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  1. #1
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    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?

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    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.


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    plc vs. microcontroller

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

    Anyone uses them extensively here ?



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    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.



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    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.



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    plc using microcontroller

    Quote Originally Posted by amrstaha
    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.
    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.


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    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.



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    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



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    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.



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    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.



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    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.


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    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@yahoo.com



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    PLC vs Microcontrollers.

    PLC is most simple than M.Processor ...

    because we can programme PLC in running if we need ..but not for M.P ....

    the LADDER logic programming is so simple than C or other language ...

    PLC is sutible for Temp and other envoirments....



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    Re: PLC vs Microcontrollers.

    Another important issue is the noise immunity of PLC's. You may design a uC project and run it without problems in your lab environment, but don't expect it to run correctly in an industrial plant.



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    Re: PLC vs Microcontrollers.

    PLC's and microprocessors can both have a role in an industrial environment.

    As has been previously stated, PLC's are robust, reliable control system components well suited to integration into an industrial environment.
    The programming software for PLC's is almost always designed to make fault finding of large, complex machines relatively easy by allowing real time, online monitoring of the program, I/O and other integrated components.

    Microprocessors generally require additional hardware to integrate with "real world" control voltages and currents and microprocessor programming generally requires more specalised training.

    I use several brands of PLC as well as several types of microprocessor. I have found that deciding which is more suitable is generally a question of what you are trying to achieve.

    For a large, complex machine/system in an industrial environment, where there will inevitably be maintenance issues and breakdowns, using a PLC assists skilled trades like Industrial Electricians to fault find and repair day to day faults because of the way runtime information can be displayed by the programming software. It is also relativly easy to make code changes to adjust or "tune" the running of the machine.
    Many modern PLC's can also be "safety rated" so that safety functions such as e-stops and access control can be easily integrated into the control system and still comply with Occupational Health and Safety laws.

    For small, commercial "stand alone" or "off the shelf" systems (including many systems that may be integrated into a larger, PLC controlled complex), a microprocessor is often more suitable. In this case, the device is usually treated as a "black box" by the industrial technician. There may be a number of adjustments that the technician can make during installation and often a HMI of some sort for displaying of status and fault messages, but the device not considered "user serviceable" and is either replaced when it fails, or returned to the manufacturer for repair. Industrial items in this catagory include DC drives, AC Variable Speed Drives and other special purpose, stand alone sub-systems.

    As an industrial electrician with an interest in electronics, I personally use microprocessors for applications where failure will not result in lost time costing many thousands of dollars per hour and where I cannot justify the cost of even a small and cheap PLC: Specialised data logging, interactive signs, wireless comunication and similar, small scale projects.

    C, BASIC and other microcontroller programming languages are extremely efficiant and powerful, but no where near as intuitive as Ladder Logic, Sequential Function Charts, Function Block Diagrams and other PLC programming languages.

    So, to my mind:
    Microprocessor = embedded control for devices made and tested exhaustively to suit a specific purpose and installed in thousands of production units which are not generally ment to be substantially modified by the end user.
    PLC's = flexible, widely understood, "user friendly" control system suitable for an industrial environment where modifications to the machine functions may need to be made routinely by skilled trade-level technicians.


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    Re: PLC vs Microcontrollers.

    curious post.

    What do you suppose is inside a PLC?

    jack



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    Re: PLC vs Microcontrollers.

    curious post.

    What do you suppose is inside a PLC?
    Micro-controllers obviously.

    PLCs are robust systems meat for the harsh industrial environments, I suppose you haven't been to some chemical plants, tire factories etc. Any system you design has to take into account the elevated temperatures, high level of electrical noise (generated when motors start and stop) generated in the power lines. So PLCs are preffered over other control systems.

    Micro-controllers used in an industrial system are of Industrial grade, the power systems are robust and can withstand a reasonably high level of voltage fluctuations, the I/O interfaces are optically isolated (wherever possible). Low voltage lines (like 5 VDC) are not longer than a few inches, as low voltage lines can pick up noise from surrounding environment.

    thanks
    a



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    Re: PLC vs Microcontrollers.

    The question of the O.P. could be rephrased like "what is the difference between an Intel Core i5 and a PC"? A PLC is a system that includes the MCU such as the PC is a system that includes a CPU from Intel, AMD ... Why is anybody still buying PCs when the when the CPU is so much cheaper and more flexible in programming

    Couldn't help it, Bob

    Sorry guys, just saw I followed a troll who revived a 3 year old post. Let's give it a rest!
    Last edited by bobsanjose; 3rd October 2010 at 07:06.


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    Re: PLC vs Microcontrollers.

    Perhaps I should have used an exclamation mark instead of a question mark.
    And... oddly enough I have indeed done all the things listed - although not recently.

    jack



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    Re: PLC vs Microcontrollers.

    Quote Originally Posted by bobsanjose View Post
    The question of the O.P. could be rephrased like "what is the difference between an Intel Core i5 and a PC"? A PLC is a system that includes the MCU such as the PC is a system that includes a CPU from Intel, AMD ... Why is anybody still buying PCs when the when the CPU is so much cheaper and more flexible in programming

    Couldn't help it, Bob

    Sorry guys, just saw I followed a troll who revived a 3 year old post. Let's give it a rest!

    hey i was xpecting this kinda answer.. bt is it all correct... ?? i mean is it how is it??

    can plc perform complex operations like equation solving and other looping and if else conditions?? OR is it meant to be jst operated as in ladder forms of programing?? actually i have worked on AVR and jst came to noe about plc.. so wass wondering the differnces and area of uses of both of them??



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