All the arcs are circular on the Proportion DLCJ Drive. It is a one-way drive. (Simpson just need to extend the arm. Gravity will do the work to contract.) You drive it by a block and tackle between the holes I have put in the picture. It is possible to do a two-way drive without fanciness. I am working on a mathematically equivalent linkage that will be able to demonstrate the two-way drive.
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