BHMbot: A new type of ultra-fast insect-scale microrobot that runs faster than cockroaches

8 min readAug 23, 2024

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:

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 proces: 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.

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.

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

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.

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