The machine

Twelve joints, a printed chassis and knees driven through pushrods. Designed and built with Emin Alp Arslan across 2021 and 2022, and apart on a shelf since.

Specification / as recorded

Status: Robot built 2021 to 2022, software rebuilt from scratch since August 2026
Role: Co-designed and co-built with Emin Alp Arslan. Simulation and learning work is mine
Joints: 12 servos, three per leg, 1.96 N.m each
Mass: 3.1 kg
Brain: Raspberry Pi, PWM at 50 Hz, three IMUs on the trunk

Sheet 02 of 03/The machine

What the robot is

Twelve joints, three per leg. Each one is a DS3218MG hobby servo with 1.96 N.m of torque and 270 degrees of travel. The whole robot weighs 3.1 kg. A Raspberry Pi drives the servos through a PCA9685 board.

Three facts about the hardware decide almost everything about the software.

Only four servos sit in the body. They swing the hips out and in. The other eight live in the hip modules. The thigh and the calf hold no servo at all, so a leg is light to swing. That layout has been misread twice from photos and CAD alone, which is why it is now written down in the repo as a fixed fact.
The knee is not direct drive. A servo in the hip module pushes a ball jointed metal rod, and the rod moves the lower leg. So the servo angle is not the joint angle. You cannot command a knee position and assume you got it.
The servos take PWM at 20 ms. That means 50 Hz is the ceiling for control, no matter what else I add.

Overhead view of the assembled quadruped showing the computer, wiring and servos packed inside the body — Everything the robot carries has to fit between the two spine rods.

Sheet 03 of 03/Building it

Printing and assembling it

The body is a printed chassis with two steel rods running its length. Everything the robot carries has to fit between those rods, so the packaging problem was set before any electronics existed.

I printed the parts, cut and fitted the pushrods, and wired twelve servos into one driver board. I also built the handheld controller: two thumbsticks, a microcontroller and four 18650 cells on one board.

3D printing was the right call for this. When a part came out wrong the fix was a reprint overnight, not a week waiting on a shop.

SolidWorks isometric view of the quadruped assembly with all four legs and twelve servos — Four servos sit in the body and swing the hips; eight sit in the hip modules and drive the thigh and the calf.

A bare rectangular robot chassis with four servos mounted and two steel rods running its length — The four body servos and the two spine rods set every other dimension.

Close up of a 3D printed robot leg with a thin metal pushrod running from a servo to the lower leg — The knee is driven by a ball jointed pushrod, so the servo angle is not the joint angle.

A Raspberry Pi stacked with a blue servo driver board, jumper wires and a power lead attached — It drives all twelve servos at 50 Hz, which is the control ceiling.

An open handheld controller board with two joysticks, a microcontroller and four cylindrical batteries — Built on its own board: two thumbsticks, a microcontroller and four 18650 cells.