Nature-Inspired Robotics: How the M4 Robot Mimics Animal Locomotion

Modern-day robotics (especially soft robotics) take a lot of inspiration from nature. A lot of these inspirations come from trying to mimic how animals move to give robots a better degree of locomotion. Many animals can navigate tricky terrain by using multi-functional appendages that adapt to different situations. These have naturally evolved over time to provide a benefit to the local environment for these animals, but these principles can also be adapted for robots.

Many birds and aquatic animals have multi-functional appendages that allow them to navigate changing terrain. This is especially true for aquatic animals that also go on land (such as seals), as their appendages need to be efficient in both the water and on land. Similarly, many birds not only fly but also walk in a lot of rough terrain and encounter many predators, so their locomotive appendages (wings and feet) need to work together to avoid hazardous situations.

This level of multi-functional appendage locomotion can be mimicked and adapted for man-made robots. The ability to mimic the natural locomotion mechanisms seen in nature could help to create new multi-functional robots that have far more capabilities. A new robot has been created called the M4, which can negotiate multiple unstructured environments on land and in the air using an interchangeable combination of wheels, thrusters, and legs.

The M4 robot, also known as the Multi-Modal Mobility Morphobot, was developed by a team of experts led by Mory Gharib, the director of Caltech's Center for Autonomous Systems and Technologies, and Alireza Ramezani, assistant professor of electrical and computer engineering at Northeastern University. According to Gharib, the robot's flexibility of motion, coupled with artificial intelligence, allows it to choose what form of locomotion will be most effective based on the terrain ahead of it.

What is the M4 Multi-Modal Robot?

Multi-Modal Mobility Morphobot (M4) is a very versatile robot with a high degree of modal diversity. The design has been inspired by animals that have large locomotion plasticity—such as birds. The M4 robot can perform different modes of locomotion by repurposing its appendages and can switch between an unmanned ground vehicle (UGV), mobile inverted pendulum (MIP), unmanned aerial system (UAS), thruster-assisted MIP, legged locomotion, and loco-manipulation in MIP modes.

The M4 has a number of multi-functional components in its system. Powered by a lithium-polymer (Li-Po) battery, the M4 contains 8 hip servos that provide up to 55 kg cm torque and wheel motors that allow a large degree of locomotion—and the combination of these components means that the appendages can be moved in sideway, swing, and wheel rolling motions. 

The ability to change the locomotion of these appendages means that the M4 can fly, roll, crawl, crouch, balance, tumble, scout, and loco-manipulate. Using the different locomotion modes, the M4 robot can navigate rough terrain and slopes up to 45°. There are many sensors and computers on the M4 that can balance its different appendages autonomously across difficult terrains. 

Multi-modal locomotion is achieved in the M4 through body and appendage morphing, where a combination of rigid and soft links and actuated joints allow the robot to shapeshift into different modal forms. The shapeshifting ability of the M4 means that the appendages can also change their function, from wheels to legs or thrusters.

Taking Inspiration from Nature to Build a Scalable Design

One of the main aims of the M4 project was to design a robot that was scalable and had a large degree of locomotion plasticity—much like those seen in nature—to be able to navigate tricky terrain. The design is considered to be scalable if the payload capacity can be increased without affecting the mobility of the robot.

To achieve a scalable design, the M4 copied the locomotion enhancement strategies utilised by many animals rather than trying to mimic the shape of animal appendages—as it was believed that this approach would be much more scalable. 

The scalable design was achieved by repurposing the appendages for other functions in a process called appendage redundancy (which aims to make the design as simple as possible by removing unnecessary, redundant components). This is primarily achieved by any added mass from components being shared by all modes. The M4 specifically achieves appendage redundancy by morphing, which maximises the locomotion plasticity and ensures that the components are used for all modes.

The design of M4 was heavily influenced by nature. For example, the team was inspired by how chukar birds use the flapping of their wings to give them leverage while running up steep inclines and how sea lions use their flippers for different types of locomotion on sea and land. "When encountering unknown environments, only robots that have the ability to repurpose their multi-modal components aided by artificial intelligence can succeed," says Gharib.

When it comes to the modes available, the M4 can morph into:

• A four-legged robot for quadrupedal locomotion.
• A four-thruster robot that flies.
• A two-thruster, two-wheeled robot that uses wing-assisted incline running (WAIR) over 45° inclines.
• A two-thruster, two-wheeled robot that can tumble over large obstacles.
• A two-wheeled, two-hand robot that can perform loco-manipulation tasks.
• A two-wheeled robot for mobile inverted pendulum (MIP) modes, enabling it to stand on two wheels.
• A four-wheeled robot that can crouch.
• A four-wheeled robot that can be used as an unmanned ground vehicle (UGV).

As far as the movements of the appendages go, a lot of inspiration has been taken from the behaviour of animals. This movement of the appendages has enabled the various modes mentioned above to become possible with a single robot.

For example, it is the aerodynamic lift forces that manipulate the contact friction and traction forces of the wheels to enable the robot to achieve a high degree of plastic locomotion so that it can traverse steep slopes. This is a process that has been inspired by wing-assisted incline running (WAIR) manoeuvres in birds—such as Chukars and Hoatzins—who achieve quadrupedal motion using both their wings and feet to climb up rough terrain and evade predators.

Likewise, there are many animals that repurpose their limbs to adapt to many different environments. Aquatic examples include turtles and sea lions who repurpose their front flippers for both swimming and walking on land. They adapt when they need to get a mobility advantage. Meerkats also eliminate any redundancy in their locomotion by standing on their hind legs. They can’t walk on two legs, but it provides a better field of view to monitor their surroundings. It’s these natural locomotion and redundancy principles that have been mimicked and adapted for the M4 so that it can morph into different configurations to better monitor and traverse its surroundings.

The Potential for Search and Rescue Operations

The ability to adapt to and traverse difficult terrains opens the M4 robot up to a number of potential application areas. One of the main ones is search and rescue efforts. In the aftermath of natural disasters—be it earthquakes, floods, landslides, or hurricanes—the landscape gets changed with a lot of damage that can make it difficult to traverse. 

In any of these natural disaster scenarios, the M4 could use its modal versatility to move across different landscapes or fly above it if ground movement is not possible to search for survivors. Additionally, because the M4 contains a range of sensors, there’s also the possibility to perform aerial surveillance and reconnaissance over a given area to search and locate survivors.

There is also the potential for the M4 to be used in rescue efforts when it’s too dangerous to send in humans. One main example is after a natural disaster and a building has been structurally compromised. The mobility inside those buildings is often tight, with collapsed stairways and corridors, and it is often too unsafe for humans to go looking in on a whim. So, there’s the potential in these scenarios to have the M4 handle the rough terrain and collapsed obstacles inside collapsed buildings to search for any survivors.

The M4 robot has been tested outdoors and has successfully navigated the terrain of Caltech's campus. With its autonomous capabilities and ability to adapt to various environments, the M4 robot represents a significant advancement in the field of robotics.

References:

Ramezani A. et al., Multi-Modal Mobility Morphobot (M4) with appendage repurposing for locomotion plasticity enhancement, Nature Communications, 14, (2023), 3323, https://www.nature.com/articles/s41467-023-39018-y

Sun J., Zhao J., An Adaptive Walking Robot With Reconfigurable Mechanisms Using Shape Morphing Joints, IEEE Robotics and Automation Letters, 4, (2019), 724-731, https://ieeexplore.ieee.org/document/8613926

Caltech, New Bioinspired Robot Flies, Rolls, Walks, and More, https://www.caltech.edu/about/news/new-bioinspired-robot-flies-rolls-walks-and-more