trying to maybe jog some memory.

I do not know why the case shim bottom driver had to have too much force. This might have been cycle time related in order to move the bottom rapidly and an unwillingness to reduce pressure after the driver was up. There were always major problems with balancing the side ram forces. There were ideas I had on ways to reduce the side ram forces, but I could not get anyone to put up the money to work on anything different. There are lots of small changes that can improve repeatability from part to part but it is very difficult to get people to experiment.

.

gar:

They would always rather limp along then be down for a repair to run perfect later!