PLC vs Microcontrollers.

EDAboard.com Forum Index -> Microcontrollers -> PLC vs Microcontrollers.

Author

Message

Lord Loh.

Joined: 19 Jun 2004
Posts: 199
Location: Texas

08 Apr 2007 19:37 plc vs microcontroller

I am being taught PLC as a part of out course.

However, I am not able to appreciate PLC at all... When we have such low cost microcontrollers, Why on earth do we need PLCs?

I read that PLC programmes are easier.... How?

Why do we need PLCs today? Or are they just a relic from the pre-microprocessor days?

eehero

Joined: 23 Nov 2005
Posts: 232
Helped: 9
Location: Libya

08 Apr 2007 21:26 microcontroller vs plc

yes, PLC could be programming using ladder diagram, it is easy for technicals to be dealing with ladder more than C or assembly or other prog. lang.
PLCs more suitable for industrial applications, they can bearing the dust and hits.

vsmGuy

Joined: 26 Jun 2006
Posts: 489
Helped: 28

08 Apr 2007 21:49 plc vs. microcontroller

Ah. For all practical purposes, they just a relic from the pre-microprocessor days.

Anyone uses them extensively here ?

jhbbunch

Joined: 21 Feb 2006
Posts: 222
Helped: 16

09 Apr 2007 2:56 ladder for microcontroller

At the heart of all PLC's is a microprocessor. The ladder logic is a very simple way of interfacing to it to do straightforward things like turning motors on and off based on a set of inputs. Anyone want to show me the code written in C for a mircrocontroller to control an industrial motor. Anyone want to try modifying that code? In PLC the logic is obvious and easily modifiable. Industrial control systems are almost always done with PLC's. In fact, you could make the claim that they are becoming more popular as microcontrollers advance. I've seen $80 PLC's based on ATMega chips. Twenty years ago you would not have found a system starting under several thousands of dollars.

amrstaha

Joined: 29 Aug 2005
Posts: 141
Helped: 6
Location: Egypt

11 Apr 2007 1:32 plc versus microcontroller

As our friend just mentioned, PLC use microprocessors or microcontrollers as their CPUs. PLCs are simpler to use especially for non-electrical engineers. PLC hardwares are robust as they come from well-recognized international manufacturers like Siemens, Allan Bradly, GE, and etc.
PLCs are easily programmed using Ladder, Function Blocks, or even Statement List (a language that is very similar to Assembly).
The main disadvantages are: PLCs are exepensive, bulky, and are not easily adapted to high speed I/O. You can't put a PLC in a mobile phone for example and you can't output high frequncy PWM for motor control from a PLC.

fazan83

Joined: 11 May 2006
Posts: 124
Helped: 1
Location: Gurun, Kedah, Malaysia

11 Apr 2007 2:58 plc using microcontroller

amrstaha wrote:

PLC votage range is input output is higher. It suitable to use with relay that have higher coil voltage usually 24VDC and sensor or conponents that work with higher voltages (usually 24VDC).

Meanwhile Micro C usually use with components with smaller voltages typical 5V and below. But we still able to interface it with higher voltages components depend on how we design the circuit as Micro C is a part of PLC.

jhbbunch

Joined: 21 Feb 2006
Posts: 222
Helped: 16

11 Apr 2007 3:13 plc vs microprocessor

Another important point I forgot to mention is that PLC's are very linear. The state of the systems inputs are all taken at one time and saved. It's like a snapshot of the inputs. The logic ladder is then processed sequentially. All rungs and all elements of a rung are executed. This allows for precise timing of execution and you never get into things like endless loops and code inside conditional statements that never get executed. This is a crucial concept because now the time it takes to execute the logic ladder has meaning and if the WDT times out, you know that you have a real problem and need to execute the timeout sequence (you probably need to shut the system down). This is not easily done in computer programs unless you try to emulate ladder logic. WDT stuff is very important in industrial process control. So PLC code tends to be much less buggy and safer.
As fazan said PLC is not amenable to small stuff or very complicated stuff, but most industrial stuff is pretty straightforward. And stuff like PWM, and servo control can be done by adding real time modules designed to do those things. That is another thing about PLC's, they are very modular. They usually have power and mounting rails that you clip your modules to.

Satti

Joined: 15 Sep 2009
Posts: 2
Location: Ludhiana

15 Sep 2009 13:39 plc microcontrollers

plc is easy to program. with ladder and it is suitable with industrial application as compare to microcontroller

Added after 1 minutes:

its easy to install and its easy to troubleshoot and programing is easy then c or assembly

Shinnster

Joined: 15 Sep 2009
Posts: 34
Location: Hello World!

16 Sep 2009 0:11 disadvantages of programmable logic controller

In terms of programming I'd say that PLC is more user friendly and easier to learn.
Easy for others to read as well unlike having to pick up a datasheet to read for that particular PIC16x or PIC18x.

phophollety

Joined: 28 Aug 2007
Posts: 3
Location: Santo André, São Paulo, Brazil

17 Sep 2009 2:51 microcontroller ladder

There is one more issue.

A goo PLC ( such as an AB or an Mitsubishi ), are very very very debugged platforms, those companies researched a lot to develop and produce those platforms, will you put on your name on a machine wich could injury a person just to save few bucks? Its clear that a uC is cheaper than a PLC but does your uC can reach easily all the specs and test that a PLC is burned-in? Also, for small volumes, it is not viable to develop all a platform to assembly few pieces of them if you can put on a debugged and know PLC.

anotherbrick

Joined: 10 Jan 2009
Posts: 25
Location: Istanbul , Turkey

17 Sep 2009 18:21 ladder microcontroller

hello

I am selling my own designed PLC for 39.- US$ ( starting from )

http://www22.brinkster.com/anotherbrick/indexe.htm

I need distributors

Google
AdSense
Google Adsense

17 Sep 2009 18:21 Ads

stan4

Joined: 19 Aug 2009
Posts: 10
Location: Nicaragua

03 Oct 2009 4:17 Re: PLC vs Microcontrollers.

Interesting question.

Why do we use PLC's now?

I agree that anyone can put a uC, a crystal, a few optocouplers and relays in a PCB, and you might think, eureka, why am I the first to discover this?

I've developed projects professionally using PLC's, I've used OMRON, SIEMENS, MITSUBISHI, YOKOGAWA, TELEMECANIQUE, TOSHIBA, AB and FANUC and I'd like to share my opinion.

First, what is the point here? is it price? ok, uC's are cheaper than PLC's, I agree, but, let's say that I want to use my microcontrollers, instead of buying someone else's PLC's, I won't be able to design and make a specific unit from one day to another so it should be convenient to stock a few of my uC's ready for any application that should come up.

- I must take some time to design, debug and test, and test again, the hardware, software and firmware for the basic CPU.
- Same with the product itself, which includes the plastic casing, for a decent multimodule PLC a mounting rack, even the packaging box.
- Now let's move to the modules, to have a competitive brand I must carry many useful modules, DC input, AC input, relay output, triac output, transistor output, analog voltage input/output, analog current input/output, temp (TC, PT100) input, RS232 comm, RS485, CAN, PROFIBUS, more recently ETHERNET, wifi.
- I must design a handheld programmer for my product (to stand beside PLC's brands).
- I must provide my product with a complete family of HMI's.
- Finally let's don't forget that each and everyone of my products must comply with various international standards, like UL, CE and FCC.

Until then, I will be one of all the fools that buy PLC's to someone else, instead of making their own, yet the amount of time and money it'd take me as a person is quite high.

So, I think, using someone else's PLC saves time and money*.

Even so, you will never get rid of using someone else's platform, many times you will have to troubleshoot an existing system and so, you need to have the softwares and the cables for various brands; if there is a damaged PLC in a network, you have to replace it with the same model PLC, not your uC; some big customers** will ask you for a specific brand of PLC, either because they like it, trust it, or they want to keep uniform their plant's PLC's.

Personally I use PLC's and uC's, everything depends on the application isn't it? now that i've thinked about it, the question seems a little bit superficial.

stan4.

* If you manage to grab a distribution from most brands, they will give you a percentage of discount from 20 to 50% of the list price.

** Coca-cola requested for a SIEMENS PLC in a recent project, I also noticed they had over 90% SIEMENS PLC's. Nestle requested for an AB PLC in a recent project, the maintenance manager told me they had an alliance with AB.

samukelo_shezi

Joined: 13 Oct 2009
Posts: 1
Location: polokwane

13 Oct 2009 0:30 Re: PLC vs Microcontrollers.

I think the microntroller languages, are improving at a fast pace. Has anyone seen the ladder logic that is free to download from Jonathan Westhues'website, www.cq.cx? Soon, the programming language ease of PLC's point, will be the thing of the past in this unending debate! Please, download this brilliant piece of software, and immidiately start making ladder programs for some PICs and AVRs.

sfshezi(at)yahoo.com