Jumping is an important ability for robots to expand their range of motion, overcome obstacles, and adapt to many unstructured environments.
Continuous jumping and directional adjustment are the basic characteristics of the ground robot with multi-modal motion. However, at present, only a few soft jumping robots are able to achieve fast and continuous jumping and control the steering movement to achieve obstacle crossing.
Now, the Chinese team has come up with an electrostatically driven tethered legless soft jumping robot based on a flexible electrostatics bending actuator. Weighing just 1.1 grams, 6.5 centimeters long and 0.85 millimeters thick, the robot can achieve a jump height of 7.68 times body height and a continuous forward jump speed of 6.01 times body length per second. Combined with two actuator units, it achieves a steering speed of 138.4° per second.
The paper is published in the scientific journal Nature Communications, and its lead authors are from Chongqing University, Harbin Institute of Technology, Shanghai University and Beijing University of Aeronautics and Astronautics.
Researchers will also be other functional electronic devices (such as sensors) are integrated into the brake, so as to realize a variety of applications, including detection of environmental change, and suggests the future structure optimization to improve the software of the robot jump performance, in the future or the further study of wireless solution or can increase the generality of this kind of robot software.
Innovate electro-hydraulic static driving mode
Two major engineering challenges facing soft jumping robots are to improve the single jump performance (jump height JH and jump distance JD) to improve the obstacle surmounting ability and to accelerate the jump frequency to improve the navigation efficiency.
At present, the industry has been developed to be able to forward soft or partial soft jumping robot navigation, driving mode is also very diverse, has integrated spring, shape memory alloy (SMA), magnetic actuator, photodynamic actuator, dielectric elastomer actuator (DEA), pneumatic actuator, chemical actuator, motor and polyvinylidene fluoride (PVDF) actuator drive, etc.
Some are energy-storage jumping robots, which typically have strong single-hop performance but require additional elastic energy-storage processes at the expense of navigation efficiency.
In addition, although the lengthening of the storage process increases the jump height, it decreases the landing stability and the jump frequency. Soft jumping robots driven by pneumatic actuators, chemical actuators and motors usually require complex navigation strategies and structures; 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 fast jumping frequencies, but their JHs and JDs are not sufficient for jumping over obstacles (<0.25 body height).
Hydraulic amplifier self-healing electrostatic (HASEL) actuator can change the distribution of internal liquid by electrical hydrostatic to achieve linear motion, the electro-hydraulic driven approach can jump in a very short period of time the amount of energy needed, without the need for a complex process of energy storage is fast obstacle robot potential solution, but still faces three big challenges:
(1) Improve single-hop performance without stacking;
(2) Fast recovery;
(3) produce forward jump and turn jump.
After in-depth analysis of the advantages, disadvantages and performance of these robotic solutions, the researchers used fast bending and springback based on electrostatics principles and frames to enhance the jumping performance of the actuator.
The researchers named the new solution LSJR: an electrohydrostatically driven legless tethered soft jumping robot based on a flexible electrohydrostatic bending actuator (sEHBA) with fast, continuous, steering jump and obstacle jumping capabilities.
Preliminary experiments show that the fast response characteristics of sEHBA lead to a short startup time (~10 ms), LSJR can be used to achieve JH of 7.68 body height, a single jump of 1.46 body length, and a continuous forward jump speed of 390.5 mm/s (6.01 body length per second). The frequency is 4 Hz.
They also demonstrated that the dual-bodied LSJR is capable of turning at a speed of 138.4° per second, making it the fastest soft-jumping robot in existence.
In experimental scenarios, the LSJR can leap over a variety of obstacles (some larger than the robot), including slopes, power lines, single-step, continuous steps, circular obstacles, gravel mounds, and cubes of different shapes.
Explore the best performance parameters
LSJR consists of two plastic semicircular bags of bidirectional polypropylene (BOPP) film material printed with flexible electrodes 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. The flexible plastic (PVC) ring frame is fixed to the edges and pre-stretched. By applying high pressure to the two electrodes, the LSJR bends itself electrically to generate forward jumping force and energy. The rear air sac has an animal-like tail to balance the jumping and landing positions. It plays an important role in the LSJR's overall structure.
From the point of view of design concept and motion principle, the researchers heat sealed a HasEL-type actuator into a semicircular separate HASEL (SCS-HasEL) actuator consisting of two semicircular liquid bags based on a zipper mechanism. Then, Replace the dielectric liquid in the scS-Hasel actuator's rear semicircle pocket with an equal amount of air, and remove the covering electrode of the rear semicircle bag so that the dielectric liquid can flow anisotropic with respect to the whole actuator.
As expected, special liquid-gas layouts were found to make the liquid-gas actuator jump forward, even as the airbag was bouncing on the ground. This is because the electrodes squeeze the liquid dielectric, causing it to flow rapidly, and the LSJR is energized to bend itself, allowing it to gain initial kinetic energy. To further improve the LSJR's jumping performance, the air in the bag can be replaced with helium or another less dense, non-explosive gas. The lightweight robot is designed to make jumps and landings stable without capsizing.
Figure | LSJR moving forward test (source: Nature Communications)
JD and JH are two important performance indexes, which 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 generates greater JD and JH. Excessive R (such as R =2:1) will affect the flexibility of 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 characteristic of the continuous forward jumping robot. At 10 kV and 4 Hz, the average rotating speed (TS) =138.4°/s, which is described in the paper as the fastest among the existing soft jumping robots, which is the result on the board. However, on different substrates, the continuous jumping ability is greatly affected. On the glass plate with smooth surface, the average TS is only 27.9°/s under the same conditions of 10 kV and 4 Hz. Sufficient substrate surface roughness can not only prevent the robot from skidding in continuous motion, but also hinder the motion of the unpowered LSJR, thus affecting steering behavior.
LSJR is capable of surmounting obstacles and is expected to perform exploration, inspection and reconnaissance missions in complex and unstructured environments. At an applied voltage of 10 kV and a drive frequency of 4 Hz, the single LSJR climbed the glass plate (tilt Angle of 3°) with a CFJS of 16.3 mm/s (0.25 body length/second), across 6.3 mm diameter wires, across 8 mm high steps, and across continuous steps.
The LSJR traversed a maximum height of 14 mm (cuboids), 18 mm (trigonometric prisms and cylinders), and successfully traversed a gravel mound containing a large amount of gravel (size: 3 to 6 mm) with obstacles spaced 4 mm apart.
There are more interesting developments
Overall, LSJR has the advantages of low profile, lightweight, modularity and cost efficiency. With a simple control strategy, the robot is capable of fast, continuous and steering jumps, loads and obstacles.
The use of a special liquid-gas layout and a fixed edge pre-bending frame to achieve rapid and continuous forward and steering jump motion caused by periodic saddle bending and anisotropic fluid flow compensated for some limitations of HASEL actuators, including :(1) unfeasible forward and steering jump; (2) The single-hop performance without stacking is weak; (3) Unable to recover quickly. In the continuous forward jumping movement, 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 the various moving substrates. At the same applied voltage (10 kV, 4 Hz), the glass substrate with the smoothest surface provides the lowest friction of all substrates resulting in the lower CFJS of 95.6 mm/s (1.47 body length/second). For now, this limits the robot's use for jumping on relatively smooth surfaces.
LSJR can be used to detect and record environmental changes, such as temperature and UV, by connecting light and soft temperature sensors, creams and photochromic dyes, and by integrating other sensors to detect more environmental factors, such as contaminants in industrial environments and civil buildings, the researchers said.
Next, they will focus on scalability and parameter optimization of sEHBA to achieve better jumping performance, development of unconstrained LSJR and applications, and other soft robots based on sEHBA, such as wall climbing robots, swimming robots, and flapping robots.
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