SOLVED PT100 Temp fluctuation

Discussion in 'Calibration, Help, and Troubleshooting' started by Nicu Damse, Apr 29, 2016.

  1. Old_Tafr

    Old_Tafr Well-Known Member

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    The actual signal (resistance value voltage whatever... ) from the PT100 thermocouple is processed by the Rumba board, I think the PT100 board translates the reading to one that the Rumba will understand as it is designed for a thermistor. What is displayed on screen and in Octoprint is produced by the processor chip using settings in the firmware on the Rumba, if part of the process of calculating the actual temperature to be displayed is to average/smooth the displayed value then it is going to remove spurious values caused by interference. Someone with knowledge of the PID processing would be able to say if there was any smoothing/averaging taking place and how the PID tuning parameters affected the output value.

    Dr Jeep is I think going to look at the actual voltage from the thermocouple ("I will graph the voltages") to see if there is interference on the actual signal on the wires between the sensor and the PT100 board. The makers of the PT100 would not have bothered with a screen if it wasn't necessary as it would be cheaper and simpler to make with just two wires and no screen. A without connectors and with connectors and no continuous screen measurements would confirm this. I have not put in connectors yet, have not done PID tuning either but I will have a look with a scope to see what the signal looks like. As is in Octoprint I get a smooth curve varying like a sine wave like the red curve in post number 6 above and without the noisy sections.
     
  2. Stefan

    Stefan Well-Known Member

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    I think the screened cable is for temperature protection of the wires... PT100 goes like 1000°c or something and these screened silverish versions are only included in the early bird BigBoxes as far as I know....
    E3D shown in the new way video that they have now shorter unshielded plug able cables.

    @Nicu Damse ... what was the problem? Would be interesting to know.
     
  3. Old_Tafr

    Old_Tafr Well-Known Member

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    I would agree but we also have to take into account that the screen is connected to the circuit (good practice being to only ground it at one end) at the PT100 board end. I'm not sure that the new leads don't have individual screening (again maybe only for heat resistant purposes) too.
     
  4. Stefan

    Stefan Well-Known Member

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    Really? I have to check mine, when I cut the wire to add plugs I think I checked and I only have a 2 pin connector on the amplifier board and the cable had only 2 thin red wires go into the amp plug.


    I still want to know what solved the problem :)
     
  5. Old_Tafr

    Old_Tafr Well-Known Member

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    On the connectors on my PT100 cables there is a very thin silver coloured wire going from the blue heat shrink to the white connector which attaches to the PT100 board.

    I can post a photo of it in-situ or remove it and photograph it, but the connectors a real pain to remove and re-assemble through the hole in the printer floor. I also don't think that the silver wire would show very well on a photograph.

    "I still want to know what solved the problem :)"

    Same.. after my first PID Autotune (failed first time as the Brexit wind blew strongly through the open window) I could see very faint noise on the decreasing temperature curve....... so not likely to be electrical noise from the (switched off) heater
     
  6. Stefan

    Stefan Well-Known Member

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    OK I checked.... and you are right.... but...

    IMG_20160627_173026.jpg IMG_20160627_173031.jpg

    I installed a plug and I have not connected the silver wire only the 2 red on separate pins.... and the sensor works. very strange.
     
  7. Old_Tafr

    Old_Tafr Well-Known Member

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    If the metal screen round the two red wires is designed to prevent electrical interference affecting the temperature reading then it should (probably) not be connected to either of the red wires but to either ground or signal ground at the PT100 board. There would be no point to connect it to ground at the heater block as "ground" at that point does not exist.

    If the screen round the two wires is intended as a heat shield then if it is metal then it still needs to be grounded.

    There is a possibility that the silver wire connected to the shield may be connected to ground at the PT100 board, I have not seen the circuit to check.

    However in either of the three cases above removing the connection between the silver wire and the PT100 board will not stop the temperature signal reaching the Rumba. Removing the silver wire may give rise to electrical interference affecting the temperature reading, and in a totally unpredictable way. It would be best to reconnect it. :D
     
  8. Stefan

    Stefan Well-Known Member

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    Yes I will do that. What I did not mention yet is that the silver wire must go al the way up into the sensor. I cut it about 10cm from the sensor ti install the plug and I had the 2 red and silver wires. I think I measured the Resistence and decided the value between the two red cabels looked nice for about 20 degrees.

    I think it is Kind of a 3 wire Setup like here
    But the red cabels must be at opposite sides Not the same. Or I would not get a Temp reading.... is the sensor Amp thing open source? I will write to E3D Support.
     
  9. Stefan

    Stefan Well-Known Member

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    Ok the e3d page from the Video Link above has a link to e3d wiki about the pt100 and the Amp Board... it has an ina826 Chip datasheet which is Kind of a operation amliphire I think but this goes way beyond me electronics skills.
     
  10. Old_Tafr

    Old_Tafr Well-Known Member

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    Best to get the data sheet on the PT100 sensor first. I suspect that the PT100 board is just changing the signal level to what the Rumba (built for a thermistor not a thermocouple) understands, I think e3d say that somewhere.

    If it works ok on the two red wires then as before the silver wire just connects to the shield which is almost certainly connected at just one end, the datasheet will tell us that. The datasheet just shows an amplifier which would be used to change the signal level.

    PS My BB is a Kickstarter one too (see post below on screen PT100 sensor cable)
     
    #30 Old_Tafr, Jun 28, 2016
    Last edited: Jul 2, 2016
  11. Dr Jeep

    Dr Jeep Well-Known Member

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    Hmm so this is interesting, you guys have screened cables for the PT100 sensors....my kickstarter edition BB does not (just red/black 2 core cable) and I think I have just successfully determined that the noise on my temp sensor varies depending on whether the steppers are energised or not (which does after all make sense given the low voltage of PT100 and the switched current pulses on the stepper cables0
     
    Old_Tafr likes this.
  12. Old_Tafr

    Old_Tafr Well-Known Member

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    The screen is not immediately obvious, other than the outside of the cable is clear plastic and you can see the braid through it, plus the connection to the PT100 board where the silver coloured wire is very hard to see once installed in the base.

    Did you do any specific tests with the stepper motors enable/disabled ? Like the heating, if the stepper motor signal is a square wave then high frequency noise is a given.
     

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