3.4 Legged locomotion

Legged robots mimic animal locomotion, using articulated legs to move. They come in various designs, from bipedal humanoids to insect-inspired hexapods. These robots excel at traversing uneven terrain and adapting to complex environments, making them ideal for challenging tasks.

Despite their advantages, legged robots face challenges in stability, energy efficiency, and mechanical complexity. Researchers tackle these issues through advanced control systems, gait optimization, and bio-inspired designs. Applications range from search and rescue to space exploration, showcasing the versatility of legged locomotion.

Types of legged robots

• Legged robots are a class of mobile robots that use articulated legs for locomotion, mimicking the movement of animals or insects
• The number and configuration of legs in a legged robot can vary depending on the specific design and intended application

Bipedal vs quadrupedal robots

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• Bipedal robots have two legs and are designed to mimic human-like walking and balance (ASIMO, Atlas)
• Quadrupedal robots have four legs, offering increased stability and load-carrying capacity compared to bipedal designs (Spot, ANYmal)
• The choice between bipedal and quadrupedal designs depends on factors such as the desired level of mobility, stability, and the intended operating environment

Hexapod and octopod designs

• Hexapod robots have six legs, often inspired by the locomotion of insects like ants or cockroaches (RHex, Weaver)
• Octopod robots feature eight legs, providing even greater stability and redundancy in case of leg failure (Dante II)
• These multi-legged designs excel in traversing uneven terrain and maintaining balance in challenging environments

Advantages of legged locomotion

• Legged robots offer several advantages over wheeled or tracked robots, particularly in terms of adaptability and mobility in complex environments
• The ability to navigate uneven terrain and overcome obstacles makes legged robots well-suited for applications such as search and rescue or exploration

Traversing uneven terrain

• Legged robots can adapt their gait and foot placement to navigate over rough, uneven surfaces (rubble, stairs)
• By independently controlling each leg, legged robots can maintain stability and traction on inclined or slippery surfaces
• The ability to step over obstacles allows legged robots to access areas that would be challenging for wheeled or tracked vehicles

Adaptability in complex environments

• Legged robots can adapt their posture and gait to fit through narrow spaces or navigate around obstacles
• The high degree of articulation in the legs enables legged robots to perform tasks such as climbing or crawling
• In cluttered environments, legged robots can take advantage of discrete footholds to maintain stability and progress

Challenges in legged locomotion

• Despite the advantages of legged locomotion, several challenges must be addressed to ensure reliable and efficient operation of legged robots
• These challenges include maintaining stability, optimizing energy efficiency, and managing the increased mechanical complexity compared to wheeled designs

Stability and balance control

• Maintaining balance and stability is crucial for legged robots, especially in dynamic situations or on uneven terrain
• Legged robots must constantly adjust their posture and foot placement to keep their center of mass within a stable region
• Advanced control algorithms, such as zero-moment point (ZMP) control or model predictive control (MPC), are used to ensure stability

Energy efficiency considerations

• Legged locomotion typically requires more energy compared to wheeled locomotion due to the need to constantly lift and lower the legs
• Optimizing gait patterns and utilizing compliant actuators can help improve energy efficiency in legged robots
• Techniques such as passive dynamics and elastic energy storage can be employed to reduce the overall energy consumption

Mechanical complexity vs wheeled designs

• Legged robots have a higher degree of mechanical complexity compared to wheeled robots, with multiple joints and actuators per leg
• The increased complexity can lead to higher costs, maintenance requirements, and potential points of failure
• However, the added complexity enables legged robots to perform tasks and navigate environments that are impossible for simpler wheeled designs

Gait analysis and generation

• Gait analysis involves studying the patterns and characteristics of leg movements during locomotion
• Gait generation refers to the process of creating and optimizing gait patterns for legged robots to achieve stable, efficient, and adaptable locomotion

Static vs dynamic stability

• Static stability is achieved when the robot's center of mass remains within the support polygon formed by its legs at all times
• Dynamic stability allows for the center of mass to temporarily leave the support polygon, enabling faster and more agile locomotion
• Legged robots can utilize both static and dynamic stability depending on the gait pattern and the desired speed or terrain

Gait patterns for different speeds

• Different gait patterns are employed for legged robots to achieve various speeds and navigate different terrains
• Common gait patterns include walking (statically stable), trotting, pacing, bounding, and galloping (dynamically stable)
• The choice of gait pattern depends on factors such as the desired speed, energy efficiency, and stability requirements

Central pattern generators (CPGs)

• Central pattern generators are neural networks that produce rhythmic output signals to control the movement of legs in legged robots
• CPGs can generate coordinated gait patterns without the need for explicit programming of each leg's trajectory
• By modulating the parameters of the CPG, different gait patterns and adaptations to terrain can be achieved

Actuators for legged robots

• Actuators are the components responsible for generating force and motion in legged robots
• The choice of actuator type depends on factors such as power density, control precision, and compliance requirements

Electric motors vs hydraulic actuators

• Electric motors, such as brushless DC motors or servomotors, are commonly used in legged robots due to their high precision and ease of control
• Hydraulic actuators, powered by pressurized fluid, offer high power density and force output, making them suitable for larger legged robots or those requiring high payload capacity
• The choice between electric and hydraulic actuators depends on the specific requirements of the robot and its intended application

Series elastic actuators (SEAs)

• Series elastic actuators incorporate an elastic element, such as a spring, in series with the actuator output
• SEAs provide compliance and force control, allowing for safer interaction with the environment and better shock absorption
• The elastic element also helps to reduce the impact of high-frequency disturbances and improves energy efficiency

Compliance and force control

• Compliance refers to the ability of an actuator to yield to external forces, allowing for better adaptation to uneven terrain and safer interaction with the environment
• Force control enables legged robots to regulate the amount of force applied by each leg, which is essential for maintaining stability and traction
• Compliant actuators, such as SEAs or variable stiffness actuators (VSAs), facilitate compliance and force control in legged robots

Sensors for legged locomotion

• Sensors play a crucial role in legged locomotion, providing feedback on the robot's state, its interaction with the environment, and enabling autonomous navigation
• Different types of sensors are used to gather proprioceptive, tactile, and visual information

Proprioceptive sensing

• Proprioceptive sensors measure the internal state of the robot, such as joint angles, velocities, and torques
• Examples of proprioceptive sensors include encoders, potentiometers, and inertial measurement units (IMUs)
• Proprioceptive information is essential for controlling the robot's posture, stability, and gait execution

Tactile and force sensors

• Tactile sensors, such as force-sensitive resistors or capacitive sensors, measure the contact forces between the robot's feet and the ground
• Force sensors, like strain gauges or load cells, measure the forces acting on the robot's legs or joints
• Tactile and force information help the robot adapt its gait, maintain traction, and detect obstacles or uneven terrain

Vision systems for navigation

• Vision sensors, such as cameras or LiDAR, provide the robot with information about its surroundings
• Stereo cameras enable depth perception and obstacle detection, while LiDAR provides high-resolution 3D point clouds
• Visual information is used for mapping, localization, and path planning, enabling autonomous navigation in complex environments

Control architectures

• Control architectures define the organization and flow of information between the sensors, processors, and actuators in a legged robot
• Different control approaches, such as hierarchical, reactive, or learning-based, can be employed depending on the specific requirements and complexity of the robot

Hierarchical vs reactive control

• Hierarchical control architectures divide the control problem into multiple levels, with higher levels responsible for planning and lower levels for execution
• Reactive control architectures use a direct mapping between sensory inputs and motor outputs, enabling fast response to changes in the environment
• Hybrid architectures combine elements of both hierarchical and reactive control to balance planning and real-time adaptation

Model-based control strategies

• Model-based control strategies rely on mathematical models of the robot and its environment to predict the robot's behavior and optimize its performance
• Examples of model-based control include computed torque control, model predictive control, and whole-body control
• Model-based approaches can provide precise control and optimal performance but require accurate models and significant computational resources

Learning-based approaches

• Learning-based control approaches use machine learning techniques, such as reinforcement learning or neural networks, to automatically learn and adapt the robot's behavior
• These approaches can enable the robot to learn complex locomotion skills, adapt to new environments, and improve its performance over time
• Examples of learning-based approaches include deep reinforcement learning for gait optimization and imitation learning for acquiring new locomotion skills

Bio-inspired legged locomotion

• Bio-inspired legged locomotion takes inspiration from the movement patterns and mechanisms found in animals and insects
• By studying and mimicking the locomotion strategies of biological systems, researchers aim to develop more efficient, adaptable, and robust legged robots

Insect-inspired hexapod robots

• Hexapod robots, such as RHex and Weaver, are inspired by the locomotion of insects like cockroaches and ants
• These robots often feature compliant legs and simple control strategies that enable them to traverse rough terrain and overcome obstacles
• Insect-inspired robots demonstrate high stability, adaptability, and robustness in challenging environments

Mammal-inspired quadruped designs

• Quadruped robots, like Spot and ANYmal, take inspiration from the locomotion of mammals such as dogs, cats, and horses
• These robots exhibit dynamic stability, agility, and the ability to navigate complex terrains
• Mammal-inspired designs often incorporate advanced control strategies, such as model predictive control or reinforcement learning, to achieve sophisticated locomotion behaviors

Humanoid bipedal locomotion

• Humanoid robots, such as ASIMO and Atlas, are designed to mimic human-like bipedal locomotion
• These robots face the challenge of maintaining balance and stability while walking, running, or climbing stairs
• Humanoid locomotion requires advanced control techniques, such as zero-moment point control or whole-body control, to ensure stable and efficient movement

Applications of legged robots

• Legged robots have a wide range of potential applications, leveraging their ability to navigate complex environments and perform tasks that are challenging for wheeled or tracked robots
• Some key application areas include search and rescue, inspection and maintenance, and space exploration

Search and rescue operations

• Legged robots can assist in search and rescue operations by navigating through rubble, collapsed buildings, or other hazardous environments
• Their ability to climb, crawl, and adapt to uneven terrain makes them well-suited for locating and assisting victims in disaster scenarios
• Examples of legged robots used in search and rescue include the RHex hexapod and the Legged Squad Support System (LS3) quadruped

Inspection and maintenance tasks

• Legged robots can be employed for inspection and maintenance tasks in industrial settings, such as power plants, offshore platforms, or pipelines
• Their ability to navigate stairs, ladders, and narrow passages allows them to access hard-to-reach areas and perform visual inspections or sensor measurements
• Legged robots like ANYmal and Spot have been used for autonomous inspection and monitoring in various industries

Legged robots in space exploration

• Legged robots have the potential to enhance space exploration by providing increased mobility and adaptability on extraterrestrial surfaces
• The ability to traverse rocky, uneven terrain and climb obstacles makes legged robots well-suited for exploring planets, moons, or asteroids
• NASA's All-Terrain Hex-Limbed Extra-Terrestrial Explorer (ATHLETE) is an example of a legged robot designed for lunar exploration and payload handling