SG90 Tendon-Driven Continuum Trunk Arm

This is a desktop continuum robot. A flexible trunk about 110 mm tall curls smoothly in any direction instead of bending at discrete elbow joints. Eight identical vertebra discs are threaded onto a 4 mm PTFE backbone. Each disc rocks on a domed hub that sits under the disc above it. Three braided tendons run through every disc at 120° spacing. At the base, three SG90 (or MG90S) servos wind those tendons onto capstan drums. Pull one tendon and let the other two out, and the trunk bends toward the tendon you pulled. Blend all three and it sweeps in a full circle. It makes a good pointer or inspection arm, a soft-robotics teaching rig or an animatronic tentacle. It's also a clear demonstration of tendon kinematics.

The base deck is printed upside down so the servo skirt grows up from the bed, and it holds the three servos in drop-in pockets. The hollow tower column bolts to the deck's centre, and the root plate bolts to the top of the tower. The root plate carries the backbone clamp, the three Bowden sockets and the first dome. Each tendon leaves its capstan, passes through a swivelling guide block on the deck and runs inside a PTFE Bowden tube through a tower window up to the root plate. From there it climbs through the vertebrae and ties off at the tip cap. The tip cap also has two heat-set inserts for mounting a payload such as a camera, LED or small gripper.

Assembly order:

1. Put M3 nuts in the four hex pockets under the base deck. Bolt the tower column onto the deck (I1).
2. Drop the three SG90s into the deck pockets with their shafts up. Screw them through the tab slots with M2x8 screws and nuts (I2).
3. Centre every servo at 90°. Seat a capstan drum on each single-arm horn and fix it with an M2x10 screw into the spline (I3).
4. Put M3 nuts under the tower top flange. Bolt the root plate on (I7).
5. Push the 127 mm PTFE backbone into the root plate so its lower end sits flush with the plate's underside. Lock it with the M3 set screw (I9).
6. Thread the eight vertebrae onto the backbone with their domes up, keeping the tendon holes lined up. Push the tip cap onto the top and lock it with its set screw.
7. Loosely bolt the guide blocks into the deck slots (I4). Cut three Bowden tubes, each running from a guide block through a tower window to a root plate socket (I8), and seat both ends against their steps.
8. Run each tendon from the tip cap down through the vertebrae, root plate, Bowden tube and guide block, then onto its capstan. Knot it at the tip counterbore and through the capstan cross hole.
9. Swivel and slide each guide block so its tendon feeds tangentially onto the drum, then tighten it.

Tuning: tension all three lines with the servos at 90° so the trunk stands straight and no line is slack. A 180° servo sweep gives about 31 mm of tendon travel, which is roughly 90° of total trunk bend. Drive the tendons as an antagonistic set, so when one servo winds in, the other two pay out. Fighting servos stall and brown out. A dab of dry PTFE lube in the Bowden tubes makes the motion noticeably smoother.