To quickly test the feasibility of a rolling robot, the team constructed an initial prototype out of a large PVC tube, with smaller silicone tubes fixed around the outside of the PVC, oriented along the axis of the large tube.
Initial PVC prototype with silicone tubing to test rolling
This test showed that expanding tubes set around the outside of a cylinder would be capable of producing motion. It also highlighted the fact that a completely round external profile would allow for smoother rolling, which would eliminate the losses in momentum seen above when the robot gets caught between silicone tubes. A smooth profile would be more efficient and accommodate faster rolling.
The second initial prototype, incorporated several new features to test, which are listed and described below.
PVC Prototype with Balloon Channels
- Long party balloons
The silicone tubes were replaced with party balloons to increase the amplitude of inflation of the driving channels. The silicone tubes only expanded about 1.5 times their original diameter, while the balloons increased over 8 times. Larger amplitude inflation of the driving channels will result in a larger force applied to the ground, and therefore more rolling.
- Acrylic rings to simulate smooth exterior rolling surface
To simulate a completely circular exterior rolling surface, we fixed the acrylic rings to the ends of the PVC. These rings held the ends of the balloons within the diameter of the rolling cylinder. In this position, the balloons did not interfere with rolling while they were deflated, but would expand enough upon inflation to breach the outer diameter of the cylinder, and cause rolling.
- Two rows of balloons along length of PVC
Two rows of driving channels were attached, each row with 9 balloons going around the perimeter. This was the first test in turning the cylinder left and right while rolling. The theory was that with one row of 9 channels all inflated, the cylinder would be tilted at an angle to one end. Then, when actuating the other side of channels in the forward rolling motion, it would be rolling on a curve away from the entirely inflated side.
- Multiple balloons attached to a single valve
The team also started experimenting with a single solenoid valve actuating multiple channels at a time. Valves were one of the more costly items in this project, and they would take up space inside the robot. To reduce cost and save space, we tried splitting the output of a single valve to three balloons. The three balloons were 120° apart, so that the ones opposite the one touching the ground would not interfere. Here we encountered an issue with inconsistencies from one balloon to the next. With pressure from one valve being distributed to three different balloons, only the balloon with the least resistance would inflate, while the other two remained virtually unchanged. Balloons had different resistances to inflating for a number of reasons. Different colors and variation from manufacturing accounted for some differences. Also, after a balloon is inflated once, it stretches out, and then becomes easier to inflate. This caused a compounding problem, because once a balloon inflated more than its other two partners, it became less resilient, and the difference between the three would get worse with every actuation. They attempted to calibrate each balloon by inflating them individually to stretch them evenly, but this was unsuccessful.
The team built a very simple deflated version of the robot to test undulating capability. After this quick test, they focused all energy on rolling, and planned to come back to this should there be time at the end of the semester.