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PID Time Proportionaing Control Analogue Output to SSR

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  • PID Time Proportionaing Control Analogue Output to SSR

    I need help getting a working PID loop to control a simple resistive heating element using PWM (Time Proportioning Control). Here are some of the details on my setup and testing apparatus.

    PLC: DL06 (D0-06DR-D)
    Analogue Output Module: F0-04DAH-2
    Temperature Input Module: F0-04THM
    SSR: 3-32VDC Input with 3-200 VDC Load

    What I have is the DL06 with an analogue output module connected to send a voltage signal to a SSR to the relay and turn it on/off. On the load side of the relay is a 12 VDC power supply providing power to a simple resister. A k-type thermocouple is being used to read the temperature of the resister to measure the process variable (temperature).

    Here is where I am stuck, I have tried the example program from the DL06 user manual in chapter 8 page 69 for the time proportioning control. The issue I believe is that I need to control my set point through the analogue module to turn the SSR on/off to effect the temperature of the process, but the example is showing a Y0 out. I do not want to wear out my output relays on my PLC. I can not figure out the ladder logic to use for Time Proportioning Control with an analogue output module sending a voltage signal to the SSR to effect the process variable.

    I also need pointed towards the correct rungs to include for the initial memory configuration.

    I did however create a successful program turning the SSR on/off with the analogue output with basic TMR controls, but I cant get the Time Proportioning Control to work.

    Any help is greatly appreciated as this has me stumped but I hope it is a simple solution.

  • #2
    Here is a setup I used in a Subroutine though it doesn't have to be.

    As for the analog out, if you use a C0 instead of Y0, then if C0 is on, move a value for max voltage out to the analog output. When C0 is not on, move a zero to the analog output.

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    • #3
      Time proportioning doesn't apply with an analog output (and standard SSRs don't take an analog control voltage, they're on /off). Time proportioning allows a DO to act as an analog over its time base (could be something like one to 10 seconds). And yes, if you use relay outputs, they'll wear out doing time proportioning at a rate appropriate for SSRs, so get a solid state (transistor) DC DO module.


      • #4
        Hi...i am a new user here. I will suggest if you use a C0 instead of Y0, then if C0 is on, move a value for max voltage out to the analog output. When C0 is not on, move a zero to the analog output. I faced the same issue and it worked for me.


        • #5
          But the entire point of time proportioning is to have a functionally analog control signal from a discrete physical output. Doing both at the same time (time proportioning AND a physical analog output) is redundant. It's like having stop signs in a traffic circle (and yes, I realize that some places actually do that).

          If you think about it, even a discrete output programmed for on-off control is time proportioned in practice. For example, if the load on a heating system is 43% of available control effort, then if the temperature averages at the setpoint over some period of time like an hour, then you'll find that the output was on 43% of that hour (or else by definition the long-term mean temperature would have changed). The control system may not be actively aware that it's hitting a certain time proportion output, but it will just do so automagically.


          • #6
            In regards to the redundancy of using a continuous analogue output to switch on/off a SSR over, time I absolutely agree with what your saying ControlsGuy. The main issue is in regards to cost of a power supply that will control continuous power through an analogue input signal. from what I understand, I could use a programmable power supply that applies a constant voltage to the load source and voltage can be controlled continuously through an analogue input signal. To the best of my knowledge these types of power supplies are relatively not cheap compared to a SSR, which is much more economical solution.

            I am open to any ideas or suggestion for economical solutions on how to continuously vary temperature of a resistive heater using PID control without applying on/off power.


            • #7
              I think more specifically the area that I could use help in is how to build a electrical circuit using a DC DO transistor controlled by an analogue voltage input and powering a 500W heating element. Ultimatley the goal is to use ramp soak PID control to control a heating cure cycle for composite oven.


              • #8
                Your heating element isn't an IR lamp, is it? With a Calrod, ceramic radiant heater, nichrome wire, etc, you can achieve PID with a DC discrete output and an SSR that does "on-off" control, but the on-off is several times per minute. 10 seconds is a popular time base. Then do a PID loop as usual, but there is one extra step. Rather than take the output of the PID and send it directly to an analog output, you use it as a timer setting for the DC DO. For example, if the PID output is 60%, and you've chosen 10 seconds as your time base, then turn the DC DO on for six seconds and off for four, on for six and off for four, and so on. The amount the temperature will change during the on and off periods is so insignificant this will work fine.

                The issue with IR lamps and that style control is mechanical fatigue. They expand and contract enough in that kind of service they will fail prematurely. What you have to do with those is to use an analog signal and for 500w you can buy an analog SSR that will take an analog input and fire by phase angle. Large systems use real SCRs and a control card and also accept an analog signal from you that determines the phase angle.

                Why are you thinking of using a programmable DC supply? Do your heaters for some reason want DC current? That would be unusual.


                • #9
                  The heating element that I am using is a 3d print bed heating element, basically a PCB board with thin copper foil etched away to make a long wire pattern, a simple resistive heater. The DC current was chosen arbitrarily, a simple 12V/10Amp xbox power supply was cheap and easy to modify to provide power to the heat bed.

                  The reason I mentioned a programmable power supplies is only due to my own ignorance. My knowledge of electrical engineering and electrical hardware is limited, I did not know an analogue input SSR existed.

                  ControlsGuy your input has been immensely helpful! I really appreciate it, would you by chance have pictures of an example program demonstrating the timer setting for a discrete output PID program?

                  Any help is appreciated.


                  • #10
                    Can you provide the complete part number of your SSR?

                    If it is an analog type then you can totally ignore the 'Time Proportioning' application. Just apply the numerically output of your PID to the analog output channel. Wire the analog output module to the input of the analog SSR.



                    • #11


                      • #12
                        OK, now I understand. It is an ordinary on/off SSR. It would be best to get a DC (not relay) output card for the signal to this SSR.