02 Eta (η)
Date of Fabrication
02-eta was the 3rd attempt at making a 1 DOF fourbar satisfying the
There were 2 previous attempts:
- resonant frequency ≥ 150 Hz
- Amplitude at resonance ≥ 120 degrees
- Q ≤ 3
The salient features in the construction of 02-eta were
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.
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 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.
- 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
See solidworks drawings for complete dimensions.
|Link lengths (mm)
||6, 6, 6, 1
|Flexure lengths (μm)
||50, 50, 50, 50
|Flexure widths (mm)
|Flexure thicknesses (μm)
||6.25, 6.25, 3.6, 3.6
|Slider crank dimensions
||Attachment point (mm)
||(10 + 6) x 3
|DC displacements (μm @ 150V)
|Blocking force (mN @ 150V)
|Resonant Frequency (Hz)
|Q (= Xac/Xdc*150/Vac)
|DC motion of actuator w/o fourbar (@ 150V)
(NOTE: this is slightly different from Mimmo's measurement of the
|DC motion of actuator w/ fourbar (@ 150V)
|DC motion of fourbar output link (@ 150V)
|Resonant frequency of actuator + fourbar (w/o wing)
(NOTE: this might be inaccurate since the measurement was done
before a fix)
|Resonant frequency of actuator + fourbar + wing
|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
- 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.
- 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.
This structure needed a fix. When it was first constructed, there were 2
The combination of these 2 problems caused the structure (actuator +
fourbar + wing) to have a resonant frequency of 100Hz.
- The alpha flexure which was badly peeling causing the alpha link to
move rather than rotate (the classic symptom of a bad alpha joint).
- 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.
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
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.