Dialogue@ZJU: The ingenious design of ZJU’s jumping robot

【Editor’s Note: Imagine a robot made not of cold, hard steel, but supple, flexible materials that allow it to leap with the grace of a gazelle. At Zhejiang University, Prof. ZHAO Peng and Dr. ZHANG Chengqian from the School of Mechanical Engineering have turned this vision into reality. Their innovative creation, a magnetically-driven bistable soft jumper, can soar over 100 times its own height in mere milliseconds, setting a new benchmark for robotic agility and speed. This research, detailed in Science Robotics in an article titled “Bistable soft jumper capable of fast response and high takeoff velocity”, offers a new perspective on efficient movement for soft robots. We delve into the fascinating world of soft jumping robots with the research team.

The moment a soft robot leaps out of the water 

Q1: What exactly are soft jumping robots?

A: Soft jumping robots, as the name suggests, are crafted from pliable materials specifically designed for jumping. Compared to their rigid counterparts, these robots exhibit remarkable impact resistance, allowing them to absorb and disperse energy during collisions to minimize damage. Moreover, they primarily use jumping as their mode of movement, which provides exceptional environmental adaptability, enabling them to navigate and operate adeptly in challenging terrains. 

Yet, soft materials typically pose a significant challenge: their low elastic modulus makes them prone to deform under stress, which can hinder their ability to generate enough force for high power output. On the other hand, existing soft jumping robots often struggle with low drive rates and extended reset times after deformation, severely limiting their response speed.

Q2: Why do traditional soft robots respond relatively slowly?

A: Soft jumping robots are usually made from soft polymers or gels, whose stress-strain properties lead to delayed force transmission and response. Additionally, various drive methods for soft robots — pneumatic, hydraulic, and photothermal systems — also face intrinsic limitations, such as the time needed to build pressure or increase temperatures, further decelerating the response speed of soft robots.

Q3: What materials and structures are used in your jumping robot? What stands out as its biggest innovation?

A: The magic behind this groundbreaking robot lies in its pyramid structure, where each of the four panels comprises magnetic materials with a high Young’s modulus, ensuring both rigidity and drive power. The creases, made from silicone with a lower Young’s modulus, harness their superb elasticity for folding.

Schematic diagram of the working principle of a bistable jumping robot 

The biggest innovation is the creation of a bistable structure driven by a magnetic field. By combining three-dimensional folding with magnetization, this design makes rapid, explosive jumps possible, storing energy efficiently and releasing it swiftly. This bistable mechanism has set new benchmarks in both jumping height and response time.

Q4: Where did the inspiration for this structural innovation originate?

 Snap-through transition behavior captured by a high-speed camera

A: The principle of using a bistable structure to achieve explosive jumping was inspired by the Venus flytrap, which swiftly closes its leaves to seize prey. By meticulously optimizing the robot’s design, we replicated this natural efficiency in our soft jumper.

Q5: How does magnetism factor into the robot’s design?

A: The four panels of this jumping robot are made of magnetic materials, and their magnetization direction is designed to align with an external pulsed magnetic field. This alignment generates magnetic torques that can cause the three-dimensional folding structure to flap, driving the bistable jumping robot to switch rapidly between different stable states, allowing for energy storage and release. In other words, magnetism serves as the powerhouse for the jumping robot.

Q6: How is bistability achieved?

A: Bistability is realized through the elastic folding of the pyramid structure. As the robot flipsflaps, it undergoes both stretching and bending strains at the creases, which means that the elastic potential energy accumulated through strain does not monotonically increase with the flapping angle; instead, there are local minima during the flapping process. When at a local minimum of elastic potential energy, the structure maintains a fragile stable state. The local minima of elastic potential energy and the initial zero point together form the two stable points of the bistable structure.

Q7: The video in your paper shows the robot jumping out of the water. What was the purpose of that scene?

Inter-medium jumping demonstration of a bistable jumping robot

A: The dramatic leap out of the water is the ultimate testament to the robot’s capabilities. For one thing, it demonstrates the robot’s high output power, enabling it to overcome underwater resistance during takeoff. For another, it showcases the robot’s maneuvarability; we use a magnetic field to precisely adjust the robot’s posture as it emerges from the water, thereby minimizing resistance and achieving a clean exit from the water.

Compared to jumping in an atmospheric environment, inter-medium jumping involves more factors and faces greater resistance, which hasn’t been addressed in previous research on jumping robots. By designing this scene, we further demonstrate the superior performance and immense potential of the jumping robot.

Q8: Why is this robot capable of changing its jumping direction?

Directional jumping demonstration

A: We guide the jumping direction of the robot by changing the magnetic field direction. When jumping in place, the magnetic field is applied perpendicularly to the horizontal plane of the jumping robot, driving it to jump vertically. When the magnetic field is angled, the entire structure of the jumping robot also undergoes deflection during the explosive jumping process, altering the angle and direction of contact with the ground, leading to jumps in different directions. Experimental results show that the jumping robot can achieve omnidirectional jumps up to 360 degrees depending on the applied magnetic field direction.

Q9: What records has this robot set?

A: This remarkable robot leaps over 108 times its own height and responds in under 15 milliseconds, setting new records for both jumping height and response time among soft robots.

Q10: What potential application scenarios do you foresee for this type of robot?

A: Our paper highlights the size effect of bistable jumping robots; even if the overall size is reduced, the jumping robot still possesses salient jumping capabilities. In the future, with advanced manufacturing technology, these robots could revolutionize biomedicine by enhancing the movement and output power of micro-robots for internal medical use. Its structure is ideal for integrating probes, drug capsules, or other payloads. Furthermore, the bistable jumping robot we proposed is wirelessly driven by an external magnetic field, providing greater mobility compared to traditional tethered robots, allowing it to perform tasks in confined environments, such as complex narrow pipelines. It is expected to play a crucial role in pipeline inspection in the future.

Apart from magnetic drive, the bistable structure is also suitable for soft robots using other drive methods, opening up new avenues for the development of high-performance soft robots.

Q11: What issues does the field of magnetic functional device shaping and manufacturing primarily focus on?

A: At its core, this field revolves around the intricate design and production of magnetic pole distribution. Functional devices composed of magnetic materials play a vital role in modern society, such as optimizing the distribution of internal magnetic poles in permanent magnet motors to achieve the highest electric drive efficiency, where the precise shaping of different pole distributions is key to performance outcomes. Thanks to the unique non-contact, rapid response, and high precision of magnetic functional materials, they provide new ideas for the development of novel flexible devices. In addition to achieving high output power for the soft jumping robot using flexible structures and magnetization design, our team has also devised an ingenious magnetization of films, theoretically proving its three-dimensional decoupling characteristics of magnetic fields and displacement, and developed a folding magnetization forming process to create a new soft tactile sensor with three-dimensional force decoupling. It is evident that new theories and forming methods for magnetization design can effectively empower the development of new magnetic devices.

Group photo of the main members of the research team

Looking ahead, our team will further explore and investigate the manufacturing and shaping technologies of magnetic functional devices, with a focus on dynamic control methods for magnetic particles. The goal is to achieve more flexible and precise designs of three-dimensional magnetization structures. Based on these magnetization design principles, we aim to develop magnetically-driven devices and sensors with novel properties.

Adapted and translated from the article written by the research team

Translator: FANG Fumin

Photo: The research team led by Prof. ZHAO Peng

Editor: TIAN Minjie