Here's the problem;
Say you have a robot arm which has several joints along it's length, each one is driven by a pair of steel cables. The other ends of the cables are connected to gearmotors which are located off the arm, to keep the weight down. So the cables which drive the farmost joints need to run through the preceding moveable joints, but the path length of each cable must not change. ie, if the cable were to stretch or become slack 'round the bends' it would affect the positioning of joints further along the arm.
It can be done using bowden-type cables of course, with an inner cable and an outer sheath, but this is not a very elegant solution. You can also try and route the cables through the centre of each joint, but unless they will bend to a very tight radius, the length still changes.
Is there a simple geometric solution to this, whereby cables can be routed around a joint, and as it flexes the path length remains constant? The Barrett Technology WAM arm appears to work this way, but I can't see how. I have thought up one solution using a pair of counter-rotating rollers, but I don't think the WAM works this way, and I'm sure there's a better way.
Any descriptions or links, especially with diagrams, much appreciated.