Legged robots are mobile machines that use articulated limbs for movement, much like animals and humans. Their purpose is to navigate complex and uneven terrain where traditional wheeled or tracked robots would falter. This allows them to access difficult-to-reach environments for a wide array of demanding applications.
How Leg Robots Walk and Balance
The locomotion of a legged robot is a complex interplay of stability, sensory input, and mechanical action. A primary concept governing their movement is stability, which can be either static or dynamic. Statically stable robots maintain their balance at all times, even when stopped, by keeping their center of gravity within the polygon of support created by their feet on the ground. This method is common in robots with more legs, like hexapods, which can move a few legs while the others form a stable base.
Dynamic stability, on the other hand, requires constant motion to stay upright, much like a person riding a bicycle. These robots, particularly bipeds and some quadrupeds, continuously shift their center of gravity and adjust their footing to prevent falling. This approach allows for faster and more agile movements, such as running and jumping, but demands sophisticated control systems.
To achieve either form of stability, legged robots rely on a suite of sensors. Inertial measurement units (IMUs), which contain accelerometers and gyroscopes, provide data on the robot’s tilt and orientation. Vision systems like cameras and LiDAR help the robot perceive its environment, identify obstacles, and plan foot placements. Force sensors in the legs provide feedback on the contact with the ground, which helps in adjusting the force exerted by each leg.
This sensory information is processed by an onboard computer, which then sends signals to actuators—typically electric motors—located at the joints of each leg. These actuators convert the electrical signals into precise physical movements, generating the desired gait. Gaits are patterns of leg movements, such as walking, trotting, or galloping, and are selected based on the robot’s design and the terrain it needs to traverse.
Diverse Forms of Legged Robots
Legged robots are categorized based on the number of limbs they possess, with each design offering a unique combination of stability, mobility, and complexity. Bipedal robots, with two legs, are designed to emulate human locomotion. This form allows them to navigate environments built for people, like stairs and narrow hallways, but maintaining balance is a significant challenge, requiring advanced control systems.
Quadrupedal robots, which have four legs, offer a greater degree of stability compared to their two-legged counterparts. Modeled after animals like dogs, these robots can move efficiently across rough and unstructured terrain. While they can achieve static stability by moving one leg at a time, they often use dynamic gaits like trotting for faster movement, where two diagonal legs are lifted simultaneously.
Hexapod robots, with six legs, are often inspired by insects and are highly stable. They can easily maintain a tripod gait, where three legs are always on the ground, forming a stable base of support well-suited for tasks that require a steady platform. Robots with more legs, like octopods, offer even greater stability for very challenging environments.
Leg Robots in Action: Real World Uses
The ability of legged robots to navigate difficult environments has led to their use in various real-world scenarios. One of the most prominent applications is in remote inspection and monitoring, particularly in locations that are hazardous or inaccessible to humans. For example, quadrupedal robots are deployed in industrial sites and nuclear facilities to check for gas leaks or survey for contamination. They are also used in construction to survey terrain and monitor progress by creating 3D maps of the site.
In the field of search and rescue, legged robots can traverse rubble and unstable structures in disaster zones to locate survivors. Equipped with thermal cameras and other sensors, they can provide first responders with situational awareness without putting human lives at risk. The ANYmal robot, for instance, has been used in simulated disaster scenarios to perform tasks like delivering payloads.
Beyond industrial and emergency applications, legged robots are also being developed for logistics, security, and exploration. They can act as “automated pack mules” to transport materials around factories or across uneven ground. In security, they can patrol large areas and alert authorities to intruders. Furthermore, their potential for use in space exploration is being investigated, with robots like NASA’s Valkyrie designed to one day assist humans on other planets.
Inside the Machine: Engineering Legged Robots
Constructing a legged robot requires balancing strength, weight, and agility through careful consideration of materials, joint design, power, and control systems. The robot’s frame and legs are often built from lightweight yet strong materials like aluminum alloys or carbon fiber composites. This choice helps to minimize the robot’s inertia and the amount of energy required for movement.
Each leg is composed of multiple links connected by joints, which allow for a specific number of degrees of freedom (DoF), dictating the leg’s range of motion. The joints are powered by actuators, which are most commonly electric motors paired with gearboxes. Designing actuators is a challenge, as they must be powerful, lightweight, and energy-efficient. Some designs incorporate elastic elements to store and release energy, mimicking the function of tendons in animals.
A rechargeable battery pack provides the power needed for sustained operation. All of these physical components are managed by an onboard computer that runs the complex control software.