04 Assembly: print, build, wire

Finished-unit users can skip this page: the factory has already printed, assembled, wired and inspected the unit (see the factory-state section in index). Follow this page only when building from parts or replacing components.

4.1 Printing and inspection

1. Print the bottom plate, shoulder servo mount, elbow servo mount and top strut (1 each);
2. Check the base outline (~88 × 82 mm) and total height (~68 mm) with calipers;
3. Test-fit the 25T servo horns in their slots; fix with screws only when there is no bind;
4. Weigh every printed part and record it against hardware/v1/meshes/print_mass_model.json.

4.2 Servo preparation (firmware zero and horn installation)

1. Wire and flash first: wire both servos, the IMU, servo power and the common ground per the table in 4.4 (mechanical parts not mounted yet; the main firmware needs the IMU to boot), then flash the main firmware per page 03, section 3.1;
2. Power on to the firmware zero: open the 115200 serial port (serial tools / the UartAssist download are on page 03, section 3.3) and power up; after boot the firmware drives both servos to q=0 (center pulses 1681 / 1456 µs). When the log shows servo start q: 0.00 0.00 rad and ready; send g to run (or autorun on: ...), the servos are at the zero;
3. Power off: cut the servo power (USB may stay connected) and confirm both servos are unloaded;
4. Install the horns: mount the shoulder horn so the upper arm is vertical, and the elbow horn so the forearm is in line with it; tighten and check the full travel by hand;
5. Direction convention (matches the simulator): shoulder +q moves the big arm toward −Y, elbow +q moves the forearm toward +X; firmware defaults are SERVO_SHOULDER_SIGN=-1, SERVO_ELBOW_SIGN=-1.

The firmware centers 1681 / 1456 µs already include the horn re-index from the factory calibration - do not count teeth: mount the horns at the firmware zero with the arm vertical. Alternatively (equivalent, optional) type e in the serial send box and press Enter to put both servos at the 1520 µs electrical center and mount there (see the command table on page 03); either way the page 05 z command does the final precise zero.

Mounting at the firmware zero is only a coarse centering (the horn spline is discrete). The precise zero must be set after full assembly and main-firmware flashing (page 03) with the page 05 zero tool: declare the visually vertical pose q=0 (z does not use the IMU and works in any pose), then bake the printed pulses into config.h and reflash. Even if the horn was not mounted perfectly, this zeroing fixes it; only when one side lacks travel (the pulse clamps at 500/2500 µs) do you need to pull the horn and re-index. The horn index sets how the travel splits between the two sides; the firmware manual limits (Q_SHOULDER_MIN/MAX) must match the real travel - do not widen them. After zeroing, shift all sequence waypoints as described on page 05, otherwise the physical get-up poses drift.

4.3 Base and arm assembly

1. Fix the bottom plate and the shoulder servo mount together with M3 screws to form the base (~68 mm tall);
2. Install the shoulder servo in the shoulder mount, horn facing up;
3. Attach the elbow servo mount to the shoulder horn and tighten (it forms the upper arm); install the elbow servo in the elbow mount;
4. Connect the top strut (forearm) to the elbow horn;
5. Move the arm to vertical by hand (q≈0) and confirm it matches the servo center.

4.4 Wiring

Device	ESP32-S3 pin	Adapter	Notes
Adapter 3V3	3V3	S1 signal pin	other end to IMU VCC (3.3 V)
Shoulder signal	GPIO4	S4	50 Hz PWM
Elbow signal	GPIO5	S5	50 Hz PWM
IMU SDA	GPIO6	S6 signal pin	other end to IMU SDA; I²C 0x23, 100 kHz
IMU SCL	GPIO7	S7 signal pin	other end to IMU SCL; I²C 0x23, 100 kHz
IMU GND	GND	— (direct)	common ground
LED (optional)	GPIO2	—	status LED
5 V power input	5V / GND	—	1-to-2 cable: one branch to the ESP32, one to the adapter V+/GND for the servos
Servo power	—	V+/GND terminal	5 V / ≥5 A; higher voltages must use this terminal only, never the ESP32 5V
S2 / S3	—	—	not connected

Notes:

• The adapter is a passive pass-through: S1/S6/S7 carry 3V3 / SDA / SCL on their signal pins, S4/S5 carry the servo signals; confirm the V+/GND/signal order of every header - reversed polarity destroys servos or the board;
• Keep I²C wires short and away from servo wires; module pull-ups are 3.3 V;
• With a 5 V supply you may feed the ESP32 5V/GND and branch the same supply to the adapter V+/GND for the servos; higher voltages (6 V / 7.4 V) must feed the adapter V+/GND only and the ESP32 must be powered from USB;
• With a 5 V battery feeding the ESP32 you may keep the Type-C plugged in: in the official DevKitC-1 schematic each USB VBUS reaches the 5V net through a Schottky diode, so there is no back-feed and USB works as data (plus auxiliary power). On a clone board that omits the diode, or if you cannot confirm it, unplug the 5 V branch to the ESP32 while flashing and power it from USB (option 2);
• Power while debugging / flashing (both are normal; pick one):
  1. the 5 V supply feeds the ESP32 and the servos via the splitter, USB carries data only;
  2. easier to reason about: unplug the 5 V branch to the ESP32, power the ESP32 from USB alone and let the 5 V supply feed only the servos. Use option 2 for flashing, serial monitoring and c calibration; option 1 for standalone operation;
• On first power-up leave the servos disconnected, verify V+ polarity and voltage with a multimeter, then test one servo, then both;
• Wiring order: ground and signals → 5 V supply (ESP32 and servos together) → USB for the serial port when needed. Reverse when powering down.

4.5 IMU mounting orientation

The IMU is fixed to the base with screws (its orientation is not adjustable), so the firmware enables IMU_YAW90 and converts the IMU-frame up vector into body coordinates:

up_body = (up_imu.y, -up_imu.x, up_imu.z)

Mechanical orientation check (possible once the servos are mounted, before power-up): using the axes printed on / documented for the IMU module,

• the shoulder servo output shaft (the shaft the horn mounts on; also called the horn shaft or gear shaft) points toward the IMU −Y;
• the elbow servo output shaft points toward the IMU +X.

From bottom to top the two servos are shoulder (lower) → elbow (upper); do not swap them when mounting the horns or wiring.

These two directions are the body's shoulder and elbow axes; if they do not match the hardware, the IMU or a servo mount is installed the wrong way - fix it before powering up.

After power-up, re-check on page 06: upright gives up≈(0, 0, 1); the four lying poses are listed in the table in 6.5.

---

← Back to the tutorial contents