Free Humanoid Centaur — an open hardware, open firmware, open data quadruped-base humanoid reference design. Third morphology in the OpenIE humanoid family alongside the bipedal Free Humanoid Platform and the wheeled Free Humanoid Wheeled. Designed for the shoal dock-A wetlands service mission and the rough-terrain mobile-manipulation deployment band more broadly. Shielded by the Free Humanoid Corpus.
A ~120 kg rough-terrain humanoid with a 12-DoF wheel-leg-hybrid quadrupedal base and the same upper body as the bipedal Free Humanoid Platform. Expressed as a single OpenLoco UDD descriptor that compiles to URDF, MJCF, STL meshes, BOM, and assembly through the existing OpenLoco toolchain. Every non-trivial design choice cites a specific entry in the Free Humanoid Corpus by id.
Phase 1 status (2026-05-07): descriptor v0.1.0 vendor-pinned to commercial-off-the-shelf components (T-Motor AK80-9 leg actuators × 12 + AK60-6 wheel-foot actuators × 4 driven by mjbots Moteus n1 controllers; Livox MID-360 solid-state lidar; u-blox ZED-F9P RTK GPS; VectorNav VN-100 IMUs; ATI Mini45 6-axis F/T sensors per foot; NVIDIA Jetson Orin Nano + Raspberry Pi 4 safety supervisor; 48V/30Ah Li-ion baseline pack with Phase 2 split to hot-swap modules). Quadruped-base leg + wheel-foot BOM is ~$16-46k depending on production-vs-budget alternates. The wetland-substrate IP-rating is the centaur’s distinguishing constraint vs. the bipedal/wheeled siblings — Phase 1 ratings are manufacturer claims, to be verified before deployment. Phase 2 work begins with the first leg subassembly + wheel-foot prototype + IP67 sealing characterization on wetland-relevant duty cycles. Full quadruped-base prototype Phase 3, shoal dock-A wetlands open-site trial Phase 4. Every non-trivial design choice cites a specific entry in the Free Humanoid Corpus by id, with dstar-zarrouk-2026, amphistar-zarrouk-2023, rstar-zarrouk-2019, star-fearing-2013, and caltech-leonardo-2021 as the wetland-and-amphibious lineage anchors recently added (round-10) that resolve previously-italicised citations on this morphology.
This document is the canonical full-system spec for the centaur morphology. It cross-references — and does not duplicate — the bipedal sibling’s ARCHITECTURE.md for everything inherited (upper body, sensing, compute, safety, manipulation, learning policy), the wheeled sibling’s ARCHITECTURE.md for the shoal cartridge-swap and hot-swap battery patterns, and the submersible sibling’s ARCHITECTURE.md for the dock-A subsurface complement (the centaur services dock-A surface; the submersible services dock-A subsurface). Decisions that diverge from any sibling are argued in full here.
Six principles, the same six as the wheeled sibling with the load-bearing one (“mission-specific, not general-purpose”) sharpened for this morphology’s wider design domain.
Open everything. Hardware, firmware, descriptors, BOM, control software, simulation models, training data, documentation. CERN-OHL-S 2.0 / Apache 2.0 / CC-BY-SA 4.0 / CC0 1.0 by artifact class.
Inherit the upper body verbatim. The 21-DoF upper body of the bipedal sibling — torso, arms, hands, head — is taken unchanged. Composition at the descriptor level: replace the bipedal legs block (or the wheeled sibling’s wheeled_base + tower_stand block) with this morphology’s quadruped_base block. The upper body, the sensing additions (with one outdoor-rugged supplement), the compute, the safety supervisor architecture, the manipulation stack, and the learning policy above the locomotion layer are all unchanged. This is the OpenLoco substrate-and-tenant pattern in action: a third morphology with no upper-body fork.
Mission-specific, not general-purpose. Like its siblings, the centaur is shaped to a specific mission band — rough-terrain mobile manipulation in environments wheeled platforms cannot reach but where a full-bipedal humanoid is operationally fragile. The shoal dock-A wetlands deployment is the v0.1 target; disaster response, agriculture, and civil infrastructure are the broader band.
Prior-art-shielded by deep quadruped chain. The wheeled-base subsystem of the wheeled sibling is shielded by the wheeled-mobile-manipulation lineage; the quadruped-base subsystem of this morphology is shielded by the deep quadruped chain back to the 1968 GE Walking Truck. Every load-bearing decision shields back to a corpus entry. The centaur’s quadruped chain is talos-bronze-giant (-250 BCE legged guardian) → ge-walking-truck (1968) → raibert-hopping-1leg (1983) → mcgeer-passive-walker (1990) → mit-cheetah (2009) → cheetah-cub-epfl (2013) → mit-cheetah-2 (2014) → anymal (2016) → hyundai-boston-dynamics-spot (2015) → ghost-robotics-vision-60 (2018) → mini-cheetah (2019) → anymal-d-eth-rsl-2022 (2022) plus gundam-rx-78-2 and mazinger-z for the piloted leg-base composite-mecha fictional anchors and centaurs-greek-myth-equivalent quadruped+humanoid composite anchors via Talos.
Fail-safe to inert. Inherited verbatim from both siblings. Every subsystem has a defined fail-safe state. On the quadruped base specifically, the fail-safe is “drop into a four-leg tripod stance with brakes latched and wheel-feet locked, freeze upper body in tucked posture, kill drive rails.” Slope-tilt and foot-fall-validation invariants are added to the bipedal sibling’s safety supervisor invariant set.
Mode switching is a hard transition, not a blend. The wheel-leg hybrid feet operate in one of two modes at a time — wheel mode (legs in stance, wheel-feet driving) or leg mode (wheel-feet locked, legs articulating). Mode transitions go through a defined intermediate “stand-still” state with both wheels and legs locked. The supervisor refuses any motion command that would attempt a blended mode. This is shielded by anymal-wl and ghost-robotics-vision-60, both of which document hard-mode-switch architectures rather than continuous blending.
Everything above the quadruped base. Specifically:
| Subsystem | Inherited section in bipedal ARCHITECTURE | What we take |
|---|---|---|
| Torso, arms, hands, head | §2.2 Kinematic topology, §2.4 Actuators | 21-DoF upper body topology: 1-DoF waist + 2-DoF neck + 2× 7-DoF arms + 2× 1-DoF underactuated synergy hands. Inherited verbatim. |
| Sensing | §2.5 Sensing | IMU at every major link, force-torque at each wrist, GelSight-style tactile at fingertips, head stereo camera. Plus quadruped-specific additions in §2.4 below (per-leg IMU, per-foot force-torque, outdoor lidar, GPS, JANUS-receiver). |
| Compute | §2.6 Compute | Joule SOM as recommended-default, with abstract HAL boundary. Verbatim. |
| Safety supervisor | §2.7 Safety supervisor | MathGround Simplex pattern. Verbatim, plus quadruped-specific invariants in §2.6 below (slope limit 30°, foot-fall validation, balance-margin hard-stop, leg-collision avoidance, mode-transition gate). |
| Manipulation | §2.9 Manipulation | Underactuated 5-finger hand. Verbatim. |
| Learning policy (above locomotion) | §2.10 Learning policy | Whole-body MPC + IL manipulation. Verbatim above the locomotion layer. The locomotion layer itself is replaced with dual-mode quadruped locomotion (§2.8 below). |
| Comms | §2.11 Comms | ROS 2, CAN-FD, gRPC, WebRTC. Verbatim, plus 5G + JANUS-receiver additions matching the wheeled sibling. |
| End-of-life | §2.13 End-of-life | Same posture. |
| Governance | README §Governance, CONTRIBUTING.md | Verbatim. |
The descriptor in descriptor/free-humanoid-centaur.udd.json inherits the bipedal links/joints/actuator_slots for the upper body and replaces the leg blocks with the four-leg quadruped-base blocks.
The wheeled sibling is not the upper-body donor — that’s the bipedal — but it is the source for two narrower commitments that ride on top of the inherited upper body:
| Subsystem | Inherited from wheeled ARCHITECTURE | What we take |
|---|---|---|
| Hot-swap battery commons interface | §2.10 Power | The CC0 hot-swap mating geometry; centaur uses three modules (1 kWh each) instead of the wheeled’s two. |
| shoal cartridge-swap protocol surface | §2.9 Mission integration with shoal | The cartridge-rotation protocol stack; centaur targets dock-A (wetlands) rather than dock-B (urban) but the protocol is the same. |
| Outdoor sensing additions (lidar, GPS, 5G, JANUS-receiver) | §2.4 Sensing additions | Same kit, with weather-rating bumped to IP67 for wetland deployment. |
The quadruped base is sourced from OpenLoco’s existing quadruped-morphology disclosures, specifically:
The corpus is morphology-agnostic; this morphology cites a different overlapping subset than its siblings. The new corpus chain leans heavily on the quadruped academic and industrial anchors:
| Corpus id | Year | What it gives us |
|---|---|---|
talos-bronze-giant |
-250 BCE | The deepest fictional / mythological anchor in the corpus for a legged guardian humanoid. The Talos myth disclosed an autonomous leg-based humanoid 2200+ years before any modern academic patent claim could possibly anticipate it. For a quadruped+humanoid composite, the Talos anchor is read for its “legged + autonomous + service-deployed” properties. |
ge-walking-truck |
1968 | General Electric’s hydraulic Walking Truck: 58-year prior art on legged loadbearing vehicles. Foundational for any legged-platform-with-payload claim. |
raibert-hopping-1leg |
1983 | Marc Raibert’s one-leg hopper: foundational dynamic-balance disclosure. 43-year prior art. |
mcgeer-passive-walker |
1990 | Tad McGeer’s passive-dynamic walker: energy-efficient passive-stable walking. |
mit-cheetah |
2009 | MIT Cheetah 1: the QDD-quadruped lineage origin. |
cheetah-cub-epfl |
2013 | EPFL Cheetah-Cub: open-source bio-inspired quadruped with CPG control. The open-source quadruped anchor. |
mit-cheetah-2 |
2014 | MIT Cheetah 2: foundational QDD architecture publication. |
hyundai-boston-dynamics-spot |
2015 | Boston Dynamics Spot: commercial quadruped, hot-swap battery, IP-rated outdoor deployment. |
anymal |
2016 | ETH ANYbotics ANYmal: industrial quadruped lineage origin. |
ghost-robotics-vision-60 |
2018 | Ghost Robotics Vision 60: commercial wheel-leg hybrid (recent variant). |
mini-cheetah |
2019 | MIT Mini-Cheetah: open QDD reference, the most-cited modern QDD anchor. |
anymal-d-eth-rsl-2022 |
2022 | ETH/ANYbotics ANYmal-D, sim2real era. |
hwangbo-anymal-sim2real |
2019 | Sim-to-real RL for quadruped locomotion; anchors the leg-mode learned-policy choice. |
dlr-justin |
2009 | Wheeled-mobile-manipulation pattern (also cited by wheeled sibling). |
spot-fuel-cell |
2024 | Spot fuel-cell variant — shielding for any extended-endurance power architecture if added later. |
gundam-rx-78-2 |
1979 | RX-78-2 Gundam: 47-year fictional anchor for piloted humanoid leg-base mecha; reads as composite-form prior art. |
mazinger-z |
1972 | Mazinger Z: 54-year fictional anchor for piloted humanoid mecha with leg-form locomotion. |
star-fearing-2013 |
2013 | Berkeley STAR (Sprawl Tuned Autonomous Robot): wheg-foot lineage origin. The earliest in-corpus disclosure of a curved-spoke “wheel-leg” mechanism that handles both rolling and stepping locomotion modes — exactly the wheel-leg-hybrid foot’s mechanical principle, at small scale. Round-10 corpus addition. |
rstar-zarrouk-2019 |
2019 | Zarrouk lab Rising-STAR (RSTAR): adaptive sprawl-mechanism quadruped with terrain-reconfigurable foot geometry. Establishes the academic chain for outdoor terrain-adaptive sprawl. Round-10 corpus addition. |
amphistar-zarrouk-2023 |
2023 | Zarrouk lab AmphiSTAR: amphibious sprawl-mechanism quadruped, the canonical academic anchor for amphibious wetland-substrate legged locomotion. Most-relevant corpus entry for the centaur’s wetland-substrate IP-rated wheel-leg-hybrid foot. Round-10 corpus addition. |
dstar-zarrouk-2026 |
2026 | Zarrouk lab DSTAR (Decoupled-FBEM STAR): decoupled-foot-block-end-mechanism sprawl quadruped, the most recent academic disclosure in the STAR lineage. Documents wetland-similar terrain deployment with a decoupled foot mechanism. The closest published academic prior art to the centaur’s wheel-leg-hybrid wetland foot. Round-10 corpus addition. |
caltech-leonardo-2021 |
2021 | Caltech LEONARDO: bipedal-quadruped composite robot with leg-thruster hybrid locomotion; relevant for the composite-mode-switching architectural pattern even though the modes are different (the centaur composes wheel-mode + leg-mode rather than thruster + leg). Round-8 corpus entry, re-cited here as a composite-mode anchor. |
The fictional/mythological chain is genuinely deep here — talos-bronze-giant provides a 2200+-year-deep anchor that the bipedal and wheeled siblings cannot match in the same way. For a centaur-class composite (humanoid torso + quadruped base), the closest existing fictional anchors in the corpus are the piloted-mecha lineage (gundam-rx-78-2, mazinger-z); a dedicated centaurs-greek-myth corpus entry is a recommended future addition flagged in docs/.
The wetland-substrate prior-art chain is the centaur-distinguishing addition over the wheeled and bipedal siblings: star-fearing-2013 → rstar-zarrouk-2019 → amphistar-zarrouk-2023 → dstar-zarrouk-2026 is a 13-year academic chain documenting curved-spoke / decoupled-foot legged mobility on amphibious / wetland substrates, terminating in a 2026 disclosure of a mechanism with explicit wetland-similar terrain deployment. This chain shields the centaur’s wheel-leg-hybrid wetland foot at strength comparable to the anymal-wl shielding for the wheel-leg architecture itself.
The mission-integration spec for dock-A is taken directly from shoal ARCHITECTURE:
| shoal section | What the centaur inherits |
|---|---|
| §2.2 The gut cartridge | Cartridge mechanical / fluid / electrical interface spec, used as the manipulation target for cartridge-rotation tasks (same as wheeled sibling) |
| §3.2 The dock-A (autonomous benthic) | The dock-A topology — autonomous benthic dock for remote rivers, marine, and wetlands. This is the centaur’s load-bearing deployment target, the wetlands counterpart to the wheeled sibling’s dock-B. |
| §4 The protocols | The dock-burst optical protocol; the platform receives status telemetry over this channel as a node on the dock LAN |
| §5 The cognitive layer | shoal-fleet’s cartridge-rotation scheduling; the platform takes work orders from this layer |
| §9 Safety and ethics | Indigenous and community consent for outdoor / public-space deployment; same posture |
| Quantity | Value | Notes / shielding |
|---|---|---|
| Total system mass | ~120 kg | 60 kg quadruped base (4 legs × ~12 kg per leg + chassis + battery) + 60 kg upper body (the bipedal sibling’s 50 kg + 10 kg for the more rugged outdoor-rated sensing additions and IP67 enclosures). Shielded by the corpus quadruped class: anymal-d-eth-rsl-2022 ~50 kg robot dog alone (centaur is heavier because of upper body), hyundai-boston-dynamics-spot ~33 kg base, bigdog and alphadog 70–150 kg load-carrying class. The 120 kg target is the mass at which 50 kg payload at extended reach is statically stable on the four-leg support polygon. |
| Shoulder height (head-up posture, leg mode) | ~1.50 m | Comparable to a small horse at withers. Shoulders sit ~1.50 m above ground when the legs are extended in standing posture; head sits ~1.85 m above ground. In low-stance / wheel mode the shoulders drop to ~1.10 m. |
| Total length (nose to tail) | ~1.20 m | Footprint length; the upper-body waist is forward of center on the chassis. |
| Total width (across leg stance) | ~0.80 m | Stance width; clears most outdoor pedestrian gates and wide-doorway industrial entries. |
| Sustained payload at extended reach | 50 kg | Possible because (a) four-leg support polygon is large, (b) low-mounted battery distributed across the chassis lowers the COM, (c) wheel-mode allows static manipulation with all four wheels braked. Shielded by bigdog and alphadog (load-carrying class quadrupeds, 70–150 kg payload demonstrated), and anymal-d-eth-rsl-2022 (10 kg per arm at extended reach precedent on a quadruped base). |
| Peak ground speed (wheel mode) | 2.0 m/s | Paved-surface travel with all four wheel-feet driving. Below the 2.5 m/s threshold widely used for shared-space mobile-robot operation in service environments. Shielded by anymal-wl (~2 m/s wheel-mode demonstrated), ghost-robotics-vision-60. |
| Peak ground speed (leg mode) | 1.0 m/s | Walking/trotting gait. Shielded by anymal, mini-cheetah, hyundai-boston-dynamics-spot. The bipedal sibling’s leg-mode top speed is the same; the centaur has no fundamental advantage over a biped here, just more lateral stability. |
| Maximum slope (leg mode) | 30° | Hard supervisor invariant. Below bigdog’s reported 35° rough-terrain limit. Shielded by bigdog, anymal-d-eth-rsl-2022, hyundai-boston-dynamics-spot. |
| Stair height (leg mode) | 220 mm | Standard residential stair tread. Shielded by anymal, hyundai-boston-dynamics-spot. |
Topology: 4 legs × 3 DoF each = 12 DoF in the quadruped base. Each leg has hip yaw, hip pitch, and knee pitch — the standard 3-DoF quadruped configuration shielded by anymal, mit-cheetah-2, mini-cheetah, hyundai-boston-dynamics-spot, ghost-robotics-vision-60. No ankle DoF; the wheel-foot rolls passively in leg mode via the wheel itself (the wheel provides the sole degree of foot-ground accommodation needed for walking on irregular surfaces, and is mechanically simpler than an articulated ankle).
Why 4 legs and not 6. Hexapod (6-leg) topologies offer redundancy and slow-static-stable gaits but at significant mass and complexity cost. Four legs is the minimum for static-stable stance during manipulation without active balancing, and is the entire commercial and academic standard in the modern era. Shielded by the entire mit-cheetah → anymal → spot lineage, none of which use 6 legs.
Wheel-leg-hybrid foot. Each leg ends in a small powered wheel (rather than a pad / rubber boot / passive ball). The wheel runs on its own QDD direct-drive motor (Phase 1: T-Motor AK60-6, ~24 N·m peak, with integrated brake). In wheel mode, the legs adopt a low-stance posture and the four wheel-feet drive the platform forward; the leg joints hold pose and the wheels carry the locomotion task. In leg mode, the wheel-feet are locked (electromagnetic brake or active velocity-zero hold) and the legs articulate to walk over terrain that defeats wheels — stairs, curbs, rocks, mud, vegetation. This is the anymal-wl architecture (recommended-future-corpus-addition; the canonical wheel-leg-hybrid academic reference), with ghost-robotics-vision-60 as the in-corpus commercial anchor. Wheel size: ~150 mm diameter (chosen for the smallest size that lifts a foot over a 60 mm obstacle while in wheel mode). Wheel tread: molded polyurethane with cleated profile per the star-fearing-2013 → rstar-zarrouk-2019 → amphistar-zarrouk-2023 → dstar-zarrouk-2026 wheg-and-decoupled-foot lineage, sized for wetland-substrate (mud, vegetation) grip rather than paved-mode rolling efficiency.
Mass distribution. Battery modules (3× 1 kWh) mounted low and centered in the chassis, between the leg attachment points. Compute and safety supervisor mounted high in the chassis, near the upper-body interface. The heavy elements stay over the support polygon’s center of pressure. Shielded by bigdog’s reported mass distribution and hyundai-boston-dynamics-spot’s demonstrated layout.
Stability margin under 50 kg payload at extended reach. With both arms forward at full extension carrying 50 kg, the COM excursion is ~0.30 m forward. With a 1.20 m × 0.80 m support polygon and the chassis mass distributed as above, the COM stays within the support polygon by a margin of ~0.30 m (i.e., the platform is stable with substantial margin). In wheel mode with the legs in stance, the support polygon is fixed and the margin is static-computable. In leg mode during a step, the support polygon shrinks to a triangle (3 feet down) and the supervisor’s balance-margin invariant (§2.6 below) hard-stops any motion that would push the COM outside the triangle.
Joint actuator type: cycloidal-class QDD at hip/knee with smaller QDD at the wheel-foot. Per architectural commitment #1 below, the leg joints use cycloidal-class quasi-direct-drive actuators with planetary reducers rather than pure-direct-drive QDD. Cycloidal-class at the hip/knee gives higher torque-density and longer actuator life under outdoor / wetland duty cycles where a pure-direct-drive QDD motor would suffer from heat dissipation. The wheel-foot uses smaller QDD direct-drive (corpus: mit-cheetah-2, mjbots-moteus) because the wheel sees lower continuous torque than the leg joints. Phase 1 vendor pinning: T-Motor AK80-9 (~144 N·m peak, ~$650 each, ~1.6 kg, manufacturer-claimed IP65; 12 units = $7,800) at the leg joints, T-Motor AK60-6 (~24 N·m peak, ~$300 each, integrated brake; 4 units = $1,200) at the wheel-feet, mjbots Moteus n1 controllers on a shared CAN-FD bus (16 controllers = ~$3,184). Phase 1 budget alternate at 1/3 scale: AK60-6 throughout (12 leg + 4 wheel = $4,800). Architectural-call resolution (cycloidal vs. harmonic-drive at the leg joints): cycloidal-class for v0.1 — shielded by sumitomo-cyclo (cycloidal reducer prior art) and the entire anymal industrial-quadruped lineage. Harmonic-drive variant held as a future descriptor variant for indoor / non-shock-loaded centaur applications.
Inherited from Free Humanoid Platform ARCHITECTURE §2.2 (kinematic topology), §2.4 (actuators), §2.5 (sensing), §2.9 (manipulation), §2.10 (learning policy). 21 DoF, hybrid actuator distribution (harmonic-drive at shoulder/elbow, tendon at wrist/hand, QDD at neck/waist), underactuated 5-finger hands, GelSight tactile fingertips, head stereo camera, IMU at every major link.
The corpus citations for the upper body are unchanged from the bipedal sibling and are recapitulated in prior-art/INDEX.md for indexing convenience — but the canonical statement of those citations is the bipedal sibling’s prior-art/INDEX.md.
The upper-body mass increases from the bipedal’s 50 kg to ~60 kg in this morphology, attributable to (a) IP67 environmental sealing on outdoor-deployed components, (b) the heavier outdoor-rated head-mounted lidar (16-channel rotating, vs. the bipedal sibling’s stereo-only head), (c) ruggedized cabling and connectors at the upper-body / quadruped-base interface.
In addition to the inherited bipedal sensing stack, the centaur morphology adds:
dlr-justin, pr2, the entire mobile-robot lidar lineage, and hyundai-boston-dynamics-spot for the outdoor IP-rated quadruped variant.anymal, mit-cheetah-2.anymal, hyundai-boston-dynamics-spot.Same Joule SOM commitment as the bipedal and wheeled siblings (see Free Humanoid Platform ARCHITECTURE §2.6). The compute distribution is unchanged: safety supervisor on a separate compute domain from the high-performance controller (Simplex pattern); perception runtime on its own compute domain. The leg-mode locomotion controller is heavier than the wheeled-mode differential-drive controller, and consumes more of the high-performance compute budget than the wheeled sibling does.
Inherits the bipedal sibling’s MathGround pattern verbatim, plus five quadruped-base-specific invariants:
Slope tilt limit (30° hard / 25° soft). Body pitch and roll are continuously monitored. Soft limit at 25°: the supervisor warns and limits motion-command authority. Hard limit at 30°: the supervisor latches all leg joints in current pose, latches all wheel-foot brakes, drives the upper body to a low-COM safe pose, and reports a slope violation upstream. Shielded by bigdog’s reported 35° rough-terrain limit and anymal-d-eth-rsl-2022’s reported tilt thresholds.
Foot-fall validation. During leg-mode swings, each foot’s expected touchdown is validated against the per-foot F/T sensor. If a foot reports < 5 N normal force after the planned touchdown time + 200 ms tolerance, the supervisor latches the next swing and forces a four-leg quadrupedal stance recovery. This is the “the ground was not where we expected it to be” invariant. Shielded by anymal, hyundai-boston-dynamics-spot, hwangbo-anymal-sim2real.
Balance-margin hard-stop. During leg-mode swings, the supervisor continuously computes the COM projection onto the support polygon (3 feet on the ground in single-leg-swing, 2 feet in trot). If the COM projection exits the support polygon by more than 50 mm in any direction, the supervisor latches the swing and forces a four-leg recovery stance. Shielded by raibert-hopping-1leg, mcgeer-passive-walker, the entire dynamic-balance literature.
Leg-collision avoidance. Each leg’s planned trajectory is continuously checked against the other three legs’ current poses. If a planned swing would cause leg-on-leg contact, the supervisor rejects the plan and forces a four-leg-stance pause. Shielded by anymal, mit-cheetah-2.
Mode-transition gate. Wheel-mode-to-leg-mode and leg-mode-to-wheel-mode transitions must pass through a defined intermediate “stand-still” state with both wheels and legs locked. The supervisor rejects any motion command that attempts a blended transition (wheels driving while legs articulate). This is the hard-mode-switch architectural commitment (#6 below) enforced as a runtime invariant. Shielded by anymal-wl, ghost-robotics-vision-60.
The shielding chain for the safety supervisor as a whole is unchanged from the bipedal sibling: asimov-positronic-robots (1940) → williamson-folded-hands (1947) → forbidden-planet-robby (1956) → magnus-robot-fighter (1963) → b-9-lost-in-space (1965) → hal-9000 (1968) → silent-running-drones (1972) → asimovs-zeroth-law (1985) → data-tng (1987) → robocop-1987 (1987) → sherman-simplex-architecture (1995) → reachability-analysis-safe-control (2005) → iso-10218-collaborative-robots (2006) → control-barrier-functions (2007) → runtime-assurance-rta (2010) → shielding-rl (2018), plus the quadruped-specific anchors bigdog, anymal, hyundai-boston-dynamics-spot, hwangbo-anymal-sim2real for the new invariants.
Inherited from Free Humanoid Platform ARCHITECTURE §2.9. 5-finger underactuated synergy-driven hand, GelSight tactile fingertips, force-torque at the wrist. Corpus: pisa-iit-softhand, shadow-dexterous-hand, dlr-hand-ii.
The shoal-mission-specific manipulation targets — cartridge handle, hopper handle, dock-A maintenance fixtures — are designed to be compatible with the underactuated 5-finger hand without specialized end-effector swaps. Same posture as the wheeled sibling; the centaur version of the cartridge-swap protocol uses the same hand spec.
The locomotion-control stack is the largest divergence from both siblings.
Dual-mode policy. The platform operates in one of two modes at a time:
anymal-wl, ghost-robotics-vision-60, the broad skid-steer literature.mit-cheetah-2, mini-cheetah, anymal, hwangbo-anymal-sim2real, hyundai-boston-dynamics-spot.Hard mode-switch. The two modes do not blend. A mode-switch command sequences: (1) bring linear and angular velocity to zero in current mode; (2) latch all actuators in current pose; (3) verify body IMU pitch and roll are within tolerance (< 5°); (4) reconfigure leg / wheel-foot actuator roles for the new mode; (5) release brakes for the new mode and accept motion commands. The supervisor’s mode-transition gate (§2.6 #5) enforces this sequence as a hard invariant. Shielded by anymal-wl, ghost-robotics-vision-60 — both of which document hard-mode-switch architectures.
Mode selection. Mode is selected by either (a) perception (terrain-classification on the lidar’s ground plane labels the surface as paved vs. unpaved; flat smooth surfaces select wheel mode, anything else selects leg mode); or (b) explicit operator override (mission-plan parameter or runtime command). The operator override is always available; perception-driven selection can be disabled. Shielded by anymal-wl’s reported terrain-classification mode-selection.
High level: behavior tree (via OpenLoco’s SkillGraphRuntime), with five primary modes:
Curbedge / cliff detection. Same pattern as the wheeled sibling, but extended for both modes. In wheel mode, an “edge ahead” signal latches all four wheel-foot brakes. In leg mode, an “edge ahead” signal forces the next foothold to be reselected onto the safe side of the edge.
This is the load-bearing original engineering for the centaur morphology. The protocol stack is largely the same as the wheeled sibling’s dock-B integration (cartridge-swap, waste-hopper retrieval, biology-reservoir handshake) but the operating environment is different:
shoal-fleet directly and from operator-side mission-planning tooling, not a municipal utility EAM.| Component | Spec | Notes |
|---|---|---|
| Primary battery | 3 kWh total, 48 V nominal | Three ~1 kWh modules, mounted low and centered in the chassis. 1.5× the wheeled sibling’s 2 kWh; the increased capacity offsets the leg-mode duty cycle’s higher continuous power draw. Estimated runtime: 4–6 hours of mixed mission depending on mode mix. |
| UPS | 0.3 kWh, 24 V nominal | Mounted high in the chassis, near compute. Dedicated to compute, sensors, and the safety supervisor; isolated from the actuator power bus. Holds the safety-supervisor invariant hot for ≥ 30 minutes after a primary-battery fault, sufficient to drive the platform to a safe stance and report. |
| Hot-swap interface | Inherited from free-humanoid-wheeled commitment #4 | Same CC0 commons spec contributed to OpenLoco UDD. Three-module geometry means hot-swap is single-module-at-a-time — two modules remain live during swap. |
| Shore-power umbilical | Optional at dock-A | dock-A is autonomous benthic — there is no shore-power umbilical at the dock itself. The centaur charges from a transport vehicle, a portable solar / generator setup, or a sister dock-B at a municipal facility before mission deployment. |
Shielded by hyundai-boston-dynamics-spot (hot-swap quadruped, IP-rated outdoor), spot-fuel-cell (extended endurance variant), and the wheeled sibling’s commitment chain.
Inherited from the wheeled sibling’s §2.11. On-robot CAN-FD; off-robot ROS 2, gRPC, WebRTC. WiFi 6 onboard. 5G modem for remote-site fleet coordination. JANUS acoustic-receiver for in-water shoal fish coordination. No additional comms primitives over the wheeled sibling.
Same posture as both siblings. See Free Humanoid Platform ARCHITECTURE §2.13.
Centaur-specific end-of-life concerns:
The single source of truth is descriptor/free-humanoid-centaur.udd.json. It is OpenLoco UDD-compliant. The OpenLoco compiler produces:
free-humanoid-centaur.urdf — ROS 2 / RViz / Gazebofree-humanoid-centaur.xml — MJCF for MuJoCo / MJXbom.csv — bill of materialsassembly.md — assembly instructionsTo compile:
cargo run -- validate /path/to/free-humanoid-centaur.udd.json
cargo run -- generate /path/to/free-humanoid-centaur.udd.json --all --output-dir out/
cargo run -- bake /path/to/free-humanoid-centaur.udd.json --output-dir out/meshes/
Naming choice for morphology_family. Two reasonable names exist for this morphology family:
humanoid_quadruped — descriptive, parallel to humanoid_wheeled and humanoid_bipedal. Tells you what’s there mechanically.centaur — short, evocative, pulls the mythological anchor forward.The descriptor uses humanoid_quadruped as the canonical morphology_family value (parallel naming with the siblings) and tags centaur as an alias in meta.alias. The repo and project name carry the evocative form (free-humanoid-centaur); the machine-readable form carries the descriptive name. This split is intentional: human-facing communication uses the mythological anchor, machine-facing tooling uses the parallel naming convention.
The descriptor structure:
meta: morphology_family: humanoid_quadruped, alias: centaur. corpus_anchors lists the quadruped-shielding chain.links: 13 base links — chassis, four *_hip_yaw_link, four *_thigh_link, four *_shank_link, four *_wheel_foot (16 leg links total + chassis = 17, but consolidated naming gives 13 unique link types) plus three battery_module_* — and 32 upper-body links inherited from the bipedal sibling.joints: 12 base leg joints (4 legs × {hip_yaw, hip_pitch, knee_pitch}) + 4 wheel-foot joints (continuous revolute, one per leg) + 3 battery-module fixed joints + 1 chassis-to-pelvis fixed joint, plus 33 upper-body joints inherited from the bipedal sibling, with the bipedal leg joints replaced by the chassis-to-pelvis fixed joint.actuator_slots: 12 leg-joint slots (cycloidal reducers + BLDC motors per architectural commitment #1) + 4 wheel-foot slots (QDD + Moteus controllers) + the bipedal sibling’s 19 upper-body slots = 35 total.modes: 6 modes — safe_pose (four-leg stance, all brakes latched, upper body tucked), wheel_mode_drive (legs in low-stance, wheels driving), leg_mode_walk (wheels locked, legs articulating, walk gait), leg_mode_trot (wheels locked, trot gait for faster leg-mode transit), manipulation_static (mode-agnostic; whichever mode entered the state, all locomotion actuators hold pose, upper body in manipulation), mode_transition (the intermediate stand-still state during a mode switch).The descriptor currently uses Phase-1-pinned mass / inertia estimates derived from T-Motor AK80-9 / AK60-6 datasheets, Livox MID-360 datasheet, VectorNav VN-100 datasheet, and reasonable structural estimates for the chassis monocoque. Iterating these to physical-build accuracy remains a load-bearing follow-up task for Phase 2, gated on the first-leg-subassembly fabrication outcome (T-Motor AK80-9 vendor pinning resolves the cycloidal vs. harmonic-drive architectural call; wheel-foot diameter at 150 mm is committed; per-leg battery distribution is deferred to Phase 2 hot-swap split per architectural commitment #5).
free-humanoid-centaur/
README.md
ARCHITECTURE.md (this document)
CONTRIBUTING.md
LICENSE-HARDWARE CERN-OHL-S 2.0
LICENSE-SOFTWARE Apache 2.0
LICENSE-DOCS CC-BY-SA 4.0
LICENSE-DATA CC0 1.0
descriptor/
free-humanoid-centaur.udd.json the source of truth
README.md
base/ quadruped-base subsystem
upper-body/ inherited from free-humanoid-platform
electronics/ PCB pointers, power tree, Joule SOM
firmware/ HAL, behaviors, MathGround, comms
control/ dual-mode locomotion (wheel + leg), manipulation policy
sim/ MuJoCo / Gazebo / Drake model pointers
docs/ additional design notes
prior-art/
INDEX.md corpus entries, by subsystem
Not covered by existing prior art and requiring genuine new disclosure:
The wheel-leg-hybrid foot at the centaur’s mass class. The biggest novel piece. anymal-wl documents the wheel-leg architecture at ~50 kg robot dog scale. The centaur is ~120 kg with a 50 kg payload at extended reach — the foot must support a ~170 kg loaded configuration on a single foot during a leg-mode swing recovery. Wheel selection, motor sizing, brake mechanism, IP67 sealing for wetland deployment, and tread for both paved-mode rolling and leg-mode mud-traction are joint engineering concerns. The mode-transition mechanical sequence (lock-wheel + release-leg-brake) needs a physical fixture that doesn’t exist in any open quadruped today.
The centaur quadruped-base UDD descriptor. OpenLoco supports quadruped morphologies (mit-cheetah, mini-cheetah, anymal, anymal-wl in its corpus); the humanoid_quadruped descriptor that also inherits a humanoid_bipedal upper body via descriptor-level composition is a new artifact. The composition pattern (substituting one base for another while keeping the upper body identical) is the third worked example after the wheeled sibling — and is the point where the substrate-and-tenant pattern’s value as a design-time pattern becomes load-bearing.
The wetland-traversal protocol. New to this morphology. The cross-product of (quadruped + outdoor IP-rated + wetland substrate + autonomous benthic dock) is not anticipated by any single corpus entry. Specifically: (a) sinkage modeling for wetland soils on the wheel-foot; (b) leg-mode foothold planning with substrate-strength uncertainty; (c) wetland-environment-specific safety invariants beyond the slope and edge invariants of the wheeled sibling (water-depth limit, vegetation-entanglement detection); (d) IP67 sealing across all leg joints with maintainability for field service.
The mode-transition runtime invariant + supervisor implementation. The hard mode-switch architectural commitment (§0 #6, §2.6 #5, §2.8 dual-mode policy) needs formal specification + verification in the MathGround framework. anymal-wl and ghost-robotics-vision-60 document the hard-switch pattern at the architecture level but their supervisor implementations are not open. The centaur’s open implementation is genuinely new.
The shoal dock-A integration. The wheeled sibling’s dock-B integration is the closest existing prior art. dock-A’s autonomous-benthic deployment is operationally different — no municipal infrastructure, no shore-power umbilical, work orders from shoal-fleet only. The integration surface is real engineering work shared between this repo and shoal.
Everything else is fork, port, or integration of existing open work, with corpus citations.
humanoid_quadruped morphology that also inherits a humanoid_bipedal upper body.Same posture as the bipedal and wheeled siblings. See Free Humanoid Platform ARCHITECTURE §7.
Centaur-morphology-specific:
bigdog-class quadrupeds have a documented military adoption history that the centaur does not inherit and explicitly disclaims.clean_fish ARCHITECTURE §9 and as both siblings. Wetland sites — riverbanks, marshes, marine shores, agricultural margins — are particularly likely to involve Indigenous land claims and rural-community consent processes. The community-consent protocol is a deployment-time gate, same as for shoal fish themselves.iso-10218-collaborative-robots and the broader mobile-robot consent-signaling literature.Same three reasons as the bipedal and wheeled siblings. See Free Humanoid Platform ARCHITECTURE §8. Trust, distribution, compounding.
The centaur-morphology-specific argument: the deployment domain — environmental remediation, disaster response, agricultural inspection, civil infrastructure — is dominated by public-good missions where closed-source physical AI is, more than for the wheeled sibling, fundamentally inappropriate. A municipal flood-response agency cannot deploy a closed centaur to inspect a flooded school; an agricultural-extension agency cannot deploy a closed centaur to monitor row crops on Indigenous land; a state DOT cannot deploy a closed centaur to inspect a bridge underside without public records of what the system is doing. The open-everything posture is the deployment vehicle that fits these missions; closed alternatives are operationally and politically excluded.
The ten load-bearing decisions for this morphology, resolved. Each commitment is shielded by the cited prior-art chain.
| # | Decision | Commitment | Shielding chain (corpus ids) |
|---|---|---|---|
| 1 | Base topology — 4-leg quadruped (not 6-leg hexapod or other) | Four legs × 3 DoF (hip yaw + hip pitch + knee pitch) = 12 DoF in the quadruped base. No ankle DoF; the wheel-foot provides the foot-ground accommodation directly. | mit-cheetah (2009), mit-cheetah-2 (2014), mini-cheetah (2019), cheetah-cub-epfl (2013), anymal (2016), anymal-d-eth-rsl-2022 (2022), hyundai-boston-dynamics-spot (2015), ghost-robotics-vision-60 (2018). The entire commercial-and-academic-modern-quadruped lineage uses 4 legs × 3 DoF; hexapod is not represented in modern dynamic quadrupeds. Shielded back to ge-walking-truck (1968, four-leg hydraulic loadbearing, foundational legged-vehicle prior art) and talos-bronze-giant (-250 BCE, mythological legged guardian) for the deeper anchor. |
| 2 | Wheel-leg hybrid foot | Each leg ends in a small powered wheel (~150 mm diameter, QDD direct-drive, electromagnetic brake for leg mode). Wheel mode for paved travel, leg mode for terrain. Mode is selected per platform (not per leg), hard-switched, never blended. | anymal-wl (the strongest published wheel-leg-hybrid reference), ghost-robotics-vision-60 (2018, commercial wheel-leg-hybrid). The commitment is the most novel piece of original engineering — the cross-product of wheel-leg-hybrid + 120 kg + 50 kg payload + IP67 wetland deployment — but the architectural pattern itself is shielded. |
| 3 | Mass class — ~120 kg / 50 kg payload at extended reach | 120 kg total (60 kg quadruped base + 60 kg upper body), 50 kg sustained payload at extended reach. Heavier than the wheeled sibling (legs are heavier than wheels, four legs heavier than two), well within the shielded payload class. | bigdog (70–110 kg loadbearing class), alphadog (~150 kg loadbearing class), hyundai-boston-dynamics-spot (~33 kg base + payload class), anymal-d-eth-rsl-2022 (~50 kg robot dog), ge-walking-truck (1400 kg historical bound, way above this morphology). The 120 kg target is the corpus median for a quadruped that carries a 50+ kg payload at extended reach. |
| 4 | Inherit upper body from bipedal sibling verbatim | 21-DoF upper body — 1-DoF waist + 2-DoF neck + 2× 7-DoF arms + 2× 1-DoF underactuated synergy hands. Hybrid actuator distribution: harmonic-drive at shoulder/elbow, tendon at wrist/hand, QDD at neck/waist. No deviations at v0.1. | All of the bipedal sibling’s §9 commitments #1, #4, #8, #10. The inheritance is governance-load-bearing: a divergence in the upper body would fork the descriptor and the corpus citations, which is exactly what the OpenLoco substrate-and-tenant pattern is designed to avoid. The wheeled sibling’s commitment #6 establishes the same inheritance posture; this morphology is the second worked example. |
| 5 | Hot-swap battery interface inherits from bipedal commons spec | 3 kWh primary (3× 1 kWh modules) + 0.3 kWh UPS. Hot-swap is single-module-at-a-time using the CC0 commons mating geometry from the bipedal sibling and the wheeled sibling. | The bipedal sibling’s §9 commitment #6 and the wheeled sibling’s §9 commitment #4. Plus hyundai-boston-dynamics-spot (commercial hot-swap quadruped), spot-fuel-cell (extended endurance variant). |
| 6 | Dual-mode locomotion — wheel mode + leg mode, hard-switch | Mode-switch passes through a defined “stand-still” intermediate state. The supervisor refuses any motion command that attempts a blended mode (wheels driving while legs articulate). Mode selection by perception (terrain-classification on the lidar’s ground plane) or operator override. | anymal-wl (the canonical hard-mode-switch wheel-leg-hybrid reference), ghost-robotics-vision-60 (commercial confirmation). The hard-switch architecture is the shielded pattern; continuous blending is not in the corpus and is therefore not the chosen architecture — choosing it would forfeit the shielding. |
| 7 | Outdoor sensing — stereo + GelSight + outdoor lidar + GPS + 5G + JANUS-receiver + per-leg IMU + per-foot F/T | Inherited bipedal sensing stack (head stereo, IMU at every link, force-torque at wrist, GelSight tactile fingertips) plus 16-channel rotating lidar (head-mounted, IP67), GPS, 5G modem, JANUS acoustic-receiver, per-leg IMU, per-foot 6-axis F/T, body tilt sensor. | Bipedal sibling’s §9 commitment #5 for the inherited stack; wheeled sibling’s §9 commitment #7 for the outdoor additions; anymal and mit-cheetah-2 for the per-leg / per-foot quadruped additions. |
| 8 | Mission integration with shoal dock-A as v0.1 deployment target | Designed against the shoal dock-A wetlands service mission as the v0.1 deployment target. Cartridge-swap, waste-hopper retrieval, and biology-reservoir handshake are first-class subsystems. The wetland-traversal protocol is novel original engineering. | shoal ARCHITECTURE §2.2 (cartridge), §3.2 (dock-A), §4 (protocols), §5 (cognitive layer). Plus the wheeled sibling’s §9 commitment #9 for the cartridge-swap and waste-hopper-retrieval surface (re-used). |
| 9 | Reference design vs. specific build | Reference design — the descriptor (free-humanoid-centaur.udd.json, CC0) is the canonical artifact. Specific physical builds with chosen vendor parts live in tenant repos. Same posture as both siblings. |
The bipedal sibling’s §9 commitment #9 and the wheeled sibling’s §9 commitment #10. Matches OpenLoco’s substrate-vs-tenant pattern. |
| 10 | Joule SOM as recommended-default with abstract HAL | Joule SOM as recommended-default compute SOM, with an abstract HAL boundary that allows alternate carriers (Jetson Orin, Pi 5 + Coral, Intel NUC) to swap in at the same physical interface. | Bipedal sibling’s §9 commitment #10. Joule SOM commitment is a recommendation, not a mandate; the abstract HAL is a hard architectural commitment. |
These commitments are this document’s defaults. Subsystem sections (§2) reflect them in detail. Future amendments require a corpus-citation update demonstrating that any new shielding chain is at least as deep as the chain it replaces.
Recommended-future-corpus-additions (called out in docs/):
anymal-wl — ANYbotics’ wheel-leg-hybrid ANYmal variant, the canonical academic wheel-leg-hybrid reference (the in-corpus commercial anchor ghost-robotics-vision-60 covers the commercial case; anymal-wl is the missing academic case)bigdog, alphadog — Boston Dynamics legged loadbearing class quadrupeds for the 70–150 kg payload chain (currently cited prose-side in §2.1 / §9 #3 without in-corpus entries)nabtesco — cycloidal-reducer industrial supplier for the actuator-class cite in §2.2centaurs-greek-myth — explicit mythological anchor for the centaur composite-form (humanoid torso + quadruped base); deeper-than-talos-bronze-giant for this specific morphologyt-motor-ak-series / CubeMars — vendor cite for the Phase-1-pinned leg actuatorslivox-mid-360 — solid-state lidar subclass cite for §2.4u-blox-zed-f9p, vectornav-vn-100 — sensor-vendor cites for §2.4nvidia-isaac-lab-2024 — sim-to-real successor to nvidia-isaac-gym-2021 for the leg-mode RL chainFree Humanoid Centaur — third morphology in the OpenIE humanoid family — designed for the shoal dock-A wetlands service mission and the rough-terrain mobile-manipulation deployment band — Phase 1 v0.1 — vendor-pinned descriptor + BOM, 2026-05-07.