In an ideal cable transmission system, the length of a pathway from drive motor to driven joint should remain constant, even though the path traverses several flexing joints.
If the length varies significantly, two problems can occur :
cable tension variation - Too taught causes wear and power-killing friction, and too loose causes joint "slop".
phase error - unintended joint movement caused by other joints along the path flexing.
One of the key technical innovations of this series of robots is called a BENDER. It is a device that provides a constant length pathway for a group of cables through a hinge-type joint. The white object in the middle of the shoulder joint pictured here is a bender that carries 10 cables which will power joints farther out on the arm.
To approximate this ideal in a hinge-type joint, the cables must be fanned out into a row and carried through a path that is BENDABLE, yet INCOMPRESSIBLE along its axis. This is achieved by creating a planar arrangement of Teflon carrier tubes alternating with stainless steel cables which provide the axial stiffness.
The first step to constructing such an object is to unify the tubes and cables with a heavy thread weave. This is done with the simple loom at left. About a foot of the weave is made and then cut into pieces.
This is a closer view of the weave being made on the loom. There are 5 tubes and 6 cables. Even though this type of cable is quite flimsy by itself, when integrated into the weave it resists axial compression well. The 49 individual strands of wire that constitute a cable are so fine that they do not suffer from fatigue failure with repeated bending.
Here, lengths of the weave are shown being fit into their aluminum anchorages.
The bender-to-be is bolted into a structure that serves as both mold and geometry fixer. The junctions of the weave pieces and the anchorages are epoxy sealed.
Silicone Rubber is here being injected from below very slowly. The rubber encapsulation serves to distribute the bending evenly through the device, and also protects the weave. Next, the mold is stood on end and epoxy is poured at the exits to fix correct curvature of the Teflon tubes.
Two finished benders are shown here mounted into their joints - a shoulder and an elbow.