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Caltech Morphobot M4 (prototype)

Caltech Morphobot M4 scalable all-electric VTOL and roadable subscale prototype vehicle


Morphobot M4 (prototype)
Pasadena, California, USA

The M4 was designed by Mory Gharib, a professor of aeronautics and bioinspired engineering at Caltech, in partnership with Alireza Ramezani, an assistant professor of electrical and computer engineering at Northeastern University. The purpose of the project was to use highly resilient and fault-tolerant locomotion strategies inspired from nature (such as birds, mammals and aquatic animals) to navigate complex terrains by developing multi-functional appendages for a subscale prototype electric vertical takeoff and landing (eVTOL), walkable and roadable vehicle.

Definitions and Some Descriptions of M4's Capabilities:

  • A* Algorithm: A searching algorithm that is used to find the shortest path between an initial and final point
  • AI: Artificial intelligence
  • CPU: Central processing unit (or computer)
  • Crouching: Tilting the four wheels outward to lower the entire fuselage, or crouch, allowing the vehicle to pass under a low object
  • Loco-manipulation: Able to use two wheel trends as hands to pick up and let go of objects
  • Microcontrollers: Simple miniature computers designed to control small features of a larger component
  • MIP: A mobile inverted pendulum (MIP) is where the two wheels of the robot are traveling forward or reverse on the ground and the fuselage is balancing to almost the vertical because its two top propeller wheels are active and keeping the vehicle in a stable vertical position
  • Multimodal: Using components that can have multiple functions
  • Object manipulation: The robot can turn its wheels and use the tread of the wheel to hold and let go of objects
  • Scalability: The inventors affirm their vehicle design can be scaled to a larger size where it can carry passengers, cargo or both
  • SLAM: Autonomous vehicle navigation in a complex and unknown environment to determine the best path to travel and to also avoid objects is called simultaneous localization and mapping or SLAM
  • Quadrupedal locomotion: The formal definition is walking using four feet. In this case, four "walking" wheels that move the robot across the ground or any hard surface.
  • Tumble: Using two active propellers and two wheels to tumble over large obstacles
  • UGV: Four wheels for unmanned ground vehicle (UGV) travel (or also called ground or road travel)
  • WAIR: Wing-assisted incline running (WAIR) over 45 degree slopes. Using two front active propellers and two rear wheels to move the vehicle up a 45 degree slope.

Morphobot M4 Subscale Prototype eVTOL, Walkable and Roadable Scalable Vehicle
The Multi-Modal Mobility Morphobot M4 is a remote controlled or autonomous scalable working eVTOL, walkable and roadable subscale prototype vehicle. The M4 stands for Multi-Modal Mobility Morphobot (or four "M"s, therefore M4). From the start of the project, the ultimate goal was to make a vehicle that can not only travel just about anywhere but is also scalable for passenger and air cargo transportation without allowing the performance of the full scale vehicle to decline.

Another main mission objective of the M4 is to use artificial intelligence (AI) to find the best paths from its departure point to its final destination and use the least amount of energy to accomplish the task. Included in the AI mission is for the M4 to be able to perform search and rescue, and again most efficiently, in an unknown environment and to save people in need. In addition, another main goal is to provide this information to any inventor who would like to use their research.

General Components:
The subscale multi-modal mobility morphobot has a small main computer, microcontrollers, electronics, motors, servo-motors, gears, large wheels, propellers, sensors, a chassis, fixed skid landing gear and more. The chassis superstructure and shrouded propeller-wheel assembly is primarily made of carbon fiber and 3D-printed parts. The use of these materials were used due to their great strength-to-weight ratio.

The M4 has a Jetson Nano CPU (an affordable robotics-focused miniature computer made by Nvidia) used for high-level decision-making for autonomous multi-modal path planning. The computer can decide what locomotion configuration it should transform to, to get from its starting place to its final destination. If there is an obstacle, the M4 robot can either stand up and look over the object or it can fly up and look over the object and decide what action it needs to take to get to its destination. The robot utilizes two microcontrollers for low-level locomotion control. One microcontroller is used for posture and wheel motion control while the other microcontroller is used to regulate thrusters.

M4 Capabilities:
The M4 has four wheels and four propellers inside the wheels allowing for both ground and air travel in almost any rugged or challenging environment. The wheels are oversized which keeps the fuselage higher from the ground than an automobile and allows the vehicle to easily travel off-road. For example, the M4 can travel on paved roads, in grassy fields or on dirt trails. The wheels can move in multiple directions allowing the M4 to lower itself, walk, fly, hold objects and more. When a road or off-road area becomes impossible to travel on, the aircraft will easily convert to a multicopter because its wheels will turn to a horizontal position and the propellers, located inside the rim of the wheels, allows the M4 fly.

The vehicle can crawl using its four wheels like arms and legs and the trend on the wheels will not move allowing the vehicle to crawl like a person. The M4 can also lower its fuselage to crawl under an object. The vehicle can stand on its rear wheels with its fuselage almost to the vertical by using its opposite wheels that are also propellers, to keep the M4 in the vertical position. In addition, the vehicle can roll across a flat surface or roll up a 45 degree surface and stay in a vertical position with the use of its propellers.

M4 Capabilities Summary:

  1. Autonomous vehicle navigation in unknown environments to travel from a departure point to a final destination, including the ability to search and rescue people on its own, in unknown environments
  2. Balance vertically while stationary or while moving using its wheels and propellers (mobile inverted pendulum)
  3. Crawl (using the wheels like legs and arms)
  4. Crouch (lower the entire fuselage)
  5. Fly
  6. Object manipulation
  7. Roadable (unmanned ground vehicle)
  8. Scout (search and rescue)
  9. Tumble over large obstacles
  10. Walk using the wheels
  11. Wing-assisted incline running (WAIR) over 45 degree slopes

Scalability Information:
The inventors call a mobile robot design scalable, if its payload capacity can be increased so its entire system of actuators, mechanisms, motors, propellers, computers and power source does not severely effect its mobility in any configurations. When the payload capacity does not allow the vehicle to move in every configuration, the scale is too large. Using high strength and light weight components is a key to scalability.

Minimizing Total Consumed Energy:
The inventors report the objective of path planning is to minimize the total energy consumed by the autonomous vehicle and optimize the choice of locomotion. For example, after a natural disaster that destroys the landscape, roads and buildings, a full scale M4 could fly, walk or drive into the an area and search for people who need rescued and transport them to safety. Multiple tests have taken place to validate the ability of the robot to perform all the functions listed above.

Abstract Available To Anyone:
The inventors created a detailed abstract and it is online available for any inventor to use. The abstract web page also includes author information, an introduction, mathematical equations, discussions, methods, experimental results, pictures, graphics, videos, acknowledgements, references, ethics declarations, supplementary information, a peer review file, and rights and permissions information. The abstract can be read online or it can be downloaded as a 15 page PDF here: Abstract: Multi-Modal Mobility Morphobot (M4) with appendage repurposing for locomotion plasticity enhancement, Nature, June 27, 2023

The inventors envision their vehicle will be used for air taxi service, emergency medial service, search and rescue, private use, surveillance, space exploration and air cargo service.

The inventors point out that any text or graphics used must be attributed and credit given to them. If any changes were made to their graphics, that this must be noted as well. This entire web page is attributed to the inventors Mory Gharib, professor of aeronautics and bioinspired engineering at Caltech and Alireza Ramezani, assistant professor of electrical and computer engineering at Northeastern University. Here are two links to the creative commons web pages, Link 1 and Link 2.

We used several graphics and most were resized and made with to 2000X1333 pixel size, for easy viewing and so that all pictures have the same aspect ratio when viewed as a thumbnail or as the full size picture. That is, we wanted to make sure all the pictures in a row have the same height and width. Several pictures have added or deleted space to the sides to keep the aspect ratio the same on all the graphics and pictures.

The graphic with a photograph of the M4 at the top of this web page and M4 locomotion abilities graphic below this paragraph is cropped. We cropped the top off from color picture of the M4, to use the M4's multi-model capability graphic (with the light blue background) by itself. The M4 locomotion abilities graphic has the lowercase "b" in the upper left-hand corner. To make the graphic cleaner, we deleted the lowercase "b".

Sihite, E., Kalantari, A., Nemovi, R. et al. Multi-Modal Mobility Morphobot (M4) with appendage repurposing for locomotion plasticity enhancement. Nat Commun 14, 3323 (2023). Abstract website link here. Abstract as a PDF link here.

M4 capabilities diagram

M4 capabilities diagram

More M4 capabilities diagram

More M4 capabilities diagram

M4 search and rescue example diagram

M4 search and rescue example diagram


  • Aircraft type: Open source scalable passenger or cargo eVTOL, roadable and walkable subscale prototype vehicle
  • Piloting: Remote or autonomous (using artificial intelligence)
  • Propellers: 4 propellers
  • Electric motors: 4 electric motors for the propellers
  • Power source: Batteries
  • Fuselage: Carbon fiber composite
  • Road travel: 4 wheels for road travel
  • Landing gear: Fixed skid landing gear
  • Safety features: Distributed Electric Propulsion (DEP), provides safety through redundancy for its passengers and/or cargo. DEP means having multiple propellers (or ducted fans) and motors on the aircraft so if one or more propellers (ducted fans) or motors fail, the other working propellers (or ducted fans) and motors can safely land the aircraft. There are also redundancies of critical components in the sub-systems of the aircraft.