Technical Specifications

Overview

This page provides comprehensive technical specifications for the Asimov v0 bipedal leg platform, including joint ranges of motion, peak torque values, motor models, and physical parameters.

All angle measurements are provided in degrees for readability. The MuJoCo simulation model uses radians internally.

System Configuration

Total Degrees of Freedom

12 DOF - 6 per leg (Hip: 3, Knee: 1, Ankle: 2)

Pelvis Height

739 mm - From ground to pelvis link in neutral standing position

Total System Mass

~11.5 kg - Both legs including all actuators and structure

Manufacturing

MJF 3D Printing - All structural components optimized for additive manufacturing

Left Leg Joint Specifications

The left leg provides six degrees of freedom with the following ranges of motion and torque capabilities:

Joint Name	Function	Min Angle	Max Angle	Range	Peak Torque	Motor Model
L_Hip_Pitch	Hip flexion/extension	-120°	+57°	177°	120 Nm	EC-A6416-P2-25
L_Hip_Roll	Hip abduction/adduction	-45°	+45°	90°	90 Nm	EC-A5013-H17-100
L_Hip_Yaw	Hip rotation	-45°	+45°	90°	60 Nm	EC-A3814-H14-107
L_Knee_Pitch	Knee flexion/extension	0°	+86°	86°	75 Nm	EC-A4315-P2-36
L_Ankle_A	Ankle dorsi/plantarflexion	-70°	+70°	140°	36 Nm	EC-A4310-P2-36
L_Ankle_B	Ankle inversion/eversion	-70°	+70°	140°	36 Nm	EC-A4310-P2-36

Left Leg Joint Details

Hip Pitch (L_Hip_Pitch)

Axis: Oblique axis (0, 0.70711, -0.70711) - 45° angled joint
Range: -120° to +57° (-2.094 to 1.0 rad)
Function: Primary forward/backward leg swing
Armature: 0.01 kg⋅m²

Hip Roll (L_Hip_Roll)

Axis: X-axis (1, 0, 0)
Range: ±45° (±0.785 rad)
Function: Lateral leg movement for balance and side-stepping

Hip Yaw (L_Hip_Yaw)

Axis: Negative Z-axis (0, 0, -1)
Range: ±45° (±0.785 rad)
Function: Leg rotation for turning and directional changes

Knee Pitch (L_Knee_Pitch)

Axis: Y-axis (0, 1, 0)
Range: 0° to +86° (0 to 1.5 rad)
Function: Leg bending for walking gait and crouching

Ankle A & B (RSU Mechanism)

Type: Coupled revolute joints controlling pitch and roll
Range: ±70° each in motor space
Actual ankle motion: Kinematically coupled through parallel linkages
See Ankle Mechanism for coupling equations

Right Leg Joint Specifications

The right leg is a mirror of the left leg with corresponding ranges and capabilities:

Joint Name	Function	Min Angle	Max Angle	Range	Peak Torque	Motor Model
R_Hip_Pitch	Hip flexion/extension	-57°	+120°	177°	120 Nm	EC-A6416-P2-25
R_Hip_Roll	Hip abduction/adduction	-45°	+45°	90°	90 Nm	EC-A5013-H17-100
R_Hip_Yaw	Hip rotation	-45°	+45°	90°	60 Nm	EC-A3814-H14-107
R_Knee_Pitch	Knee flexion/extension	-86°	0°	86°	75 Nm	EC-A4315-P2-36
R_Ankle_A	Ankle dorsi/plantarflexion	-70°	+70°	140°	36 Nm	EC-A4310-P2-36
R_Ankle_B	Ankle inversion/eversion	-70°	+70°	140°	36 Nm	EC-A4310-P2-36

Right Leg Joint Details

Hip Pitch (R_Hip_Pitch)

Axis: Oblique axis (0, -0.70711, -0.70711) - Mirrored 45° angle
Range: -57° to +120° (-1.0 to 2.094 rad)
Note: Range is inverted compared to left leg due to mirrored geometry

Hip Roll (R_Hip_Roll)

Axis: X-axis (1, 0, 0)
Range: ±45° (±0.785 rad)
Identical to left leg

Hip Yaw (R_Hip_Yaw)

Axis: Negative Z-axis (0, 0, -1)
Range: ±45° (±0.785 rad)
Identical to left leg

Knee Pitch (R_Knee_Pitch)

Axis: Negative Y-axis (0, -1, 0) - Mirrored
Range: -86° to 0° (-1.5 to 0 rad)
Note: Negative range due to mirrored joint orientation

Ankle A & B (RSU Mechanism)

Same specifications as left leg
Coupling equations apply identically to both legs

Motor Specifications

Asimov v0 uses high-performance quasi-direct drive motors from Encos:

Motor Models by Joint Type

Hip Pitch Motor - EC-A6416-P2-25

Application: Both L_Hip_Pitch and R_Hip_Pitch
Peak Torque: 120 Nm
Usage: Highest torque motor in the system for primary leg swing
Quantity: 2 (one per leg)

Hip Roll Motor - EC-A5013-H17-100

Application: Both L_Hip_Roll and R_Hip_Roll
Peak Torque: 90 Nm
Usage: Lateral balance and side-to-side movement
Quantity: 2 (one per leg)

Hip Yaw Motor - EC-A3814-H14-107

Application: Both L_Hip_Yaw and R_Hip_Yaw
Peak Torque: 60 Nm
Usage: Rotational movements and turning
Quantity: 2 (one per leg)

Knee Pitch Motor - EC-A4315-P2-36

Application: Both L_Knee_Pitch and R_Knee_Pitch
Peak Torque: 75 Nm
Usage: Leg bending for walking and stance control
Quantity: 2 (one per leg)

Ankle Motors - EC-A4310-P2-36

Application: All ankle joints (L_Ankle_A, L_Ankle_B, R_Ankle_A, R_Ankle_B)
Peak Torque: 36 Nm each
Usage: Combined to provide ankle pitch and roll through RSU mechanism
Quantity: 4 (two per leg)
Note: Same motor model used for both A and B positions

Total Motor Count

Motor Model	Quantity	Application
EC-A6416-P2-25	2	Hip Pitch (left + right)
EC-A5013-H17-100	2	Hip Roll (left + right)
EC-A3814-H14-107	2	Hip Yaw (left + right)
EC-A4315-P2-36	2	Knee Pitch (left + right)
EC-A4310-P2-36	4	Ankle A & B (2 per leg)
Total	12	6 DOF per leg

Ankle Mechanism

The Revolute Spherical Universal (RSU) ankle is a key innovation in Asimov v0, using a coupled linkage mechanism instead of direct drive.

Mechanism Overview

The ankle uses two motors (A and B) connected via parallel linkages to a bar. By controlling motors A and B, the system achieves both pitch (dorsi/plantarflexion) and roll (inversion/eversion) movements.

Forward Kinematics: Pitch/Roll → Motor A/B

To convert desired ankle pitch (θ_p) and roll (θ_r) to motor positions:

// Using small angle approximation
theta_A =  K_P * theta_p - K_R * theta_r
theta_B = -K_P * theta_p - K_R * theta_r

// Where:
// K_P = d / r  (pitch coupling constant)
// K_R = c / (2 * r)  (roll coupling constant)
// d = distance from pivot point to bar
// c = length of the coupling bar
// r = radius of motor mount (r_A = r_B)

Example Code:

float right_A = ANKLE_COUPLING_K_PITCH * right_pitch - ANKLE_COUPLING_K_ROLL * right_roll;
float right_B = -ANKLE_COUPLING_K_PITCH * right_pitch - ANKLE_COUPLING_K_ROLL * right_roll;

float left_A = ANKLE_COUPLING_K_PITCH * left_pitch - ANKLE_COUPLING_K_ROLL * left_roll;
float left_B = -ANKLE_COUPLING_K_PITCH * left_pitch - ANKLE_COUPLING_K_ROLL * left_roll;

Due to the mirrored rotation axis on the pitch ankle and asymmetrical linkages, the PR → AB equations are the same for both legs.

Inverse Kinematics: Motor A/B → Pitch/Roll

To read actual ankle pitch and roll from motor positions:

// Direct calculation (not an inverse of forward kinematics)
theta_p = (theta_A - theta_B) * (r / (2 * d))
theta_r = -(theta_A + theta_B) * (r / c)

// Alternatively:
theta_p = (theta_A - theta_B) / (2.0 * K_PITCH)
theta_r = -(theta_A + theta_B) / (2.0 * K_ROLL)

Example Code:

float right_pitch = (right_A - right_B) / (2.0f * K_PITCH);
float right_roll  = -(right_A + right_B) / (2.0f * K_ROLL);

float left_pitch = (left_A - left_B) / (2.0f * K_PITCH);
float left_roll  = -(left_A + left_B) / (2.0f * K_ROLL);

Mechanism Advantages

Reduced Distal Weight

Motors are positioned higher on the leg, reducing rotational inertia at the ankle.

High Torque Capacity

Mechanical advantage provides strong torque output for both pitch and roll.

Compact Design

Achieves 2-DOF ankle control in a smaller package than dual direct-drive design.

Force Distribution

Load is shared across parallel linkages, improving durability.

Articulated Toe

Each foot includes a passive articulated toe joint with the following specifications:

Toe Joint Parameters

Parameter	Left Toe	Right Toe
Joint Type	Passive revolute hinge	Passive revolute hinge
Range	-60° to 0° (-1.047 to 0 rad)	0° to +60° (0 to 1.047 rad)
Stiffness	4.5 Nm/rad	4.5 Nm/rad
Damping	0.5 Nm⋅s/rad	0.5 Nm⋅s/rad
Spring Reference	0° (neutral)	0° (neutral)
Friction Loss	0.02 Nm	0.02 Nm
Mass	0.112 kg	0.112 kg

Toe Benefits

The articulated toe acts as passive compliance during the toe-off phase of walking, improving energy efficiency and enabling more natural gait patterns similar to human walking.

Energy Return: Spring stores and releases energy during push-off
Natural Gait: Allows heel-strike to toe-off transition
Improved Stability: Additional ground contact point during stance phase
Passive Compliance: No actuation required, reducing system complexity

Physical Dimensions

Key dimensional parameters extracted from the MuJoCo model:

Leg Segment Lengths

Segment	Length (mm)	Description
Hip to Knee	247.7	Combined hip linkage (hip offset + upper leg)
Knee to Ankle	294.7	Lower leg length
Ankle to Ground	~50	Foot height including toe
Toe Length	~65	From toe joint to tip
Total Leg Length	~592	Hip joint to ground (approx.)

Hip Configuration

Hip Width: 113.1 mm (center-to-center between left and right hip pitch joints)
Hip Offset: 56.6 mm (lateral offset from pelvis centerline)
Hip Vertical Offset: 105.4 mm (pelvis to hip pitch joint)

Foot Dimensions

Foot Length: ~145 mm (heel to toe tip)
Foot Width: ~70 mm (at widest point)
Ground Clearance: 22 mm (sole thickness)

Mass Distribution

Approximate mass breakdown per leg:

Component	Mass (kg)	Percentage
Hip Assembly	2.59	45%
Upper Leg (Thigh)	1.28	22%
Lower Leg (Shin)	2.25	39%
Ankle & Foot	1.16	20%
Total per Leg	~5.75	100%
Both Legs	~11.5	-

Mass values are approximate and based on the MuJoCo simulation model. Actual hardware masses may vary slightly depending on manufacturing tolerances and assembly.

Inertial Properties

Key inertial properties from the simulation model are available in the MuJoCo XML file. These include:

Center of mass positions for each link
Diagonal inertia tensors for all bodies
Armature values for joint dynamics (0.01 kg⋅m² for all actuated joints)

Accessing Full Inertial Data

The complete inertial properties are defined in the MuJoCo simulation model at:/sim-model/xmls/asimov.xmlEach body includes:

<inertial> tags with position, quaternion, mass, and diagonal inertia
Precise COM locations relative to parent frames
Full mass and inertia values for accurate dynamics simulation

Control Interface

Joint Naming Convention

All joints follow a consistent naming pattern:

[Side]_[Location]_[Type]_joint

Examples:

left_hip_pitch_joint
right_knee_joint
left_ankle_pitch_joint

Actuator Configuration

Actuators are configured programmatically using position control with PD gains. All damping comes from the actuator configuration (KD term), not from joint damping parameters.

Key points:

Control Type: Position control (built-in position actuators)
Joint Friction: 0 (removed to prevent double-damping)
Joint Damping: 0 for motors (all damping via actuator KD)
Armature: 0.01 kg⋅m² for all actuated joints

Manufacturing Notes

3D Printing Specifications

Technology

Multi Jet Fusion (MJF)Nylon PA12 material optimized for strength and durability

Post-Processing

Standard MJF finishing:

Bead blasting
Optional dyeing
No support removal needed

Assembly Considerations

Standard metric fasteners throughout
Press-fit bearings in several locations
Motor mounting uses standard bolt patterns
Cable routing channels integrated into printed parts

Simulation Model

The MuJoCo simulation model provides:

Full kinematic chain with accurate joint placement
Collision geometries using capsules for efficiency
Visual meshes (STL files) for realistic rendering
Contact exclusions between adjacent links
Sensor suite: IMU (gyro, accelerometer, orientation), contact sensors

View 3D Model

Interactive 3D visualization of the complete assembly

Reference Resources

GitHub Repository

Complete source files, CAD models, and simulation code

Mechanical Design

CAD files, assembly instructions, and manufacturing details

Ankle Mechanism Details

Complete mathematical derivation of RSU ankle kinematics

Simulation Model

MuJoCo XML files and assets for physics simulation

Overview

Hardware

Simulation

Resources

Documentation Index

​Overview

​System Configuration

Total Degrees of Freedom

Pelvis Height

Total System Mass

Manufacturing

​Left Leg Joint Specifications

​Hip Pitch (L_Hip_Pitch)

​Hip Roll (L_Hip_Roll)

​Hip Yaw (L_Hip_Yaw)

​Knee Pitch (L_Knee_Pitch)

​Ankle A & B (RSU Mechanism)

​Right Leg Joint Specifications

​Hip Pitch (R_Hip_Pitch)

​Hip Roll (R_Hip_Roll)

​Hip Yaw (R_Hip_Yaw)

​Knee Pitch (R_Knee_Pitch)

​Ankle A & B (RSU Mechanism)

​Motor Specifications

​Motor Models by Joint Type

​Total Motor Count

​Ankle Mechanism

​Mechanism Overview

​Forward Kinematics: Pitch/Roll → Motor A/B

​Inverse Kinematics: Motor A/B → Pitch/Roll

​Mechanism Advantages

Reduced Distal Weight

High Torque Capacity

Compact Design

Force Distribution

​Articulated Toe

​Toe Joint Parameters

​Toe Benefits

​Physical Dimensions

​Leg Segment Lengths

​Hip Configuration

​Foot Dimensions

​Mass Distribution

​Inertial Properties

​Control Interface

​Joint Naming Convention

​Actuator Configuration

​Manufacturing Notes

​3D Printing Specifications

Technology

Post-Processing

​Assembly Considerations

​Simulation Model

View 3D Model

​Reference Resources

GitHub Repository

Mechanical Design

Ankle Mechanism Details

Simulation Model

Build docs developers (and LLMs) love

Overview

System Configuration

Left Leg Joint Specifications

Hip Pitch (L_Hip_Pitch)

Hip Roll (L_Hip_Roll)

Hip Yaw (L_Hip_Yaw)

Knee Pitch (L_Knee_Pitch)

Ankle A & B (RSU Mechanism)

Right Leg Joint Specifications

Hip Pitch (R_Hip_Pitch)

Hip Roll (R_Hip_Roll)

Hip Yaw (R_Hip_Yaw)

Knee Pitch (R_Knee_Pitch)

Ankle A & B (RSU Mechanism)

Motor Specifications

Motor Models by Joint Type

Total Motor Count

Ankle Mechanism

Mechanism Overview

Forward Kinematics: Pitch/Roll → Motor A/B

Inverse Kinematics: Motor A/B → Pitch/Roll

Mechanism Advantages

Articulated Toe

Toe Joint Parameters

Toe Benefits

Physical Dimensions

Leg Segment Lengths

Hip Configuration

Foot Dimensions

Mass Distribution

Inertial Properties

Control Interface

Joint Naming Convention

Actuator Configuration

Manufacturing Notes

3D Printing Specifications

Assembly Considerations

Simulation Model

Reference Resources