The School of Energy and Power Engineering at Beihang University has developed a new type of insect-level legged microrobot, called BHMbot, which achieves ultra-fast cable-free running speed. BHMbot is equipped with an advanced drive mechanism based on electromagnetic drives, which solves the problem of severe drop in running speed after carrying the necessary load. The robot can achieve a high-speed jumping movement mode similar to some mammals through a specific running gait.
The robot has two independently controlled front legs that can achieve a variety of movement trajectories, such as circles, rectangles, letter shapes, and can cross obstacles.
The BHMbot is able to reach a relative speed of 17.5 body lengths per second (BL/s) without a cable, a remarkable achievement considering its 2 cm length. In tests of an optimized prototype, the BHMbot reached a maximum speed of 50 cm/s (33.3 BL/s) without a payload.
The design of BHMbot is mainly inspired by fast-running animals in nature, especially mammals and insects that have efficient jumping and running abilities. Researchers have observed that these creatures achieve high-speed movement through a specific running gait and jumping momentum. This movement pattern includes the swinging of the legs and the impact of the ground, which allows the body to temporarily become airborne, thereby greatly increasing the speed of movement.
California mite (Paratarsotomus macropalpis): This mite can run at extremely high speeds, reaching a maximum speed of 192.4 body lengths per second (BL/s). Its high-speed running ability inspired researchers to design BHMbot, a micro-robot that can achieve ultra-high-speed operation.
Australian tiger beetle (Cicindela eburneola): This tiger beetle also has extremely high running speed, reaching a maximum speed of 171 BL/s. Its movement pattern provided a reference for the design of BHMbot, especially in terms of how to achieve efficient movement with a simple leg structure.
Cockroach (Nauphoeta cinerea): Common cockroaches run at a speed of about 13 BL/s. Although the speed of BHMbot is slightly lower than that of some specific insects, it still surpasses the movement performance of common cockroaches, demonstrating its success in mimicking insect movement.
Running mammals: BHMbot's bouncing motion mechanism is inspired by the running posture of mammals. When mammals run, their bodies experience a "bouncing" process, that is, the body is lifted into the air when the limbs leave the ground. This bouncing motion generates propulsion through the coordinated swinging of the limbs and contact with the ground. By imitating this bouncing pattern, BHMbot achieves an efficient movement method similar to running mammals.
Design and Mechanics
The design focus of BHMbot (Beihang Micro Robot) lies in its unique driving mechanism and structural layout, which enables high-speed cable-free operation while carrying a load. The following is a detailed introduction to the design and mechanism of the robot:
BHMbot: detailed introduction to the design and mechanism of the robot
Drive mechanism
Core components: The driving system of BHMbot consists of an electromagnetic drive, a four-bar linkage mechanism and front legs. Each electromagnetic drive includes a cantilever, a permanent magnet and an air-core coil. When the coil is energized, an alternating electromagnetic force is generated on the magnet, causing the cantilever to vibrate.
Implementation of bouncing motion: BHMbot uses a bouncing motion that is achieved through periodic impacts between the front legs and the ground. The front legs and the ground generate a downward impact force, which causes the robot to obtain an upward reaction force. This reaction force enables the robot to bounce on the ground, thereby achieving high-speed forward motion. This mechanism imitates the running gait of mammals, allowing the robot to maintain high speed without a cable.
Electromagnetic actuators: The robot is equipped with two electromagnetic actuators, each consisting of a cantilever, a permanent magnet, and an air-core coil. The actuator outputs a vibration motion, which is converted into a swinging motion of the front legs through a four-bar linkage. The actuator has a high power density (over 200W/kg) and can be operated at a low operating voltage (less than 2V), avoiding the need to design a high-voltage boost module.
Driving process: When an alternating voltage is applied to the coil, the current in the coil generates alternating attractive and repulsive forces on the magnet. This causes the magnet to vibrate back and forth on the cantilever, and this vibration is converted into the swing of the front legs through the four-bar linkage. The swing of the front legs will push the robot forward or turn.
BHMbot: Driving process
Structural design
Design of front legs and hind legs: In order to generate upward bouncing force, the front legs of BHMbot are designed to be longer than the hind legs, forming an upward tilt angle (θ0). This design enables the front legs to effectively push the robot upward during the swinging process, thereby achieving high-speed forward movement.
Movement mode: BHMbot's operation is divided into multiple stages, including front leg swinging, body bouncing, air flight and landing. The coordinated movement of each stage enables the robot to achieve continuous forward movement. By controlling the operating frequency and swing amplitude of the electromagnetic drive, BHMbot can adjust its forward speed and steering angle.
Materials and Components: The robot is made of a variety of lightweight materials, including a carbon fiber frame, microcircuit boards, and lithium batteries. Its structure is designed to be very compact to reduce weight and improve operating efficiency.
Movement Analysis
Running gait: BHMbot's running gait is achieved by swinging its front legs at high frequency. This gait is similar to the running style of mammals. The robot will go through multiple consecutive bouncing cycles during operation. Each bouncing cycle includes four main stages: front leg swinging, bouncing, flying in the air, and landing.
Relationship between running speed and jumping frequency: The running speed of BHMbot depends on the combination of jumping length and jumping frequency. The optimized design enables the robot to maintain a high jumping frequency even when carrying a load, thus achieving a higher speed.
BHMbot: Relationship between running speed and jumping frequency
Control strategy
Independent drive control: BHMbot's two front legs can be controlled independently, which enables it to achieve a variety of complex motion trajectories. When both front legs are driven at the same time, the robot moves in a straight line; when only one side of the front leg is driven, the robot turns around the other side. This independent drive design allows BHMbot to easily achieve a variety of complex path planning, such as circular, rectangular and letter-shaped trajectories.
Wireless Control: BHMbot is equipped with a wireless microcontroller unit (MCU) that can receive control instructions from a computer or smartphone via Bluetooth. This allows operators to remotely control the robot and adjust its movement path and speed in real time.
Energy Management and Optimization
Low voltage operation: BHMbot's electromagnetic driver is designed to operate at a lower voltage (less than 2V), which eliminates the need for a high voltage boost module. This low voltage operation not only improves the stability of the circuit, but also reduces power consumption.
Optimized design: By optimizing the drive and mechanical structure, such as adjusting the length ratio of the front and rear legs and the distance between the drive and the magnet, BHMbot has achieved the ability to maintain high speed while carrying a load. This optimization ensures the efficient operation of the robot, especially when it needs to carry sensors or other electronic devices in actual application scenarios.
Performance
Performance of BHMbot
Operating speed
Maximum operating speed: BHMbot is able to reach a relative operating speed of 17.5 body lengths per second (BL/s) without a cable, a remarkable achievement relative to its 2 cm body length. In optimized prototype testing, BHMbot reached a maximum speed of 50 cm/s (33.3 BL/s) without a payload.
Effect of load on speed: The study showed that the running speed of BHMbot increased after the load increased to a certain extent (i.e., reaching the optimal load mass), because the increase in jump frequency offset the decrease in jump length. This discovery allows BHMbot to maintain a high movement speed while carrying control electronics and power supply.
Turning Agility
Relative centripetal acceleration: The maximum relative centripetal acceleration of BHMbot on the paper surface is 65.4 BL/s² (counterclockwise turning), and the relative centripetal acceleration when turning clockwise is 39.4 BL/s². Such an acceleration level shows that the robot has extremely high turning agility, far exceeding similar micro robots.
Turning radius: By adjusting the frequency difference between the two actuators, BHMbot is able to achieve turns of different radii. The larger the frequency difference, the smaller the turning radius. In extreme cases, the robot can complete a 300-degree clockwise turn and a 320-degree counterclockwise turn in 0.4 seconds, with turning radii of 1.0 cm and 0.7 cm respectively.
Energy efficiency performance
Cost of Transportation (COT): The cost of transportation (COT) of BHMbot is used to measure its energy efficiency. The value of COT is 303.7, indicating that its overall energy efficiency is high. In addition, the COTM of BHMbot's driving mechanism is 9.3, which is close to the lowest value reported among insect-scale microrobots, showing the high efficiency of its driving mechanism.
Power consumption: The BHMbot consumes 1.77 watts at top speed. Although the high current on the circuit board and the inefficiency of the electromagnetic drive have some impact on its overall energy efficiency, the drive mechanism itself still shows efficient energy utilization.
Various surface adaptability
The BHMbot was tested on different surfaces, such as glass, wood, paper, and plastic, and showed high speed and acceleration performance. Testing on these surfaces further verified the robot’s wide adaptability and high performance.
The robot can also maintain operation on complex surfaces such as plastic plates with shallow water and circular pipes, demonstrating its ability to adapt to a variety of environments.
Compared with other micro robots
Compared to other insect-scale microrobots, BHMbot excels in relative running speed and turning agility, surpassing many other robots even at a larger mass level. Although its speed is slightly slower than the fastest cockroach (50 BL/s), it has surpassed common cockroaches and some other insects (such as the marathon scorpion and the bumblebee).
Use case
The high performance and miniaturized design of BHMbot make it have wide potential in multiple practical application scenarios. The following are several main application scenarios of this robot:
Use case of BHMbot
Search and rescue mission
Search and rescue: BHMbot is able to pass through narrow spaces, reach the location of trapped people, and perform search and rescue missions. In a simulated scenario, BHMbot was used to move in an environment full of obstacles and approach a simulated collapsed structure to collect SOS signals from a built-in Bluetooth speaker. The micro-microphone (MEMS) integrated on the robot can capture sound signals and transmit data to a smartphone or computer for analysis and decoding. After completing the signal collection, BHMbot is also able to return to the starting point along another path, which provides an effective technical means for actual search and rescue missions.
Internal structure inspection
Engine inspection: The high maneuverability of BHMbot makes it very suitable for structural inspection inside complex machinery, such as internal inspection of aircraft turbofan engines. In the experimental demonstration, BHMbot successfully passed through the narrow channel between the stator blades of the turbofan engine and moved quickly inside the turbine tail cone. This capability shows that BHMbot can be used to inspect the internal structure of complex machinery and potentially equipped with a micro camera to capture internal images.
Collaboration with drones
Transportation and deployment: Since BHMbot has limited battery life and cannot climb to high altitudes, it can collaborate with drones (such as quadrotors) when it needs to move long distances or reach high geographical locations. BHMbot can be carried by a drone to a location close to the target area and recovered by the drone after the mission is completed. In the experimental demonstration, BHMbot was installed in the pod of a quadrotor, transported from the ground to the desktop, and autonomously detached from the pod after arriving. After completing the mission, it returned to the pod and waited for the drone to transport it back to the starting point. This collaborative approach shows the potential of BHMbot in tasks such as search and rescue.
Navigation in complex environments
Obstacle avoidance and path planning: BHMbot is able to navigate in complex environments full of obstacles. Through real-time control and pre-programmed path instructions, the robot can move accurately on different surfaces and can pass through narrow tunnels or through complex terrain. In the experiment, BHMbot successfully passed through an area scattered with stones and leaves, and smoothly passed through a tunnel that was only 2.5 cm wide and 4 cm high, demonstrating its adaptability in complex environments.
