Industrial control systems

Prog logic is good for production of machine controls where limited function is acceptable. PC control allows for a more flexible solution but takes up more space and requires trained people to use it.

The PLC may be smaller, unless you use a PC-104 form factor for the PC controller.

To make the distinction even more difficult, the PC-104 could run a soft PLC program and behave mostly like a PLC.

Sme PLC's have quite a lot of features, and in fact can be as large and complex as PCs.

To me, the main difference is that the PLC is a closed (proprietary) platform, and the PC may be an open platform.

not too different from the two sysstem.

the front is considered as a special tool for control engineering, the later as universal tool.

They are all based on cpu inner. ram, rom and i/o termilal.

I was told the front is envoloped from relay program, and easy, flexible, use. more i/o terminers.

and pc can be used for system control

both of them are using in China engineering.

most devices are bought from usa, germany.

PC based systems are upgradeable and / or flexible in application.

Maintenance cost is less as PC maintenance is cheaper. Faulty PC itself can be replaced by another in shortest time.

A PC is expected to only have a lifetime of a couple of years where a PLC can easily last ten to twenty.

A new PLC can be installed in its rack and the program downloaded in a matter of a minute or two where it can take a half hour plus to install just a new PC's operating system let alone the application; ever hear of down-time?.

A PC's real-time characteristic change with its CPU clock speed and I've seen many a application behave erratically when installed in a newer faster PC than what it was developed in. Again, ever hear of down-time?

A PC is susceptible to viruses, a PLC isn't.

Many PLCs support a hot backup feature where a backup PLC module takes over for a failed primary CPU without a hiccup. I've yet to see a PC rated for for any type of Safety Integrity Level; many industrial applications can't be handled with anything less.

There are many reasons PLCs are used in manufacturing instead of PC. I agree, PC are great data collectors for SCADA. I've yet to see any longevity out of a PC based system (Steeplechase as an example) and have replaced many of these system with PLCs over the last 25 years.

I would argue the exact opposite.

PLCs are designed from the ground up to operate in rugged, noisy, hot, humid, high vibration industrial applications. You can change out components and swap operating memory modules without ever looking at a screen or finding a boot disk or accessing a keyboard. You can add analog modules, motion modules, all kinds of I.O modules and industrial communication systems without having to reconfigure the CPU with drivers etc,. that may or may not be compatible with whatever you already have installed.

PCs were designed to view pornography.

Go to Opto22.com and check out the white papers. You might find something helpful there.

mitchc1970@yahoo.com

At the risk of stating the obvious, the traditional PLC is a special purpose system used for building industrial quality machines. They have industrially toughened inputs & outputs that tolerate electrical noise, static, humidity, temperature, vibration and other insults. The programming language, while usually limited, is designed for real machines, so it's easy to write a robust program that always works (and if done properly, easy to fault find). Most manufacturers produce a wide range of sizes speeds and display types. Oh yes, and they don't get stolen. In short, a PLC is tough, deterministic, easily adapted to most tasks and incredibly reliable, a good choice for most machines. However if you need large data handling, statistical comps or a pretty GUI go for a PC based system. There's no global solution, it depends on the project and the available money/time, so I design and build machines using both systems. Jeff

In general I would say:

PLC is deterministic in time and relyable. I would definitively use a PLC if my sampling time is smaller than one second, and if there are safety related matters associated to your control system.

A Pc is not deterministic in time, and relyability depends on the sysop. I would suggest ussing a PC only for SCADA purposes.

http://en.wikipedia.org/wiki/SCADA

Statistical post-treatment of the data is better handled by a PC, and automatic reporting is not always feasible with a PLC :)

I hope this will help in with your table

Everyone has given good advice and the choice isn't always easy to make because the line differentiating PLC and PC controls has blurred in recent years. They have both borrowed each other's strengths and many of the larger installations are a combination of each.

My advice is to total your point count for analog I/O used for proportional control compared to what you need for digital or discrete I/O. If the system is heavy on proportional and the discrete are mostly permissives, the PC is probably going to be less expensive and the OS more attuned. On the other hand, if it's more ladder logic, a PLC may be better. It greatly depends on the overall size, purpose, redundancy requirements. Is it a distributed system? And operator interface requirements are key too.

Really, the "merits" can only be analyzed in reference to a specific purpose.

Why are you discriminating against DCS and Hybrid control systems?

different people like different tools. so you cannt say which one is better than another one? it depends on workers favourite. I think.

plc is also a cpu basiced. inside it there is a micro centre control chip as well. and it was designed as a special tools, and easy use in practice.

industry pc can also use in bad circumstance and work condition

some survey shows some country's people like plc and others like pc or simple microchip or dsc etc.

I prefer to design a control system by myself with microchip, I think its also convenient to expand its i/o with IC

All of you have made a good point on PLC vs. PC. I just want to add one more: the setup cost of PLC is high compare to a PC. However, a PLC can last a lot longer (in general) than a PC.

MidniteFighter

Don't forget that a PC doesn't have to run Windows XX... The are other OS's which enable real-time processes. PLC's are great, but just like every other piece of electronics, they are subject to change. I have yet to understand why my company will spend lots of money on destop computers about every 5 yrs, but will not upgrade the computerized equiptment that makes the products/profits. Some of our controls are 80's vintage!

Bosses only count their money with the production being generated, otherwise the engineers or the production heads get the stick and sometimes some cooking! They don't know about the machines or controls after they have bought them to do production and only production of finished products that they can count cares. Probably the PLC manufactureres did too well to have manufacturted reliable, dependable and durable logic and control components.

Engineers or not, we all have accept the norm of that PCs are excusable to be less lasting less reliable and they are probably manufacutured with a predated life span. The manufacturers got away mainly they are going to the consumers and not engineers. There was never a good durable engineering in PCs....

Any one really building a reliable, robust, reliable industrial PC?... Have yet to hear or know an industrial PC that last the threshold of 5 years.

Will some one do it ? .. then we all may slowing going into PCs from PLCs.

Until now, PLC is still bloddy good!!!

YT Lim