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Hydraulic and Pneumatic Systems

Introduction

Beyond electric actuation, hydraulic and pneumatic systems are two other important drive methods for robots. Hydraulic systems are renowned for their extremely high power density, while pneumatic systems stand out for their simplicity and compliance. This article covers the principles of both fluid power systems, their typical applications, and comparisons with electric approaches.

Hydraulic Systems

Pascal's Law

The fundamental principle of hydraulic systems comes from Pascal's Law — a change in pressure at any point in an enclosed fluid is transmitted equally to all parts of the fluid:

\[ P = \frac{F}{A} \]

Force Amplification

Force is amplified through pistons of different cross-sectional areas:

\[ \frac{F_1}{A_1} = \frac{F_2}{A_2} = P \]
\[ F_2 = F_1 \times \frac{A_2}{A_1} \]

When \(A_2 > A_1\), the output force \(F_2\) exceeds the input force \(F_1\).

Conservation of Energy

Force is amplified, but piston stroke is correspondingly reduced: \(F_1 \cdot d_1 = F_2 \cdot d_2\). The work (energy) remains constant.

Hydraulic Cylinders

A hydraulic cylinder converts hydraulic energy into linear motion:

Types

Type Structure Features
Single-acting cylinder Oil supply on one side, spring return Simple, unidirectional force
Double-acting cylinder Alternating oil supply on both sides Bidirectional force
Telescopic cylinder Multi-stage nested sleeves Long stroke, compact retracted length

Key Parameters

Parameter Formula
Push force (extend) \(F_{push} = P \times \frac{\pi D^2}{4}\)
Pull force (retract) \(F_{pull} = P \times \frac{\pi (D^2 - d^2)}{4}\)
Piston velocity \(v = \frac{Q}{A}\)
Power \(P_{power} = P \times Q\)

Where \(D\) is the bore diameter, \(d\) is the rod diameter, and \(Q\) is the flow rate.

Hydraulic Power Unit

The Hydraulic Power Unit (HPU) supplies high-pressure oil to the system:

Motor → Hydraulic pump → High-pressure oil → Control valve → Cylinder/Motor
                            ↑                                │
                            └── Tank ← Filter ←──────────────┘
                                 (Return oil)

Components

Component Function
Hydraulic pump Converts mechanical energy to hydraulic energy (gear/piston/vane pump)
Tank Stores hydraulic oil, dissipates heat, settles contaminants
Relief valve Limits maximum system pressure, safety protection
Directional control valve Controls oil flow direction (switching valve)
Proportional/servo valve Precisely controls flow and pressure
Filter Removes particles from the oil
Accumulator Stores hydraulic energy, smooths pulsations

Hydraulic System Parameters

Parameter Typical Range
Operating pressure 10–35 MPa (industrial), 70 MPa (special)
Hydraulic fluid Mineral oil (most common), synthetic ester, water-based
Oil temperature 30–60°C (normal operating range)
Filtration precision 10–25 um

Boston Dynamics Atlas (Hydraulic Version)

Atlas is the benchmark for hydraulic-driven humanoid robots:

Parameter Value
Height 1.5 m
Weight 89 kg
DOF 28
Actuation Hydraulic servo (custom electro-hydraulic servo valves)
Power source Electric hydraulic pump
System pressure ~21 MPa
Features Extremely high power density, outstanding dynamic performance

The hydraulic Atlas could perform backflips, parkour, and other extreme dynamic maneuvers, demonstrating the enormous advantages of hydraulic actuation in power density.

Electric Atlas

In 2024, Boston Dynamics released an all-new electric Atlas, transitioning to an electric drive approach. This indicates that as motor technology advances, electric drives are becoming increasingly competitive in the humanoid robot domain.

Advantages and Limitations of Hydraulics

Advantages:

  • Extremely high power density (can be 10x or more than electric motors)
  • Natural overload protection (relief valve limits pressure)
  • Can achieve extremely large forces/torques
  • Fast response (oil is nearly incompressible)

Limitations:

  • Complex system (pump, valves, piping, tank)
  • Oil leakage risk
  • High maintenance cost
  • Loud noise
  • Oil temperature affects performance
  • Not suitable for clean environments

Pneumatic Systems

Operating Principle

Pneumatic systems use compressed air as the working medium. Unlike hydraulics, air is compressible, which gives pneumatic systems inherent compliance.

Pneumatic Cylinders

Type Features
Standard cylinder Double-acting, linear motion
Compact cylinder Short stroke, space-saving
Rodless cylinder Piston drives external slider via magnetic coupling, long stroke
Rotary actuator Outputs rotational motion
Guided cylinder Built-in guide rail, prevents rotation

Output Force

\[ F = P \times A - F_{friction} \]

Typical operating pressure: 0.4–0.8 MPa (4–8 bar)

This is an order of magnitude lower than hydraulics, so pneumatic cylinders are generally larger to compensate for the lower force.

Proportional Valves

Traditional pneumatic valves have only ON/OFF states; proportional valves provide continuous adjustment:

Type Function Accuracy
Proportional pressure valve Continuously adjusts output pressure ±1% FS
Proportional flow valve Continuously adjusts flow rate ±2% FS
Servo valve High-precision position/force control <±0.5% FS

Pneumatic System Components

Compressor → Air tank → FRL (Filter+Regulator+Lubricator) → Solenoid/Proportional valve → Cylinder
                                                                                          │
                                                                                     Exhaust silencer

FRL (Filter-Regulator-Lubricator):

  • F (Filter): Removes moisture and contaminants
  • R (Regulator): Adjusts working pressure
  • L (Lubricator): Adds oil mist for lubrication (modern cylinders are often lube-free)

McKibben Artificial Muscle

Structure

The McKibben Pneumatic Artificial Muscle (PAM) is a bioinspired actuator:

  • Inner layer: Rubber tube (elastic membrane)
  • Outer layer: Braided mesh (nylon or Kevlar fiber)
  • Both ends sealed, one end connected to an air tube

Operating Principle

When inflated, the rubber tube expands radially, and the geometric constraints of the braided mesh convert radial expansion into axial contraction:

\[ F = \frac{\pi D_0^2}{4} \cdot P \cdot \left(\frac{3}{\tan^2\alpha} - \frac{1}{\sin^2\alpha}\right) \]

Where \(D_0\) is the initial diameter and \(\alpha\) is the braid angle.

Characteristics

Feature Value
Contraction ratio Up to ~25–30%
Force-to-weight ratio Extremely high (>100x own weight)
Response time ~50–100 ms
Muscle-like behavior Contracts to produce pulling force (unidirectional)
Compliance Naturally compliant, safe for human interaction

Limitations

  • Can only pull, not push (like muscles, requires antagonistic pairing)
  • Severely nonlinear, difficult to control precisely
  • Hysteresis effects
  • Low efficiency of compressed air

Applications

  • Bioinspired robots (simulating musculoskeletal systems)
  • Exoskeletons (lightweight, compliant)
  • Soft robotics research

Soft Robot Actuators

Pneumatic Soft Actuators

Actuators made from flexible materials (silicone PDMS, TPU) that undergo controlled deformation when inflated:

Types

Type Motion Mode Application
Bending type Inflation causes one-sided expansion → bending Soft fingers, tentacles
Extension type Inflation causes axial elongation Soft arms
Twisting type Inflation produces rotation Rotary joints
Vacuum type Vacuum causes contraction Grasping, locomotion

PneuNet (Pneumatic Network)

A classic soft actuator design proposed by Harvard University:

  • Multiple air chambers arranged on a flexible substrate
  • Inflation expands the chambers; the non-expanding side constrains the structure, causing bending
  • Different chamber geometries enable different motion modes
Before inflation:        After inflation:
┌─┬─┬─┬─┐          ╭─╮╭─╮╭─╮
│ │ │ │ │          │ ││ ││ │
│ │ │ │ │   →      ╰─╯╰─╯╰─╯
└─┴─┴─┴─┘            ╲    ╱
(Straight)             ╲  ╱ (Bent)

Fabrication Methods

  • Mold casting: Silicone poured into 3D-printed molds
  • Direct 3D printing: TPU/silicone 3D printing
  • Lost-wax method: Sacrificial core for complex internal channels

Electric vs. Hydraulic vs. Pneumatic

Feature Electric Hydraulic Pneumatic
Power density Medium Very high Low
Precision High High Medium-low
Response speed Fast Fast Medium
Efficiency 80–95% 60–80% 10–30%
Control complexity Medium High Low-medium
Cleanliness Clean Oil leakage risk Clean
Noise Low High Medium
Maintenance Low High Medium
Cost Medium High Low-medium
Compliance Requires control implementation Requires control implementation Inherent
Overload protection Requires additional design Relief valve natural protection Natural pressure limiting

Selection Guide

graph TD
    A[Drive Method Selection] --> B{Extreme force/power needed?}
    B -->|Yes| C{Precise control needed?}
    B -->|No| D{Compliance/safety needed?}
    C -->|Yes| E[Hydraulic Servo]
    C -->|No| F[Hydraulic / Pneumatic]
    D -->|Yes| G{High force needed?}
    D -->|No| H[Electric Drive]
    G -->|Yes| I[Pneumatic Artificial Muscle]
    G -->|No| J[Pneumatic Soft / Electric]

    style E fill:#fbb,stroke:#333
    style H fill:#bfb,stroke:#333
    style I fill:#bbf,stroke:#333
    style J fill:#fbf,stroke:#333

Typical Robot Drive Choices

Robot Drive Method Reason
Industrial 6-axis arm Electric (AC servo) High precision, low maintenance
Atlas hydraulic version Hydraulic Extreme dynamic performance
Collaborative robot Electric (BLDC) Safe, precise, clean
Spot quadruped Electric (BLDC + QDD) Power density is sufficient
Soft gripper Pneumatic Compliance
Industrial press Hydraulic Large force, simple
Sorting robot Pneumatic gripper Fast, reliable

Electrification Trend

In recent years, robot actuation has shown a clear trend toward electrification:

  1. Motor technology progress: High-energy-density permanent magnets and advanced winding techniques improve power density
  2. Driver progress: FOC and high-bandwidth current loops improve control performance
  3. QDD maturation: Low gear ratio + high-torque motors balance force control and power
  4. Maintenance advantage: Electric systems are nearly maintenance-free
  5. Cost reduction: Mass production lowers motor and driver costs

Domains Where Hydraulics Remain Irreplaceable

  • Heavy-duty operations (excavators, large construction robots)
  • Extreme environments (underwater, high temperature)
  • Extreme dynamic performance requirements

New Opportunities for Pneumatics

  • Flourishing soft robotics development
  • Human-robot interaction safety requirements
  • Bioinspired actuation research
  • Low-cost rapid prototyping

Summary

  • Hydraulic systems are based on Pascal's Law, offering extremely high power density for high-force scenarios
  • Pneumatic systems use compressed air and are naturally compliant, suitable for safe interaction
  • McKibben artificial muscles mimic the contraction behavior of biological muscles
  • Soft robot actuators achieve motion through controlled deformation of flexible materials
  • Electric drive is the current mainstream trend for robots, but hydraulics and pneumatics retain irreplaceable advantages in their respective domains
  • Drive method selection requires holistic consideration of force, precision, speed, cost, maintenance, and safety

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