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Accelerometer output confusion

  1. Feb 28, 2010 #1
    I've been playing around with an accelerometer I've got hold of, yet I'm having a little difficulty understanding it's output. Electronics isn't my strong point.

    I understand the output is a voltage, but the I'm finding the values hard to interpret.

    When connected it outputs data in the format [z x y]. I clamped the device in a clamp and recorded the data at each rotation so I knew what axis each direction was (i didnt have access to the data sheet at the time)

    the values I got from this for 1 direction were when averaged [4556 4017 4021]. This represented [1 0 0] so Zout was @ 1g.

    I guess what I'm asking is, how does the [4556 4017 4021] values relate to the output voltage? Ive tried reading through the data sheet but it really doesnt make that much sense to me. Can anyone point me in the right direction?

    EDIT: http://www.freescale.com/files/sensors/doc/data_sheet/MMA7260QT.pdf?fsrch=1 is a link to the data sheet.

  2. jcsd
  3. Mar 1, 2010 #2
    Are you sure that the accelerometer is properly calibrated? (Look at pg. 6 for expected values), Are you sure that you averaged the readings properly? Have you filtered properly? (Do the setup described on page 3 and test) The numbers are within the acceptable g range (+-5000), but are on the high side. This makes me think you're getting some bad g values. Have you tested your setup with a multimeter to check that the circuits alright?

    The data sheet is actually really useful, you've just gotta follow the steps listed in it.
    Last edited: Mar 1, 2010
  4. Mar 1, 2010 #3
    Ahh looking back to pg.6 I just realised that for the side view where [x y z] = [ 0 0 1] that is the same as [1.6V 1.6V 2.45V]

    So for my values I would simply divide [4017 4021 4556] by the respective voltages to get
    [2.5e3 2.4e3 1.8e3] which I'm assuming as you suggested is the output voltage (just V in this case)
    and this could then be applied to the other results I have to work out the rest. Obviously there is some error in the system where x and y should equal the same values but this is expected.

    If I do this for all sides I can then take an average of the 0 and 1 values for each axis so that when it comes to looking at my experimental data I then just divide the recorded value buy the averaged 0/1 value to get how many g's are acting in that direction?

    As for calibration of the device , I believe as long as the correct voltage is passed to it, it should be fully calibrated.
  5. Mar 1, 2010 #4
    No idea, but you have to pass the output through a filter (pg. 3) before you do anything with it.
  6. Jan 8, 2011 #5
    I have been given this question as a part of my assignement but I am not at all able to solve it as I am a student of mechanical engineering..
    can any1 please help , I have to submit it by wednesday 12 / 01 / 2011

    the question is

    A beam is vibrating sinusoidal at 40 Hz. The displacement at the centre of the beam is set to 0.60 mm peak-to-peak. A piezoelectric accelerometer attached to the centre of the beam, has a sensitivity of 2.5 V/g and an output impedance of 200 . However, there is a requirement to measure the velocity of the motion at the centre of the beam.

    (a) Calculate the output of the accelerometer in Volts rms.

    (b) Draw a circuit incorporating an operational amplifier, which will allow a velocity signal to be obtained from the accelerometer signal.

    (c) Identify appropriate component values for the velocity measurement system to give a sensitivity of 2 Vs/m. Clearly identify how you arrive at your chosen values.
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