plc

Hey ,I am trying to find 4 common problems associated with a plc system.

Unregistered,

There are so many common problems associated with a PLC system, that I hesitate to limit them to four, but here would be my four:

1. Finding circuit problems on a PLC system cannot be easily done without access to the software and ladder logic, where the old relay logic circuits could be visually inspected without any other aids other than an eyeball and a brain!

2. PLC systems are subject to failures from logic faults, power blinks, low backup batteries, operator error, and loss of network communications.

3. If you buy equipment containing a PLC, if you do not get a copy of the software and ladder program, then you have only half a machine. If for any reason you need to access or change the logic, you would have to go back to the original vendor.

4. PLCs, like any computer-related device, become obsolete rapidly. You could be in a situation where the PLC on your old machine is no longer supported by the manufacturer.

Lancie1 said:

4. PLCs, like any computer-related device, become obsolete rapidly. You could be in a situation where the PLC on your old machine is no longer supported by the manufacturer.

Click to expand...

Lancie1,
I have a slight disagreement with this statement. While it's true that PLC manufacturers (like their PC counterparts) consistently strive to introduce new products and whiz-bang features, the established players (Rockwell, Siemens, et. al.) can and do support legacy products for a long time. PLCs are built for the long haul, and I believe the main reason manufacturers push for upgrades is if the benefits achieved from the features in the new PLC outweigh the costs associated with making such a change. As a Rockwell guy, I know that while the marketing push is focused on the Logix controllers, the other PLCs (PLC-5, SLC500, and, in rare cases, even the PLC2/3 controllers) can be supported in some shape or form.

Regards,
Rick

P.S. When was the last time Bill Gates supported Windows 3.1?

I agree with Rick. PLC (lack of) obsolescence is a strong benefit over most PC based control systems.

Rockwell has a program called silver series with a goal of 1-2 years advanced notice before they stop manufacturing a product, and then up to 7 years where that product is still actively supported . . .

See here and here for some details . . .

Marc

I am in a dilemma on this one.

Many of the older units by the "major" brands do have some support or product life left. AB, Omron, Siemens, GE and AD(Koyo) still provide for many legacy products to a point.

I have to "kind of" agree with Lancie though, obsolescence can be a problem.

I had to deal with a couple of really nice re-winders that used Klockner-Moeller PS306 and PS316. They both did the job but they were obsolete and hard to find. The PS316 could use digital I/O from the newer PS416 but not the analog cards, which is what went bad. The machine basically got scrapped because of this.

In many cases the "cost" associated with a legacy or obsolete unit because of interfacing methods to other devices can make it hard to replace...again IT DEPENDS but I have to agree that it can be a problem.

Four most common problems, in my experience:

1) Process or machine functions not clearly defined or process equipment not adequate for accomplishing the required objective

2) Program logic not correct to accomplish the defined functions

3) Field device malfunction or field wiring not correct

4) PLC hardware failure, often caused by Item 3

1) Human error on the specifier's part.
2) Human error on the designer/programmers's part.
3) Human error on the panel builders part.
4) Human error on the installers part.

After we correct all the Human Errors, everything seems to be fine.

Rick,

I know that Allen-Bradley does a good job gennerally in supporting the old units. From the suppliers viewpoint I am sure that it seems that the old units will never go away. However from a users viewpoint, it seems that sometimes I am forced to scrap a perfectly good PLC because I cannot make it work with other devices.

P.S. When was the last time Rockwell brought out a new version of the PLC2 software?

Touche! You are right as to the fact that certain products are not updated once they reach maturity (or "Silver Series, as we prefer to call it in the RA world )

I see your point regarding the fact that some new devices are not supported by legacy controllers; however I would personally categorize this as part of the equation when you are deciding if upgrading the PLC hardware is worth the added costs...

Rick

Support of older products is an interesting point. The biggest problem I see is that quite often the chip/component manufacturers discontinue a component that cannot be replaced with anything else.

This is not the PLC manufacturers fault. When they find out a component is being discontinued, they can buy up a quantity of them to keep the older PLC's going, but the numbers are usually finite.

This has happened to the older Omron C200H with DC power supplies. Some components were discontinued by the manufacturer and now are impossible to obtain. Makes life difficult sometimes.

Have you considered these problems with the PLC System

1. Mechanical engineer
2. Electrical Engineer
3. Programmer
4. Project Manager.

PLCs are NOT Real-Time!

PLCs have an inherent Latency (Scan-Time)!

PLCs are late in detecting events when they happen!
(The exception, sorta, being Immediate Inputs... Better is Interrupt Signal followed by Immediate Output... still, there is latency involved.)

PLCs can MISS events when they happen and then go away!
(There are some ways around this problem... sorta, maybe, sometimes.)

PLCs are late is responding to events when they happen!
(The exception, sorta, being Immediate Outputs.)


With respect to the question, the bottom line on PLCs is Latency... in detection and reaction.

failing i/o. biggest non-people related issue i've seen.

Terry makes an excellent point with scan time latency. You get the monkeyhead seal of approval for that.... which i'm sure is exactly what you were hoping for.

Interesting objective, to identify problems is the main one for decides them.

Terry, detaching: Truth, but Scan-teams is not a villain.

The manufacturers justify that launching CPU with high speed, will be capable to solve the problems, salesmen use this argument. But never, the performance multiplies or reduces the problems (of timed factor), if project not to disponible the sequence of signals order and synchronized appropriate for the PLC, exactly in system in CPU of immediate processing. Nothing,never it will influence.

A reply to coincide with Elevmike, it would be:
People who work without knowing because they make

Tom Jenkins, clarified well.Now, needs to know as prevents them.

1. Main error- the capacity human being to include limitations for solution. If not to include itself, never would finish...

I do not know because they insist and to include marks if it was one generic question...

The most common problem I encounter with PLC is the size of the brains of people running industrial plants.

They want Fast AND Cheap, Versatile but Standardized, Complex yet User-friendly, functions that need math but that will be serviced by personal they will not spend a penny to traine, and the list goes on...

Besides that all them transistors are great.