5.2 Pneumatic and hydraulic artificial muscles

Artificial muscles are revolutionizing robotics by mimicking biological muscle function. Pneumatic and hydraulic muscles use pressurized fluids to generate force and movement, offering high power-to-weight ratios and inherent compliance.

These bio-inspired actuators come in various types, like the McKibben muscle. They can be arranged in antagonistic pairs or other configurations to achieve complex motions. While challenges exist, their potential in robotics and prosthetics is immense.

Pneumatic and Hydraulic Muscles

Principles of Fluidic Artificial Muscles

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• Pneumatic artificial muscles (PAMs) utilize compressed air to generate force and movement
  • Consist of an elastomeric tube surrounded by a braided mesh
  • When pressurized, the muscle expands radially and contracts axially
• Hydraulic artificial muscles (HAMs) operate on similar principles but use incompressible fluids
  • Offer higher force output compared to PAMs due to fluid incompressibility
  • Require more complex sealing and containment systems
• Fluidic muscle serves as a general term encompassing both PAMs and HAMs
  • Mimics the contraction behavior of biological muscles
  • Provides a high power-to-weight ratio (up to 400:1 compared to natural muscle)

Characteristics and Performance

• Pressure-force relationship in fluidic muscles follows a non-linear curve
  • Initial pressure increase results in rapid force generation
  • Force output plateaus at higher pressures
• Contraction ratio of fluidic muscles typically ranges from 20-30%
  • Depends on factors such as initial braid angle and material properties
• Fluidic muscles exhibit inherent compliance
  • Allows for safer human-robot interaction
  • Provides shock absorption capabilities

Applications and Advantages

• Used in various robotic systems (prosthetic limbs, exoskeletons)
• Offer advantages over traditional actuators
  • Lightweight construction
  • High power-to-weight ratio
  • Inherent compliance for safer operation
• Challenges include non-linear behavior and hysteresis
  • Require complex control algorithms for precise positioning

McKibben Muscle

Structure and Operation

• McKibben muscle represents the most common type of fluidic artificial muscle
  • Invented by Joseph L. McKibben in the 1950s for orthotic applications
• Consists of an inner elastomeric bladder surrounded by a braided mesh sleeve
  • Bladder materials include rubber or silicone
  • Mesh typically made of nylon or other strong, flexible fibers
• Operation principle based on the pneumatic artificial muscle concept
  • Pressurized air causes radial expansion and axial contraction
  • Braided mesh constrains radial expansion, amplifying axial contraction

Performance Characteristics

• Contraction ratio of McKibben muscles typically ranges from 20-30%
  • Influenced by initial braid angle and material properties
  • Higher initial braid angles result in greater contraction but lower force output
• Force output depends on muscle diameter and operating pressure
  • Larger diameters and higher pressures generate greater forces
  • Force decreases as the muscle contracts (non-linear force-length relationship)
• Exhibits natural compliance due to the compressibility of air and elastomeric materials
  • Provides inherent safety in human-robot interaction scenarios
  • Allows for energy storage and release, similar to biological muscles

Advantages and Limitations

• Advantages of McKibben muscles include
  • High power-to-weight ratio (up to 400:1 compared to natural muscle)
  • Inherent compliance for safer operation
  • Simple construction and low cost
• Limitations and challenges
  • Non-linear behavior complicates control
  • Hysteresis effects due to friction between bladder and mesh
  • Limited contraction ratio compared to some other actuator types

Muscle Configurations

Antagonistic Pair Arrangement

• Antagonistic pair configuration mimics the arrangement of biological muscles
  • Consists of two artificial muscles working in opposition
  • One muscle contracts while the other extends
• Provides bidirectional movement and force generation
  • Allows for precise control of position and stiffness
  • Enables more complex and natural-like motions in robotic systems
• Control strategies for antagonistic pairs
  • Co-contraction used to adjust joint stiffness
  • Differential pressure control for position and force regulation

Alternative Configurations

• Parallel muscle arrangements
  • Multiple muscles connected in parallel to increase force output
  • Useful in applications requiring high force generation (exoskeletons, heavy-duty robotic arms)
• Series muscle configurations
  • Muscles connected end-to-end to increase overall contraction range
  • Applied in scenarios requiring extended range of motion (robotic fingers, flexible manipulators)
• Hybrid configurations
  • Combine parallel and series arrangements for optimized performance
  • Allow for customized force-displacement characteristics in complex robotic systems

Design Considerations

• Muscle selection and sizing
  • Based on required force output, contraction ratio, and operating pressure
  • Consider trade-offs between force generation and contraction range
• Attachment methods and end fittings
  • Critical for efficient force transmission and durability
  • May include crimped fittings, mechanical clamps, or specialized connectors
• Integration with structural components
  • Design of mounting points and load-bearing structures
  • Consideration of overall system weight and compactness