Hydraulics for Robotics: The Ultimate Guide

Hydraulics is how you get a two-tonne excavator arm to curl a bucket of wet clay, and it is how the hydraulic-era Atlas did a backflip. The physics is almost embarrassingly simple: fluid does not compress, so pushing on it at one place moves it at another, and if you push hard enough on a large enough piston you get a force that no electric motor of the same weight can match. Everything else in a hydraulic system (the pump, the reservoir, the valves, the hoses, the accumulators) exists to generate that pressure, meter it precisely, and get the spent fluid back to the tank.

For decades hydraulics owned every job that needed brute force in a small package: construction machines, aircraft flight controls, forging presses, and the first generation of dynamic legged robots. The force density is real and it is large. A hydraulic cylinder running at 21 MPa (about 3,000 psi) makes roughly ten times the force per unit of actuator mass that a comparable electric actuator makes, and it does it while tolerating shock loads that would strip a gear train. That is why the heavy end of robotics grew up hydraulic.

The catch is everything around the cylinder. You need a power unit that is heavy and hot, valves that cost more than the motor they replaced, seals that eventually weep, and a fluid that makes a mess when a hose lets go. The efficiency is poor, the maintenance is real, and the whole system runs at pressures that will inject oil through skin. Those downsides are exactly why robotics, including the humanoid programs that started hydraulic, has been migrating to electric actuation. This guide covers how hydraulics works, how to size it, where it still wins, and why the industry is walking away from it anyway.

The take: Hydraulics buys you the highest force density and shock tolerance available in a compact actuator, through one law (force = pressure x area at thousands of psi) applied to an incompressible fluid. You pay for it with a heavy, hot, inefficient power unit, expensive servo valves, seals that leak, and a fluid that contaminates. For legged robots and general robotics, electric quasi-direct-drive actuators have closed most of the force-density gap while erasing those costs, so new designs go electric unless the load is genuinely in the multi-tonne, high-shock, or high-power-density regime where nothing else fits.

Companion reading: robot actuators, pneumatics for robotics, legged & quadruped hardware, construction robotics, real-time control systems, and power electronics & motor drives.

Table of contents

1. Key takeaways
2. The physics: Pascal, incompressibility, and force density
3. Anatomy of a hydraulic system
4. Pumps and the power unit
5. Valves: from on-off to servo
6. Actuators: cylinders and rotary
7. Accumulators and energy storage
8. Servo-valve control and its physics
9. Sizing a hydraulic actuator: worked numbers
10. Hydraulics in robots: legged, construction, aerospace
11. The downsides and failure modes
12. Electrohydraulic actuators and the shift to electric
13. How to choose
14. Frequently asked questions

The physics: Pascal, incompressibility, and force density

Hydraulics rests on Pascal's law: pressure applied to a confined fluid transmits equally in all directions. Confine oil in a cylinder, push on it with a small piston, and the pressure appears undiminished on a large piston somewhere else, multiplying force by the ratio of the two areas. That is the hydraulic lever, and it is exact for an ideal fluid.

The force a cylinder produces is the pressure times the piston area:

F = P * A
   F = force (N)
   P = gauge pressure (Pa)
   A = effective piston area (m^2)

Put real numbers in. A cylinder with a 50 mm bore has a piston area of A = pi * (0.025)^2 = 1.963e-3 m^2. At a working pressure of 21 MPa (3,000 psi, a common mobile-hydraulics figure) it makes:

F = 21e6 Pa * 1.963e-3 m^2 = 41,200 N  (about 4.2 tonnes-force)

That is a 4.2 tonne push from a steel tube you can hold in one hand. No electric motor plus gearbox of that mass comes close. Raise the pressure to 35 MPa (5,000 psi, used in aerospace and high-density mobile systems) and the same cylinder makes 68 kN. This linear scaling of force with pressure is why the field chases high pressure: force density rides directly on the pressure you can safely contain.

Why incompressibility matters

The other half of the story is that hydraulic fluid barely compresses. Its resistance to compression is the bulk modulus:

beta = -V * (dP / dV)
   beta ~ 1.5 to 2.0 GPa for typical mineral hydraulic oil

A bulk modulus near 1.5 GPa means that raising the pressure by 15 MPa (150 bar) compresses the oil by only about 1 percent in volume. For most purposes the fluid moves as a solid rod that happens to flow around corners. Two consequences follow. First, the actuator is stiff: apply a load and the piston holds position with minimal give, which is what you want for precise force and position control. Second, a small pump displacement produces a proportional, predictable actuator motion, because almost none of the delivered volume is lost to squashing the fluid.

Air is the opposite. It is thousands of times more compressible, so a pneumatic actuator is a soft spring you cannot precisely position under varying load. That single difference is why hydraulics does force and stiffness while pneumatics does cheap, fast, compliant motion. (The pneumatics side is covered in the pneumatics guide.)

Entrained air ruins this. A few percent of undissolved air bubbles drops the effective bulk modulus by an order of magnitude, because the gas compresses first. A system with air in the fluid feels spongy, oscillates, and loses stiffness. Bleeding air and keeping the reservoir above the pump inlet are basic to making a hydraulic system feel rigid.

The mass balance: where the density goes

The force density is at the cylinder, and only at the cylinder. A hydraulic cylinder is close to a solid steel forging with a rod, so it packs enormous force into little mass. But that cylinder cannot do anything without a pump to pressurize the fluid, a prime mover (electric motor or engine) to drive the pump, a reservoir to hold the fluid, valves to direct it, hoses to carry it, and usually a cooler to shed the heat. Those parts are heavy and they do not shrink with pressure. So the honest figure of merit depends on where you draw the boundary: the actuator alone is spectacular, the full power unit plus a single actuator is mediocre, and the full power unit driving many actuators from one pump is good again. Hydraulics wins when one power unit feeds many high-force actuators, which is exactly the excavator and the aircraft, and loses when you need one clean actuator on a battery, which is the modern robot.

Anatomy of a hydraulic system

Every hydraulic system, from a log splitter to a humanoid leg, is the same block diagram:

• Reservoir (tank). Holds the fluid, lets air and water separate out, dissipates heat, and provides a settling volume. Sized at roughly 2 to 3 times pump flow per minute for stationary systems, far smaller and pressurized or bladder-type for mobile and robotic systems where volume is precious.
• Prime mover. An electric motor (industrial) or an engine (mobile) that turns the pump. In a robot this is a compact brushless motor.
• Pump. Converts mechanical rotation into fluid flow at pressure. The heart of the power unit.
• Relief valve. Caps the maximum pressure by dumping flow to tank when pressure exceeds a setpoint. The system's pressure fuse; without it a deadheaded pump would burst something.
• Directional and control valves. Steer flow to and from the actuators and meter how much. This is where control lives, from a simple on-off solenoid valve to a precision servo valve.
• Actuators. Cylinders (linear) and motors (rotary) that turn fluid power back into mechanical force and motion.
• Filters. Remove contamination. Placed on the pump inlet (suction), the pressure line, and the return line depending on the design. Non-negotiable for servo systems.
• Accumulator. A pressure vessel that stores fluid under pressure to absorb shocks, supply peak flow, and hold pressure with the pump off.
• Cooler and conditioning. Heat exchanger, plus sometimes water removal, because all the inefficiency ends up as heat in the oil.

Fluid flows from tank, through the pump, out at pressure, through the control valves to the actuator, does work, and returns through the return filter to the tank to start again. The plumbing is a closed loop for the oil and an open one for the energy: electrical or engine power in, mechanical work and a lot of heat out.

Pumps and the power unit

The pump sets the pressure ceiling and the flow the system can deliver. Pumps are positive-displacement: each revolution moves a fixed volume, so flow is proportional to speed and displacement, and pressure rises to whatever the load demands (up to the relief-valve limit). The three families that matter:

Pump type	Pressure range	Efficiency	Traits
Gear pump	to ~25 MPa (3,600 psi)	~80 to 88%	Cheap, robust, noisy, fixed displacement, tolerant of dirt
Vane pump	to ~17 MPa (2,500 psi)	~80 to 88%	Quiet, moderate pressure, some variable-displacement designs
Piston pump (axial/radial)	to ~35 to 70 MPa (5,000 to 10,000 psi)	~90 to 95%	High pressure, high efficiency, variable displacement, expensive, needs clean oil

Axial-piston pumps are the workhorse of high-performance hydraulics. A swashplate tilts to change piston stroke, so the pump can vary its displacement from full flow to zero while running, which is how a load-sensing system delivers only the flow the actuators need and saves the energy a fixed pump wastes across the relief valve.

The flow a pump delivers is straightforward:

Q = D * n * eta_vol
   Q       = flow (L/min)
   D       = displacement (L/rev, i.e. cc/rev / 1000)
   n       = pump speed (rev/min)
   eta_vol = volumetric efficiency (~0.9 to 0.97)

A 10 cc/rev pump at 3,000 rpm delivers Q = 0.010 * 3000 * 0.95 = 28.5 L/min. The power the pump draws from its motor is the hydraulic power plus the losses:

P_hydraulic (kW) = P (bar) * Q (L/min) / 600
P_input     (kW) = P_hydraulic / eta_overall

At 210 bar and 28.5 L/min the hydraulic output is 210 * 28.5 / 600 = 10.0 kW, and with an overall pump efficiency around 0.9 the motor must supply about 11 kW. Every kilowatt of difference becomes heat in the oil, which is why the cooler exists and why the power unit runs hot.

The power unit is the weight

For a robot, the power unit is the problem. A pump, its drive motor, the reservoir, the valves, and a cooler together weigh far more than the cylinders they feed and run at a fraction of electric efficiency. The hydraulic-era Atlas got around this with a compact, high-speed onboard pump and a custom integrated valve-and-actuator design that kept plumbing short, but the machine still carried a hot, thirsty power core and, in its earliest form, an external tether for hydraulic and electrical power. That power-unit tax is the single biggest reason walking robots left hydraulics.

Valves: from on-off to servo

Valves are where you control a hydraulic system. They range from crude to exquisite, and the precision you buy determines what the system can do.

• Directional control valves. Spool valves that connect actuator ports to pressure or tank. A simple 4/3 solenoid valve (four ports, three positions) drives a cylinder extend, retract, or hold. On-off, cheap, no fine control.
• Proportional valves. A spool positioned proportionally to a solenoid current, so flow varies smoothly with command. Bandwidths of 10 to 80 Hz, good for speed and position control where microsecond response is not required. The workhorse of modern mobile and industrial motion control.
• Servo valves. The precision instrument. A torque motor drives a hydraulic pilot stage (flapper-nozzle or jet-pipe) that positions a main spool with very low hysteresis and high bandwidth, 100 to 250 Hz or more. This is what closed-loop force and position control at high dynamics requires, and what the dynamic robots used.

How a two-stage servo valve works

The elegant part is the pilot stage. In a flapper-nozzle valve, a milliampere-level current in a torque motor deflects a flapper between two nozzles. That tiny mechanical deflection unbalances the pilot pressures, which drives the much larger main spool, which meters the full flow to the actuator. A feedback spring or electrical spool-position feedback closes the loop so spool position tracks the input current. The result is that a fraction of a watt of electrical signal controls tens of kilowatts of hydraulic power with a linear, low-lag response.

The price of that precision is fragility to contamination. The flapper-nozzle gaps and spool clearances are 2 to 5 micrometers. A particle that size jams the pilot or scores the spool, so servo systems demand fluid cleaned to tight ISO 4406 cleanliness codes (often 16/14/11 or better) and run continuous filtration. A servo valve costs a few thousand dollars and will not forgive dirty oil, which is a large part of why servo-hydraulic robots were expensive to build and maintain.

Rule of thumb: pick the cheapest valve that meets the bandwidth you actually need. On-off solenoid for clamps and simple sequencing, proportional for smooth speed and position at tens of hertz, servo only when you need force control above 100 Hz. Every step up in valve class multiplies cost and cleanliness demands.

Actuators: cylinders and rotary

The actuator converts fluid power back into mechanical work. Two families cover almost everything.

Cylinders (linear)

A hydraulic cylinder is a bore, a piston, a rod, and seals. Pressurize one side and the piston pushes; pressurize the other and it retracts. Key variants:

• Double-acting. Pressure on either side, so the cylinder drives in both directions. The default for controlled motion.
• Single-acting. Pressure extends, a spring or the load retracts. Simple, for jacks and clamps.
• Differential (rod) effect. The rod occupies area on one side, so the extend and retract forces differ for the same pressure. A cylinder pushes harder than it pulls, and moves faster on retract because it has less area to fill. You must account for the annulus area on the rod side:

F_extend  = P * A_piston
F_retract = P * (A_piston - A_rod)

For a 50 mm bore with a 28 mm rod at 21 MPa, extend force is 41.2 kN but retract force is only P * (A_piston - A_rod) = 21e6 * (1.963e-3 - 0.616e-3) = 28.3 kN. Robotic joints that must be symmetric use double-rod cylinders or account for the asymmetry in control.

Rotary actuators and motors

For continuous rotation or large-angle joints, a hydraulic motor (essentially a pump run backward: gear, vane, or piston) turns flow into torque and speed. For limited-angle joints, a rotary vane actuator or a rack-and-piston gives high torque over a bounded arc. Torque scales with displacement and pressure:

T = D * P / (2 * pi)
   T = torque (N.m)
   D = displacement (m^3/rev)
   P = pressure (Pa)

Cylinders dominate robotics because most joints are revolute and a cylinder driving a lever arm is compact, stiff, and gives excellent force density over the working range. The hydraulic-era Atlas and Boston Dynamics' BigDog and its successors used custom cylinders on lever linkages at the joints for exactly this reason.

Accumulators and energy storage

An accumulator is a pressure vessel that stores hydraulic energy by compressing a gas (usually nitrogen) behind a bladder, diaphragm, or piston. It does several jobs a pump alone cannot:

• Peak flow. A pump sized for average flow can be small if an accumulator supplies the surge during a fast move. This lets a robot use a modest pump and still deliver a burst of high-power motion, exactly the profile a jumping or kicking leg needs.
• Shock absorption. The gas cushions pressure spikes from sudden load changes or valve closures, protecting the plumbing.
• Energy recovery. In some designs, energy from a decelerating or lowering load charges the accumulator instead of burning across a relief valve, then discharges on the next move. This is one lever hydraulic machines use to claw back some of their poor efficiency.
• Hold pressure with the pump off. The accumulator maintains system pressure so the pump can idle or cycle, saving energy and heat.

The gas follows the gas law, so stored energy depends on the precharge pressure and the working pressure band:

p1 * V1 = p2 * V2   (isothermal, ideal gas)
   precharge p0 typically ~0.9 * minimum working pressure

For dynamic robots, the accumulator was a key trick: it decouples the average power the pump must produce from the peak power a fast motion demands, so a compact power unit can still deliver an explosive move. The cost is stored energy that must be handled safely, because a charged accumulator is a spring holding real energy even when the machine is off.

Servo-valve control and its physics

Closed-loop hydraulic control is what made hydraulic robots and aircraft flight controls possible. The controller commands the valve, the valve meters flow, the flow moves the actuator, a sensor measures the result, and the loop closes. Understanding the plant it controls explains both the strengths and the tuning headaches. (For the broader control-loop treatment, see the real-time control systems guide.)

Flow, not force, is what the valve commands

A servo valve is fundamentally a variable orifice, and flow through an orifice follows the square-root law:

Q = Cd * A_valve * sqrt(2 * dP / rho)
   Q       = flow through the valve
   Cd      = discharge coefficient (~0.6 to 0.7)
   A_valve = valve opening area (set by spool position, i.e. by command current)
   dP      = pressure drop across the valve
   rho     = fluid density

Two things fall out. First, flow sets actuator velocity (velocity = Q / A_piston), so a servo valve is naturally a velocity command, and you get position by integrating and force by the pressure that develops. Second, the square-root dependence on pressure drop makes the plant nonlinear: the same spool opening passes different flow at different loads, so a simple linear controller performs unevenly across the operating range unless you compensate.

The hydraulic resonance

The actuator and the fluid form a spring-mass system. The oil's bulk modulus acts as a stiff spring on either side of the piston, and the load mass sits on that spring, giving a hydraulic natural frequency:

omega_h = sqrt( (beta * A^2) / (V * m) )
   beta = fluid bulk modulus
   A    = piston area
   V    = trapped fluid volume (one side)
   m    = load mass reflected to the piston

This resonance, often in the tens to low hundreds of hertz, sets the ceiling on control bandwidth. You cannot close the loop faster than roughly this frequency without exciting the resonance and making the actuator ring. Short hoses, small trapped volume, and a stiff mount all raise omega_h and let you control harder, which is why high-performance hydraulic actuators integrate the valve directly onto the cylinder to shrink the trapped volume. Entrained air, by softening beta, drops omega_h and wrecks the achievable bandwidth, another reason air in the fluid is the enemy.

Servo-hydraulic control gives outstanding force fidelity: because pressure is directly readable and force is P x A, you can close a high-bandwidth force loop that an electric-plus-gearbox actuator struggles to match without a torque sensor. That force controllability, plus the shock tolerance, is precisely what the dynamic legged robots wanted.

Sizing a hydraulic actuator: worked numbers

Size a hydraulic actuator in the same order every time. Suppose a robot leg joint needs to deliver 3,000 N of linear force through a cylinder, at a peak actuator speed of 0.5 m/s, and we run a 21 MPa (210 bar) system.

Step 1: bore from force. Required piston area:

A = F / P = 3000 N / 21e6 Pa = 1.43e-4 m^2
bore diameter = sqrt(4 A / pi) = sqrt(4 * 1.43e-4 / pi) = 13.5 mm

Round up to a standard 16 mm bore, which at 21 MPa makes P * A = 21e6 * pi * 0.008^2 = 4,220 N, giving comfortable margin over the 3,000 N requirement. Always size the bore so working pressure sits below the relief setting with headroom.

Step 2: flow from speed. The flow needed to move a 16 mm piston at 0.5 m/s:

Q = A * v = (pi * 0.008^2) * 0.5 = 1.005e-4 m^3/s = 6.0 L/min

Step 3: peak power. The hydraulic power at the actuator:

P_hyd = P * Q = 21e6 Pa * 1.005e-4 m^3/s = 2.11 kW

That is the peak for one joint. A quadruped or humanoid with a dozen actuators moving at once could demand tens of kilowatts of peak hydraulic power, which is why the power unit and accumulator sizing dominate the machine design.

Step 4: pump and prime mover. If the pump feeds this joint plus others, sum the worst-case simultaneous flow, add margin, pick a displacement and speed to deliver it (Q = D x n x eta), and size the electric motor for P_hyd / eta_overall. Use an accumulator to cover the peaks so the pump can be sized nearer the average.

Step 5: relief, cleanliness, cooling. Set the relief valve above working pressure with margin, specify filtration to the servo valve's cleanliness requirement, and size the cooler for the total loss power (input minus useful output), which at 30 to 60 percent system efficiency is a large fraction of the input.

War story: A team building a hydraulic quadruped sized every cylinder from peak joint force and got beautiful actuators, then discovered the onboard pump and cooler they needed to feed all four legs during a trot weighed more than the rest of the robot. They had sized the actuators and forgotten that the power unit is the real budget. They switched to accumulator-buffered peak flow and a smaller pump, and even then the thermal load and noise pushed the next revision to electric actuators. Size the power unit first; the cylinders are the easy part.

Hydraulics in robots: legged, construction, aerospace

Legged robots, the hydraulic era

The first generation of dynamic legged robots was hydraulic because nothing else delivered the force density and shock tolerance a running leg needs. Boston Dynamics' BigDog, LS3, WildCat, and the original hydraulic Atlas used compact onboard pumps feeding custom servo valves and cylinders at the joints. Hydraulics gave them the ability to absorb a hard foot strike without destroying a gear train, to make explosive jumps by dumping accumulator flow, and to control joint force at high bandwidth by reading cylinder pressure. The hydraulic Atlas famously did a backflip on that muscle. (More on legged machines in the legged & quadruped hardware guide.)

The costs were equally famous. The machines were loud (the pump whine), hot, thirsty, and prone to weeping fluid at fittings. Building and maintaining the custom servo valves was expensive, and the power unit ate the energy budget. When electric quasi-direct-drive actuators matured enough to deliver comparable joint torque with backdrivability and clean torque control, the whole field pivoted. The current Atlas is fully electric, and essentially every new humanoid and quadruped (Unitree, Boston Dynamics' electric Atlas, Tesla, Figure, and the rest) is electric.

Construction and heavy machines

At the multi-tonne scale, hydraulics is unchallenged. An excavator's boom, arm, and bucket cylinders, a crane's outriggers and telescoping sections, a forging press, and a road machine's blade all run hydraulic because the forces are in the tens to hundreds of kilonewtons and often hundreds of tonnes, where no electric actuator is remotely practical. One engine-driven pump feeds many high-force cylinders, which is the case where hydraulic system efficiency and mass are actually reasonable. Robotic and autonomous versions of these machines keep the hydraulic muscle and add electronic control valves and sensors on top. (See the construction robotics guide.)

Aerospace flight controls

Aircraft primary flight controls (ailerons, elevators, rudders) have historically been hydraulic because of the force density and the proven reliability of servo-hydraulic actuation. Modern aircraft are shifting toward electrohydraulic and electromechanical actuators under the "more-electric aircraft" push, but hydraulics remains widespread where a compact actuator must move a large aerodynamic load quickly and reliably.

The downsides and failure modes

The reasons robotics is leaving hydraulics are all real and mostly unfixable at the system level.

• Leaks. Every fitting, seal, and hose is a potential leak. Seals wear, hoses chafe and burst, and a system at thousands of psi weeps oil that fouls the machine and the floor. For a robot working around people or products, this alone is often disqualifying.
• Weight and volume of the power unit. The pump, motor, reservoir, valves, and cooler are heavy and bulky, and they do not benefit from the actuator's force density. On a mobile robot this is dead weight and lost battery range.
• Efficiency. System efficiency from electrical input to useful mechanical work is often 30 to 60 percent, far below a well-designed electric drivetrain that can hit 80 to 90 percent. Throttling flow across valves, relief-valve dumping, and pump losses all become heat.
• Heat. All that inefficiency ends up in the oil, so the system needs a cooler, and the fluid viscosity changes with temperature, which shifts the control behavior. Thermal management is a constant chore. (See the broader thermal-management context for how electric drives compare.)
• Maintenance. Fluid changes, filter changes, seal replacement, leak chasing, and fluid analysis are ongoing. Servo valves demand tight cleanliness or they fail.
• Contamination sensitivity. Particles score spools, jam pilots, and abrade seals. A single ingress event can kill a servo valve.
• Safety. Pinhole leaks at high pressure can inject oil through skin, a serious injury. Accumulators store energy that must be discharged before service. The fluid is often flammable.

One thing hydraulics does have going for it: it is electrically quiet. The actuators generate no electromagnetic interference and tolerate harsh environments (heat, radiation, vibration) that stress electronics, which is part of why aerospace kept it. It is EMI-free but messy.

Common failure modes

• Seal wear and weeping. Gradual, expected, and the usual maintenance driver. Rod seals and piston seals are wear items.
• Hose failure. Chafing, aging, and pressure cycling burst hoses, sometimes suddenly. Routing and abrasion protection matter.
• Servo-valve silting and jamming. Contamination lodges in the pilot stage, causing null shift, hysteresis, or lockup.
• Cavitation and aeration. Low inlet pressure or air ingestion makes the pump cavitate (erosion, noise) and softens the fluid, wrecking stiffness and control.
• Fluid degradation. Oxidation, water contamination, and viscosity breakdown change the fluid's behavior and its lubricity, accelerating wear.

Electrohydraulic actuators and the shift to electric

The compromise that keeps hydraulic force density while ditching the central plumbing is the electrohydraulic actuator (EHA): a self-contained unit with an electric motor driving a small bidirectional pump that feeds an integrated cylinder, all in one package. You run an electrical wire to it, not a hydraulic line. The motor speed and direction command the actuator directly, so there is no servo valve, no central pump, no long hoses, and no shared reservoir. Aerospace adopted EHAs to reduce the aircraft-wide hydraulic system, and some robots use them to get hydraulic-like force in a discrete, wire-controlled joint.

EHAs recover several hydraulic downsides: no central power unit, far less plumbing to leak, and much better efficiency because the motor delivers only the flow the actuator needs instead of throttling a constant pump. What remains is the local seal and fluid maintenance and the added mass of the motor and pump on each joint. For a small number of very high-force joints, an EHA can beat a purely electric actuator on force density while staying clean enough to use.

Why the field is going electric

For general robotics the trend is decisive. Electric quasi-direct-drive actuators (a low-Kv brushless motor, a small single-stage gearbox, field-oriented control, and an encoder) now deliver joint torque densities and shock tolerance close enough to hydraulics for legs and arms, while giving:

• Clean operation with no fluid to leak.
• 80 to 90 percent efficiency versus 30 to 60 percent.
• Direct, precise torque control from motor current, no servo valve.
• Simple wiring instead of pumps, hoses, and reservoirs.
• Quiet operation and easy integration with battery power.

That package is why the humanoid programs that started hydraulic went electric, and why almost no new robot chooses hydraulics unless the load is genuinely beyond electric reach. The details of the electric alternative are in the robot actuators guide and the power electronics & motor drives guide.

How to choose

The decision comes down to a few questions about the load and the environment.

1. How large is the force, really? Below a few kilonewtons per joint, electric actuators win on every axis except peak shock. Above tens of kilonewtons, and especially in the tonne-to-hundred-tonne range, hydraulics is often the only practical answer.
2. How many high-force actuators share a power source? One power unit feeding many big cylinders (excavator, press) is where hydraulics is efficient and sensible. One actuator on a battery is where it is worst.
3. How bad is the shock loading? Hard impacts that would strip a gear train are absorbed gracefully by a fluid column. If the duty cycle is full of impacts and you cannot backdrive an electric actuator fast enough, hydraulics still has an edge.
4. Can you tolerate leaks and maintenance? Near people, food, cleanrooms, or on a machine that must run untended, hydraulic leaks and upkeep are often disqualifying.
5. What is the power budget? On a battery-powered mobile robot, the 30 to 60 percent efficiency and the power-unit mass are usually fatal. On a plugged-in or engine-driven machine they matter less.

Rule of thumb: default to electric actuation for any new robot. Choose hydraulics only when the load is in the multi-tonne or high-shock regime, when one power source feeds many high-force actuators, or when you inherit a hydraulic platform (construction, aerospace) where the force density and proven reliability still pay for the mess. Where you want hydraulic force but hate hydraulic plumbing, look at a self-contained electrohydraulic actuator before a central power unit.

Frequently asked questions

Why do hydraulics make so much more force than electric motors of the same size? Because force equals pressure times area, and hydraulic systems run at thousands of psi. A small cylinder at 21 MPa makes tonnes of force from a steel tube you can hold, while an electric motor makes torque limited by magnetic saturation and then needs a heavy gearbox to turn it into linear force. The catch is that the hydraulic force density is at the cylinder only. The pump, motor, reservoir, and cooler that feed it are heavy and inefficient, so the whole-system density is far less impressive than the actuator alone.

Why did legged robots like Atlas start hydraulic and then switch to electric? Early dynamic robots needed force density and shock tolerance that only hydraulics delivered, so BigDog and the original Atlas used onboard pumps, servo valves, and cylinders. They were powerful but loud, hot, thirsty, leaky, and expensive to maintain. When electric quasi-direct-drive actuators matured to comparable joint torque with backdrivability and clean torque control, the field switched. The current Atlas and essentially every new humanoid and quadruped are electric.

What is the bulk modulus and why does it matter? Bulk modulus is the fluid's resistance to compression, around 1.5 to 2 GPa for hydraulic oil. A high bulk modulus means the fluid barely compresses under load, so the actuator is stiff and holds position, and a small pump displacement produces predictable motion. Entrained air drops the effective bulk modulus dramatically, which is why air in the fluid makes a system feel spongy and oscillate. It is the property that lets hydraulics do precise, stiff force control while pneumatics cannot.

Why are servo valves so expensive and fragile? A servo valve controls tens of kilowatts of hydraulic power from a milliampere signal using a hydraulic pilot stage with clearances of 2 to 5 micrometers. That precision gives high bandwidth and low hysteresis, but a particle the size of the clearance jams or scores it. So servo valves cost thousands of dollars and demand fluid cleaned to tight ISO cleanliness codes with continuous filtration. That cost and cleanliness burden is a big part of why servo-hydraulic robots were hard to build and maintain.

How efficient is a hydraulic system compared to an electric drivetrain? A hydraulic system typically converts 30 to 60 percent of electrical input into useful mechanical work, because flow is throttled across valves, the relief valve dumps excess, and the pump has losses, all of which become heat in the oil. A good electric drivetrain reaches 80 to 90 percent. That efficiency gap, plus the power-unit mass, is decisive against hydraulics on battery-powered mobile robots and a major reason the field is going electric.

What is an electrohydraulic actuator (EHA) and when would I use one? An EHA is a self-contained package: an electric motor drives a small bidirectional pump feeding an integrated cylinder, controlled by a single electrical wire with no central pump or servo valve. It keeps hydraulic force density while eliminating central plumbing and the throttling losses, so it is much more efficient than a valve-controlled system. Use it when you need very high force at a few joints but want clean, wire-controlled integration, which is why aerospace and some robots adopt EHAs instead of a central hydraulic power unit.

Where does hydraulics still clearly win? At the heavy end: excavators, cranes, forging presses, and aircraft flight controls, where forces run from tens of kilonewtons to hundreds of tonnes and one engine-driven pump feeds many high-force cylinders. At that scale no electric actuator is practical, and the shared power unit makes the system mass and efficiency reasonable. Hydraulics also tolerates harsh, high-temperature, high-radiation, and high-vibration environments that stress electronics, and it generates no electromagnetic interference.

How do I size a hydraulic cylinder for a joint? Work in order. Get the bore from force and pressure (A = F / P, then bore = sqrt(4A/pi)), round up to a standard bore so working pressure sits below the relief setting with margin. Get the flow from the required speed (Q = A x velocity). Get peak power from P x Q. Then sum the worst-case simultaneous flow across all joints to size the pump and prime mover, use an accumulator to cover peaks so the pump can be smaller, and size the cooler for the loss power. The cylinders are easy; the power unit is the real budget.

Is hydraulic fluid a hazard? Yes. High-pressure pinhole leaks can inject fluid through skin, a serious injury that needs immediate surgery. Charged accumulators store real energy and must be discharged before service. Many hydraulic fluids are flammable, and leaks foul machines, floors, and products. These hazards, plus the ongoing leak and maintenance burden, are why hydraulics is often disqualified for robots working around people, food, or cleanrooms.