Weighing only 1.1 grams, the fastest soft jumping robot is made in China!

Jumping is a very important ability for robots, which enables them to expand their range of activities, overcome obstacles, adapt to many unstructured environments, etc.

Continuous jumping and directional adjustability are essential properties for ground robots with multimodal locomotion. However, currently only a few soft jumping robots can achieve rapid continuous jumping and controlled steering motion to traverse obstacles.

Now, a Chinese research team has proposed an electrostatically driven tethered legless soft jumping robot based on a soft electrostatic bending actuator. The robot weighs only 1.1 grams, is 6.5 centimeters long, and 0.85 millimeters thick. It can achieve a jumping height of 7.68 times its body height and a continuous forward jumping speed of 6.01 times its body length per second. Combined with two actuator units, it can achieve a turning speed of 138.4° per second.

The relevant paper was published in the scientific journal Nature Communications. The main authors of the paper are from Chongqing University, Harbin Institute of Technology, Shanghai University, Beijing University of Aeronautics and Astronautics, etc.

The researchers also integrated other functional electronic devices (such as sensors) into the actuator to enable a variety of applications, including detecting environmental changes. They also suggested that future structural optimization should be carried out to improve the jumping performance of soft robots. Further research on tetherless solutions may enhance the versatility of this type of soft robot.

Innovative electro-hydraulic static drive

Improving the single-jump performance (jump height JH and jump distance JD) of soft jumping robots to improve their obstacle surmounting ability, while increasing the jumping frequency to improve their navigation efficiency, are two major engineering challenges facing soft jumping robots.

At present, the industry has developed soft or partially soft jumping robots that can navigate forward, and the driving methods are also very diverse, including integrated springs, shape memory alloys (SMA), magnetic actuators, photodynamic actuators, dielectric elastomer actuators (DEA), pneumatic actuators, chemical actuators, motors and polyvinylidene fluoride (PVDF) actuators.

Some of them are energy storage jumping robots, which usually have strong single-jump performance, but because they require additional elastic energy storage processes, they sacrifice navigation efficiency.

In addition, although the extension of the energy storage process increases the jumping height, it reduces the landing stability and the jumping frequency; soft jumping robots driven by pneumatic actuators, chemical actuators and motors usually require complex navigation strategies and structures; while lightweight soft jumping robots based on DEAs and PVDF actuators can perform simple jumps by bending body parts without additional energy storage, which leads to a fast jumping frequency, but their JHs and JDs are not sufficient to meet the requirements of crossing obstacles (<0.25 body height).

Hydraulically amplified self-healing electrostatic (HASEL) actuators can achieve linear motion by changing the distribution of internal fluids through electrohydrostatics. This electro-hydraulic drive method can generate the energy required for jumping in a very short time without the need for a complex energy storage process. It is a potential solution for fast obstacle-crossing robots, but it still faces three major challenges:

(1) Improve single-hop performance without stacking;
(2) Achieve rapid recovery;
(3) Produce forward jumps and turning jumps.

After an in-depth analysis of the strengths, weaknesses and performance of these robotic schemes, the researchers used electrostatic principles and rapid bending and rebound of the frame to enhance the actuator's jumping performance.

Figure | LSJR detailed design and motion principle (Source: Nature Communications)

The researchers named their new solution LSJR: an electrohydrostatically driven tethered legless soft jumping robot based on a soft electrohydrostatic bending actuator (sEHBA) with rapid, continuous, steering jumping and obstacle traversal capabilities.

Preliminary experiments showed that the fast response characteristics of the sEHBA resulted in a short start-up time (~10 ms) and that the LSJR could be used to achieve a JH of 7.68 body heights, a single jump of 1.46 body lengths, and a continuous forward jump velocity of 390.5 mm/s (6.01 body lengths per second) at a frequency of 4 Hz.

They also demonstrated that the dual-body LSJR can turn at a speed of 138.4° per second, the fastest among existing soft jumping robots.

Figure | The principle and effect of robot bouncing (Source: Nature Communications)

In the experimental scenario, LSJR's rapid continuous jumping motion can cross a variety of obstacles (some of which are larger than the robot), including ramps, wires, single steps, continuous steps, circular obstacles, gravel hills, and cubes of different shapes.

Explore optimal performance parameters

LSJR consists of two plastic semicircular bags, which are biaxially oriented polypropylene (BOPP) film materials with flexible electrodes printed on them for potential wire connections. The front of the bag is filled with dielectric liquid and the back is filled with the same volume of air. A flexible plastic (PVC) ring frame is fixed on the edge and pre-stretched. By applying high voltage to the two electrodes, LSJR is energized to bend itself, generating force and energy for jumping forward. The rear airbag has a tail similar to that of an animal, which is used to maintain balance in jumping and landing postures, and plays an important role in the entire structure of LSJR.

In terms of design concept and movement principle, the researchers heat-seal a HASEL-type actuator into a semicircular separated HASEL (SCS-HASEL) actuator, which consists of two semicircular liquid bags based on a zipper mechanism. Then, they replace the dielectric liquid in the rear semicircular bag of the SCS-HASEL actuator with an equal amount of air, and remove the covering electrode of the rear semicircular bag, so that the dielectric liquid can flow anisotropically relative to the entire actuator.

As expected, it was found that the special liquid-gas layout allows the liquid-gas actuator to jump forward, even if the airbag jumps on the ground. This is because the electrodes squeeze the liquid dielectric, making it flow quickly, and the LSJR is energized to bend itself, allowing it to gain initial kinetic energy. To further improve the jumping performance of the LSJR, the air in the bag can be replaced with helium or other non-explosive gases with lower density. The lightweight robot design makes jumping and landing stable without capsizing.

Figure｜LSJR mobile forward test (Source: Nature Communications)

JD and JH are two important performance indicators that can be used to characterize the jumping performance of LSJR. r = electrode area: non-electrode area. Experiments show that when r = 1:1, the robot produces greater JD and JH. Excessive r (e.g. r = 2:1) will affect the flexibility of the BOPP film, hinder the normal bending of the frame, and reduce the vertical ground reaction force.

In addition, the continuous forward jumping speed (CFJS) is an important performance feature of the continuous forward jumping robot. At 10 kV and 4 Hz, the average rotation speed (TS) = 138.4°/s. **According to the paper, this is the fastest among existing soft jumping robots, and this is the result on a wooden board. **On different substrates, the continuous jumping ability will be greatly affected. On a glass plate with a smooth surface, under the same conditions of 10 kV and 4 Hz, the average TS is only 27.9°/s. Sufficient substrate surface roughness can not only prevent the robot from slipping during continuous motion, but also hinder the motion of the unpowered LSJR, thereby affecting the steering behavior.

LSJR has good obstacle-crossing ability and is expected to perform exploration, inspection, and reconnaissance tasks in complex and unstructured environments. Under an applied voltage of 10 kV and a driving frequency of 4 Hz, a single LSJR climbed a glass plate (with an inclination angle of 3°) with a CFJS of 16.3 mm/s (0.25 body length/s), crossed a wire with a diameter of 6.3 mm, crossed an 8 mm high step, and crossed continuous steps.

In the obstacle crossing test with an obstacle height interval of 4 mm, the maximum height that the LSJR can cross is 14 mm (rectangle), 18 mm (triangular prism and cylinder), and it can also smoothly cross a gravel hill containing many gravels (size: 3 to 6 mm).

Figure｜LSJR mobile forward test overcoming obstacles (Source: Nature Communications)

There are more interesting evolutions

In general, LSJR has the advantages of low profile, lightweight, modularity and cost-effectiveness. Through simple control strategies, the robot can achieve fast, continuous and steering jumping, carrying and obstacle traversing capabilities.

By adopting a special liquid-gas layout and an edge-fixed pre-bent frame, the robot realizes rapid and continuous forward and turning jumping motion caused by periodic saddle-shaped bending and anisotropic liquid flow, which makes up for some limitations of HASEL actuators, including: (1) unachievable forward and turning jumping; (2) weak single jump performance without stacking; (3) inability to recover quickly. In the continuous forward jumping motion, the angle deviation of each jump can be controlled within 8°, and the maximum jumping height of the robot can reach 18 mm.

The jumping performance of LSJR depends not only on the applied voltage but also on the surface texture of various moving substrates. Under the same applied voltage (10 kV, 4 Hz), the glass substrate with the smoothest surface provides the lowest friction among all substrates, resulting in a lower CFJS of 95.6 mm/s (1.47 body lengths/s). Currently, this limits the application of the robot to jumping on relatively smooth surfaces.

The researchers said that LSJR can be used to detect and record environmental changes such as temperature and ultraviolet light by connecting light and soft temperature sensors, pastes and photochromic dyes, and can also be used to detect more environmental factors such as pollutants in industrial environments and civil buildings by integrating other sensors.

In the next step, they will focus on the scalability and parameter optimization of sEHBA to achieve better jumping performance, develop unconstrained LSJR and applications, and other soft robots based on sEHBA, such as wall-climbing robots, swimming robots, and flapping-wing robots.

References:
https://www.nature.com/articles/s41467-021-27265-w

Written by: Cooper Edited by: Kou Jianchao Layout by: Li Xuewei

Source: Academic Headlines