SG90 Sawtooth-Foot Inchworm Crawler

This is a beginner locomotion robot with no wheels. It uses one SG90 (or MG90S) micro servo, sitting sideways on a raised pedestal at the front of a rear chassis deck. The servo swings a two-armed front yoke up and down. That flexes the body at a hinge, like an inchworm, and pulls the two feet together and apart. Each foot has sawtooth ridges on its sole. They slide forward easily and catch when pushed backward, so every flex moves the robot forward. It suits Arduino and ESP32 classes: one PWM pin, a 4xAA pack and a Nano, and you have a crawler.

The rear chassis is the base. It carries the servo pedestal, the battery and controller grid, and one traction foot. The servo lies on its side between two cradle end walls, and a bridge strap clamps it down, so the build does not depend on the servo's mounting-tab dimensions. The front yoke has three parts: a drive arm clamped to the stock single-arm horn, an idler arm on a 4 mm steel pin in the other direction, and a crossbeam that ties both arms to the second, identical foot. The idler post is slotted in X, and the crossbeam's arm slots are adjustable in Y. Together these absorb the SG90's shaft offset and horn stack height, which vary between servo makers, so nothing has to be forced into place.

Assembly order:

1. Press M3 heat-set inserts into both feet (I1) and into the two pedestal bosses (I4).
2. Bolt one foot under the rear chassis with 4x M3x8 screws (I1/I2).
3. Lay the SG90 in the cradle with the shaft facing +Y and the wire toward the open lower corner. Clamp it with the strap and 2x M3x16 screws (I4).
4. Centre the servo (90°). Fit the single-arm horn so it points forward, and fix the horn to the drive arm with 2x M2x8 screws and nuts through the radial slot (I7). Then fit the horn back on the spline with its screw through the arm's 7 mm access hole.
5. Press the 4 mm pin into the idler post, slide the idler arm on, and bolt the post loosely to the deck (I5). Slide it in X until the pin is in line with the servo shaft, then tighten.
6. Bolt the crossbeam onto the second foot with 4x M3x10 screws (I1/I8). Then drive 4x M3x16 screws up through the counterbored slots into both arm pads (I6/I8), adjusting the slots in Y so neither arm is pre-loaded.
7. Strap a 4xAA holder and a controller to the M3 grid, and connect the servo.

Tuning: sweep the servo between about 60° and 120° at 0.5–1.5 Hz. A bigger swing gives a longer stride but more foot slip. The steep faces of the sawtooth face the rear (-X). On slick desks, add a wide rubber band or a strip of grip tape across the rear half of each sole. On carpet, bare teeth work best. Keep the battery over the rear foot: the weight there helps the rear foot grip while the front yoke reaches forward.