In order to maintain balance and traction, the motion paths and cyclical timings of a wall climber's legs must be fine tuned. The leg's end effector, or foot, must maintain a steady and prolonged contact with the wall. Push in too much and the linkage will stall, push in too little and the foot will slip, so a near linear motion profile is desired. A compliant foot, such as one made of rubber or springs, could allow for more flexibility in the system, but excessive compliance might diminish the foot's traction against the wall. With these challenges in mind, our group has chosen a leg geometry that we believe strikes the balance between contact force and compliance. A sketch of this linkage is shown below.
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LP = 65mm
Offset angle theta = 56°
Foot angle delta = 80°
L1 is the ground link that will be connected statically to the rest of the climber body. L2 is the driving link that will be rotated around the origin point at a constant angular velocity. The ground link is offset by theta = 56° from the X axis so that the outer path of position P is as parallel to the Y axis as possible. To help visualize this motion, an animation of the linkage is shown below.
Figure 5: Animation of the leg linkage and the end effector point P. Dotted arrows represent the velocities at each point.
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It should be noted that for our final design, we changed our configuration from vertical to horizontal because the assembly was growing heavier than expected. The mechanism would no longer be working against the force of gravity, and so we decided to simplify our assembly and swap the rubber-ended spring feet for foam feet with less friction. Although the leg force calculations above no longer apply to our final design, they should still be considered if our project were to ever be adapted back to a vertical scenario.