Starfish Project: Feet Actuation – Servo Attachment and String System

Servo-String Coupling Attachment:

Figure 16: Servo – String Coupling Attachment

Figure 17: Servo – String Coupling Attachment Design Drawing at 2:1 Scale

We 3D printed two iterations of four servo attachments – one for each servo. This piece has an extruded portion that fits right on the servo gear. The design consists of two circles with one smaller circle between them that could provide a point of attachment around which the string was wrapped. It also has eight rectangular dividers that went around the inner circle in which our seven or eight strings could be tied to as seen in Figure [16]. Figure [17] illustrates the dimensions of our servo attachment. We printed these pieces on the resin printer. When we implemented them in our final product we had to wrap duct tape around the strings in the center to ensure they didn’t break off. In the future, we would suggest making the open center portion of the attachment wider so that the strings could be wrapped underneath each of the eight dividers.


Stringing System: 

Figure 18: String System for a Single Servo

The stringing system was one of the major factors in the reduction from fifty feet to thirty feet. This is because both the problems that we believed would arise with string orientation and spacing at this higher value, and because the size the servo motor coupling piece would need to be to allow for 12 or 13 strings to be attached. At our current thirty string model, two servo motors control seven feet, and two control eight feet. These feet are separated amongst the five legs to disperse the force across the entire starfish body. The stringing  system for one servo is shown in Figure [18]. To move the feet all in the same direction, the string was oriented through eyelets to align them properly. Eyelets are all placed perpendicular to the slot in the front of each slot (Figure [19]).

Figure 19: Close up photo of eyelet placement

Using nuts and bolts to hold up our servos was effective in also helping prevent our string from getting tangled in one another as shown in Figure [20]. Once the strings were tied, it was difficult to make adjustments because it required unscrewing all the servos and there was limited space to work with. Also, since the ends of the strings were cut shorter, it was hard to tie and untie all of them. The bolts acted as additional pivot points to wrap our string around to try and help orient motion as they got pulled. At one end the string was tied to our servo attachment, and at the other it was wrapped and knotted around the hole in the foot approximately three times. 

Figure 20: Close up of using of nuts and bolts to ensure strings did not tangle or intersect

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