SG90 Wheeled Serpentine Snake Robot

This is a desktop snake robot about 440 mm long: a head, four servo segments and a battery tail, joined by four yaw joints. A Raspberry Pi Zero 2 W drives a PCA9685 board, which sends a travelling sine wave down the joints. Each segment rides on a pair of free-spinning wheels, so the body slides easily forward but grips the floor sideways. That difference is what turns side-to-side wriggling into forward motion, the way a real snake moves. It's aimed at hobby roboticists who want a multi-servo project that actually moves across the floor, using parts they probably already own: 9 g servos, 624 bearings and M3/M4 screws.

Each joint is a printed U-yoke. The yoke's top arm screws onto the servo horn, and its bottom arm turns on a 624 bearing that sits on an M4 bolt in the housing floor. Because the joint is supported above and below, the servo only has to supply twisting force; it doesn't carry the snake's weight. The yoke's back plate bolts to the front face of the next housing. All four body segments, the head and the tail share one M3 bolt pattern (a 16 × 20 mm rectangle), so the parts go together in a straight chain. Each servo drops in from the top, rests on its mounting tabs and is held down by a clamp frame. The clamp presses on the tabs, so the design doesn't depend on the servo's tab hole spacing, which isn't reliably published. The M4 pivot bolt runs in a 10 mm slot in the floor, so you can line it up exactly under the servo shaft. The height of the horn above the tabs varies between servo brands, so the 4 mm gap under the housing floor is filled with M4 washers until the yoke turns without binding.

Assembly order:

1. Press M3 heat-set inserts into every housing (front face, clamp pillars, floor) and into the head and tail fronts. Press M4 inserts into the wheel bridges.
2. Screw a wheel axle bridge under each housing. Press a 624 bearing into each wheel and fit the wheels with M4×16 bolts and washers.
3. Set each servo to its centre position, drop it into its housing and screw the clamp frame down over the tabs.
4. Press a 624 bearing into each yoke's bottom arm. Screw a two-arm horn to the top arm with M2 screws, horn aligned with the arm.
5. Put the horn on the servo spline. Pass an M4×20 bolt up through the bearing, add washers to fill the gap to the floor, slide the bolt along the floor slot until it sits right under the shaft, and tighten the nylock nut.
6. Bolt the yoke's back plate to the next housing (or to the tail). Bolt the head to the front of housing 1.
7. Mount the Pi Zero in the head, and strap the battery pack, PCA9685 and UBEC into the tail. Run the servo leads along the top.

Tuning: the joints can physically swing ±60°, but limit them to ±50° in software. A good starting gait is angle_i = 35°·sin(ωt − i·70°) at about 0.5–1 Hz. Stretch rubber bands or O-rings into the wheel tread grooves for sideways grip. MG90S metal-gear servos are recommended; SG90s work on smooth floors.