SG90 3-DOF Ball-Balancing Platform

This is a compact 3-RSS parallel-manipulator ("mini Stewart") platform. Three SG90 micro servos sit at 120° around a round base. Each servo swings a 20 mm crank, and a linkage rod runs from each crank up to a 150 mm tray. Driving the three cranks together gives roll, pitch and heave, which is enough to balance a ping-pong ball with a PID loop, level a small camera, or teach parallel-kinematics inverse kinematics on a desk. Every joint is printed: the cranks and the tray knuckles carry integral 10 mm ball studs, and the rods have double-cone snap sockets that press over them. The only metal hardware is the three servos plus M3 screws, nuts and inserts.

The printed parts share one layout. The three spokes are at 90°, 210° and 330°. Each servo cradle bolts to the base with two M3 screws into captured nuts. The stock single-arm horn screws under the slotted crank. The rods snap onto the crank balls at the bottom and the knuckle balls at the top. The knuckles bolt with M3 screws into heat-set inserts in bosses on the underside of the tray. Tray, base, cranks and rods all print flat. The cradle prints standing on its foot. The knuckle prints lying on its web with the ball pointing up.

Assembly order:

1. Press 6 M3 hex nuts into the pockets under the base. Bolt the three servo cradles on with M3x12 screws, wall face toward the +tangential side.
2. Push each SG90 through its cradle window from the horn side. Fix it with its own tab screws through the vertical slots, then centre the servos (90°) electrically.
3. Fit the stock single-arm horn to each servo. Lay the crank over it and screw the horn to the crank through the crank slots with the horn's M2 screws. At centre, the crank should point horizontally outward.
4. Heat-set 6 M3 inserts into the bosses under the tray. Bolt on the three knuckles with M3x10 screws, all balls facing the same way as the crank balls.
5. Snap one end of each rod onto a crank ball. Lower the tray and snap the top sockets onto the knuckle balls. Slide the knuckles along their slots until each rod hangs in a vertical plane.
6. Wire the servos to a PCA9685 or Arduino and supply 5 V at 3 A or more.

Tuning notes: The servo shaft offset inside the SG90 body is not a verified dimension. The cradle therefore uses slotted tab holes, and the knuckles slide ±2 mm, so every build can be trued up by hand. Measure the crank angle at level in software rather than assuming 90°. Limit crank travel to about ±60° so the ball end never reaches the base. If the snap sockets are too stiff, open the split slot slightly with a knife. If they are loose, print the rods at 100% flow. A 40 mm ping-pong ball with an overhead camera or a 150 mm resistive touch panel taped to the tray gives the position feedback.