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Ideal foot output position profile
Due to shipping delays, the actual output from the nitinol actuator has not been determined yet. In order to make progress with analysis, a slider crank mechanism was designed to generate an elliptical shape to stand in for the nitinol actuator. The output of this mechanism was then coupled to the input of a modified Klann walking mechanism. Figure 3, below, shows the first iteration of the walking mechanism with the slider crank ellipsoid generator as the input.
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First iteration walking mechanism design
Mobility analysis of this mechanism shows there are 7 linkages, 8 joints, 1 half joint, and 1 grounding. Using equation 1 yields that this system has 1 degree of freedom
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In figure 6, the positional set of each joint is mapped to their corresponding x and y position. It can be seen when comparing to the first design iteration, that the only notable change is in Point C and the foot tip. As mentioned above, the positional profile of the foot tip matches more closely with the desired profile; however, to change the foot tip’s profile, the position of point C, located at the obtuse angle of the leftmost ternary link, and the position of point E had to be altered, as the position of the foot tip corresponds with these two positions. Since point E’s profile can only change in magnitude and size, but cannot change geometry, as it is linked to the ground, the main difference comes from Point C, and this is evident when contrasting it to the previous position profile.
Figure 6.
Velocity Profile:
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Figure 7.
Foot velocity plot
This velocity profile is not ideal since the velocity during ground contact is not relatively constant. This would lead to an inconsistent movement of a vehicle using a walking mechanism of this design.
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Mechanical advantage plot
Physical Prototype:
Prototype Mechanism
The Prototype currently consists of the major links and joints for the walking motion based on the Klann Linkage. The Klann linkage was chosen over the Jansen linkage due to the reduced small range of motion for the joints, which better lends itself to compliant mechanisms. The prototype is composed of rigid links with swappable compliant rolling contacts (CRCs). The joints were chosen to be swappable as breakage in the joints was a known risk factor.
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Currently, we are still missing the nitinol actuator. The biggest blocker to success for this is getting the materials in time for us to iterate on the design. Hopefully we will be able to receive the materials soon to begin the cycle of designing, building, and testing that has made the prototype so far. The compliant rolling joints, while performing much better than expected, still need optimization to reduce out-of-plane motion. In addition, we need to incorporate the design changes from the analysis into the prototype, optimizing for stride pattern, force profiles, and other considerations. This can be done by altering the geometry of the oval to yield the desired end-effector behavior.
Bill of Materials:
Item: | Quantity: | Price: |
PETG Filament | 1 kg | $24.99 |
M3x10 Flathead screws | 10 | $7.81 |
M3 hex nuts | 10 | $5.99 |
M3 heat set inserts | 10 | $9.40 |
Super Glue (gel) | 1 oz | $2.86 |
Kapton Tape | 1 roll | $29.99 |
Return Spring (tension spring) | 1 | $10.99 |
Nitinol wire | 1 meter x 1 roll | $15.99 |
Nichrome wire 26 AWG | 2 meters x 1 roll | $21.99 |
Power Supply (30V 10A) variable | 1 | $69.99 |
Arduino Mega | 1 | $43.56 |
Mechanical Relay | 1 | $7.39 |
Total | - | $250.95 |