3.2 Design Process
We did our preliminary iteration using a python script that calculates and plots the motion profile with the linkages visible. This script also includes sliders to adjust all necessary values. Initially, the motion profile was much more circular and not flat at the bottom, which would have led to unstable walking motion.
After tweaking the values, we ended up with a shape that is much more efficient as there is a flat section on the bottom of the motion profile that exists for a significant duration of the input crank’s rotation.
After this, we doubled the link lengths to result in the animation and the velocity and acceleration plots above. We used those measurements directly in CAD to mock up our first physical iteration. This consisted of slot shapes that all have center to center distances set equal to the correct link lengths.
However, the bearings we are using for our prototype are very unstable, and the inner race is able to twist, so our first physical iteration had much more lateral movement than we desired. On our next iteration, we doubled up the bearings in order to geometrically constrain their rotation to purely radial. Additionally, because the first iteration was only slot shapes, the actual end point of point P (according to the motion plot) was not on a vertex of the triangular shape, so we modified that link to perfectly match the profile from the python script. Additionally, the base was modified so that there is a stand whose bottom aligns with the flat portion of the motion profile. This allows for easier visualization of the motion and allows us to see how smoothly it can actually walk. A rubber band was then placed along the outside of the ground contact linkage to increase friction, leading to our final prototype:
We liked both the smooth stride motion and scale of our prototype, so we maintained the same geometry for our final product and kept the same fourbar linkage system as it reduced tolerance stackup. In order to ensure that our linkages would be as rigid as possible, we opted to manufacture the final linkages out of 6 mm thick acrylic as it was the stiffest material we had available. Additionally, we scrapped the bearings and used screws with locknuts and low friction nylon washers at our pin joints. 3D printed TPU was used as the feet to ensure the necessary friction for walking and add compliance to the system if needed. An individual linkage is shown below:
We also wanted to ensure that all 8 legs would rotate in sync with each other, so we utilized a single motor and shaft with custom 3D printed gearboxes. This allowed us to control the final output RPM of our motor as the motor already had the necessary torque to articulate the legs under normal expected load. An image of the gearbox is shown below:
Manufacturing 8 legs and 4 sets of gears left us with our final product: an 8-legged walking robot that utilizes 4 bars for smooth motion with enough vertical clearance to traverse small objects.