Robot Actuators: Electric, Hydraulic & Pneumatic

An actuator is the thing that actually moves; everything else in a robot is a spectator to it. Sensors perceive, controllers decide, structure holds it all together, but the actuator is where electrical or fluid power crosses the line into 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; you cannot write software that beats the second law of thermodynamics or the yield stress of a gear tooth. Pick the right one and a mediocre controller still does useful work. Actuator selection is the one hardware decision a robot never fully recovers from: it sets the ceiling on force, bandwidth, efficiency, and safety before a single line of control code is written.

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 rather than 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 is 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. For how actuators fit the wider discipline, see the robotics canon.

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.

One number worth internalizing: skeletal muscle generates only about 0.2 to 0.35 MPa of stress across its cross-section. A hydraulic cylinder beats it by two orders of magnitude, and an electric motor beats it on continuous power density and controllability. Yet nothing engineered matches muscle's combination of compliance, silence, energy recovery, and self-repair. That gap is why biomimetic actuation is still an open research program.

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. There's a scaling law worth knowing: motor torque goes as τ ∝ σ · r² · L, the magnetic shear stress σ at the airgap (~20-60 kPa for well-cooled machines, two to three orders of magnitude below hydraulic pressures), times the square of the airgap radius r, times stack length. That r² is why "pancake" large-diameter motors give so much torque for their mass, the physical reason QDD works: you buy torque with radius instead of gear ratio.

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 to 95% wall-to-shaft. Hydraulic systems are 40 to 60% wall-to-work after pump, valve throttling, and leakage losses. Pneumatic is brutal: 10 to 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 to 4,000. A servo-valve is $1,000 to 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 to 500 W BLDC motor wants to spin at 3,000 to 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: roughly 3:1 to 10:1 per stage (stack 2 to 3 stages for ~100:1), 90 to 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 to 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 to 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 high continuous torque the motor is thermally limited. Here's the physics, the most useful thing to carry out of this section. Torque is set by the torque constant Kt: τ = Kt · I. But the loss heating the windings is ohmic: P_loss = I² · R. Combine them: P_loss = (τ / Kt)² · R, so loss scales with the square of torque: hold twice the torque, dump four times the heat. The winding-independent figure of merit is the motor constant Km = Kt / sqrt(R) (units N·m/√W): torque per square-root-watt of heating, roughly invariant to turn count (more turns raises Kt and R together). When comparing frameless motors for a joint, compare Km, not the datasheet peak torque.

The reason this becomes the binding limit is the thermal time constant. A small robot-joint motor has a winding-to-housing thermal constant of seconds to a minute; housing-to-ambient is minutes. So a motor survives a 200 ms torque spike far above its continuous rating (the copper's heat capacity soaks it up) but cooks itself in steady state at a fraction of that. The continuous rating is just the torque at which steady-state winding temperature settles at the insulation limit (a class-F/H limit of 155 to 180 °C) for some assumed ambient and mounting; change the heatsink and the number moves. That thermal wall, not the peak torque on the datasheet, is what kills electric actuators in real duty cycles.

War story: A pick-and-place cell passed every bench test and hit its cycle-time target. Two hours into the first production shift, joints started throwing over-temperature faults. Peak torque was fine, but nobody had computed the RMS torque over the real 1.1-second cycle, and the continuous rating was quietly a third of the peak. The fix was slowing two aggressive mid-cycle moves until the duty-cycle RMS dropped back under the continuous line. Size for the integral, not the spike.

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 to 35,000 kPa (50 to 350 bar), with mobile and aerospace systems pushing 21,000 to 35,000 kPa (210 to 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.

Where the stiffness (and the bandwidth ceiling) come from

The reason hydraulics feel so crisp is the near-incompressibility of oil, its bulk modulus β ≈ 1.4-1.8 GPa. A trapped oil column behaves like a very stiff spring: k_hyd ≈ β · A² / V, with A the piston area and V the trapped fluid volume between valve and piston. That stiffness is why hydraulic joints hold position under load without the sag an electric drive shows.

But that same equation sets the bandwidth ceiling. The oil column plus moving mass resonate at a hydraulic natural frequency ω_h = A · sqrt(β / (V · m)). Two consequences burn people: (1) minimize dead volume V: long hoses between valve and cylinder tank the natural frequency and your force loop with it, which is why serious hydraulic robots mount the servo-valve on the actuator. (2) Entrained air is poison: just 1% air by volume can collapse the effective bulk modulus by an order of magnitude, because you're now compressing bubbles instead of oil. The actuator goes spongy and a loop tuned on a stiff plant oscillates. Bleeding air is a core part of tuning.

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 to 1,000 kPa (6 to 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 to 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. Quantify it: a gas-filled chamber has stiffness k_air ≈ γ · P · A² / V, with γ ≈ 1.4 the adiabatic index. Versus the hydraulic β · A² / V, you've swapped a 1.5 GPa bulk modulus for an effective γ·P ≈ 0.8 MPa, roughly 2,000× softer. That drops the natural frequency to a few hertz, and worse, a double-acting cylinder's two chambers form a nonlinear spring whose stiffness changes with piston position, so a controller tuned mid-stroke goes unstable near the endstops. Add seal stick-slip and you have a plant that's nonlinear, low-frequency, and lightly damped, the trifecta a position loop hates. 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 to 20% wall-to-work) because you pay full compressor energy and then throw most of it away on expansion and exhaust, 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 to 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 to 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.

The asymmetry falls out of the screw geometry. Driving and backdriving efficiencies for a power screw are:

η_drive     = tan(λ) · (1 − μ·tan λ) / (tan λ + μ)
η_backdrive = tan(λ) · (tan λ − μ) / (1 + μ·tan λ)

λ = lead angle,  μ = thread friction coefficient

Note the sign flip on μ·tan λ. The self-locking condition is tan(λ) ≤ μ: when the lead angle is shallow enough that friction exceeds the helix's tendency to unwind, η_backdrive goes to zero and the screw holds any load with the motor off. A ball-screw's rolling contact gives μ ≈ 0.003-0.01, so it's effectively always backdrivable, hence the brake on vertical axes. An ACME lead-screw with a bronze or polymer nut runs μ ≈ 0.1-0.2, above typical lead angles, so it holds dead. You trade the same friction that wastes 50 to 80% of your input power for never needing a holding brake, the entire ball-vs-lead decision in one inequality.

The take: The screw is the actuator's personality. The motor sets how much power is available; the lead angle and thread friction decide whether it comes out as speed or force, whether the axis holds itself or falls under gravity, and whether the thing is efficient or a space heater. Choose the conversion mechanism first, the motor second.

(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.

The idea and the name come from Gill Pratt and Matthew Williamson's 1995 paper "Series Elastic Actuators" (IEEE/RSJ IROS), out of the MIT Leg Lab. The spring stiffness k is the central knob, and it cuts both ways. Force resolution improves as k drops: with a deflection sensor of resolution Δx_min, the smallest resolvable force is F_min = k · Δx_min, so a softer spring feels finer forces (why rehab and haptic SEAs run soft). But force-control bandwidth is capped by how fast the motor can wind that spring against its reflected inertia, ω_bw ≈ sqrt(k / J_reflected), so softer also means slower. You can't get both maximum resolution and maximum bandwidth from one spring. VSAs exist because that tension is fundamental. Choosing k is choosing where your task lives on that curve.

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 Pisa VSA-II, DLR's VS-Joint/FSJ, and the VUB 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 quantitative case was made by Wensing, Wang, Kim et al. in "Proprioceptive Actuator Design in the MIT Cheetah" (IEEE Transactions on Robotics, 2017). The design principle is "gap-radius scaling": grow the airgap radius (recall τ ∝ r²) to hit the needed torque at low ratio rather than raising the ratio. The payoff: torque you can sense from current is limited by friction masquerading as torque. Reflected inertia scales as N², but the Coulomb (dry) friction that actually corrupts the current-based estimate reflects only as N (viscous damping scales as N²; see below). A 6:1 drive reflects just 6× the motor's already-tiny dry friction, keeping current-based force estimates usable; push N to 100:1 and it reflects ~100×. The signal drowns and you're back to a physical torque sensor. That linear-in-N friction term is why QDD chose radius over ratio.

The lineage

The MIT Cheetah (Sangbae Kim's lab) productionized QDD: custom high-torque "gap-radius" motors with ~5 to 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 to 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 scales with the square of the gear ratio (as does viscous damping); Coulomb friction scales linearly:

J_reflected        = J_motor × N²
friction_reflected ≈ friction_motor × N   (Coulomb/dry; viscous damping scales as 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. This is regulated: ISO 10218-1/-2 govern industrial-robot safety, and ISO/TS 15066 sets the biomechanical force and pressure limits for power-and-force-limited collaboration (transient contact limits of a few hundred newtons, part-of-body dependent). Meeting them during an impact is an actuator-transparency problem: peak collision force in the first milliseconds is F_peak ≈ v · sqrt(k_contact · m_eff), and m_eff is dominated by reflected rotor inertia J_motor · N². A high-ratio arm commanded soft in software still slams the operator with its reflected inertia before any loop can react: you can't filter your way out of physics faster than your sample rate. Low reflected inertia (low N, i.e. QDD) is the honest path to passing ISO/TS 15066.
• 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 to 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).

If your motion is acceleration-dominated, there's an optimum: load acceleration is maximized when the reflected load inertia equals the motor's rotor inertia, the classic inertia-matching result, N_opt = sqrt(J_load / J_motor). Below it the motor can't get the load moving; above it, torque goes to accelerating its own reflected rotor. Real designs run an "inertia ratio" of 5:1 to 10:1 for control margin, but that anchor tells you whether you're in the right neighborhood before iterating.

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, ingress protection (IP rating per IEC 60529, two digits, solids then liquids, so IP67 is dust-tight and survives temporary immersion), EMC (a FOC drive switching tens of kHz is a radio transmitter, bound by IEC 61800-3 emissions limits for adjustable-speed drives), functional safety if the joint has a safety role (IEC 61508 / ISO 13849 SIL/PL levels, e.g. a Safe Torque Off input), and whether you can 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 to 300	300 to 600 (actuator)	50 to 150	100 to 250	150 to 400	low (high BW, tiny stroke)	low
Force/torque density	Medium	Very high	Low	Medium	Medium to high	High (tiny stroke)	High (tiny stroke)
Working "pressure"/source	DC bus 24 to 800 V	5,000 to 35,000 kPa	600 to 1,000 kPa	DC bus	DC bus	100s of V	I²R heating
Efficiency (wall→work)	85 to 95%	40 to 60%	10 to 20%	80 to 90%	85 to 93%	high (static)	<10%
Bandwidth	10s to 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 to 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 to 95%	Yes (needs brake)	Medium	Low	Medium	Efficiency + heavy load
Lead-screw (ACME)	20 to 50%	No (self-locking)	Low to 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 to 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 to 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 to 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.