Octobot

An eight-legged walking robot with 24 servos, built end to end. The chassis and legs are CAD. A custom ATmega32U4 board drives every servo. A browser workbench rigs the CAD model and solves its inverse kinematics.

The workbench runs live in the browser: https://deivid-negoita.github.io/octobot/. It needs no install and no build step.

The robot carries environmental-monitoring, 3D-scanning and speleological payloads. Those payloads need the legs to place feet accurately on uneven ground, which a fixed gait cannot do. The kinematics workbench in this repository exists to solve that.

---

What is here

Folder	Contents
kinematics/	Browser workbench and locomotion sandbox. Three.js, no build step.
hardware/	KiCad 7 project for the 24-channel servo control board.
mechanical/	STEP exports of the chassis and a single leg.
docs/	Hardware notes, kinematics notes and the bill of materials.

The embedded firmware lives in a separate repository and is not included here.

---

Run the kinematics workbench

The workbench runs in the browser with nothing to install: https://deivid-negoita.github.io/octobot/. It is a static site, published from kinematics/ by .github/workflows/pages.yml on every push to main. A fork gets its own copy after enabling Settings → Pages → Source → GitHub Actions once.

To run it locally instead: the pages load ES modules, so they need an HTTP server. Opening index.html from the filesystem gives you a blank screen and a CORS error in the console.

cd kinematics/tools
python serve.py          # or double-click start.bat on Windows

Open http://localhost:8123. The chassis mesh is about 12 MB, so the first load takes a few seconds. A progress bar reports the download.

Two pages share the model:

index.html, the IK workbench. The CAD model rigs itself on startup. The loader finds the feet from the mesh geometry, then builds a hip-yaw → shoulder-pitch → knee-pitch chain per leg on the real servo shaft axes. It binds every servo, bracket and casing to the link it is screwed to. You get eight chains named LEG-1 to LEG-8. Drag the cyan target and that leg solves toward it in real time while the servo angles update. Press Space to run a gait.

playground.html, the locomotion sandbox. Drive the robot with W A S D over terrain blocks you spawn and drag. The feet land on the block surface or the ground by analytic sampling, and the body rides at the average support height. Live servo angles for all 24 joints stream into the telemetry panel.

Key	Action
1 / 2	JOINTS / SOLVE mode (workbench)
Space	start or stop walking (workbench)
H	open the help card (workbench)
P	show or hide the panels (both pages)
W A S D	walk (playground)
Q / E	lower or raise the body (playground)
Y	toggle the onboard camera view (playground)

Read docs/kinematics.md for the solver, the collision guard and the gait model.

---

The control board

One board drives all 24 servos. An ATmega32U4 talks over I²C to two PCA9685 16-channel PWM drivers, which gives 32 channels for the 24 servos and leaves headroom for sensors. An SY8303 synchronous buck regulator supplies the servo rail from the battery, and a P-channel MOSFET on the input protects against reverse polarity.

The 24 servo headers sit on two separate power rails of 12, so the inrush of one group of legs cannot brown out the other. An HC-12 radio module plugs into a 4-pin header on D8 and D9. Running the link on a software UART keeps the hardware USART free and lets the antenna sit away from the servo wiring. The board is a 4-layer design with 120 placements, an AVR-ISP header, a DIP switch and two tactile buttons.

Building 24 servo channels from discrete drivers would have cost more and taken more board area than the two PCA9685s do.

hardware/fabrication/ holds the package the board house was sent. It carries gerbers for all four copper layers, both drill files, the IPC netlist, the placement file and the BOM. Read docs/hardware.md for the rail split, the power path and the pin assignment of every header.

Bill of materials

120 placements over 33 lines: 87 SMT and 33 through-hole. The table is read out of the layout by hardware/tools/gen_bom.py rather than typed. --check re-groups the fabrication BOM the same way and compares it designator by designator, so this list cannot quietly go stale:

python hardware/tools/gen_bom.py --check
120 placements match octobot-controller-bom.csv exactly.

Qty	Value	Manufacturer part	Package	Ref	Function
2	PCA9685PW/Q900_118	NXP	SOP65P640X110-28N	U1–U2	16-channel 12-bit PWM generator. 12 channels used each, at 0x40 and 0x41.
1	ATmega32U4-M	Microchip	QFN-44-1EP_7x7mm_P0.5mm_EP5.2x5.2mm	U3	Host MCU. Native USB, 16 MHz crystal, I2C master.
1	AVR-ISP	—	AVR-ISP	U4	ISP header on D14/D15/D16 and RESET.
1	SY8303AIC	Silergy	SOT65P280X110-8N	IC1	Synchronous buck. VIN down to the 5.0 V logic rail set by R11/R12.
3	LED	—	LED_0201_0603Metric	D1, D3–D4	Power, RX and TX indicators.
1	4,7 uH	Wurth Elektronik 7447713047	7447714220	L1	Buck output inductor.
1	Crystal_GND24	—	CRYSTAL-SMD-5X3.2-4PAD	Y1	MCU clock, loaded by C1/C3.
1	HPZR-C10X	Nexperia	HPZRC10X	D5	10 V regulator diode. Clamps the pass FET gate.
1	RQ3E075ATTB	ROHM Semiconductor	RQ3E075ATTB	Q1	P-channel MOSFET. Reverse-polarity pass element on the input.
28	Conn_01x03_Pin	—	PinHeader_1x03_P2.54mm_Vertical	J2–J8, J10–J14, J17–J30, J32–J33	24 servo headers (GND / rail / signal), 2 rail feeds (J29/J30), 2 analog headers (J32/J33).
4	1x2	—	PinHeader_1x02_P2.54mm_Vertical	J9, J16, J31, J34	Input power (J9), 5 V taps (J16/J34), I2C breakout (J31).
1	47346-0001	Molex	MOLEX_47346-0001	J1	USB micro-B receptacle. Programming and 5 V input.
1	Conn_01x04_Pin	—	PinHeader_1x04_P2.54mm_Vertical	J15	HC-12 radio header: 5 V, GND, D8, D9.
2	TL3365AF180QG	E-Switch	SW_TL3365AF180QG	S1–S2	Reset (S1) and user button on D12 (S2).
1	DS04-254-2-03BK-SMT	CUI Devices	DS04254203BKSMT	S3	3-position DIP switch on D14/D15/D16.
24	220 Ohm	—	R_0603_1608Metric	R31–R54	Series resistor on each servo signal line.
20	10K	—	R_0603_1608Metric	R1, R5, R8, R10, R14, R16–R30	PWM address and OE straps, I2C pull-ups, buck enable.
2	1K	—	R_0402_1005Metric	R7, R9	LED series resistors.
2	22 Ohm	—	R_0603_1608Metric	R2–R3	USB D+/D- series termination.
1	100K	—	R_1206_3216Metric	R15	Pass FET gate pull-down.
1	100K Ohm	—	R_0805_2012Metric	R13	Buck switching-frequency set resistor.
1	10KOhm	—	R_0603_1608Metric	R4	RESET pull-up.
1	110K Ohm	—	R_0805_2012Metric	R11	Buck feedback divider, upper leg.
1	15K Ohm	—	R_0805_2012Metric	R12	Buck feedback divider, lower leg.
1	1KOhm	—	R_0603_1608Metric	R6	LED series resistor.
6	10uF	—	C_0805_2012Metric	C4–C5, C18–C19, C22–C23	Rail decoupling.
2	0,1uF	—	C_0603_1608Metric	C17, C21	Decoupling.
2	18pF	—	C_0603_1608Metric	C1, C3	Crystal load capacitors.
2	1uF	—	C_0805_2012Metric	C2, C8	USB UCAP and buck input decoupling.
2	875075161013	Wurth Elektronik	CAPAE1030X1240N	C6, C9	Aluminium electrolytic. Bulk storage, one per servo rail.
1	0.1uF	—	C_0805_2012Metric	C7	AREF decoupling.
1	10nF	—	C_1206_3216Metric	C20	Buck bootstrap.
1	12 pF	—	C_1206_3216Metric	C16	Buck feedback feed-forward.

120 placements over 33 lines: 87 SMT and 33 through-hole.

Distributor part numbers are not tracked in the KiCad project, so an assembly house needs those added at order time.

---

Mechanical

mechanical/ holds STEP exports of the assembled chassis and of a single leg. Every part is 3D printed. The chassis was modelled in Autodesk Fusion 360, and kinematics/models/octobot.glb is the mesh export the web app loads.

STEP files are text and compress to roughly 15% of their size, so the repository carries the zips and ignores the loose exports. The chassis is 52 MB unpacked and 7.7 MB zipped. Unzip before opening in CAD:

cd mechanical
unzip octobot-chassis.step.zip

---

Tooling

kinematics/tools/ holds the development scripts:

python serve.py          # static server with caching disabled
python verify.py         # drive both pages in Chromium, fail on any console error
python shots.py          # regenerate the README screenshots

verify.py and shots.py need Playwright:

pip install playwright && playwright install chromium

verify.py boots each page and asserts that the rig produced eight legs. It then exercises the help modal, toggles the panels, runs the gait, spawns a terrain block and walks the robot. It exits non-zero if the browser logged an error.

---

Built with

Three.js 0.160 loaded from a CDN, plain ES modules, no bundler and no dependencies to install. KiCad 7 for the board. Autodesk Fusion 360 for the mechanics.

License

MIT. See LICENSE.