Starfish Project: Foot Design

How do tube feet help echinoderms move? + Example

We discussed many different ideas while designing the feet of our starfish, but all iterations kept the same idea of pulling a string until the foot tilts onto a sliding heel to create motion. The thought behind this idea was that the flat portion of the foot would have a higher static friction coefficient than the angled heel, so when pulled by the string, the flat portion would resist motion until eventually tilting onto the heel which would then slide forward until the tension in the string was released by the motor. We called this angle that the foot was pulled by the string our sweep angle. We decided it should match the heel angle to ensure that the sliding would only occur at the maximum tension applied by the string. As seen in Figure [7], when the feet are fully pulled by the string, the heel would be in contact with the ground in order to initiate sliding.   

Figure 7: Feet pulled by string (left) and tension released (right)

To restore this now tilted foot to vertical, the feet were going to be grouped randomly and pulled at different time increments by the motor. This would achieve both the randomness of feet movement that the starfish exhibits naturally and also allow for the tilted feet to feel a restoring force while the other groups were initially resisting their tilting. At the end of our project, once the model was constructed, it became apparent that we needed a larger restoring force to make the legs vertical again. A future design may utilize a rubber band attached to the wall via a similar eye screw and wrapped around each leg to snap the leg back to vertical once tension is released. 

Figure 8: Initial Foot Design

Originally, our foot design was the same diameter all the way through as seen in Figure [8]. This design was too short in length for any productive movement, too thin for our body holes, and the heel angle was too large. The bottom of the foot also didn’t cover enough surface area to be able to hold the starfish’s body up properly. In our next iteration of the foot design, we made the bottom portion of it larger and increased the diameter as a whole (Figure [9]).

Figure 9: Second Iteration

Figure [10] shows dimensions of our final foot design.

Figure 10: Final Foot Design Drawing at 1:1 Scale

We printed different variations of the heel angle at 20°, 25°, 30°, and 35° to see which would be best. For our third iteration, we decided on a 20° heel angle and to add a rocker, to prevent rotation, and a cap to prevent our feet from falling out of our slots in the body. This rocker length was calculated using the basic trigonometry equation 2*L*sin(20). We took the length from the string hole to the bottom of the starfish body, where the rocker is located, L, and found the projection of the arc length which is our rocker length. These variables are visually demonstrated in Figure [11].

Figure 11: Sweep Angle Diagram

In the end, our foot height, diameter, top hole diameter, and thicker bottom designs from iteration two were utilized. The rocker dimensions can also be seen in Figure [11] and the cap dimensions can be seen in Figure [12].

Figure 12: Foot Cap Design Drawing at 5:1 Scale

In our initial foot designs, we attempted to put 3D printed grooves at the bottom of the feet, however these were too small to print as seen in Figure [13]. As an alternative for our final design, we cut pieces of Fearless Body and Clothing double sided tape from Amazon and stuck them to the bottom of our feet as shown in Figure [14].

Figure 13: Initial friction groove design

Figure 14: Fashion Tape on flat part of foot used in final design

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