Ladder Logic Controlling Analog

Discussion in 'Embedded Systems and Microcontrollers' started by BR-549, Jan 7, 2016.

  1. BR-549

    Thread Starter Well-Known Member

    Sep 22, 2013
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    I was recently on a post that was over 3 years old and only 8 responses. Yet it had almost 8000 views.

    When I was programming ladder logic, there were only digital outputs and you had to have a DOS operating system. We did have a timer and a counter.

    What are the different ways to control an analog output with ladder logic? Are there any preferred ways or guide lines or standards?

    Or is it PLC dependent? Do they have latter functions for this now? Or just an I/O hardware module extension?

    And is ladder logic still the preferred programming language for PLCs?
     
    Last edited: Jan 7, 2016
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Ladder logic is still the stuff, at least for us. One of our PLC is 22 years old so I am not sure of the modern solution.
     
  3. JohnInTX

    Moderator

    Jun 26, 2012
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    I thought it should be 'ladder' too. Changed name of thread.
     
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  4. BR-549

    Thread Starter Well-Known Member

    Sep 22, 2013
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    Thank you kindly sir.
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    At work I am building a machine that has:
    - 2 three phase motors (each motor is controlled/powered by A-B PowerFlex 525 motor controller)
    - 3 signal lights
    - 1 ultrasound sensor
    - 6 relays
    - a few other things
    and not a single PLC is to be found.
     
  6. BR-549

    Thread Starter Well-Known Member

    Sep 22, 2013
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    I've often wondered about that, and asked, why PLCs are still so popular with industry. I was always told it's because everyone can understand ladder logic.

    I never did buy that. Have you ever seen a true relay controlled process, hardware wise? The first factory I worked in, had a foundry, large machine shop and several large chrome platers, plus large shipping and packaging warehouse. Not a single PLC.

    And the maintenance dept. had no ladder or wiring diagrams. It was educational.

    But of course large industry(auto) had and used such things. But by the nineties, I would have thought industry would go to c. Especially the big boys.

    I always thought networking and central control was very slow in coming with PLCs. But maybe that was just the environment I worked in.

    One would think that after a couple of decades of graduating programmers, ladder would be replaced with something a little more versatile.

    Heck...with these new chips, ladder is probability just an software option. The real meat is probably in c, wouldn't you think.
     
    Last edited: Jan 7, 2016
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    To be fair, the panel next to the one I described above has PLC and that PLC has 5 cards, some input, some output.

    To OP. All the schematics that I use to build the electrical panels for the machines that we build, they are all in ladder logic format.
     
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  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In my opinion the PLC is still one of the most practical and useful ways to replace relay logic systems, and the main reason for ladder display is for the benefit of maintenance electricians etc, when it is displayed in the familiar schematic format to aid in on-floor trouble shooting .
    To program it is often quicker to use mnemonic listing then display the ladder when done, for many decades there has been module add on's for the majority consisting of Servo, Analogue and step dir etc
    The analogue module, whether input or output, is usually quite simple and the analogue value is loaded or read/stored in a binary value register.
    Also the networking feature has also been around for >30yrs.
    Also Modbus link.
    Max.
     
  9. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    The old Dos based software that I used all had analog functionality. TI505, Fanuc, etc. I remember one of my first encounters with relay logic in a German injection moulder with pictorial style prints. Forget that. As mentioned, Electricians can generally decipher ladder. PLCs are evolving, with distributed software networking to smart end points leading on the larger scale. Much of our programming is done in structured text, similar to a c format. Function block is also popular.
     
  10. dannyf

    Well-Known Member

    Sep 13, 2015
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    "why PLCs are still so popular with industryindustry"

    Reliability, simplicity, and a large installed base of relay based controls. The fact that any field tech can program it on the fly is a huge plus.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The added feature of a PLC ladder display over a schematic print is that the on/off/status of all the I/O can be observed on screen, which I credit with being a Huge help when trouble shooting.
    Max.
     
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  12. panic mode

    Senior Member

    Oct 10, 2011
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    plcs are still stuff that breaths life into machines and - that is not about to change any time soon.
    of course it is possible to create logic using alternative means (relays, pneumatic etc) but nothing comes even close to efficiency and ease of use of PLCs, specially when you need to modify or duplicate some system or interface to something else. many PLCs support multiple languages so one can choose whatever is most suitable for the problem at hand, but ladder logic is still the king.

    edit:
    to stay on topic, you can simply write value to an output value using MOV instruction for example. how you get the value is up to you, you can compute it any way you like (scaling, pid, whatever). different plcs have different instruction sets. basic instructions are the same but advanced instructions are often different, though similar.
     
    Last edited: Jan 10, 2016
  13. dannyf

    Well-Known Member

    Sep 13, 2015
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    I assume an outboard comparator would do it: the PLC would output a digtal signal (pwm better but regular digital IO would work, assuming sufficiently fast cycle time), filtered through a low-pass filter and that signal is compared to a target by the outboard comparator. The output from the comparator is then fed back to the PLC to control the digital IO (or pwm). Negative feedback assumed.

    If the target is an analog input, this system can output an analog signal that tracks the analog input.
     
  14. dwayne1776

    New Member

    Apr 3, 2016
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    I work for a huge company and we use plcs/hmi on every job. its a big demand because reliable and fast. NYC can not run without it. Its up to us to keep NY running.
     
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