Inverse Parallelogram Mechanism = Ellipse Gear
Ellipse's major axis = b
Ellipse's minor axis = sqrt(b.b-a.a)
a: crank length; b: coupler link length
Ellipse's foci are centers of crank's rotary joints.
For details see:
http://meslab.org/mes/threads/18359-Diem-chet-trong-truyen-dong-co-khi
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