II-Design Process

I began this project exploring the Strider four bar mechanism shown in the previous section, but upon realizing it was a bit oversimplified, I researched more complex and smoother mechanisms. In doing so, I came across a paper by Mehdigholi and Akbarnejad that looked to optimize the standard Watt's Six Bar Linkage for walking. However, the paper does not feature a diagram with linkages to scale, so I had to start by simulating various link lengths and gauge the correct lengths to use for the assembly process. To simulate various iterations, I made use of the online Planar Mechanism Kinematic Simulator, which allowed me to vary link lengths using an XY-coordinate system fairly easily.

Before using the software though, I needed a starting point. Based on the diagram given in Mehdigholi and Akbarnejad's paper, I made links of approximate lengths in SOLIDWORKS and using the measure feature, was able to map out the XY coordinates of each link and joint with respect to an origin that I set. This diagram is shown below:

With this rough coordinate system in play, I used PMKS to map out the overall trajectory of a "step." Here, the position of interest would be link 5 because the "foot" of the mechanism would be mounted directly to it and follow its motion. Thus, the position box was checked and monitored for link 5 in the PMKS simulations shown below. With each iteration, I used trial and error to see the effect of varying various links on the output motion path's shape–my goal was to arrive at the trajectory shown in the previous section. (NOTE: the resolution in the coordinate system was much higher than could be simulated in PMKS, so each dimension was rounded to the thousandths place for the iterations shown below).

• Iteration 1: original links shown in CAD model above
• Iteration 2: Links 3 and 4 are shortened to get desired semi-circle type shape, but still not the smooth flat gliding motion that is desired.
• Iteration 3: Link 5 lengthened to flatten the base of the step's trajectory; great shape but slightly offset by an angle.
• Iteration 4: Link 4 lengthened to adjust the lean of the path in the previous iteration; acceptable trajectory.

Based on PMKS, the final link lengths were set to be: 

• L1 = 2"
• L2 = 2.375"
• L3 = 4.625"
• L4 = 4.625"
• L5 = 3.875"
• L6 = 1.875" (distance between grounding points) 

From the planar simulation, I could not determine the arrangement of all the links. I needed to work with some tangible links, so I bought balsa wood and cut strips of the correct link lengths. Although not the most accurate method, I burned holes for the joints in roughly the right points and used screws and posts from a hardware store as my joints. The screws and posts were a perfect choice for joints as they could be tightened fully, yet still leave a smooth surface to serve as a "bearing" with the wood. This helped me better understand where links would collide with one another and how to space each of them apart.

Six-Bar Walker Mk1

The biggest concern from this first prototype was the collision associated with grounding the input crank. A shaft would have to go through the hole in the ternary link shown above. However, regardless of the arrangement of the other links, the shaft would collide with at least one link when built like this. To modify the design, I took inspiration from an engine's piston and crankshaft assembly, wherein multiple links, pistons, can move about a single input shaft out of phase with one another and not collide with the central shaft. In the subsequent prototypes, I used a bolt and locknuts to secure either side of the crank for link 4. The locknuts were necessary to ensure each side of the crank moves in sync and the original input shaft's rotation is translated to rotation in the outermost link connected to link 3. Additionally, at this point I learned how to use the lasercutter for quick adjustments and much more precise smoother holes. Initially, I laser cut this design out of acrylic, but I found that caused squeaking and was less rigid than I would like it to be. Thus, wood was used for the links and acrylic spacers were made to provide clearance for each link and bolt heads.

 Six-Bar Walker Mk2

The second prototype was significantly better than the first in that it was functional and significantly sturdier. However, the 1/4" plywood available at the Texas Makerspace is not exactly 1/4" and this tolerance contributed to a lot of wobbling and stiffness in certain points about the motion. Nonetheless, it was functional and followed the desired motion path as it was designed to; I faced some difficulty in tuning it to reach this point, though. Namely, the crank is exceptionally difficult to assemble since both sides need to be at the same position at the same time. I ended up using a shaft to align the central holes with one another and then tightened the locknuts.

Some other modifications had to be made to arrive at a final prototype, please see part six for details on the final prototype.