Actuators

Actuators make robots move. Learn electric motors (servo, stepper, BLDC), gear reducers (planetary, harmonic, cycloidal), hydraulic and pneumatic actuators, sizing torque/speed/inertia ratio, and emerging soft and series-elastic actuators.

Every joint needs an actuator that delivers the right torque at the right speed with enough precision. Most industrial robots use AC servo motors with high-ratio gearboxes; heavy machines use hydraulics; grippers often use pneumatics.

Actuator choice drives cost, weight, precision, speed and safety of the whole robot.

An analogy

Actuators are the robot's muscles; gearboxes are its tendons and bones trading speed for strength.

Electric motors

AC servo (permanent-magnet synchronous) motors with encoders: precise, dynamic — industrial robots. Brushless DC (BLDC): drones, mobile robots, cobots. Steppers: cheap open-loop positioning (3D printers) but lose steps under overload. DC brushed motors: simple, low cost.

Reducers

Motors are fast and weak; joints need slow and strong. Planetary gearboxes (efficient, some backlash), harmonic (strain-wave) drives (100:1+ in one stage, near-zero backlash — robot wrists, cobots), cycloidal/RV reducers (high stiffness and shock capacity — robot bases), ball screws and belts for linear axes.

Fluid power

Hydraulics: highest force density (excavators, heavy manipulators, Atlas humanoid prototypes) but leaks and noise. Pneumatics: cheap, fast, compliant — ideal for grippers and simple pick-and-place, hard to position precisely.

Sizing and new actuator types

Size motor for peak and RMS torque, maximum speed, and inertia ratio (large mismatch causes poor servo response). Quasi-direct-drive (low ratio, high-torque motors) enables backdrivable, force-sensitive legged robots; series-elastic actuators add a spring for safe interaction and force control; soft pneumatic actuators and artificial muscles enable delicate grasping.

Formulas

• Torque through a gearbox: τ_m = τ_L / (N · η)
• Speed: ω_m = N · ω_L
• Reflected inertia: J_reflected = J_L / N²
• Inertia ratio: J_L/N² : J_m ideally < 5–10 : 1
• Motor current: I = τ_m / K_t

Worked example: Motor for the shoulder joint

Given: Joint torque 20.6 N·m, speed 60°/s (1.047 rad/s); Harmonic drive N = 100, η = 0.8

Find: Motor torque, speed and power.

• τ_m = 20.6/(100 × 0.8) = 0.257 N·m
• ω_m = 104.7 rad/s = 1000 rpm
• P = 20.6 × 1.047 = 21.6 W at the joint

Answer: ≈ 0.26 N·m at 1000 rpm — a ~100 W servo with margin

Worked example: Inertia ratio

Given: J_L = 0.587 kg·m², N = 100; Motor J_m = 1.5 × 10⁻⁵ kg·m²

Find: Reflected inertia and ratio.

• J_ref = 0.587/10 000 = 5.87 × 10⁻⁵
• Ratio = 5.87/1.5 = 3.9 : 1 ✓

Answer: ≈ 3.9 : 1 — good servo tuning expected

Common mistakes

• Wrong: Choosing a motor on torque only. — Right: Check speed, RMS torque, inertia ratio, and thermal limits.
• Wrong: Ignoring gearbox backlash for precise tasks. — Right: Use harmonic/cycloidal reducers or preloaded designs.

Applications

• Industrial arms: AC servos + RV reducers (base) + harmonic drives (wrist).
• Cobots & humanoids: Integrated joint modules with torque sensing; quasi-direct drives for legs.
• Grippers: Pneumatic, electric servo and soft vacuum grippers.

Key takeaways

• Servos + reducers dominate robot joints.
• τ_m = τ_L/(Nη); ω_m = Nω_L; J_ref = J_L/N².
• Keep inertia ratio modest for good control.
• Hydraulics for force, pneumatics for simple grippers.

Going further

• Torque sensing: Joint torque sensors enable compliant control and collision detection in cobots.
• Thermal modelling: Motor winding temperature limits continuous torque; RMS sizing prevents overheating.
• Direct-drive motors: Eliminate gearboxes for ultra-high precision (wafer handling), at higher cost and size.