Robotic Figures: Joint Design
The Slave Zero / One robots have 3 classes of joints - HINGE (simple bending), REVOLUTE (simple twisting), and BALL AND SOCKET (tilt in 2 axes). There is only one ball and socket, but several each of revolute and hinge types.
The hinge joint is simple in overall concept, but there is some subtlety to the actual implementation. The main structure (see drawing below of a stripped-down shoulder hinge joint) consists of a nesting pair of "wishbones" which start out as 1/8" wall aluminum box. The closer in (proximal) wishbone has some material milled away on its inner mating surfaces to accomodate teflon washers - these prevent metal to metal contact. The distal wishbone has a "driven disk" fixed concentric with its pivot axis (red dashed line), and this disk gets rotated by cables (magenta lines) causing the distal wishbone (and the rest of the arm outboard) to move. The actual pivot in the shown design has ball bearings in the proximal wishbone, and shoulder bolt axles which get fixed to distal wishbone during assembly. The cable center point is knotted, and this knot locks the cable into the driven disk. The 2 free ends of this cable snake through the robot all the way back to the servo driver disk.
Much of the complexity of these hinge joints arises from the need to route cables through the center. A device called a bender serves as the conduit for the cables. The drawing (left) shows an elbow joint with a bender installed.
The bender must be somewhat free to move within the hinge joint to minimize cable and bender wear, thus the (yellow) pivots, made from brass-tip set screws which are fixed with loc-tite.
Hinge joints towards the center of the body must be frictioned to keep the robot from being too "bouncy". The friction does waste power, so it needs to be adjustable so that the best balance can be found by experiment. A pair of pull-in bolts (visible in the photo below) squeezes the proximal wishbone onto the teflon washers, allowing good adjustability. Teflon is used because it is one of the few materials which does not exhibit "stick-slip" behavior (due to the static and dynamic coefficients of friction being similar). This is crucial to smooth operation in robots like these.
The range of motion of the shoulder hinge joint is about 210 degrees, and the elbow about 130.
Revolute joints produce rotation in-line with the structure; they twist.
And, like hinge joints, they also perform structural roles.
The geomety trick here is to reroute cables 90 degrees so they can operate their respective revolute joints, this is approached in various ways as can be seen in the illustrations.
BALL AND SOCKET JOINT
Two degrees of freedom with a common pivot point (although with a limited range of motion) are achieved with this ball and socket joint. The head's pitch (forward / backward action) and roll (left / right action) are controlled by this device.
The socket is an epoxy casting with 4 conduits for the ball cables embedded. The cables form 2 opposing pairs at right angles to each other, and the conduit entries are flared.
The inner revolute joints must rotate freely as intended but must also hold the robot together. They also need adjustable friction to limit bouncing as discussed above. A system involving spring-loaded teflon cones evolved. As the axis is tightend-down by rotating a nut, the teflon cone is forced by spring washers into a cone-depression in the driven disk. This force-centers, holds flat, and frictions the joint. An exploded view of the arrangement is shown here.
The range of motion attainable with these revolute joints is primarily limited by the amount of twist one wants to subject the system to: too much twist causes friction via cable-cable rubbing and also causes major stress at the conduit entries. 180 degrees is easily achieved in this design.
This 2-frame animation shows the simultaneous operation of 2 revolute joints (Shoulder 3, Wrist 1) and a hinge joint (Elbow).