Progress 11/05/2014

Actuator CAD

Contracted

Extended

Figure 1: Leg shown with motors and ball screws mounted

Since the last progress review, I designed the motor and ball screw mounting hardware, selected appropriate bearings and couplers, and designed the torque couplings at the knee and ankle (Figure 1). The ballscrew transfers the axial load to a mounting plate through two thrust bearings, which sandwich the mounting plate (Figure 2). Nestled between the thrust bearings is a ball bearing that handles any small radial load on the ball screw. Finally, the torque is transmitted from the motor to the ball screw through a high-torsional-rigidity, low-backlash coupler. The motor side of each actuator transmits force to the joint that is being actuated. The ball nut side of each actuator attaches through a rotary joint to the shank of the leg (Overview in Figure 1).

Figure 2: Knee actuator assembly

After completing the design of the actuator mounting, I fabricated the necessary components using the Replicator 2X 3D printer. I cut several joint shafts from rod stock using a horizontal band saw and ground the ends on a grinder wheel so that the bearings slide on smoothly. I cut a demonstration foot out of ABS plastic and drilled mounting holes.

Challenges

Knee Joint Design

Figure 3: Knee dual shaft holder (highlighted in blue)

In order to maintain sufficient spacing between the actuator assembly and the shank of the leg, some creative design of the knee joint was required (Figure 3). I set the angle between the two shaft holders in the knee component at 45 degrees, halfway through the full range of rotation of the knee joint. This helps to keep the joint moment arm relatively constant, and also serves to space the actuator assembly away from the shank.