I have been experimenting with various methods of joint actuation. I have so far investigated three possible routes:
one-way link with springs
two-way bellow
pneumatic muscles
- The one-way link with springs consists of a single airbag located at the joint and springs (silicone tubing in this case) the run between the two jointed segments. The airbag extends the joint and the springs return the joint to its contracted position. This particular configuration is best suited to actuating a joint between ~45º and 180º. The 45º lower bound has only to do with the interfering geometries of the two jointed segments. The primary limitation of the design lies in the fact that the maximum force applied in the return direction is limited by the spring. The extension force is much greater by comparison.
- The two-way bellow actuator is constructed of a 3” diameter polyurethane bellow. I trimmed this bellow so there were 3 convolutions, enabling a maximum inflated extension of approximately 8”. The bellow can be actuated using both positive and negative pressure (vacuum). This enables a joint that is pneumatically actuated in both directions using a soft membrane. This joint is best suited for actuating between ~45º and 180º. Again, the ~45º lower bound has to do with link geometry, not the actuator. The main drawback of this actuation type is the various ways in which the bellow can collapse. Instead of consistently returning to its flat configuration under vacuum, the bellow collapses on itself in place. Utilizing a more stable/rigid bellow could solve this problem.
- Pneumatic muscles have not yet been integrated onto an inflatable link. However, the method for actuating a joint can be applied such that a joint was an increased range of motion (passed 180º) and also additional degrees of freedom in a single joint. The maximum displacement of pneumatic muscles is their limiting factor; a 6” muscle will contract to 4.5” at best yield only a 1.5” displacement. The force output from each muscle is very high compared to the previous forms of actuation.