Reachy 2021 Orbita Head Stuck: Diagnosing and Fixing a Mechanical Joint Jam

ByPriya Nair

Won’t move with the power off — that tells you something

When a robot joint locks up under power, the problem could be a driver fault, a stalled motor, or a software issue. When it won’t move with the power completely off, those explanations disappear. Something physical is in the way.

On the Reachy 2021, the head is driven by Orbita — a parallel mechanism Pollen Robotics designed in-house. Three Maxon DC motors independently actuate three disk assemblies, and together those disks tilt a ball joint to point the head. It’s a compact mechanism, and compact means a seized bearing or a wedged gear tooth has nowhere to hide. Any one jam locks the whole head.

What Orbita actually consists of

Each of Orbita’s three motor-disk assemblies contains a cylindrical disk with an integrated gear and internal bearings, driven by a small DC motor through a pinion. The disks roll against each other and collectively tilt the ball joint. A magnetic encoder on each motor tracks position for closed-loop control.

The parallel structure is the whole point — it makes Orbita stiffer and more accurate than a serial chain of joints would be. The tradeoff is that servicing one motor means working around the other two. You can’t just swap one component in isolation without understanding how it sits in the kinematic chain.

Start without tools: manual arm movement

Before opening anything, try moving each Orbita arm manually from its base. Grip the arm close to where it attaches at the bottom and apply controlled lateral force in the direction of the stuck movement. Pollen Robotics’ support documentation recommends this as the first diagnostic step — it can break a light jam loose.

Apply more force than feels instinctive. These are metal linkage components, not printed plastic. If one arm frees up and the others don’t, that tells you which disk assembly to focus on. If nothing moves at all regardless of force, the jam is deep inside the housing and manual coaxing won’t fix it.

Four things that cause a hard mechanical block

Seized bearing in a disk assembly

The disk assemblies each run on internal bearings. These can corrode or dry out after extended use or storage in varying humidity. A seized bearing locks one disk completely, which blocks the entire parallel mechanism. This is the most likely cause when the joint won’t yield to manual force at all.

Jammed gear or pinion

The disk gear and motor pinion mesh in a tight space. A chip of material, debris, or worn brush residue from the Maxon motor can lodge between the teeth. A gear jam tends to produce a hard stop in one rotation direction but some give in the other — consistent with a head that refuses to turn right but has limited movement left.

Cable or wiring snag

Motor and encoder cables route through the Orbita housing. Prolonged face-tracking can drive the head through repeated angular cycles that slowly wind or pinch a cable against an internal wall. A cable snag usually feels like increasing resistance rather than a sudden hard block, but a fully pinched cable can stop movement entirely.

Displaced or bent connecting rod

The rods linking the disk assemblies to the ball joint are small. A hard impact — if the robot was stopped by an external obstacle mid-tracking, or tipped — can displace or slightly bend one. Worth checking visually as soon as the housing is open, before pulling anything else apart.

Approaching disassembly

Orbita is open source hardware. Pollen Robotics publishes CAD context through their orbita3d_control GitHub repository, which includes kinematics and assembly information useful for understanding how the layers go together before you start removing fasteners. Their I-wrist-bypass repository also includes a bill of materials and assembly guide for an Orbita3D variant.

Three things to do before you open it:

  • Note the exact disk positions. Orbita’s encoders use the startup position as an absolute reference. Reassemble with the disks misaligned and the head’s zero will be wrong.
  • Take photos of the gear mesh at each stage. The orientation of each disk-to-pinion contact matters on reassembly.
  • Check the Maxon motor brushes while you’re in there. Hours of face-tracking adds up, and worn brushes shed residue that accelerates gear wear.

If the disassembly feels uncertain, Pollen Robotics maintains active support on their community Discord and forum. For a hardware fault at this level, a direct support ticket is worth sending — some replacement components can be sourced through them.

Recalibration after reassembly

Once the mechanism is back together, start the robot with the head close to the straight-ahead position. The Reachy 2021 FAQ is explicit about this: Orbita’s position encoder covers only part of the full motion range, and the power-on position sets the absolute reference. Start crooked, and face tracking will drift from the first movement.

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