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Minature accelerometer design

  1. Jul 24, 2015 #1
    Accelerometer Design: You have been asked to help design an accelerometer with a measurement bandwidth of 1000 Hz using a cantilevered design (see figure). The cantilever (c) has a rectangular cross-section and the proof-mass (m) is 1 gram. The cantilever can be made from single-crystal Silicon [E=190 GPa, ρ =2.3 g/cm3] or Aluminum [E = 75 GPa, ρ =2.7 g/cm3], your choice.
    upload_2015-7-25_8-1-56.png

    Assuming this accelerometer design can be properly modeled and analyzed as a second order system (i.e., a lumped mass-spring-damper, ignoring the mass of the cantilever), please answer the following:

    a) What is the amount of stiffness in the spring element such that the natural frequency of the device is 2 kHz. Is this a good natural frequency for an accelerometer that may be used for accelerations of frequencies up to 1 kHz?

    b) Please design the dimensions of the accelerometer cantilever given the stiffness calculated in part (a) such that the mass of the beam is less than 2% of the proof-mass. Also comment on which material (Silicon or Aluminum) you selected and explain why you selected this material. [Hints: This is a design problem so there are multiple solutions. If you want a more specific (constrained) case, consider a square cantilever cross-section. Recall that the deflection of a cantilever can be modeled as a linear spring and watch your units].

    c) How much deflection will your designed accelerometer cantilever see subjected to a 10g acceleration (g = 9.8 m/s2) resulting from an external force (F) (assume that the cantilever is massless)?

    d) Assume the requirements are that the settling time of accelerometer must be less than 20 milliseconds, what is the amount of viscous damping (or damping ratio) needed? [Hint: Assume the accelerometer is underdamped]
    solve:
    a.

    For a cantilevered beam, the 1st natural Frep is
    where B=pi/2, L=length of cantlevered, E young's moduler, I=b.h^3/12,
    b. i dont know
    c. deflection at beam delta=M.L/2EF
    d. i dont know
    can you help me to solve and edit my answer?
     

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    Last edited: Jul 24, 2015
  2. jcsd
  3. Jul 24, 2015 #2
    a. You might want to include the missing formula in your post?
    b. Can you attempt something here? It seems like it would be easy.
    c. I don't think your formula is right. You may also need to assume a time variable.
    d. Do you know a formula with damping you could use?
     
  4. Jul 25, 2015 #3
    can you show your answer more detail? please ., thanks
     
  5. Jul 25, 2015 #4

    billy_joule

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    Science Advisor

    If you want others to put in effort to help you, you should make an effort too.
    Can you answer b & d?
     
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