02 Eta (η)
  • Date of Fabrication
    May 2002

    • Fabrication Parameters     Performance    Fabrication Notes

    Description

    02-eta was the 3rd attempt at making a 1 DOF fourbar satisfying the following criteria:
    1. resonant frequency ≥ 150 Hz
    2. Amplitude at resonance ≥ 120 degrees
    3. Q ≤ 3
    There were 2 previous attempts:
    1. 02delta 02-delta used a very wimpy actuator. blocking force of 44 mN and DC motion of 256 microns. This gives a force*displacement product of only about 11 micro-joules, compared to 19 micro-joules for this structure and 25 for 02-zeta.
    2. 02zeta 02-zeta used a slightly more powerful actuator. It had a force*displacement product of about 25 micro-joules. This structure used a 19 micro-joule actuator.
    The salient features in the construction of 02-eta were
    1. The flexures for the top link were made from 3.6 micron polyester film instead of 6.25 microns as has been the case so far. This is the first time thinner flexures were used.
    2. The fourbar was made from about 5 micron steel shim. This thickness was acheived by dunking the laser cut part in FeCl3 for about 2:20 minutes.

    Fabrication Parameters

    Fourbar Dimensions
    See solidworks drawings for complete dimensions.     		Link lengths (mm) 6, 6, 6, 1
    Flexure lengths (μm) 50, 50, 50, 50
    Flexure widths (mm) 3,1.5,1.5,3
    Flexure thicknesses (μm) 6.25, 6.25, 3.6, 3.6
    Slider crank dimensions Attachment point (mm) 2
    Actuator parameters Dimensions (mm) (10 + 6) x 3
    Serial Number 05-08-02-1M
    DC displacements (μm @ 150V) 410
    Blocking force (mN @ 150V) 46
    Stiffneses (N/m) 126
    Resonant Frequency (Hz) ???
    Q (= Xac/Xdc*150/Vac) ???

    Performance Parameters

    DC motion of actuator w/o fourbar (@ 150V)
    (NOTE: this is slightly different from Mimmo's measurement of the same)    		 390 μm
    DC motion of actuator w/ fourbar (@ 150V) 351 μm
    DC motion of fourbar output link (@ 150V) 52 degrees
    Resonant frequency of actuator + fourbar (w/o wing)
    (NOTE: this might be inaccurate since the measurement was done before a fix)    		 270 Hz
    Resonant frequency of actuator + fourbar + wing 170 Hz
    Q (calculated as θ(AC)/θ(DC)
    NOTE: The measurement was made by finding the ratio of AC motion at 30V (measured as 60 degrees) and DC motion at 45V (measured as 30 degrees)    		 3.0

    NOTE

    1. The DC output motion / input voltage is highly non-linear. Motion at 45 V = 30 degrees and motion at 150V = only 52 degrees. a change from about 2/3 degrees/V to about 1/3 degrees/V. The fourbar non-linearity will definitely cause some effect like this. However, it needs to be checked whether it causes all this much of an effect.
    2. The transmission ratio corresponding to a slider crank attachment of 2 mm is 3000 rads/m (approximately). However, the observed transmission ratio at DC is about 2500 rads/m. This is a consistent observation. The tranmission ratio always turns out to be lower than the simple calculation of l_biggest/l_smallest/l_slidercrank. Kinematics need to be reviewed.

    Fabrication Notes

    This structure needed a fix. When it was first constructed, there were 2 significant problems
    1. The alpha flexure which was badly peeling causing the alpha link to move rather than rotate (the classic symptom of a bad alpha joint).
    2. Initiallly a 2 DOF wing was attached. However, it was attached in such a way that the lower tube of the wing was not connected to the top link at all. This led to the wing having a twisting resonance.
    The combination of these 2 problems caused the structure (actuator + fourbar + wing) to have a resonant frequency of 100Hz.

    These problems were easily fixed. The first was fixed by applying a liberal dose of green super-glue at the output flexure. The accelerator was also used. It should be noted that the DC motion did not decrease at all after the repair. It should therefore be re-emphasized how important a good alpha joint is. The second problem was solved by replacing the 1 DOF wing with a snipped 2 DOF wing. In the future, we will have to decrease the width of the 2 DOF wing at least near the base.

    Another thing to be noted was in the fabrication of the actuator. This actuator was initially fabricated without the U-channel. After attaching a U-channel and reinforcement, the force*displacement product increased from 13 micro-joules to about 19 micro-joules. Moral: Always use U-channels.