Legged robots are a type of mobile robot which use articulated limbs, such as leg mechanisms, to provide locomotion. They are more versatile than wheeled robots and can traverse many different terrains, though these advantages require increased complexity and power consumption. Legged robots often imitate legged animals, such as humans or insects, in an example of biomimicry. [1] [2]
Legged robots, or walking machines, are designed for locomotion on rough terrain and require control of leg actuators to maintain balance, sensors to determine foot placement and planning algorithms to determine the direction and speed of movement. [3] [4] The periodic contact of the legs of the robot with the ground is called the gait of the walker.
In order to maintain locomotion the center of gravity of the walker must be supported either statically or dynamically. Static support is provided by ensuring the center of gravity is within the support pattern formed by legs in contact with the ground. Dynamic support is provided by keeping the trajectory of the center of gravity located so that it can be repositioned by forces from one or more of its legs. [5]
Legged robots can be categorized by the number of limbs they use, which determines gaits available. Many-legged robots tend to be more stable, while fewer legs lends itself to greater maneuverability.
One-legged, or pogo stick robots use a hopping motion for navigation. In the 1980s, Carnegie Mellon University developed a one-legged robot to study balance. [6] Berkeley's SALTO is another example. [7] [8] [9] [10]
Bipedal or two-legged robots exhibit bipedal motion. As such, they face two primary problems:
Stability control is particularly difficult for bipedal systems, which must maintain balance in the forward-backward direction even at rest. [1] Some robots, especially toys, solve this problem with large feet, which provide greater stability while reducing mobility. Alternatively, more advanced systems use sensors such as accelerometers or gyroscopes to provide dynamic feedback in a fashion that approximates a human being's balance. [1] Such sensors are also employed for motion control and walking. The complexity of these tasks lends itself to machine learning. [2]
Simple bipedal motion can be approximated by a rolling polygon where the length of each side matches that of a single step. As the step length grows shorter, the number of sides increases and the motion approaches that of a circle. This connects bipedal motion to wheeled motion as a limit of stride length. [2]
Two-legged robots include:
Quadrupedal or four-legged robots exhibit quadrupedal motion. They benefit from increased stability over bipedal robots, especially during movement. At slow speeds, a quadrupedal robot may move only one leg at a time, ensuring a stable tripod. Four-legged robots also benefit from a lower center of gravity than two-legged systems. [1]
Four legged robots include:
Six-legged robots, or hexapods, are motivated by a desire for even greater stability than bipedal or quadrupedal robots. Their final designs often mimic the mechanics of insects, and their gaits may be categorized similarly. These include:
Six-legged robots include:
Eight-legged legged robots are inspired by spiders and other arachnids, as well as some underwater walkers. They offer by far the greatest stability, which enabled some early successes with legged robots. [1]
Eight-legged robots include:
Some robots use a combination of legs and wheels. This grants a machine the speed and energy efficiency of wheeled locomotion as well as the mobility of legged navigation. Boston Dynamics' Handle, a bipedal robot with wheels on both legs, is one example. [29]
Bipedalism is a form of terrestrial locomotion where a tetrapod moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped, meaning 'two feet'. Types of bipedal movement include walking or running and hopping.
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped. Quadruped animals are found among both vertebrates and invertebrates.
Walking is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an 'inverted pendulum' gait in which the body vaults over the stiff limb or limbs with each step. This applies regardless of the usable number of limbs—even arthropods, with six, eight, or more limbs, walk. In humans, walking has health benefits including improved mental health and reduced risk of cardiovascular disease and death.
Robot locomotion is the collective name for the various methods that robots use to transport themselves from place to place.
Passive dynamics refers to the dynamical behavior of actuators, robots, or organisms when not drawing energy from a supply. Depending on the application, considering or altering the passive dynamics of a powered system can have drastic effects on performance, particularly energy economy, stability, and task bandwidth. Devices using no power source are considered "passive", and their behavior is fully described by their passive dynamics.
The zero moment point is a concept related to the dynamics and control of legged locomotion, e.g., for humanoid or quadrupedal robots. It specifies the point with respect to which reaction forces at the contacts between the feet and the ground do not produce any moment in the horizontal direction, i.e., the point where the sum of horizontal inertia and gravity forces is zero. The concept assumes the contact area is planar and has sufficiently high friction to keep the feet from sliding.
Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that in water, with reduced friction being replaced by the increased effects of gravity.
BigDog is a dynamically stable quadruped military robot that was created in 2005 by Boston Dynamics with Foster-Miller, the NASA Jet Propulsion Laboratory, and the Harvard University Concord Field Station. It was funded by DARPA, but the project was shelved after the BigDog was deemed too loud for combat.
Marc Raibert is the Executive Director of the Boston Dynamics AI Institute, a Hyundai Motor Group organization that is focused on solving the most important problems in robotics and artificial intelligence to achieve fundamental advances in the engineering and science of robotics. Raibert was the founder, former CEO, and now Chairman of Boston Dynamics, a robotics company known for creating BigDog, Atlas, Spot, and Handle.
A six-legged walking robot should not be confused with a Stewart platform, a kind of parallel manipulator used in robotics applications.
Boston Dynamics is an American engineering and robotics design company founded in 1992 as a spin-off from the Massachusetts Institute of Technology. Headquartered in Waltham, Massachusetts, Boston Dynamics has been owned by the Hyundai Motor Group since December 2020, but having only completed the acquisition in June 2021.
Webots is a free and open-source 3D robot simulator used in industry, education and research.
RHex is an autonomous robot design, based on hexapod with compliant legs and one actuator per leg. A number of US universities have participated, with funding grants also coming from DARPA.
The evolution of human bipedalism, which began in primates approximately four million years ago, or as early as seven million years ago with Sahelanthropus, or approximately twelve million years ago with Danuvius guggenmosi, has led to morphological alterations to the human skeleton including changes to the arrangement, shape, and size of the bones of the foot, hip, knee, leg, and the vertebral column. These changes allowed for the upright gait to be overall more energy efficient in comparison to quadrupeds. The evolutionary factors that produced these changes have been the subject of several theories that correspond with environmental changes on a global scale.
Bio-inspired robotic locomotion is a fairly new subcategory of bio-inspired design. It is about learning concepts from nature and applying them to the design of real-world engineered systems. More specifically, this field is about making robots that are inspired by biological systems, including Biomimicry. Biomimicry is copying from nature while bio-inspired design is learning from nature and making a mechanism that is simpler and more effective than the system observed in nature. Biomimicry has led to the development of a different branch of robotics called soft robotics. The biological systems have been optimized for specific tasks according to their habitat. However, they are multifunctional and are not designed for only one specific functionality. Bio-inspired robotics is about studying biological systems, and looking for the mechanisms that may solve a problem in the engineering field. The designer should then try to simplify and enhance that mechanism for the specific task of interest. Bio-inspired roboticists are usually interested in biosensors, bioactuators, or biomaterials. Most of the robots have some type of locomotion system. Thus, in this article different modes of animal locomotion and few examples of the corresponding bio-inspired robots are introduced.
Arm swing in human bipedal walking is a natural motion wherein each arm swings with the motion of the opposing leg. Swinging arms in an opposing direction with respect to the lower limb reduces the angular momentum of the body, balancing the rotational motion produced during walking. Although such pendulum-like motion of arms is not essential for walking, recent studies point that arm swing improves the stability and energy efficiency in human locomotion. Those positive effects of arm swing have been utilized in sports, especially in racewalking and sprinting.
A walking vehicle is a vehicle that moves on legs rather than wheels or tracks. Walking vehicles have been constructed with anywhere from one to more than eight legs. There are many designs for the leg mechanisms of walking machines that provide foot trajectories with different properties.
Auke Jan Ijspeert is a Swiss-Dutch roboticist and neuroscientist. He is a professor of biorobotics in the Institute of Bioengineering at EPFL, École Polytechnique Fédérale de Lausanne, and the head of the Biorobotics Laboratory at the School of Engineering.
Elena Garcia Armada is a Spanish researcher, roboticist and business founder who has conducted research on prosthetic exoskeletons to aid people in walking.
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