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Artificial pulse train possible in a PLC?

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  • Artificial pulse train possible in a PLC?

    Has anyone attempted to generate an artificial pulse train in a PLC? What I'm asking is a way to avoid purchasing/using an encoder. I have an application where I'll be controlling motor speed via a GS3 over RS485 from a PLC (a P1K). This so functions would happen based on pulse train and not by timers. I can have a sensor to trigger the beginning of the next cycle, and a prox for this purpose is easily affordable. (OEM problems; can we make this more affordable?) Thanks!
    Why worry? If you've done the best you can, worrying won't make it any better

    - Walt Disney


  • #2
    I did years ago with one of the AB-branded plcs. do not remember which micrologix.

    [edit: It must have been a micrologix 1100 and PTO instruction(s)]
    Last edited by kewakl; 10-07-2019, 10:46 AM.

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    • #3
      The pulse train would be limited by scan time and have variance unless you used special hardware such as a PWM output that can produce the pulse train outside of scan time. I believe BRX and P1K can do this in hardware. If you want to use this artificial count as your position you will also want to feed it into a high speed counter that also runs outside of the ladder scan so you dont miss counts. Even though a BRX or a Productivity 1k can do this, BRX would be the way to go IMO since the PWM module on Productivity costs $72 and then you add the HSI on top for another $65, which is not much cheaper than an encoder anyway, and you get PWM and HSI built in with a BRX. So that would be the cheaper and more integrated option.

      Though if you do not plan on using a high speed counter, your counter will be limited by scan time too so maybe using a "software PWM" would be fine since a standard counter would run at the same speed and in sync with each other.

      Another possibility is to not use the PLC for PWM output to create a virtual encoder pulse train, but instead use the GS3 and have it generate your PWM for the virtual encoder pulses (GS3 "FO" output is a PWM frequency output based on speed reference). Write the frequency to the GS3 to make it run the speed you want, have the GS3 outputting PWM speed signal, and feed the PWM back to a high speed counter. This will save you an output on the BRX, or save $72 on a PWM module for a P1k. It also sort of gives you confirmation the GS3 is running how you want since the returned signal will be directly from the drive unit.





      Your idea is pretty interesting. Let us know how it turns out.
      Last edited by MikeN; 10-07-2019, 10:47 AM.

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      • #4
        Originally posted by MikeN View Post
        Another possibility is to not use the PLC for PWM output to create a virtual encoder pulse train, but instead use the GS3 and have it generate your PWM for the virtual encoder pulses (GS3 "FO" output is a PWM frequency output based on speed reference). Write the frequency to the GS3 to make it run the speed you want, have the GS3 outputting PWM speed signal, and feed the PWM back to a high speed counter. This will save you an output on the BRX, or save $72 on a PWM module for a P1k. It also sort of gives you confirmation the GS3 is running how you want since the returned signal will be directly from the drive unit.





        Your idea is pretty interesting. Let us know how it turns out.
        I like it! I'll investigate further and let you know.
        Why worry? If you've done the best you can, worrying won't make it any better

        - Walt Disney

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        • #5
          Several ways to do this with BRX. PWM, AXIS using velocity, etc, etc.
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          • #6
            Originally posted by Do-more PE View Post
            Several ways to do this with BRX. PWM, AXIS using velocity, etc, etc.
            I'll check this out, too.
            Why worry? If you've done the best you can, worrying won't make it any better

            - Walt Disney

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            • #7
              Originally posted by Todd Dice View Post
              Has anyone attempted to generate an artificial pulse train in a PLC? What I'm asking is a way to avoid purchasing/using an encoder. I have an application where I'll be controlling motor speed via a GS3 over RS485 from a PLC (a P1K). This so functions would happen based on pulse train and not by timers. I can have a sensor to trigger the beginning of the next cycle, and a prox for this purpose is easily affordable. (OEM problems; can we make this more affordable?) Thanks!
              Would this help? I wrote this to create simulated pulses from an encoder. By changing N7:1 it changes the speed of the pulse train. Since this was for simulation, rung 6 was ok to do, but for real hardware it would not be allowed. I used B3:1/0 to turn the pulse train on or off.

              Mike
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              • #8
                Originally posted by mikeexplorer View Post

                Would this help? I wrote this to create simulated pulses from an encoder. By changing N7:1 it changes the speed of the pulse train. Since this was for simulation, rung 6 was ok to do, but for real hardware it would not be allowed. I used B3:1/0 to turn the pulse train on or off.

                Mike
                My thinking is to avoid time base so that actions based on a pulse would react instantly to motor speed changes. Since I will be controlling motor speed via a PLC to the GS3, and assuming I had an encoder that generated 1024 pulses per revolution, I would think I could create a artificial pulse train residing in the PLC absent a physical encoder.
                Why worry? If you've done the best you can, worrying won't make it any better

                - Walt Disney

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                • #9
                  With out an encoder, the count/position error from the AC drive/motor slip and add the error generating the pulse train; would this error give you the resolution you need?

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