Lengths of yellow bars: a

Lengths of blue bars: b

Length of pink bar (input): c

Distance between centers of fixed pivots: d

Orange pen draws a portion of a circle of radius R (in orange).

Circle center is on the line that connects centers of the fixed pivots.

Distance between fixed pivot center of the pink bar and the circle center is L.

L = d.(b.b – a.a)(d.d – c.c)

R = c.L/d = c.(b.b – a.a)(d.d – c.c)

If (d - c) is chosen small, R can be very large.

This avoids the need to arrange a pivot at the circle center.

The mechanism has dead positions when all bars (except the pink one) are in line. If (d – c) = 0 the circle becomes a straight line. See:

Peaucellier linkage 1