As we continue to develop a design, we began prototyping leg designs using K'NEX. The purpose of the joints are to allow the servo to bend each leg segment using a tendon like string. The joints need to reduce the force the servos apply in order to prevent the servos from burning out. Also, the joints need to limit leg movement to two-axis. Otherwise, the leg would flex unpredictably and would be far more difficult to control. To accomplish these task, we built several prototypes of increasing complexity.
First Prototype: Pulley System
For this prototype, we focused on creating a system to reduce the amount of force needed to operate the joint. On the underside of the joint an elastic band is attached. This is what restores the joint to its natural, closed state when no force is applied to the servos. The string tendon is attached to the top of the joint in a pulley system. This system still requires the same amount of work to operate, but replaces a high force load (yet shorter amount of string) with a low force (but a lot more string to wind in). The optimal ratio that we found for this setup was 4:1. After that, the additional strands resulted in too much additional friction and prevented the joint from properly restoring itself (if different materials are used, it is likely that this result will vary). The joint in operation can be viewed below.
2nd Prototype:Double Joint
As the full leg will have a minimum of two joints on it, a system had to be devised so that each joint can move independent of the others. To achieve this, we relied on the principles of torque. The joint works because as the string tightens it exerts a force on the two struts at each joint. The force is multiplied based on the distance it is from the joint (the farther the distance, the greater the multiplication factor). This, along with the pulley system, reduces the amount of force that servo needs to apply in order to manipulate the leg. The downside of this is that any small force that is applied to the struts is then amplified and placed on the joint. Therefore, if we ran multiple lines through the strut, each line would greatly affect the joint, making the system dependent to multiple inputs. In order to counter this, the second line can be run through the center of the joint. As this is 0 distance from the joint, it applies 0 torque, leaving the joint unaffected as additional strands are tightened or loosened. This systems allows us to control each joint separately, greatly increasing our control over the leg. This system can be seen in action below.
Complete Leg
The completed leg features longer struts in order to better mimic the anatomy of the opiliones. As this added more weight, the joints had to be reinforced to prevent them from collapsing. Also, a third joint
was added at the bottom to mimic a foot that adapts to the slope of the ground surface with respect to the leg. This provides the leg with greater traction with the surface, helping to prevent slipping. The finished design appears in action below.
