This seems a pretty neat way of backlash control - its a brake cable from a motorbike, one end is mounted to quill and the other has an air cylinder on it with variable pressure. The guy that fitted it also fitted a very tidy gear reduction system in the aluminium casing on the left, 15:1 ratio, double reduction. He reports zero backlash and full quill travel :)

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I think on cost alone, it has to be worth a try to run via the quill rack, powering the knee at any speed is going to take some time to save up for.