Parallel Ankle Mechanism

An ankle joint where two motors, through linkages, jointly drive both pitch and roll instead of each controlling one axis alone.

The parallel ankle mechanism is a common ankle design in humanoid and biped robots: instead of one motor controlling pitch (the toes tipping up and down) and another controlling roll (the sole tilting side to side), two motors push and pull the foot plate together through linkages, forming a closed kinematic chain. The advantage is that the motors can be moved up the shin closer to the knee, making the end of the leg lighter and reducing the swing-leg inertia, and the two motors can combine their torque output. The tradeoff is that the relationship between motor angle and ankle angle becomes nonlinearly coupled, requiring the mechanism's own forward and inverse kinematics to convert between them, with torque also converted through a Jacobian; since URDF can only describe tree structures, simulations often simplify it into two independent series pitch and roll joints instead. Unitree's G1 SDK offers exactly this choice: PR mode issues commands as if the pitch and roll joints were in series, while AB mode directly controls the two parallel motors.

ExampleIn a typical left-right symmetric two-link ankle: when the two motors turn in the same direction, the foot pitches; when they turn in opposite directions, the foot rolls; any other combination of motor angles is a blend of the two.

Also called

Closed-Chain Ankle, Parallel Ankle

Related

Parallel Mechanism · Serial Mechanism · Unified Robot Description Format · Unitree G1 · Bipedal Robot · Reflected Inertia

Sources

unitree_sdk2 example: g1_ankle_swing_example.cpp（PR: Series Control for Pitch/Roll Joints; AB: Parallel Control for A/B Joints）
Wikipedia: Parallel manipulator