Rotary vs. Linear Actuators

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The Big Debate: Rotary vs. Linear Actuators in Humanoid Robotics The critical design choice between rotary and linear actuators in humanoid robotics shapes: – Performance characteristics, – Manufacturing constraints, – Which robots will win or lose Here’s a deep dive into @GoingBallistic5 & @MarwaEldiwiny's podcast:

INSIDE A ROTARY ACTUATOR: Rotary actuators directly generate rotation that can be applied to robot joints. Think of electric motors that create turning motion. Rotary actuators: Generate rotation directly → apply to the joint

INSIDE A LINEAR ACTUATOR: Linear actuators convert rotation into straight-line motion, then back to rotation for joints. They use mechanisms like planetary roller screws, Ball screws or lead screws. Linear actuators: Generate rotation → convert to linear motion → convert back to rotation at the joint through linkages

Let's look at the trade-offs for humanoids in a few key areas:

STRENGTH: • Rotary: Consistent, predictable torque throughout motion - what you ask for is what you get • Linear: Freakishly strong at sweet spots (Teslabot's knee can lift a grand piano!) but non-linear torque curve with diminishing effectiveness at range extremes Winner: It depends. Linear for peak power, rotary for consistent performance

SUPPLY CHAIN DYNAMICS: • China's established drone motor manufacturing creates rotary actuator ecosystem advantages • Precision planetary roller screws require specialized manufacturing capabilities with limited suppliers for linear systems Winner - Rotary. Manufacturing infrastructure creates path dependency in design decisions

VELOCITY: • Rotary: Capable of human-equivalent walking/running speeds with direct power transmission • Linear: Fundamentally limited by conversion inefficiencies and mechanical constraints Winner: Rotary No linear-actuated humanoid has demonstrated walking speeds above 1 mph

PRECISION & CONTROL: • Rotary: Uniform precision across movement range with linear control relationships • Linear: Variable precision with poor control authority at motion extremes Winner: Rotary. Non-linear behaviour of linear actuators requires complex compensation

RANGE OF MOTION: • Rotary: Can spin completely around if needed • Linear: Limited to about 135° in practice (vs. human knees at 150°+) & hits "deadlocks" at extremes where they cannot generate force Winner: Rotary.

MECHANICAL COMPLEXITY: • Rotary: Integrated design with minimal articulation points • Linear: Multiple joints, bearings, and linkages Winner: Rotary.

PACKAGING CONSIDERATIONS: • Rotary: Concentrated mass at joint locations with potential form factor challenges • Linear: Distributed along limb length, competing with electronics and power systems for internal volume Winner: It depends. You can also use remote actuation to hide motors in "meaty" parts (forearm/calf), connect to joints via rods

THE VERDICT: • Rotary actuators are gaining dominance due to simplicity and advantages across the board • Linear actuators still has their place, especially where strength is required The best strategy to make a joint-by-joint assessment considering the specific goals & requirements of your robot

Check out the full podcast here. @GoingBallistic5 @MarwaEldiwiny youtube.com/watch?v=8WwZzZcPvwM&t=4602s