Strain wave gear differential with high gear reduction

A harmonic drive, comprising:

• a ground ring;
• a first flex spline extending axially from the ground ring;
• a first input shaft that is radially exterior to the first flex spline,
• wherein the first input shaft includes a first wave generator that engages the first flex spline;
• a second flex spline, radially within the first flex spline and rotationally coupled to the first flex spline,
• a second input shaft that is radially within the second flex spline,
• wherein the second input shaft includes a second wave generator that engages the second flex spline;
• an output shaft coupled to the second flex spline,
• wherein:
• the first flex spline includes a first spline that faces radially inward;
• the second flex spline includes a second spline that faces radially outward;
• and
• the drive further includes a circular gear that includes:
• a third spline that face outwardly and engages the first spline;
• and a fourth spline that faces radially inward and engages the second spline.