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Thermocouple Module IO Error - PID Loop

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  • Thermocouple Module IO Error - PID Loop

    Hello,

    I am trying to implement a simple heating PID loop using do-more BRX BX-DM1E-18ED13 PLC and BX-08THM 8 - channel thermocouple module.

    Sometimes I get the following warning: Master 0: BRX Local I/O Master (1 base has a warning)
    These bases have warnings:
    • Base 0 (BRX Local I/O)

    "One or more IO masters are indicating problems with the module - $IOError (ST152)"

    Accordingly, the temperature readings stored in WX1 from the thermocouple module goes haywire (Eg: if room temp is around 22 degrees C, the thermocouple reading shows 45 degrees C). This seems to happen at random and somehow gets corrected when PLC is restarted the next day.

    Entering "ST152" in the data view shows "ON"

    I can't seem to find the cause for this since the documentation on this module is limited.

    Can anybody help me figure out the problem? I have also attached my ladder code (still in development).
    Attached Files


  • #2
    Further analysis showed that flags X32 and X33 are HIGH. These flags correspond to Burn Out for Channel 1 and Channel 2 respectively. The burn Out detection was configured to go HIGH. What does a Burn Out mean and how can I reset the warning?

    Comment



    • #3
      Not a thermocouple module expert, but burn out generally means the sensor is bad, or at least the reading from the sensor is out of range. Might also just be bad wiring.

      The PLC is simply passing on the errors reported by the module. Pretty sure the module clears the errors automatically when it gets good readings.

      Comment



      • #4
        Burnout means that somewhere between the module and the thermocouple there is an open condition. As BobO said, the thermocouple could be opening up at temperature, there could be a loose wire, etc.
        If you have an urgent issue, please contact AutomationDirect's Technical Support team.

        AutomationDirect.com Technical Support: 1(800) 633-0405 or (770) 844-4200 Email Tech Support

        Comment



        • #5
          BobO Do-more PE Thank you for your advice. I figured out the root cause. It looks like I had enabled 2 out of 8 channels in my thermocouple module, but was using only one channel (although I plan to use two thermocouples later). The second channel was open causing this warning.

          However, the temperature still seems to very off. I am using a J type thermocouple (red and white wires; red: - white: +) The current room temperature is about 20-degree Celsius. However, the thermocouples read 33-degree Celsius. I have attached a snapshot of my thermocouple module configuration. The thermocouple module stores temperatures values in the WX1 register. Since these temperatures have implied decimal point, I scale the value and store actual temperature values in RX1. I have also attached data view for these registers.

          Any suggestions on what else to investigate?

          (PS: I am using J-type Bolt on thermocouples from automation direct PART NO: THMJ-B01L06-01)
          Attached Files

          Comment



          • #6
            Could be any number of reasons for being slightly off.

            Might want to start off with page 8-15 of the user manual: https://cdn.automationdirect.com/sta...xuserm/ch8.pdf
            If you have an urgent issue, please contact AutomationDirect's Technical Support team.

            AutomationDirect.com Technical Support: 1(800) 633-0405 or (770) 844-4200 Email Tech Support

            Comment



            • #7
              How is RX1 being scaled?

              The setup screen shows that WX1 is 1:1 to RX1 and should have the same value, but in the data view RX1 has been scaled to a decimal place required.

              Comment



              • #8
                RogerR There is an implicit scaling during setup. The 16-bit resolution of the thermocouple module outputs -32768 to 32767. This value is the temperature reading with implied decimal. I simply set my RX1 to be in the range -3276.8 to 3276.7. This gives me actual temperature value.

                Comment



                • #9
                  The change from -32768 to 32767 to the range -3276.8 to 3276.7 in WX1 threw me off a bit.
                  I have always had the full range -32768 to 32767 in WX1.
                  Looking closer, it is because the measurement is in Celsius instead of Fahrenheit. When the switch is made, a decimal is taken off.

                  Comment



                  • #10
                    Do-more PE BobO RogerR After going through the user manual and experimenting a bit, I observe following things:

                    1. I no longer observe any "Burn-out" errors in the thermocouple module.

                    2. The "IO Error" warning of the thermocouple module is still ON. The documentation doesn't specify what this error exactly corresponds to. I have enabled only two channels out of eight in my thermocouple module. Both the channels are connected to two different "J" Type thermocouples. Should rest of the channels (channels 3 to 8) be shorted together and connected to ground, even though only two channels are enabled in software? Can somebody help me figure out what causes this IO Error?

                    3. The type-J thermocouple in channel 1 is ADC product (THMJ-B01L06-01). It is of grounded type and has a stainless steel sheath. Since it is already grounded, There is no need to ground it on the thermocouple input module side. However, I observe that when the stainless steel sheath touches a metal, the temperature readings seem to be affected.

                    4. The type-J thermocouple in channel 2 is not an Automation Direct product and has a smaller wire thickness (higher AWG). Does wire gage affect temperature readings? For this thermocouple, I have noticed that when the sheath touches anything metallic (Eg. metallic sheath of the thermocouple in channel 1 or a piece of iron) the reading goes to -1960.

                    5. A note in the user manual says: "NOTE: With grounded thermocouples, take precautions to prevent having a voltage potential between thermocouple tips. A voltage of 1.25 V or greater between tips will skew measurements". Does this refer to the voltage between "+" channel and the "-" channel at the thermocouple input module?

                    Any further pointers on troubleshooting this issue will be helpful!

                    Thanks
                    Rock

                    Comment



                    • #11
                      The unused inputs should be connected together, either shorted directly or a piece of thermocouple wire 1" long twisted together.
                      The advantage of putting at least one piece of thermocouple wire twisted is that the input serves as a cabinet temperature indicator, at least until you need the channel for something else.

                      I do not hook the unused input terminals to ground and do not use grounded thermocouples, so cant really comment on that.

                      Comment



                      • #12
                        1. Good

                        2. What are the "X" bit associated with the module doing? Are any of them on? See page 8-31 of the user manual: https://cdn.automationdirect.com/sta...xuserm/ch8.pdf

                        3. Right. You are creating a ground loop. Everything should be bonded to the same potential. Ground loops are bad. Excess current needs a good path to be removed from the system to a common ground point.

                        4. Doesn't matter unless you are running long distances.

                        5. This means that the difference between any channel needs to be less than 1.25V. So if you measure CH1+ to CH2+, or CH1+ to CH8+ they should read less than 1.25V.
                        If you have an urgent issue, please contact AutomationDirect's Technical Support team.

                        AutomationDirect.com Technical Support: 1(800) 633-0405 or (770) 844-4200 Email Tech Support

                        Comment



                        • #13
                          Originally posted by RogerR View Post
                          The unused inputs should be connected together, either shorted directly or a piece of thermocouple wire 1" long twisted together.
                          The advantage of putting at least one piece of thermocouple wire twisted is that the input serves as a cabinet temperature indicator, at least until you need the channel for something else.

                          I do not hook the unused input terminals to ground and do not use grounded thermocouples, so cant really comment on that.

                          I have shorted the + and - of unused channels using a copper wire. This doesn't seem to solve the problem. I am suspecting the second thermocouple may be of the ungrounded type and is messing up the readings. For ungrounded type thermocouples, should the "-" wire lead should also be connected to 0 V in the thermocouple module? (See the attached picture)
                          Attached Files

                          Comment



                          • #14
                            Originally posted by Do-more PE View Post

                            2. What are the "X" bit associated with the module doing? Are any of them on? See page 8-31 of the user manual: https://cdn.automationdirect.com/sta...xuserm/ch8.pdf
                            Here is the status of the "X" bits (see pictures attached)

                            X16 (Self test failed): OFF
                            X17 (Missing 24 V): OFF
                            X18 (Data not valid): OFF

                            Channel 1 "X" bits:
                            X24 (Out of range): OFF
                            X32 (Burn out): OFF

                            Channel 2 "X" Bits:
                            X25 (out of range): ON
                            X26 (burn out): OFF

                            The out of range for CH2 is ON since the reading is -1950. This happens when the sheath of the TC gets in contact with anything metallic. When I make sure sheath is free from metallic contact, the temperature readings are positive but still off from actual temperature.

                            Originally posted by Do-more PE View Post

                            3. Right. You are creating a ground loop. Everything should be bonded to the same potential. Ground loops are bad. Excess current needs a good path to be removed from the system to a common ground point.
                            I am suspecting the thermocouple in CH2 might be of un-grounded type. For an un-grounded type thermocouple, should the CH2- be connected to 0 V in the thermocouple module?


                            Originally posted by Do-more PE View Post

                            5. This means that the difference between any channel needs to be less than 1.25V. So if you measure CH1+ to CH2+, or CH1+ to CH8+ they should read less than 1.25V.
                            I measured these voltages. None of them are so high. in-fact most of them are close to 0V (0.002 to 0.004 V DC)

                            Still having trouble understanding what is causing the module "IO Error".

                            Attached Files

                            Comment



                            • #15
                              Originally posted by RockB View Post

                              Still having trouble understanding what is causing the module "IO Error".
                              The module IO Error is on because X25 is on. It is simply the ORed result of slot errors.

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