Robot Actuators: Electric, Hydraulic & Pneumatic — The Ultimate Guide

An actuator is the thing that actually moves. Sensors perceive, controllers decide, structure holds it all together — but the actuator is where electrical or fluid power becomes mechanical work, and it is almost always the component that decides what your robot can and cannot physically do. Pick the wrong one and no amount of clever control will save you. Pick the right one and a mediocre controller still does useful work.

This guide is the long version. We'll go family by family — electric, hydraulic, pneumatic — then through the things that don't fit neatly in a box: series-elastic actuators (SEA), quasi-direct-drive (QDD), pneumatic muscles, shape-memory alloy (SMA), and piezo. For each, real numbers with units, real products you can buy, and opinions with reasons attached. The goal is that you finish able to size and select an actuator for a specific job, not just recite a textbook taxonomy.

The take: For 90% of robotics built in 2026, an electric BLDC motor plus a gearbox is the right answer — it's controllable, clean, efficient, and the supply chain is mature. Hydraulics win only when you need extreme force density in a small envelope and can tolerate the mess; pneumatics win only at the gripper, where cheap compliance and speed matter more than precision. The interesting frontier isn't a new energy source — it's how we arrange the electric motor: low gear ratios (QDD) and deliberate elasticity (SEA) are what make legged and contact-rich robots work.

Companion reading: servo motors, brushless DC motors, gearboxes (harmonic & cycloidal), and end-effectors & grippers.

Table of contents

1. Key takeaways
2. What an actuator actually is
3. The tradeoff space
4. Electric actuators
5. Hydraulic actuators
6. Pneumatic actuators
7. Linear actuators deep-dive
8. Series-elastic & variable-stiffness
9. Quasi-direct-drive (QDD)
10. Soft & novel actuators
11. Backdrivability, transparency & force control
12. Sizing & selecting an actuator
13. Comparison tables & cheat-sheet
14. Frequently asked questions

What an actuator actually is

Strip away the marketing and an actuator does one job: take stored power and produce a controlled force or torque over a displacement. The "controlled" part is what separates an actuator from a motor or a cylinder bought off a shelf. A bare BLDC motor is a transducer; bolt on a gearbox, an encoder, and a drive running field-oriented control and you have an actuator — a closed-loop force/position source you can command.

The muscle analogy, used carefully

Biology is a useful frame if you don't take it too far. Muscle is a linear, contractile, compliant actuator with absurd control resolution (motor units recruited progressively) and the ability to act as both motor and brake. It's also slow to respond chemically, can only pull (never push), and has terrible peak power compared to its continuous power.

Most engineered actuators invert that: rotary, can push and pull, fast, but stiff and with poor intrinsic energy storage. The whole story of SEA, QDD, and soft actuators is the field trying to claw back muscle's good properties — compliance, impact tolerance, force control — without giving up the electric motor's controllability.

The three families plus the frontier

Electric — electromagnetic torque from current in a magnetic field. Rotary by nature (BLDC, brushed DC, stepper, AC servo), made linear with screws, belts, or by literally unrolling the motor (linear motors). Dominates by sheer breadth.

Hydraulic — pressurized incompressible fluid (oil) pushes a piston. Enormous force density, high stiffness, but needs a power unit and plumbing.

Pneumatic — compressed air pushes a piston or inflates a structure. Cheap, fast, compliant, clean, but soft and hard to position precisely.

The frontier — series-elastic (a spring in series with an electric drive), variable-stiffness (a tunable spring), QDD (low-gear-ratio electric), and the genuinely different physics of McKibben muscles, SMA, piezo, and electroactive polymers.

Rule of thumb: if you can't name the energy source, the conversion mechanism, and the control variable (current? flow? pressure?), you don't yet understand the actuator well enough to size it.

The tradeoff space

There is no best actuator, only best-for-a-job. The job is defined by where it sits in a multi-axis tradeoff space. Get fluent in these axes and selection becomes mechanical.

The axes that matter

Power density (W/kg) — how much mechanical power per unit mass. Matters for anything that moves the actuator itself: legs, arms, drones, mobile robots. Hydraulic systems are heavy because of the power unit, but hydraulic actuators at the joint are light and powerful.

Force/torque density (N/kg, N·m/kg, or N/cm²) — peak force in a given size or mass. Hydraulic cylinders are the champions: a 50 mm bore cylinder at 21,000 kPa (210 bar) makes about 41 kN of push. No comparable-mass electric drive comes close.

Bandwidth (Hz) — how fast the actuator can change force/position. Piezo: kHz. Electric direct-drive: 100s of Hz. Geared electric: 10s of Hz at the output. Hydraulic: tens of Hz, valve-limited. Pneumatic: a few Hz for controlled motion because air is compressible.

Controllability — how precisely and linearly you can command output. Electric wins outright: torque is nearly proportional to current. Hydraulic is good with servo-valves. Pneumatic is poor mid-stroke.

Efficiency — electric drivetrains hit 85–95% wall-to-shaft. Hydraulic systems are 40–60% wall-to-work after pump, valve throttling, and leakage losses. Pneumatic is brutal: 10–20% wall-to-work once you count compressor inefficiency and expansion losses. Pneumatic air is the most expensive energy in the factory per joule delivered.

Backdrivability / transparency — can the load move the actuator? Critical for contact, safety, and force sensing. Set mostly by gear ratio and friction. Direct-drive and QDD are transparent; harmonic and worm drives are not.

Cost & supply chain — a NEMA 23 stepper is $25. A Harmonic Drive actuator module is $1,500–4,000. A servo-valve is $1,000–3,000. A custom hydraulic power unit is five figures before you've moved anything.

You can't max all of them

These axes trade against each other. Adding a gearbox multiplies torque density but destroys backdrivability and adds backlash. A servo-valve gives a hydraulic actuator bandwidth but costs more than the cylinder. A series spring buys you force control and impact tolerance at the direct cost of position bandwidth. Every actuator choice is a position in this space, and the art is knowing which axis your application actually cares about.

Electric actuators

If you're building a robot in 2026 and you don't have a specific reason to do otherwise, you're using electric actuators. They're clean, controllable, efficient, quiet enough, and supported by the deepest component ecosystem of any family.

Rotary: the BLDC + gearbox stack

The workhorse is a brushless DC (BLDC) or AC servo motor driven by field-oriented control, almost always followed by a gearbox. See the BLDC deep-dive and the servo-motor guide for the motor side; here we care about the actuator as a unit.

Why the gearbox? A typical 100–500 W BLDC motor wants to spin at 3,000–8,000 rpm and makes modest torque — tenths of a N·m to a couple of N·m continuous. A robot joint wants tens to hundreds of N·m at tens of rpm. The gearbox bridges that gap. Reduction N multiplies torque and divides speed (minus efficiency):

T_out = T_motor × N × η_gear
ω_out = ω_motor / N

Common choices:

• Planetary — 3:1 to ~100:1 per stage, 90–97% efficient, some backlash (arcmin-class), cheap and robust. Good general-purpose.
• Harmonic (strain-wave) — 30:1 to 160:1 single stage, near-zero backlash, compact, but ~70–90% efficient and not cheap. The default for arm joints where precision matters (used heavily by industrial-arm and cobot makers; Harmonic Drive LLC owns this space).
• Cycloidal — 30:1 to 200:1, high shock-load capacity, low backlash, good for high-torque base joints. Nabtesco RV series dominates heavy industrial arms.

Maxon's EC-series motors with GP gearheads, Kollmorgen frameless kits, and integrated modules from Harmonic Drive (FHA/SHA series) are the components you actually buy.

Linear: turning rotation into a push

Electric linear actuators take a rotary motor and convert with a screw or belt — covered in depth in the linear section below. For now: ball-screw for efficiency, lead-screw for cost and self-locking, belt for long fast strokes, linear motor for bandwidth.

Why electric wins by default

• Torque is proportional to current — clean, fast, linear control with a cheap current sensor.
• 85–95% efficiency means modest cooling and modest batteries.
• No fluids, no compressor, no leaks, no separate power unit.
• Encoders are cheap and precise; closed-loop position control is a solved problem.
• The supply chain is enormous, so prices keep falling and availability is good.

The honest weaknesses: peak force density trails hydraulics, and at very high continuous torque the motor gets thermally limited — copper losses scale with current², so doubling torque quadruples heating. That thermal wall, not the torque rating on the datasheet, is what kills electric actuators in real duty cycles.

Hydraulic actuators

Hydraulics are about force density and stiffness, full stop. When you need huge force in a small joint envelope and you can tolerate the supporting infrastructure, nothing else competes.

How the system is built

A hydraulic system is a system, not a part: an electric or combustion-driven pump pressurizes oil, an accumulator stores energy and smooths spikes, valves (especially servo-valves and proportional valves) meter flow to cylinders (linear) or hydraulic motors (rotary). A reservoir, filters, and a cooler round it out.

Working pressures are typically 5,000–35,000 kPa (50–350 bar), with mobile and aerospace systems pushing 21,000–35,000 kPa (210–350 bar). Cylinder force is just pressure times piston area:

F = P × A
A = π/4 × D²    (D = bore diameter)

Example: D = 50 mm, P = 21,000 kPa (21 MPa)
A = π/4 × (0.050 m)² = 1.96 × 10⁻³ m²
F = 21 × 10⁶ Pa × 1.96 × 10⁻³ m² ≈ 41,200 N ≈ 41 kN

41 kN from a 50 mm cylinder. Bosch Rexroth, Parker, Moog, and Eaton supply this world; Moog servo-valves are the reference for high-bandwidth force control.

Why Atlas used hydraulics — then dropped them

For years the Boston Dynamics Atlas humanoid was hydraulically actuated, and the reason was force density: hydraulic actuators let Atlas pack the peak joint torques needed for jumps, backflips, and recovery into a human-sized envelope. Hydraulic stiffness also gives crisp force control through good servo-valves.

But hydraulics on a legged robot are a nightmare to live with. They leak (Atlas videos famously showed fluid streaks), they're loud, the power unit and plumbing are heavy and inefficient, and maintenance is constant. In 2024 Boston Dynamics retired the hydraulic Atlas and revealed an all-electric Atlas. That's the headline event of this decade in actuation: once electric drives (QDD-style, see below) got close enough on force density, the operational advantages of electric — efficiency, cleanliness, controllability, no plumbing — won decisively. Agility Robotics' Digit was electric from the start for the same reasons.

When hydraulics still win

• Heavy construction and forestry robots, excavators-turned-autonomous, large manipulators.
• Anything needing >50 kN at a single joint in a tight envelope.
• High-stiffness force application (presses, test rigs).
• Situations where a combustion engine already provides the prime mover.

If your robot fits through a normal door and runs on batteries, you almost certainly don't want hydraulics in 2026.

Pneumatic actuators

Pneumatics trade precision for cheapness, speed, compliance, and cleanliness. That trade is exactly right at the gripper and exactly wrong almost everywhere else.

How it works and what's available

Compressed air at ~600–1,000 kPa (6–10 bar) from a shop compressor feeds cylinders, rotary actuators, grippers, and vacuum generators through solenoid or proportional valves. Festo and SMC are the dominant suppliers; a Festo DSNU round cylinder or an SMC MHZ2 parallel gripper is in tens of thousands of factory cells worldwide.

Force again is pressure times area, but the pressures are 10–50× lower than hydraulic, so a 32 mm bore cylinder at 600 kPa makes only about 480 N. You get speed and softness, not brute force.

Why pneumatics own end-of-arm tooling

Walk any factory and the grippers are mostly pneumatic. Reasons:

• Cheap compliance — air is a spring. A pneumatic gripper naturally accommodates part variation and won't crush a fragile part if you regulate pressure. Getting equivalent compliance from an electric gripper means force sensing and control loops.
• Speed — open/close cycles in tens of milliseconds. Pick-and-place loves this.
• Two-state simplicity — most grippers and clamps only need open/closed. Solenoid valve, done. No drive, no encoder, no tuning.
• Cleanliness & safety — no electrical sparking at the tool (good for ATEX/explosive environments), and exhausted air is clean.
• Vacuum — a Venturi vacuum generator off the same air supply handles suction-cup picking of boxes, sheets, and glass.

Where pneumatics fail

Mid-stroke position control. Air compresses, so a pneumatic cylinder is a poorly-damped spring-mass system that wants to slam to the endstops. You can servo-control pneumatics with proportional valves and good feedback, but it's finicky and rarely worth it versus an electric actuator. Energy efficiency is also terrible — 10–20% wall-to-work — making compressed air the most expensive utility per joule in most plants.

Use pneumatics for binary, fast, compliant, clean tasks at the tool. Don't ask them to hold a precise mid-stroke position.

Linear actuators deep-dive

Lots of robotics motion is linear — Cartesian gantries, presses, Z-axes, telescoping joints. The conversion mechanism dominates the actuator's character far more than the motor does.

Ball-screw

A ground screw with recirculating ball bearings between screw and nut. 80–95% efficient, high load capacity, long life, low friction. Because of low friction it's also backdrivable — gravity or load can spin it — which means a vertical axis needs a brake. Used wherever efficiency and load matter: machine tools, heavy gantries, high-end linear actuators (e.g. Thomson, NSK, Bosch Rexroth screw assemblies).

Lead-screw (ACME / trapezoidal)

Sliding-contact thread, often with a polymer nut. 20–50% efficient — the high friction is the point: it makes the screw self-locking (non-backdrivable) so it holds position with zero power. Cheap, simple, fine for low-duty positioning and anything that must hold a load when de-energized. The efficiency penalty means more motor for the same output.

Belt drive

A toothed belt over pulleys. Lower force, but very fast over long strokes and cheap. Backlash from belt stretch limits precision. The standard choice for the long axis of a gantry or a 3D-printer-style motion system where speed beats stiffness.

Linear motor (direct drive)

No screw or belt — the motor's force acts directly on the moving stage (an unrolled BLDC). Zero backlash, very high bandwidth (100s of Hz), high acceleration, no wear parts in the drivetrain. The downsides: lower force density (you're paying for every newton with magnets and copper), heat dissipation into the structure, and cost. Used in semiconductor lithography, pick-and-place machines, and high-throughput inspection — anywhere settling time and precision dominate.

Lead/pitch, and no-load vs loaded

Screw output force and speed depend on lead (axial travel per revolution):

v_linear = (rpm / 60) × lead
F_linear ≈ (2π × η × T_motor) / lead

Smaller lead → more force, less speed (and more self-locking tendency)
Larger lead → more speed, less force, more likely backdrivable

A subtle trap: efficiency is load-dependent. A lead-screw might show a reasonable static efficiency on the datasheet but be far worse under light load and dynamic conditions. Always check efficiency at your actual operating force, and remember that backdriving efficiency is lower than driving efficiency — that asymmetry is what makes self-locking possible.

(See the comparison table for a side-by-side.)

Series-elastic & variable-stiffness

Here's the counterintuitive idea that reshaped legged and rehab robotics: deliberately make your actuator softer by putting a spring in series between the motor/gearbox and the load.

Why add a spring on purpose

A stiff geared actuator is a great position source and a terrible force source — tiny position errors create huge forces, and impacts spike loads through the gear teeth. Insert a known spring in series and three things happen:

1. Force becomes measurable from deflection. Measure the spring's compression with an encoder and you know output force exactly: F = k × Δx. The spring is your force sensor.
2. Force control becomes position control of the spring. The motor servos spring deflection, which is far more robust than trying to control force through a stiff, high-friction gearbox.
3. Impact energy is absorbed by the spring, not slammed through the gear teeth — the actuator survives footstrikes and collisions that would destroy a rigid drive.

The cost: the spring adds a low-frequency pole, so position bandwidth drops. You've traded crisp positioning for clean force control and robustness. For a leg hitting the ground, that's a fantastic trade.

Where SEAs are used

Gill Pratt's SEA work led to robots like the original Cog/M2 and, more famously, the actuators behind much of modern legged robotics. Boston Dynamics and Agility have used elastic elements in legs; rehabilitation exoskeletons and the Valkyrie/THOR-class humanoids used SEA extensively because gentle, controllable force against a human body is the whole job.

Variable-stiffness actuators (VSA)

A VSA lets you tune the series stiffness on the fly — soft for a delicate or dynamic task, stiff for precise positioning. Mechanically it's usually two motors antagonistically loading nonlinear springs (the DLR/VSA-II and "MACCEPA" designs are the canonical references). They're complex and heavy for what they deliver, so they've stayed mostly in research, but the concept — match impedance to the task — is exactly right and shows up in software form (impedance control) on QDD robots instead.

Quasi-direct-drive (QDD)

If SEA is the mechanical answer to force control, QDD is the electrical-plus-software answer, and it's the one that's actually winning in legged and humanoid robots.

The idea: skip the big gearbox

A direct-drive motor (no gearbox) is perfectly backdrivable and transparent, but to make joint-level torque it must be huge and heavy. A high-ratio geared motor is compact but stiff, non-backdrivable, and can't sense external force without a torque sensor. QDD splits the difference: a large-diameter, high-torque BLDC motor plus a single low-reduction stage, typically 6:1 to 10:1, driven by field-oriented control.

Why this works so well:

• Low gear ratio means the actuator stays backdrivable — the load can move the motor, and friction is low.
• Because torque ≈ current and the gearing is light, you can estimate output torque from motor current alone — proprioceptive force control, no extra torque sensor. This is the key trick.
• The big motor provides enough torque density that a single stage is sufficient for legs.
• FOC gives you high-bandwidth current (hence torque) control.

The lineage

The MIT Cheetah (Sangbae Kim's lab) productionized QDD: custom high-torque "gap-radius" motors with ~5–7:1 planetary stages and current-based torque estimation enabled fast, robust, contact-rich running and jumping. That architecture went commercial through Unitree (the quadrupeds, and the cheap motor modules everyone now prototypes with) and is the actuation backbone of most modern legged robots and humanoids. The all-electric Atlas, Unitree H1/G1, and many others lean on QDD-style joints.

QDD vs SEA

They solve the same problem — force control and impact tolerance — by different means. QDD does it with low gearing + current sensing (no physical compliance, so high bandwidth but it must control its own stiffness in software). SEA does it with a physical spring (intrinsic impact tolerance, lower bandwidth). The field has largely converged on QDD for dynamic locomotion because software impedance control on a transparent drive is more flexible than a fixed mechanical spring, and because removing the spring restores bandwidth. SEA persists where physical compliance is a hard safety requirement (against human bodies).

If you're building a legged or contact-rich robot today, start with QDD modules. They're now cheap enough to prototype with and give you force control "for free" from current sensing.

Soft & novel actuators

Beyond the big three lies a zoo of actuators that exploit different physics. Most are niche, but each owns a corner where conventional actuators are awkward.

McKibben pneumatic muscles

A rubber bladder inside a braided mesh sleeve. Inflate it and the braid geometry forces it to shorten and fatten, pulling like a muscle. Festo's "Fluidic Muscle" (DMSP/MAS) is the commercial example.

• Contractile (pull-only), very high peak force-to-weight (up to ~1,500 N from a 20 mm Festo DMSP muscle), inherently compliant.
• Nonlinear, hysteretic, needs air — control is harder than an electric drive.
• Used in exoskeletons, biomimetic limbs, and lightweight assistive devices where muscle-like compliance and high force-to-weight beat precision.

Shape-memory alloy (SMA)

Nitinol wire that contracts ~4–5% when heated (electrically) above its transition temperature, returning when cooled.

• Silent, tiny, high force-to-weight, no moving parts to wear.
• Slow (cooling-limited, often >1 s cycle) and inefficient (you're heating metal), with limited strain and short fatigue life if overstrained.
• Used in micro-grippers, deployable space mechanisms, medical devices, and anywhere silence and tiny scale dominate.

Piezoelectric

A piezo crystal strains a fraction of a percent under voltage — minuscule displacement but enormous bandwidth (kHz) and stiffness.

• Sub-nanometer resolution, kHz response, high force, microscopic stroke.
• Used directly for nanopositioning (microscope stages, lithography fine-stages, fast steering mirrors), and in ultrasonic/inchworm piezo motors (Physik Instrumente, Nanomotion) that accumulate tiny steps into macroscopic, high-resolution motion with zero backlash and self-locking holding.

Electroactive polymers (EAP / dielectric elastomers)

"Artificial muscle" polymers that strain under high electric fields. Large strain, soft, lightweight — but need kilovolts, suffer reliability/breakdown issues, and remain mostly a research curiosity in 2026 despite decades of promise.

Reach for a novel actuator only when a conventional one physically can't do the job — sub-micron precision (piezo), centimeter-scale silent motion (SMA), or muscle-like soft pulling (McKibben). Otherwise an electric drive is less trouble.

Backdrivability, transparency & force control

This deserves its own section because it's the property that decides whether your robot can safely touch the world — and it's the one engineers most often get wrong.

Definitions

Backdrivable — you can move the output by hand (or the load can move it) and the motor turns. Transparent — the actuator faithfully transmits forces in both directions with little distortion from friction or inertia. A direct-drive motor is both; a worm-gear drive is neither.

What sets it

Mostly gear ratio and friction, not the motor. Reflected inertia and friction scale with the square of the gear ratio:

J_reflected = J_motor × N²
friction_reflected ≈ friction_motor × N²  (plus the gearbox's own friction)

A 100:1 harmonic drive reflects the motor's tiny inertia as a large effective inertia at the output and adds its own meaningful friction — the result feels like trying to backdrive through molasses. A 6:1 QDD drive reflects 36× inertia, which is small enough that the joint stays transparent.

Why it matters

• Force control — a transparent drive lets you control force well (directly, or via current as in QDD). A non-backdrivable drive fights you and needs a separate torque sensor for clean force control.
• Safety / cobots — a backdrivable arm yields when it hits a person; a stiff geared arm transmits the full collision force. Cobots either use moderate gearing plus joint torque sensors (Universal Robots, KUKA iiwa) or accept the gearing and rely on current-based collision detection.
• Contact-rich tasks — assembly, polishing, and any task involving controlled contact need the actuator to be a good force source, which means transparency or excellent torque sensing.

The two roads to good force control: (a) make the drive transparent (QDD, direct-drive, SEA) and infer/measure force cheaply, or (b) keep the high gearing for torque density and add a dedicated joint torque sensor (Harmonic Drive + strain-gauge torque sensor, the classic industrial-arm-with-force-control approach). Road (a) is winning in mobile/legged/humanoid; road (b) still rules precise industrial arms.

Sizing & selecting an actuator

Now the practical part. Here's how to actually pick and size, in order.

Step 1 — Build the force/torque budget

Sum the worst-case loads at the actuator output: gravity, inertia (τ = J × α), friction, process forces, and a safety factor. For a rotary joint:

τ_peak = J_total × α_max + τ_gravity + τ_friction + τ_process

Size the actuator's peak torque above τ_peak with margin (1.5–2× is common), and the continuous torque above the RMS torque over the duty cycle.

Step 2 — Compute the RMS / thermal load

This is where most designs fail in the field. Motors are thermally limited; continuous torque depends on how fast heat leaves the windings. Compute RMS torque over the motion cycle:

τ_rms = sqrt( (1/T) × ∫ τ(t)² dt )

τ_rms must stay under the continuous rating at your actual ambient and cooling. A motor that handles the peak can still cook itself if the average is too high. Doubling torque quadruples I²R heating — respect that exponent.

Step 3 — Set speed and pick the gear ratio

You know the output speed and torque you need; the motor has a speed/torque sweet spot. Pick N to map one onto the other, then check that backdrivability, backlash, and efficiency are acceptable. High N for torque density (industrial arm), low N for transparency (legged/cobot).

Step 4 — Check bandwidth

Does the actuator respond fast enough for the control task? Geared electric: fine for arms and AGVs. Need >50 Hz force control at the output? You're looking at QDD, SEA, direct-drive, or hydraulic with servo-valves — not a high-ratio harmonic drive.

Step 5 — Apply the decision tree

The decision tree, compressed:

1. Need precise position/torque, clean, battery-powered, fits through a door? → Electric (BLDC + gearbox). Default.
2. Need force control, impact tolerance, transparency for legs/contact? → QDD (or SEA if physical compliance is mandatory).
3. Need >50 kN in a tight joint and can tolerate plumbing? → Hydraulic.
4. Binary, fast, compliant, clean motion at the tool? → Pneumatic.
5. Sub-micron precision? → Piezo. Silent centimeter-scale? → SMA. Muscle-like soft pull? → McKibben.

Step 6 — Don't forget the boring stuff

Connectors, encoder resolution, brake (any vertical/backdrivable axis), thermal path, IP rating, EMC, and whether you can actually buy it in volume. The actuator that's perfect on paper but has a 40-week lead time is the wrong actuator.

Comparison tables & cheat-sheet

Numbers below are representative order-of-magnitude figures for typical robotics-scale components, useful for first-pass selection — always confirm against the specific product datasheet.

Actuator family comparison

Property	Electric (BLDC+gear)	Hydraulic	Pneumatic	SEA	QDD	Piezo	SMA
Power density (W/kg)	100–300	300–600 (actuator)	50–150	100–250	150–400	low (high BW, tiny stroke)	low
Force/torque density	Medium	Very high	Low	Medium	Medium–high	High (tiny stroke)	High (tiny stroke)
Working "pressure"/source	DC bus 24–800 V	5,000–35,000 kPa	600–1,000 kPa	DC bus	DC bus	100s of V	I²R heating
Efficiency (wall→work)	85–95%	40–60%	10–20%	80–90%	85–93%	high (static)	<10%
Bandwidth	10s–100s Hz	10s Hz	few Hz	10s Hz	100s Hz	kHz	<1 Hz
Controllability	Excellent	Good (servo-valve)	Poor mid-stroke	Excellent (force)	Excellent (force)	Excellent	Poor
Backdrivable	Depends on ratio	Yes (with valve)	Somewhat (springy)	Yes	Yes	No (self-lock)	No
Cleanliness	Clean	Leaks/oil	Clean	Clean	Clean	Clean	Clean
Cost	Low–medium	High (system)	Low	Medium	Medium	High	Low
Typical use	Arms, AGVs, cobots	Heavy/construction, ex-Atlas	Grippers, EOAT, vacuum	Legs, rehab, exo	Legged, humanoid	Nanopositioning	Micro/medical/space

Linear actuator comparison

Type	Efficiency	Backdrivable	Speed	Backlash	Relative cost	Pick it when
Ball-screw	80–95%	Yes (needs brake)	Medium	Low	Medium	Efficiency + heavy load
Lead-screw (ACME)	20–50%	No (self-locking)	Low–medium	Low	Low	Cheap, must hold w/o power
Belt drive	90%+	Yes	High	Medium (stretch)	Low	Long, fast strokes
Linear motor	n/a (direct)	Yes	Very high	None	High	Bandwidth, precision, zero backlash

Selection cheat-sheet

If your priority is…	Reach for…
General-purpose robot joint	BLDC + planetary or harmonic
Precise industrial arm joint	BLDC + harmonic/cycloidal + torque sensor
Legged / dynamic locomotion	QDD modules (low ratio + FOC)
Human-contact force control	SEA, or QDD/torque-sensed cobot drive
Maximum force in tiny envelope	Hydraulic cylinder + servo-valve
Fast binary gripping/clamping	Pneumatic cylinder/gripper
Picking boxes/sheets/glass	Pneumatic vacuum (Venturi)
Long fast Cartesian axis	Belt drive
Heavy efficient linear axis	Ball-screw (+ brake if vertical)
Hold a vertical load unpowered	Lead-screw (self-locking)
Sub-micron positioning	Piezo stage / piezo motor
Silent, tiny, low-cycle motion	SMA wire
Muscle-like compliant pull	McKibben pneumatic muscle

Frequently asked questions

What's the difference between an actuator and a motor? A motor is a raw transducer that converts energy to motion. An actuator is a complete, controllable motion unit — motor plus transmission, feedback, and drive electronics arranged to produce a commanded force or position. Every actuator contains a prime mover (motor, cylinder, etc.); not every motor is an actuator.

Why are most factory grippers pneumatic if pneumatics are so inefficient? Because at the gripper you're paying for compliance, speed, simplicity, and cleanliness, not energy efficiency. A pneumatic gripper is an air spring that won't crush parts, cycles in tens of milliseconds, needs only a solenoid valve, and sparks nothing. Electric grippers match the precision but cost more and add control complexity. For binary clamping at the tool, pneumatics still win on total cost.

Why did Boston Dynamics switch Atlas from hydraulic to electric? Hydraulics gave the old Atlas the force density for explosive moves, but they leaked, were loud and inefficient, and demanded heavy plumbing plus constant maintenance. By 2024, electric (QDD-style) actuators had enough force density to do the job, so the all-electric Atlas got better efficiency, cleanliness, and controllability with no fluid system. It's the clearest signal that electric is overtaking hydraulics wherever it can.

What is a quasi-direct-drive (QDD) actuator? A large high-torque BLDC motor with a single low-reduction gear stage (about 6:1 to 10:1) driven by field-oriented control. The low ratio keeps it backdrivable and transparent, and because torque tracks motor current you can sense output force from current alone — proprioceptive force control with no extra torque sensor. It's the dominant architecture for legged and humanoid robots.

Why deliberately add a spring (SEA) — doesn't that hurt precision? It hurts position bandwidth, yes, but it buys clean force control (force = spring stiffness × deflection, so the spring is your force sensor), impact tolerance (the spring absorbs shock instead of the gear teeth), and stable interaction with the environment. For a leg hitting the ground or a robot pushing on a human, that trade is exactly right.

What makes an actuator backdrivable, and why care? Mostly low gear ratio and low friction — reflected inertia and friction scale with ratio squared. Backdrivability matters for force control, collision safety, and contact-rich tasks: a backdrivable arm yields when it hits something, while a high-ratio geared arm transmits the full collision force and needs a torque sensor to feel anything.

Ball-screw or lead-screw — how do I choose? Ball-screw for efficiency (80–95%) and load capacity, but it's backdrivable so a vertical axis needs a brake. Lead-screw for low cost and self-locking holding — its high friction (20–50% efficiency) means it holds position with zero power, at the cost of needing a bigger motor for the same output. Cheap holding axis → lead-screw; efficient working axis → ball-screw.

When should I use a linear motor instead of a screw? When you need very high bandwidth, high acceleration, zero backlash, and excellent settling — semiconductor stages, high-speed pick-and-place, precision inspection. You pay with lower force density, heat dumped into the structure, and higher cost. If raw force matters more than dynamics, a screw is cheaper and more force-dense.

How do I size an actuator so it doesn't overheat? Size peak torque above your worst-case load with 1.5–2× margin, but the binding constraint is usually thermal: compute RMS torque over the full duty cycle and keep it below the continuous rating at your real ambient and cooling. Heating scales with current squared, so a duty cycle with brief high-torque spikes can still cook a motor that's "rated" for the peak.

Are soft/McKibben/SMA/piezo actuators ready for real robots? In their niches, yes. Piezo is mature and standard for nanopositioning. SMA is used in micro-grippers, medical, and space deployables. McKibben muscles appear in exoskeletons and biomimetic limbs. They're not general-purpose replacements for electric drives — reach for them only when conventional actuators physically can't meet the precision, scale, silence, or compliance requirement.

Do hydraulics have any future in mobile robotics? Limited. They still win for very high force in a tight envelope (heavy construction, forestry, large manipulators) and where a combustion engine already supplies power. But for battery-powered, human-scale robots, electric QDD has largely closed the force-density gap, and the operational disadvantages of hydraulics — weight, inefficiency, leaks, maintenance — make them hard to justify.

What's the single most common sizing mistake? Sizing to the peak torque on the datasheet and ignoring the thermal/RMS load. Engineers see "10 N·m peak," design for 8 N·m, and then the actuator overheats because the continuous rating is 3 N·m and their duty cycle averages 4 N·m. Always size the continuous rating against RMS torque, then check peak separately.