Prototype, Initial

 Initial Prototype:

We started by designing the mechanism that will allow rotary motion in the sagittal and coronal planes. A four bar mechanism will attach to the motor to provide rotary motion in the coronal plane. The four bar mechanism has two springs to provide compliance and allow the four-bar to bend out of plane (Figure 1). The first prototype was built out of aluminum plus off-the-shelf springs, collars, and pins ordered from McMaster-Carr. The mechanical advantage of the four-bar is strictly a function of the moment arms (Equation 1). This can be adjusted by changing the attachment point of the springs. The two springs counter each other so that at least one spring is always in tension. 

Based on a review of biomechanics literature for subjects (mean mass 85 kg) walking on a 5° slope perpendicular to the direction of travel, an intact ankle generates coronal moments up to 15 Nm rotating the foot away from the body (eversion) and 8 Nm rotating the foot toward the centerline of the body (inversion) [Dixon 2010]. Some of this torque is generated by the motion of the body center of mass about the foot, but this provides bounds on the amount of torque to be applied.

Figure 1. Initial assembly of the mechanism, excluding the motor, footplate, and cuff.

Equation 1. Mechanical advantage

MA = r_in/r_out = 2 in. / 0.75 in. = 2.667