The Complete Guide to Actuator Selection for Humanoid Robotics

From Micro Actuators to Full-Scale Joint Systems

Designing a humanoid robot is an exercise in managing compromise. Every design choice sits on a sliding scale: Torque vs. Weight, Speed vs. Precision, and Power vs. Heat.

In traditional industrial robotics, these compromises are solved with massive stationary power supplies and heavy cast-iron gearboxes. Humanoid robotics does not have that luxury. The most consequential decision in the design process is selecting the correct actuator for each joint—finding the "sweet spot" where biological form meets electromechanical function.

This guide details the engineering framework for selecting actuators that solve the "Packing Problem" while delivering human-scale performance.

1. How Actuators Drive Humanoid Robots

Industrial robots rely on high-RPM rotary motors and harmonic drives. While precise, they are heavy and rigid. Human-scale robots require compliance, compactness, and distributed actuation.

The shift is moving toward linear micro-actuation, specifically using the Tendon-Driven Configuration. By utilizing FIRGELLI Micro Pen Actuators (Series FA-BS16), engineers can decouple the motor from the joint.

• Stationary Actuator Placement: Motors are mounted in the "forearm" or "calf," not the joint itself.

• Inertia Reduction: This removes weight from the distal (moving) end of the limb, allowing for faster acceleration with less power.

• Bio-Mimetic Transmission: Force is transmitted via Bowden cables or linkages, mimicking the biological flexor/extensor tendon system.

2. Force, Speed, and Control Tradeoffs

Physics imposes a strict tradeoff: Force and Speed are inversely related. You cannot optimize for both without drastically increasing motor size (and weight).

Using the FIRGELLI FA-BS16 performance data as a reference, we can categorize actuators by biological function:

Engineering Rule: Always size your actuators for the Peak Torque requirement of the joint, not the average load. It is better to have overhead than to stall a motor and trigger a thermal cutout.

3. Calculating Joint Torque Requirements

To select the right micro-actuator, you must translate "Force" (Linear) into "Torque" (Rotational).

Formula: $\tau = F_{act} \times d$

Design Example: Robotic Index Finger

• Scenario: You need a pinch force of roughly 7N at the fingertip. Through the mechanical linkage, this requires a tendon pull of 50N.

• Actuator Selection: 50N Model (11 lbs).

• Lever Arm ($d$): The tendon attaches 15mm (0.015m) from the knuckle pivot.

• Calculation: $50N \times 0.015m = \mathbf{0.75 Nm}$

Result: A 0.75 Nm torque output matches human-scale gripping requirements for handling tools, electronics, or household objects.

4. Why Micro Actuators Solve the "Packing Problem"

The human arm is a marvel of packaging, containing dozens of muscles within 2–3 liters of volume. Standard "box" servos are difficult to pack densely because of their protruding motors and gear trains.

FIRGELLI micro actuators solve this via Inline Geometry:

• Diameter: 16 mm (0.63").

• Stackability: The cylindrical shape allows them to be bundled together like muscle fibers. You can fit a cluster of 4-5 actuators (controlling an entire hand) inside a chassis no wider than a human wrist.

• Weight: With units weighing between 49g and 81g, a full 5-finger actuation bank weighs less than 350g—lighter than many single industrial grippers.

5. Proven Humanoid Design Patterns

A. Forearm Topology (Remote Actuation)

• Design: Mount a bank of 5 actuators near the elbow.

• Routing: Run Bowden cables (ptfe-lined housings) through the wrist to the fingers.

• Benefit: Keeps the hand lightweight, reducing the torque load on the shoulder and elbow motors.

B. Feedback-Driven Gripping (Haptic Intelligence)

• Logic: Drive the actuator until the Hall Sensor pulses stop changing (indicating the finger has hit an object), but the Current rises (indicating force is being applied).

• Action: The controller detects this state and automatically switches from "Position Mode" to "Force Maintenance Mode," holding the object securely without crushing it.

6. The Future of Actuation

Actuators are evolving from "dumb" movement sources into intelligent edge devices.

With the integration of high-resolution Hall Effect sensors (up to 1143 pulses per inch) and precise current-force linearity, engineers can now design robots that "feel" their environment.

Final Thought:

Humanoid robotics will not be won by better AI code alone. It will be won by the teams that master human-scale actuation—building bodies that are as capable and adaptable as the minds controlling them.