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Troubleshooting displacement measurements on an Instron 8874 testing system.

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  • Troubleshooting displacement measurements on an Instron 8874 testing system.

    During zero-compression fatigue testing of equine bone samples I encountered some troubles with the displacement recordings in recent samples. There is a large noise component in the displacement reading of recent tests which was not present in previous tests which were run under the same conditions. While in some samples the noise is larger during the second half of the test, there is also one sample in which the noise is largest in the middle third of the test. Actuator displacement is recorded through the inbuilt Dynacell loadcell.

    I am trying to find the source of the large variations in displacement readings (still talking about displacements of less than 0.2 mm).
    So far I considered the following:
    a) True changes in length of the samples: unlikely. Why would a bone lengthen when it is under a compressive load?
    b) External vibrations: unlikely. This would require an almost constant source of vibration from 10 pm to 8 am to explain the noise in one test and shorter periods during day time for other tests.
    c) Machine related ‘miss-registration’: 1) loosening of the crosshead resulting in change of frame compliance, but there are cycles with little noise after periods of much noise. 2) Poor loadcell performance – is this the most likely option?
    d) Other?
    How can I differentiate one from the other? Are there some points/issues which should be considered while trying to find the source of this problem? Is the use of an external extensometer the only possibility to monitor performance of the actuator position recordings of the inbuilt loadcell? Due to the small sample size it is difficult to attach an extensometer to the sample. Would it be valid using an external extensometer between the upper and lower platen?

    Thanks for any ideas!

  • #2
    Re: Troubleshooting displacement measurements on an Instron 8874 testing system.

    Hi Sandra,

    First of all: are you measuring the same range of displacement? I mean: is the noise larger than usual in absolute terms (something is going wrong) or in % (maybe you are measuring something hard to measure).

    From your description, it seems to be electrical noise. Did you institution install some new electric appliance that might cause electromagnetic interference? O that might induce noise in the AC power lines?
    You can:
    - check the shielding of your testing machine and the ground connections
    - ensure that nobody has a mobile on in the testing space
    - check the frequency of the noise:
    -50 Hz in Europe or 60 Hz in the US means that the AC noise gets through your signal conditioning and acquisition
    - a different frequency might be related to the mechanics of the testing machine (e.g. the dither of the servovalve)... but it would be odd to see it only some time of the day
    - high frequency could be mobile phones or other crap


    Hope this helps,

    luca

    Comment


    • #3
      Re: Troubleshooting displacement measurements on an Instron 8874 testing system.

      Hi Luca

      Thanks for these ideas! I will follow them up and keep you updated on the findings.

      Sandra

      Comment


      • #4
        Re: Troubleshooting displacement measurements on an Instron 8874 testing system.

        Originally posted by Sandra View Post
        Hi Luca

        Thanks for these ideas! I will follow them up and keep you updated on the findings.

        Sandra
        It is odd that this happens at night. Check if there is construction/renovation work going nearby. A lot of this stuff is done at night. (Along the lines of what Luca said.)

        You say something about specimen elongating during compression. This sounds different from the type of noise you are describing in other parts of your message. If there is a slippage issue, this could happen; the fact that it happens during a portion of the experiment doesn't rule it out.

        Check your cables too. If you have a damaged cable, depending on how it is positioned it may create noise. I would test the channels and the cables without loading the sample as well. Keep it on overnight without running the experiment and see what noise it records.

        Comment


        • #5
          Re: Troubleshooting displacement measurements on an Instron 8874 testing system.

          Originally posted by Sandra View Post
          During zero-compression fatigue testing of equine bone samples I encountered some troubles with the displacement recordings in recent samples. There is a large noise component in the displacement reading of recent tests which was not present in previous tests which were run under the same conditions. While in some samples the noise is larger during the second half of the test, there is also one sample in which the noise is largest in the middle third of the test. Actuator displacement is recorded through the inbuilt Dynacell loadcell.

          I am trying to find the source of the large variations in displacement readings (still talking about displacements of less than 0.2 mm).
          So far I considered the following:
          a) True changes in length of the samples: unlikely. Why would a bone lengthen when it is under a compressive load?
          b) External vibrations: unlikely. This would require an almost constant source of vibration from 10 pm to 8 am to explain the noise in one test and shorter periods during day time for other tests.
          c) Machine related ‘miss-registration’: 1) loosening of the crosshead resulting in change of frame compliance, but there are cycles with little noise after periods of much noise. 2) Poor loadcell performance – is this the most likely option?
          d) Other?
          How can I differentiate one from the other? Are there some points/issues which should be considered while trying to find the source of this problem? Is the use of an external extensometer the only possibility to monitor performance of the actuator position recordings of the inbuilt loadcell? Due to the small sample size it is difficult to attach an extensometer to the sample. Would it be valid using an external extensometer between the upper and lower platen?

          Thanks for any ideas!
          Hi Sandra,
          There could be many reasons for why the displacement signal is noisy in comparison to other samples and we know how frustrating this can be when a machine is not producing the intended results! The apparent noise, or error, may be a function of the machine, the set-up, the test demands, environmental effects (temperature, noise, air currents, etc.), surrounding equipment or even the samples themselves.

          If you would like to email me directly with some photos of the set-up and the machine in the lab, the machine details (with the machine model/serial number), the test requirements/test method files and some data files, I can take a quick look to see if there is anything obvious for you, or at least point you in the right direction!

          Kind regards,
          Toby

          Comment

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