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Introduction
In Harvard University’s family of robots, there are two bionic robots that, despite their small size, are widely regarded as among the most promising robots for future development. I believe many can guess, without me saying much, they are the renowned bionic bee robot RoboBee and the bionic cockroach robot HAMR.
Dr. Chen Yufeng is currently a postdoctoral researcher at Harvard University, participating in the follow-up research and development of these two robots, and has published relevant articles in top journals multiple times. Recently, the Robot Lecture Hall invited Dr. Chen Yufeng to give an online talk titled “Multi-gait Movement of Bionic Micro Robots in Various Environments,” which provided a detailed interpretation of these two micro robots from Harvard University and their fluid dynamics analysis.
Dr. Chen Yufeng
To facilitate communication and learning, the Robot Lecture Hall has summarized Dr. Chen Yufeng’s sharing content as follows (due to the complexity of flapping fluid dynamics, this is not introduced here. Interested friends can click to read the original text and watch the video for learning):
The Development History of the Two Robots
Tracing back to 2008, Professor Robert J. Wood, the head of Harvard University’s Micro Robot Laboratory, confirmed the development project of RoboBee, taking the bumblebee in nature (weighing 120mg, flapping frequency of 180~250Hz) as the bionic object, designing a micro robot that weighs only 80mg and flaps at a frequency between 120~260Hz, and verifying that this micro flapping robot can take off vertically.
The body and wings of RoboBee are mainly made of carbon fiber, and the actuating device is piezoelectric ceramic. Although RoboBee is currently powered externally, integrating all the tiny components (about 20) into a micro robot weighing only 80mg is clearly time-consuming and labor-intensive if operated manually under a microscope, and the precision is not high.
To address this issue, Harvard University’s Micro Robot Laboratory developed a pop-up Micro-Electro-Mechanical Systems (MEMS) manufacturing technology in 2011. It combines different materials using printed circuit MEMS technology, cuts out the corresponding profiles using laser cutting technology, and finally utilizes the elastic differences between different materials to allow the robot to form automatically.
In 2012, an article about RoboBee was published in the top journal “Science,” and after years of effort, RoboBee achieved the functionality of a true ‘drone,’ capable of stable takeoff, hovering, and path tracking actions.
In 2015, Dr. Chen Yufeng proved through fluid dynamics simulation that RoboBee can swim underwater by changing its flapping frequency.
In 2016, another article about RoboBee was published in “Science,” at which time RoboBee had an electrode resembling a ‘straw hat’ on its head, which helped the micro robot adhere to the surfaces of most objects, including glass, wood, and soft leaves, something that no previous flying device could achieve, which is significant for bionic robots.
In 2017, with Dr. Chen Yufeng’s efforts, RoboBee broke through the surface tension of water, achieving the ability to leap from water to air, and published the article in the top journal “Science Robotics.”
The bionic cockroach quadruped robot HAMR is another famous micro robot developed by Harvard University. The ordinary cockroach in nature weighs about 0.75g and can reach a maximum speed of 1.5m/s. The HAMR robot differs slightly from real cockroaches; it has only four legs and can reach a maximum speed of 40cm/s, with two weight versions of 1.2g and 2.8g. Although this speed is far from that of real cockroaches, the speed of HAMR is already 8.4 times its own length per second, which just shows that real-world cockroaches are too formidable.
The HAMR robot was initiated in 2009, when the Harvard University Micro Robot Laboratory designed a six-legged micro bionic robot similar to a cockroach, but at that time, this robot could only crawl forward at a speed of 5cm/s.
In 2013, the micro robot team completely redesigned HAMR, adopting the same pop-up manufacturing technology as RoboBee, and at this time, HAMR looked much more advanced and achieved speeds of 30-40cm/s.
With the basic design completed, the micro robot team began further exploration of HAMR’s functionality, such as in 2017, HAMR could crawl on steep slopes.
In 2017, researchers integrated battery sensors and other components into HAMR, forming a new version of HAMR weighing 2.8g. Although the weight advantage was somewhat lost, this new HAMR no longer needed an external power supply.
In 2018, under Dr. Chen Yufeng’s exploration, HAMR gained amphibious capabilities, allowing it to switch freely between land and water.
Flapping Flight Robots’ Movement in Water and Air
In the research of flapping wing robots, fluid simulation and measurement during the flapping process help effectively enhance the robot’s lift and further install more sensors on the robot to achieve more functionalities. Therefore, the study of flapping fluid dynamics is a crucial step in the development of RoboBee.
However, due to the high flapping frequency of RoboBee, challenges arise in measuring the flapping motion itself, the lift and drag generated by the flapping motion, and the changes in the flow field during the research process. Dr. Chen Yufeng elaborated on the fluid dynamics of flapping robots in the video sharing; interested friends can click to read the original text and watch the video for learning.
In the study of RoboBee, Dr. Chen Yufeng’s main work includes studying whether the flapping motion in water and air has similarities and how the flapping robot can utilize this similarity to achieve more functionalities. In the research process, challenges also arise in how to use one actuation device to enable the robot to move in both air and water, and how to overcome and utilize the surface tension of water.
Through calculations, it is found that the flapping motion of the robot in water and air is very similar, so by changing the flapping parameters of the robot in water, similar flapping flow fields, lift, and drag can be achieved in air.
Experiments also show that although the flapping frequency is different in air and water, the passive rotation of the flapping motion is very similar, which also proves that the forces obtained by the two systems are very similar, therefore, by lowering the flapping frequency, RoboBee can also swim in water.
Thus, after performing the corresponding electrical treatment on RoboBee, the robot was placed in water to test at an estimated flapping frequency of about 5Hz. However, it was observed that the lift generated at a flapping frequency of 5Hz was insufficient for RoboBee to take off, and the system was not stable.
Interestingly, when the flapping frequency was increased to 11Hz, it was found that the robot became a passively stable system. This is different from the robot’s movement in air, where RoboBee is an actively unstable system that requires image tracking and control algorithms to maintain stable flight.
As for why these two different results occur, the research found that when RoboBee is in low-frequency motion in water, its own shaking is very large, which causes instability for the robot; while in high-frequency situations, due to the water’s resistance on the body, the robot becomes passively stable.
This means that in the air, the air’s resistance on the body is very small, but in water, although its effect on the robot’s flapping is very similar to that in air, its effect on the body is very large, thus helping the robot achieve a stable effect. In this regard, Harvard University has also established a corresponding mechanical model to explain why RoboBee can achieve passive stability in water.
It is worth mentioning that after discovering that RoboBee can achieve passive stability in water, researchers at Harvard University also provided resistance to RoboBee by adding a particularly large sail, achieving the same passive stability in air.
Therefore, after a series of studies and experiments, the Harvard University Micro Robot Laboratory was the first to realize that a micro robot weighing only 80mg could fly in the air, traverse into water, and swim in water. However, while completing these tasks, new problems also arose that need to be solved:1. The process of traversing from air to water is not very smooth, and the success rate is not 100%. Many times the robot gets stuck on the water surface and cannot go down. 2. Although the robot can swim well in water, once it enters the water, it can no longer return to the air.
The reason the micro robot RoboBee faces such dilemmas is due to surface tension, which is negligible for larger robots but becomes an insurmountable barrier for them due to their low weight and lift. A careful analysis of the impact of surface tension on RoboBee reveals two problems: 1. Surface tension creates a torque that makes the robot very unstable, preventing it from maintaining a stable posture. 2. When placing the robot on a force sensor, it was found that the surface tension to overcome when emerging from the water is 12 times its weight and 4 times its maximum lift, indicating that the robot cannot rely solely on its wings to return to the air.
In the face of these issues, Dr. Chen Yufeng proposed corresponding solutions, suggesting adding devices to stabilize RoboBee’s state on the water surface and finding ways to provide instantaneous lift to help RoboBee break through the surface tension that exists only on the water’s surface.
Therefore, they designed the following idea: First, generate hydrogen and oxygen by electrolyzing water and collect these gases, using the lift generated by the gases to slowly push the robot’s delicate parts above the water surface; second, ignite the hydrogen-oxygen mixture to achieve takeoff from the water surface through a miniature explosion.
Based on these ideas, Dr. Chen Yufeng designed a new type of amphibious micro robot. Compared to the old RoboBee, it first changed the shape of the wings, reduced the area, and increased the flapping frequency, thus enhancing the lift produced during flapping; secondly, it added four buoyancy devices to ensure that the robot maintains an upward posture when on the water surface; finally, it added a gas collection chamber to RoboBee, which also acts as an explosion chamber.
The upper layer of RoboBee’s gas chamber is made of titanium alloy to withstand the pressure generated during the explosion, while the sides are made of carbon fiber. The core component, a flat metal plate, is placed in the middle of the gas chamber. At a low voltage of about 5V, the electrode on the metal plate begins to electrolyze water, converting it into hydrogen and oxygen, with the mixed gas stored in the gas chamber while expelling water. When the gas is fully collected, the voltage is raised to 200V, at which point the copper piece above the electrode ignites the hydrogen-oxygen mixture, resulting in a momentary explosion.
After completing a series of tasks, the new RoboBee can generate thrust of up to 5N within 2‰ seconds, launching the robot out of the water. This enormous thrust allows RoboBee to achieve a jump height of 37cm, with the entire jumping and landing process taking 0.55 seconds.
Additionally, since the robot is wet when it first emerges from the water, the water increases the robot’s weight, making it unable to continue flying. Therefore, RoboBee is designed to be passively stable; after emerging from the water, it can maintain an upward posture and land passively and steadily on the ground. Once the water on the robot’s body dries, it can fly again.
Thus, through a series of studies, the Harvard University Micro Robot Laboratory has designed the world’s smallest amphibious robot that can operate in both water and air, with Science noting that this robot is at least 1000 times smaller than other robots with similar functions.
Quadruped Amphibious Robot
Before discussing the quadruped robot, let’s first discuss the impact of water’s surface tension on robots. The previously introduced RoboBee is a 160mg robot that needs to overcome surface tension that is 12 times its weight, so when researching micro robots, a lot of instantaneous explosive force is needed to successfully resist surface tension.
However, when the focus shifts to humans of different scales, a person’s weight is over 1000 times the surface tension, which is why we find it difficult to feel the impact of water’s surface tension on ourselves while swimming.
In contrast, on the scale between RoboBee and humans, such as the 1g HAMR robot, its surface tension is very similar to its weight. If these two forces are very similar, we can utilize this force to achieve certain functionalities.
Based on this, Dr. Chen Yufeng raised the following questions: 1. How can we control the magnitude of surface tension to some extent? 2. If we can control the magnitude of surface tension, what new functionalities can the crawling robot HAMR achieve?
Based on these questions, the Harvard University Micro Robot Laboratory proposed another project to enable HAMR to crawl on land, swim on water’s surface, sink successfully to the bottom, crawl on the bottom, and successfully climb from the bottom to land.
This project faces three challenges: 1. How to ensure the robot can float on the surface while being controllable to enter the water when needed; 2. How to use one actuation device to achieve movement in different environments; 3. How to overcome surface tension.
To address these issues, the Harvard University Micro Robot Laboratory designed a new type of HAMR robot. Compared to the original HAMR, the robot was equipped with four foot pads and wing ribs on the soles.
The robot’s foot pads can provide two types of lift on the water’s surface: one is from the surface tension of the water (proportional to the perimeter of the foot pad) and the other is from the buoyancy generated by the foot pad (proportional to the area of the foot pad). In this new design, surface tension accounts for 25% of the total upward force, while buoyancy accounts for 75%.
This design facilitates the robot’s controllable entry into the water, utilizing the phenomenon of electrowetting to change the contact angle between the water and the foot pad by applying voltage. When a voltage is applied to the vertical surface of the foot pad, the contact angle between the vertical part of the foot pad and the water surface decreases, thereby reducing the upward surface tension; when the corresponding voltage is applied to the flat part of the foot pad, the corresponding buoyancy also decreases, since the maximum height of the water is also related to the contact angle. In summary, when the voltage on the foot pad is increased, both the buoyancy and surface tension will decrease, allowing the robot to automatically sink to the bottom.
To verify the theoretical analysis, a corresponding experimental device was designed, placing the foot pad on a force sensor and measuring its maximum upward force by applying different voltages. The changes in upward force from 0 to 600V confirmed the correctness of the theoretical analysis.
The second question is how to enable the robot to move on the water’s surface. If the robot moves on the water’s surface in the same way it crawls on land, the efficiency is very low. This is because HAMR’s forward and backward movement on land is symmetrical, and its Reynolds number is very low, resulting in minimal thrust provided by this gait when placed in water. Therefore, a new gait must be researched to allow HAMR to effectively swim on the water’s surface.
A great bionic object for this aspect is the water boatman; many biologists studied in the 1980s how the water boatman swims in water. They utilize the asymmetric motion of their hind legs, fully extending their hind legs while pushing down, and retracting them while advancing to reduce resistance, allowing the water boatman to swim forward.
Inspired by the water boatman, a similar mechanism called a flap was designed. When moving backward, the passive flap fully opens to generate thrust, while during the reset process, the flap retracts to reduce resistance. The specific approach is that this passive flap propulsion method utilizes the coupling phenomenon between water and the passive flap, mimicking the water boatman’s gait without an active actuation device.
Using the newly designed gait, HAMR can reach a speed of 2.5cm/s on the water surface, with a flapping frequency of about 5Hz. Additionally, the robot can rotate in water for 10 seconds.
After solving the above issues, HAMR also encounters significant challenges when emerging from the water. When HAMR emerges, it faces considerable surface tension from the water, which hinders the robot’s return from the bottom to land. By altering the rigidity of the robot and adding devices to its front legs to increase friction, the robot finally managed to climb out of the water from a 6.5° slope. The energy consumption for emerging from the water is approximately 3.5 times that for moving at the bottom, making the emergence process still quite challenging.
Finally, the Harvard University Micro Robot Laboratory has created the world’s first micro robot capable of freely traversing between water surface, underwater, and land.
About the Future
Although Harvard University has solved many problems related to micro robots and continually developed them to possess functionalities unattainable by larger robots, there is still much practical work to tackle regarding the future of micro robots.
For instance, are there newer flexible actuation devices? Is there better manufacturing technology? Can flying robots carry their own energy and control circuits? Can more sensors be placed? How will robots collaborate to complete complex projects in the future?
With a vision for the next 5-10 years regarding micro robots, it is hoped that a large number of micro robots will work collaboratively in a swarm, showcasing limitless potential. Just like the scene below, where numerous RoboBees replace bees’ work, completing artificial pollination…
Academician of the Engineering Academy Cai Hegao | Professor Wen Li of Beihang University | Deepwake Technology Yuan Peijiang | Shen Zhi Lan Wei Jiancang | Daran Technology Zhang Chunsong
Yifei Intelligent Control | Deepwake Technology | Fanuc | Royole Technology | YouRobot | Yushu Technology | Zhen Di Technology | iRobot
① Industrial Sewing Robots | Unmanned Intelligent Mining Robots | China’s Dumpling Production Line Automation Workshop | MIT Construction Robots
② Service Sony Robot Dog Aibos | Folding Robot FoldiMate| Japan Cycling Robot | Tactile Mechanical Hand LUKE| Da Vinci Robot | Robot Band | Flying Car | Japan Nursing Robot Collection
③ Special Toyota Humanoid Robot | Underwater Robot Exploration No. | Russian Humanoid Robot FEDOR | American Heavy Machinery Guardian GT | Boston Dynamics Atlas 360-degree Backflip | Chinese Quadruped Robot Laikago | Beihang University Four-Wheeled Robot | Florida Research Institute “Mechanical Ostrich| Daran Variable Cell Robot
④ Bionic 3D Printed Bionic Robots | Tokyo University Sweating Humanoid Robot| Flexible Batteries | Harvard Soft Muscle| Harvard RoboBee
Intel Dr. Song Jiqiang | Zhongmin International Liu Guoqing| Professor Chen Xiaoping | Yu Shi Technology Jiang Yan| Zhejiang University Professor Xiong Rong| Changjiang Scholar Sun Lining| Shanghai University Unmanned Boat Expert Group| New松总裁 曲道奎| Beihang University Professor Wang Tianmiao| 863 Expert Li Tiejun Professor| Beijing University of Posts and Telecommunications Professor Liu Wei| Tsinghua University Professor Deng Zhidong| Tsinghua University Professor Sun Fuchun| Tianjin University Doctoral Supervisor Qi Jun Tong| Harbin Institute of Technology Professor Du Zhijiang| Changjiang Scholar Wang Shuxin| Gan Zhongxue Professor | Silicon Valley Maker Zhao Sheng
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