SG90 Parallelogram-Leg Biped Walker

This is a small two-legged walker driven by four SG90 micro servos. Two hip servos swing the legs fore and aft, and two ankle servos roll the feet from side to side. Each leg is a parallelogram four-bar: a drive link on the hip servo horn and a passive follower link, both 60 mm between pivots. Because of that, the ankle block stays upright however far the leg swings, so the foot always lands flat. The gait is simple. Roll both ankles to shift weight over one foot, swing the unloaded leg forward with its hip servo, then roll back and repeat on the other side. It's for hobbyists who want a biped they can actually program in an afternoon without 12+ servos.

Everything bolts together with M3 pivots and the small M2 screws SG90s use. The torso is an open U-channel with the two hip servos mounted in its side walls. The follower pivot sits in a slot so you can set the linkage to a true parallelogram whatever your servo's shaft offset. The deck on top holds a 4xAA holder and your controller on a 10 mm M3 grid. The feet stick out inward so the stance is narrow enough to balance on one foot.

Assembly order:

1. Press 4 M3 heat-set inserts into the torso corner posts. Mount both hip servos from outside the side walls with their shafts toward the front, using M2x10 screws and nuts through the tab slots.
2. Mount each ankle servo in an ankle block's front window, shaft end up, with M2x10 screws and nuts.
3. Centre every servo (90°) and fit a single-arm horn on each.
4. Screw each hip drive link to its hip servo horn with 2 M2x10 self-tapping screws through the radial slots, with the link hanging straight down.
5. Bolt each follower link to the torso rear slot with an M3x20 screw, washers and a nyloc nut. Leave it finger-loose for now.
6. Bolt both links to the ankle block lug (drive link on the outboard face, follower on the inboard face) with M3x20 screws, washers and nyloc nuts.
7. Slide the torso slot pivot until the ankle lug sits level, then tighten it.
8. Screw each foot to its ankle servo horn through the foot's upright. Slide the horn in the vertical centre slot until the sole sits flat, then tighten.
9. Screw the deck onto the torso posts with 4 M3x8 screws. Strap on the battery holder and mount the controller.

Tuning: tighten the pivot nyloc nuts just until the links move without axial play. Start with ±15° of ankle roll and ±20° of hip swing, and move the servos slowly (at least 300 ms per move) so the weight has time to shift. If the walker tips outward, move the battery pack lower and toward the rear, or glue a 2-3 mm rubber pad under each foot. Print one foot mirrored in the slicer (mirror in Y) so you have a left and a right.