Knuckle

Knuckle

A 35 mm round FOC driver that bolts to a GBM2804 gimbal motor and answers on the Open Duck Mini's servo bus as an STS3215.

Part of HeyPCB's open robot boards. Designed end to end with the HeyPCB method on KiCad 10 (headless pipeline, Freerouting 2.4.1); every number below is measured on the files in this repository. Mentions of ./run.sh, docs/rules/, blocks/ or parts/ refer to the heypcb-kicad SDK that produced them.

Not fabricated, assembled or powered.

Knuckle is a 35 mm round field-oriented-control (FOC) driver that mounts on the back of an iPower GBM2804H-100T gimbal motor and answers on the Open Duck Mini's Feetech STS servo bus as an STS3215 (model 777). It is meant as a drop-in for the duck's head_yaw (ID 32) and head_roll (ID 33) joints. It was designed with the HeyPCB method on KiCad 10 (the headless heypcb-kicad pipeline, KiCad 10.0.6, routed by Freerouting 2.4.1) from the brief in board.json, following the research spec in robots/research/actuator/spec.md.

It has not been fabricated, assembled or powered, and its firmware is not written. The STS3215 emulation below is what that firmware must do; none of it exists yet. Every gate number below is read from the final run's report.json. What this README says about copper, pours, pads and silkscreen was read from kicad/knuckle.kicad_pcb with KiCad 10.0.6's pcbnew, and every mechanical number was measured on the solids by cad/parts.py (cad/fit.json). Numbers marked (est.) are estimates or come from the spec; they are not measurements.

• MCU: STM32G431KBU6 (UFQFPN-32, 5 x 5 mm, Cortex-M4F at 170 MHz) on its internal HSI16 clock, at the centre of the top face. SWD on a Tag-Connect TC2030-NL footprint on the bottom (nothing to fit).
• Driver: TI DRV8311H, a 3-20 V three-phase bridge (5 A peak, 210 mOhm high + low side) with three integrated low-side current-sense amplifiers. 3x PWM mode: TIM1 drives INHA/B/C, and one enable line drives all three INLx.
• 3.3 V rail: the DRV8311's own AVDD regulator. There is no buck (see Decisions).
• Encoder: MagnTek MT6701QT (QFN-16, 14-bit, SSI) exactly on the motor axis on the motor-facing (bottom) face, under the shaft magnet, with its 100 nF. Nothing else is on the bottom face inside an 11 x 11 mm square around the axis.
• Bus: two Molex Mini-SPOX 5267-03A vertical sockets (pin 1 DATA, 2 battery, 3 GND) wired in parallel for daisy-chaining, opposite each other on the vertical axis. An SN74LVC1G126 drives DATA from USART2 TX while the USART's hardware DE is high; an SN74LVC1G125 passes DATA to RX while DE is low. DATA has 100 R in series, a 10k pull-up and a 5 V ESD diode.
• Power in: the 2S battery from the bus (6.0-8.4 V) through an AO3401A reverse-polarity P-FET, an SMAJ12A TVS and a 22 uF / 25 V polymer tantalum bulk capacitor.
• Sensing: motor-rail voltage (100k / 22k) and a 10k NTC beside the driver.
• Other: a red status LED; the motor's three phase leads solder into plated holes at the right rim; four radial screw slots for the motor's base pattern.
• Board: a 35.0 mm circle (the motor's own diameter), 6 layers, 1.6 mm. F.Cu, In1 a GND plane, In2-In4 and B.Cu routed; after routing F.Cu and B.Cu are poured GND, In2 and In4 VBAT, In3 VM.

Scope: head joints only

The spec's answer, which this board follows: a 2804-class gimbal motor on a 2S pack cannot replace an STS3215 in a duck leg, direct or geared. The numbers are the spec's (sections 0 and 2; model outputs and MuJoCo static torques, not bench measurements):

At 7.4 V every gimbal winding the spec checked (5-11 Ohm) is voltage-limited to about 1.2-2.7 W of mechanical power (est.), against the STS3215's 2.6 W (datasheet) to 4.5 W (identified model). So v1 is a direct-drive head actuator: the MT6701 under the shaft magnet is the absolute joint encoder, with no gearbox, no backlash and no second encoder, and the duck's runtime keeps working unchanged because the board answers as model 777 at the same IDs and registers. The ten leg joints, neck_pitch and head_pitch stay on STS3215.

Decisions and substitutions

The MCU pins (read back from the board's U1 pads; functions from the KiCad symbol's own alternates):

Phase current and copper

Phase current, from the spec's per-phase resistance (Rs = 5.292 Ohm, ST motor profiler) at the voltage-limited stall, where SVPWM puts VM / sqrt(3) across each phase (est.):

The DC column is the winding's VM² / (2 Rs) divided by VM: 5.2 W at 7.4 V and 6.7 W at 8.4 V. A 39 g motor cannot hold that; the spec asks the firmware for an I²t limit above about 0.3 A.

Track ratings by IPC-2221 at a 10 C rise (heypcb-kicad's route.ipc2221_amps):

Where the copper ran:

• All three phases are on F.Cu, 1 oz outer copper, with no vias: PHASE_A 3.3 mm at 0.25 mm; PHASE_B 4.95 mm, of which 2.26 mm at 0.187 mm and 2.69 mm at 0.25 mm; PHASE_C 7.24 mm at 0.25 mm. Freerouting laid PHASE_B's first stretch out of pin 11 at 0.187 mm, between PHASE_A's track and pin 12, and the import's widening step, which checks 0.2 mm to other nets, could not restore 0.25 mm there. That stretch is below the 0.2-0.3 mm asked for. The narrowest phase copper, 0.187 mm, carries 0.71 A against 0.65 A RMS at 8.4 V; the 0.25 mm runs carry 0.88 A. The 0.25 mm class is the widest track that enters the DRV8311's single 0.2 mm OUTx pads at 0.4 mm pitch with 0.15 mm to the next pin, because Freerouting runs without neck-down.
• A tracks-only keepout on In2-In4 between the driver's outputs and the motor holes (phase_lane_outer_only) keeps the phases on outer copper. On the route before it, PHASE_C took 6.8 mm of In2, where 0.25 mm carries 0.27 A at 0.5 oz.
• VM is 72.25 mm of track. The bridge current leaves Q1 on 0.3 mm of F.Cu, drops through one 0.6 / 0.3 mm via into the In3 VM pour, and comes up through another next to C9, at the driver. The In3 VM pour is one island of 609.8 mm² that joins all five VM vias. Its narrowest cut between those two vias is 2.8 mm (a vertex max-flow on a 0.1 mm raster, more than 1 mm from either via, which can only undercount): about 1.5 A at 0.5 oz inner. 7.35 mm of VM's F.Cu is 0.225 mm: 4.1 mm among the driver's VM pins and capacitors, which the bridge current crosses (0.81 A on 1 oz outer, against 0.79 A DC at an 8.4 V stall), and 3.2 mm on the TVS's branch. A via's 25 um barrel around a 0.3 mm drill is about 1.8 A by the same formula with the outer constant.
• VBAT (the bus pass-through) is a 0.6 mm track on In4 from J1 pin 2 to J2 pin 2 along the left rim, inside the In4 VBAT pour; In2 is poured VBAT as well, as the spec asks. Each pour is one island (In2 659.6 mm², In4 679.6 mm²). Between the two sockets, more than 1.5 mm from either pad centre, the narrowest cut is 2.8 mm on In2 and 4.1 mm on In4: together about 3.5 A at 0.5 oz inner. At each socket the pours join pin 2 through 0.4 mm thermal spokes: 1.6 mm of copper per layer within 1 mm of the pad centre, about 2.0 A for both layers at 0.5 oz. The Mini-SPOX contact is rated 3 A (catalogue row).

STS3215 emulation the firmware must implement

Not written. From spec section 5; register facts are cited there by file and line.

• Bus: 1 Mbps 8N1 half duplex on USART2 with hardware DE (DEM mode). Parse FF FF ID LEN INSTR P... CHK with CHK = ~(ID + LEN + INSTR + sum P) & 0xFF; drop bad checksums. Answer only its own ID, and broadcast (254) only in SYNC_READ slots. Honour Return Delay Time (reg 7) and Response Status Level (reg 8).
• Instructions: PING, READ, WRITE, SYNC_WRITE (0x83) and SYNC_READ (0x82) are required; REG_WRITE and ACTION are used by the official SDK; RESET is optional.
• SYNC_READ slotting: reply after the status packets of every ID listed before its own have passed on the bus, counting them by parsing, with a timeout per missing slot (about 100 us, est.). This is critical: rustypot's set_kps SYNC_READs register 22 from all 14 IDs and fails if any one is silent.
• Registers: model 777 at 3-4; ID at 5 (flash-backed; configure_motor.py sets it); baud at 6 (at least 0 = 1 M); P / D / I at 21 / 22 / 23; mode 33 (0 = position); torque enable 40 (0 = INLx low, bridge Hi-Z); goal position 42 and present position 56 (0-4095 from the MT6701's 14 bits >> 2 plus the homing offset at 31; 2048 = 0 rad; sign-magnitude in bit 15); present speed 58 (steps/s, bit 15 sign); load 60 (bit 10 sign); voltage 62 (0.1 V, from VM_SENSE); temperature 63 (from the NTC); current 69; limits 9 / 11 / 13-15 / 16 / 48; lock 55; acceleration and max velocity 41 / 84 / 85 accepted.
• Control law: the identified STS3215 model, tau = clamp(P x 0.166 x (7.4 x 1.178 / 2.479) x (q_ref - q) - 0.56 x omega - D-term, ±tau_max), with q_ref slew-limited to 5.29 rad/s, then Iq = tau / Kt. The motor's 0.03-0.047 N·m saturates it: at P = 8 the head is stiff only within about ±0.4° (est.).
• FOC core (SimpleFOC): BLDCDriver3PWM(PA8, PA9, PA10, PB0), LowsideCurrentSense(1 V/A, PA5, PA6, PA7), MagneticSensorMT6701SSI(PA15), 7 pole pairs, 25 kHz centre-aligned PWM, sensor alignment stored in flash.
• Protection: act on nFAULT (the hardware variant retries OCP itself after 5 ms); NTC derating and a motor I²t limit; under-voltage below 6.0 V and over-voltage above 9 V; torque off on a fault with STS error bits 0 (voltage), 2 (overheat) and 5 (overload).
• What it cannot reproduce: the STS3215's 0.026 kg·m² reflected armature. A direct-drive head is far more responsive; test that in simulation first (below).

Measured gates

Final run of 2026-10-02, ./run.sh --board boards/knuckle with media, from report.json:

• The DRC warning is silk_over_copper: one stroke of D3's (the TVS's) front footprint silk crosses J3's NPTH alignment hole at (4.72, 14.96). The fab clips it.
• The bus sockets have no 3D body. KiCad 10.0.6 ships no model for the Molex 5267 series, so the renders show bare pads and fab/knuckle.step has no body for J1 and J2. cad/parts.py stands in their F.Fab outline (9.9 x 4.9 mm) at the catalogue row's 5.9 mm height.

Silkscreen

Positions are in mm from the top-left corner of the board's 35 x 35 mm square, +y down; the motor axis is at (17.5, 17.5).

• Front: KNUCKLE below J1 at (17.5, 8.4) and STS BUS above J2 at (17.5, 26.6), plus the designators that fit.
• Back (the motor-facing side, so readable only before assembly): the title KNUCKLE (1.2 mm, bold) and the lines "STS3215 drop-in", "head_yaw ID 32", "head_roll ID 33", "rev A" and "unfabricated", each placed where the silk stage found room among the parts; SWD at the Tag-Connect footprint.
• Not placed: the brief's STAT word at the LED (D4) and MOTOR word at the phase holes (J4) found no free spot. The phase holes run A, B, C from the bottom: PHASE_A at (31.3, 19.0) on the square pad, PHASE_B at (31.3, 15.0), PHASE_C at (31.3, 11.0). Which motor lead is A does not matter to FOC: the firmware's alignment finds the order.

Printed parts and fit

cad/parts.py (CadQuery 2.8, OCP, trimesh) builds four printed parts around the board's real STEP (fab/knuckle.step, solids named by reference) and the real STS3215 horn (STS3215_03a.step from SO-ARM100), and measures the fit on the solids. Frame: z = 0 is the motor's base (stator) face, +z toward the board. The motor itself (15 mm tall, Ø35, a Ø7 shaft stub protruding 1.2 mm) is the spec's (est.); see Check these before ordering.

All four STLs read back watertight (standoff 931.2 mm³, magnet holder 150.6, cover 4534.8, horn adapter 1030.3).

Previews (SVG drawn by the script's own hidden-line renderer, PNG by rsvg-convert):

• cad/exploded.svg / .png: horn adapter, motor proxy, magnet holder and magnet, standoff, board, cover;
• cad/section.svg / .png: a section through the axis, with the air gap, the cover top and the horn face marked.

The parts and assembly.step (with a motor proxy) are in cad/. The duck's head brackets must still be redrawn in the duck's own CAD for a Ø38 x 35 mm cylinder; that is not part of this project.

Check these before ordering

The board passes ERC, the netlist round trip, routing, DRC (errors, unconnected and parity), DFM and the fab gate in KiCad. It has not been fabricated, assembled or powered, and it has no firmware.

Mechanics: measure one GBM2804H-100T first

• Base hole pattern and thread. The slots take M2 or M3 screws on the 16 mm and 19 mm pairs (r = 8.0 and 9.5) and on a 12 mm square (r = 8.49). They do not take a 12 mm-diameter pattern (r = 6.0). The thread is M2 per iFlight's GM2804 answer; HobbyKing users report tapping M3. cad/parts.py takes the pattern from MOTOR["base_pairs"].
• Shaft stub, base face and diameter. The magnet holder assumes a Ø7 shaft protruding 1.2 mm (an unlicensed CAD parameter, cross-check only). That number sets the holder, and through it the standoff height and the 1.000 mm air gap; the 0.5-2.0 mm window tolerates about ±0.5 mm of error. Also check that the base face is flat where the standoff bosses sit, and the Ø35 the cover skirt assumes.
• Rotor-top holes for the horn adapter: "12 mm" centre to centre in two vendor listings, four holes on Ø16 in another model. The adapter's slots cover Ø12 to Ø16.98.
• Phase leads. The solder holes are 0.7 mm drills on 1.7 mm pads at 4 mm pitch at the +X rim, where the cover's lead notch is. Check the leads' size, length and exit angle.
• Header height. The cover's openings and roof use 5.9 mm from the catalogue row for C45581067, not from Molex's drawing.

Bus and firmware

• Logic-analyser capture of a real STS3215 chain before writing the bus layer: SYNC_READ slot timing, the units of Return Delay Time, Torque Enable at power-on, whether a Goal Position write enables torque, status packets at Response Status Level 0 and 1. Dump a real servo's registers 0-86 for the defaults.
• Test in Open_Duck_Playground first. Change head_yaw and head_roll to a direct-drive model (about zero armature, 0.03 N·m saturation) and check that the walking policy still holds before flying the board on the robot.
• Connector pin order. 1 DATA, 2 VBAT, 3 GND is the Waveshare adapter's order. Check it against a Feetech cable: a cable wired otherwise puts 8.4 V on DATA, and the 74LVC buffers are 5.5 V tolerant.
• Chain order. The VBAT class assumes the Knuckles sit at the far end of the neck/head branch. An STS3215 downstream would pass its 2.5 A stall current through J1 and J2: above the 2.0 A at the sockets' thermal spokes at 0.5 oz, inside the Mini-SPOX's 3 A.
• SWD only before assembly. The Tag-Connect footprint is on the motor-facing face at r = 13.8 mm, between the standoff and the cover skirt; once mounted, the motor covers it. Flash before mounting, or plan a bootloader over the STS bus. IDs are set over the bus afterwards with the duck's configure_motor.py.

Fab and assembly

• Via in pad. One 0.5 / 0.25 mm GND via at the board centre lies inside both the MCU's exposed pad (top) and the MT6701's (bottom), and one lies in the DRV8311's. Order them filled and capped, or expect solder to wick.
• Stackup and inner copper. The gerber job lists 35 um copper on all six layers and 0.274 mm dielectrics: KiCad's default stackup, not a fab's. The ratings above give 0.5 oz and 1 oz inner copper; choose when ordering.
• Two-sided assembly. 20 of the 41 SMD parts in the CPL are on the bottom: the MT6701 and its 100 nF, the NTC, both small diodes, 10 of the 13 resistors and 5 more capacitors. Quote it as double-sided.
• Stock (live lookups of 2026-10-02; re-check): T521B226M025ATE100 1799, STM32G431KBU6 2114, MT6701QT-STD 5361, DRV8311HRRWR 6703. Every part number in fab/knuckle_bom.csv comes from the pinned table; the Knuckle rows are dated 2026-10-02.

Thermal and electrical (estimates, not measured)

• DRV8311. The spec estimates at most 0.76 W in the driver at an 8.4 V stall (conduction 0.10-0.19 W, switching 0.02-0.09 W, quiescent 0.13 W, AVDD 0.35 W at 68 mA; this board's AVDD share is about 0.28 W), about 40-45 C above ambient inside a cover. Its exposed pad has one via into the In1 plane. Measure the case at stall. If it runs hot, the AP63203 buck takes the AVDD share off (about 18 C less, spec).
• The motor is the limit. 5.2 W at a 7.4 V stall and 6.7 W at 8.4 V in a 39 g motor; the firmware needs an I²t limit, and the VM copper at the driver (0.81 A) has almost no margin over a full 8.4 V stall (0.79 A).
• AO3401A gate. VGS is -8.4 V on a full pack, inside the part's common ±12 V rating (not read from its datasheet). A surge the TVS clamps at up to 19.9 V exceeds it; the spec's optional 10 V gate-source zener is not fitted.
• No fuse. The pass-through is unfused, like the STS3215 it replaces.

Licences

• Open Duck Mini (apirrone/Open_Duck_Mini, Apache-2.0): its MJCF, BOM, wiring diagram and docs were read for the spec. Nothing from it is in this repository.
• Open_Duck_Mini_Runtime and Open_Duck_Playground (apirrone): no licence declared. Read only; this README cites facts from them (IDs, registers, the 50 Hz loop), and no code was copied.
• STS3215 STEP (TheRobotStudio SO-ARM100, STEP/SO100/STS3215_03a.step, Apache-2.0): cad/parts.py reads it from the research folder to measure the horn. It is not copied here.
• rustypot (Apache-2.0), lerobot (Apache-2.0), FTServo_Python (MIT), Rhoban/bam (Apache-2.0): read; protocol facts and identified parameters cited. pypot (GPL-3.0): read only, register addresses cited as facts.
• Datasheets (TI SLVSFN2B DRV8311, MagnTek MT6701 Rev 1.8, Waveshare bus-adapter schematics) and the ST motor profile: facts cited, nothing redistributed.
• Knuckle's own brief, blocks, CAD script and this README are the repository's own.

Files

Committed:

• board.json: the brief the pipeline built everything here from.
• cad/parts.py: the printed parts and the fit check.
• README.md: this file. The blocks are blocks/knuckle_*.json (plus the shared gpio_led), and the new part numbers are in parts/lcsc_pinned.json.

Regenerated by the commands below, under build/knuckle/ (not committed):

• report.json: the run's gate report, the source of every gate number above.
• knuckle.kicad_pro, knuckle.kicad_sch, knuckle.kicad_pcb (routed and poured).
• fab/knuckle_gerbers.zip (14 layer and job files, the drill file and the drill map), fab/knuckle_bom.csv (24 rows, 43 parts), fab/knuckle_cpl_jlc.csv (41 SMD parts, rotations corrected), fab/knuckle_pos.csv, fab/knuckle_schematic.pdf, fab/knuckle.step.
• render/: KiCad raytraced frames: 0062_place.png (placed, in 3D), 0105_3d.png (top), 0106_3d.png (bottom) and the turntable 0107_3d.png to 0167_3d.png.
• video/knuckle_design_process.mp4: the design-process video (113.2 s, 169 frames).
• cad/: standoff, magnet_holder, cover, horn_adapter (.step and .stl), assembly.step, fit.json, exploded.svg/png, section.svg/png.

Reproduce

From the heypcb-kicad repository:

./run.sh --board boards/knuckle              # every stage, with the 3D renders and the video
./run.sh --board boards/knuckle --no-media   # stop before the 3D frames, renders and video
python cad/source/parts.py   # the printed parts and fit.json (a Python with CadQuery and trimesh)

parts.py needs the fab stage's STEP and refuses to read a board mid-run. It finds the STS3215 STEP in the research folder beside the repository, or takes --sts-step.

Placement is fully locked in the brief (45 parts, each with its reason), and Freerouting is given a Specctra file whose order is set by content alone, so the brief routes the same way every time. On 2026-10-02 it was built in full with media twice: once with In2 poured GND and the earlier class notes, and once as committed. Both routed 121 -> 0 open with the same copper (589 tracks, 55 vias, 729.7 mm, the same per-layer lengths); only the solve times differed. The copper, pour and pad figures in this README were measured on the second run's knuckle.kicad_pcb.

License

Hardware: CERN-OHL-S v2. Files under cad/ that derive from third-party CAD carry their own notice there.