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Recently, Science Advances magazine published the latest research results of researchers at North Carolina State University: an energy-saving crawling robot with two-way movement inspired by mollusks such as caterpillars.
It is reported that the soft crawling robot uses joule heating of the distributed, programmable nano-silver wire (AgNW) heater in a thermal bimorph actuator based on liquid crystal elastomer, and finally realizes the front and back movement. In addition to this, the researchers predicted and optimized the local curvature of the robot under thermal stimulation; The influence of crawling speed on bidirectional drive mode was studied. Finally, the researchers demonstrated that the robot has the ability to pass through limited-spaced obstacles that can be used for search and rescue.
First, the heat driving material simulates the movement of mollusks when heated
Soft crawling robots have received widespread attention in the fields of biomedical engineering, surgical assistance, and perception technology. People have always taken a lot of inspiration from the animal world, and there have been attempts to combine soft materials and soft robot designs, such as octopuses, snakes, caterpillars, etc. These animals have some unique features, including multimodal locomotion, movement through narrow gaps, and in complex and unstructured environments.
Researchers have been exploring different stimulation methods to drive soft crawling robots, including pressure, heat, electric field action, magnetic field, etc. Of these types of stimuli, electrical stimulation is the simplest one, of which electrically stimulated actuators are widely used. For electroactive polymers, ionic activation typically operates in an electrolyte environment, while field activation requires high voltages, so the range of use of electroactive polymers is limited.
Another type of electrically stimulating brake, the thermal dual-chip actuator, has attracted widespread attention because of its advantages of programmable operation, light weight, low drive voltage, no electrolyte, and no bondage operation.
Among the different thermal driving materials, liquid crystal elastomer, as a thermal drive material combining polymer network and liquid crystal mesogens, has attracted much attention because of its powerful driving ability, reversibility, high processability, programmability and other unique characteristics.
As the temperature rises, the liquid crystal medium changes from nematic phase to isotropic, resulting in obvious macroscopic deformation of the material. Various LCE-based drivers have been designed and manufactured, typically driven by ambient heating, photothermal effects and eletrothemal actuation. A recent study has successfully integrated strechable resistive heaters with liquid crystal elastomers to better control electrical signals.
The researchers made a two-way soft crawling robot with multiple modes of movement. It is achieved by joule heating of a distributed, programmable nanosilver wire heater in a liquid crystal elastomer-based thermal dual element driver. The researchers achieved different temperature distributions and curvature distributions by heating the already designed nano-silver wire and programmable heating. At the same time, because the robot heats the front and rear ends and produces different friction with the ground, two-way movement is realized.
To demonstrate the functionality of crawling robots in potential applications, the researchers also described the performance of forward and reverse movements and tested scenarios through closed gaps. Through practical and finite element analysis, the researchers studied how crawling robots move, crawl speed, and ability to pass through small gap obstacles.
Second, the movement of the mother-of-pearl moth inspired the crawler crawling robot
In nature, the mother-of-pearl moth (Pleurotya rural) moves bidirectionally. In the process of moving forward, the larvae of mother-of-pearl moths usually fix the front end to move the tail forward, and while doing this, the larvae will also contract several nodes at the back end, which makes the larvae produce a typical hump on the back. The larvae then release the hump as it anchors the terminal tip, and then the entire caterpillar flattens again while the larvae move one step forward.
In the opposite direction, the larva fixes the end to the ground and then engages in a forceful contraction through the middle of the body. This movement produced a huge hump that arched the entire body. Then, when the larvae anchor the anterior, release the hump, flatten again and move one step back.
The key to achieving bidirectional movement is the control of body curvature, and while larval kinematics involve more complex active control of different body parts, for soft-bodied crawling robots, only the curvature of body parts needs to be controlled to mimic bidirectional movement like a larva.
Figures 1C and D below show the forward or reverse movement of the crawling robot as it heats up in different heating channels (or modes). When the heating is turned off, the relaxation of the bent bimorph structure causes the actuator to complete a cycle of movement forward, backward and backward.
The E and F of Figure 1 correspond to the infrared image and tilt view of the actuator when channel 1 and channel 2 are heated, respectively. In C and D in Figure 1, two inner electrodes and two outer electrodes apply constant currents resulting in forward or reverse motion.
Inspiration for reptiles
Figure 2A shows the manufacturing process of a crawling robot. Because of its excellent electrical conductivity and machine compliance, nanosilver wire has been widely used as a heating material in soft devices. In this work, the researchers used nanosilver wires as heating elements, embedded under the surface of a polydimathylsiloxane matrix.
The crawling robot is a double crystal structure that bonds nano-silver wires, polydimethylsiloxanes and carbon black on a liquid crystal elastomer belt to form a composite film. The nano-silver wire has a percolation network structure, and carbon black powder is doped with polydimethylsiloxane to improve thermal conductivity, and then the two drops in the nano-silver wire network for solidification. The nanosilver wire network is semi-embedded under the composite surface of polydimethylsiloxane and carbon black.
Notably, the thermal conductivity of a 4:1 polydimethylsiloxane and carbon black mixture with a 4:1 mass ratio improved by 31% compared to pure polydimethylsiloxane, but there was no significant change in Young's modulus.
When an electric current is applied to the nano-silver wire network, the heat generated by Joule heating is transferred to a composite layer of polydimethylsiloxane and carbon black and a liquid crystal elastomer layer. At this time, the semi-embedded silver nanowire network structure is located on the surface layer of the polydimethylsiloxane and carbon black mixture and the surface layer of the liquid crystal elastomer, because the surface of the polydimethylsiloxane and carbon black mixture can form stronger bonds with the surface of the liquid crystal elastomer than the surface of the silver nanowire.
When the temperature rises, the polydimethylsiloxane and carbon black mixture expands due to thermal expansion, while the liquid crystal elastomer shrinks due to nematic-isotropic transition.
Figure 2C shows a top view of a crawling robot. Each actuator contains two conductive channels (1 and 2). Through the designed nano silver wire heating mode, the temperature distribution is customized to realize the movement of the crawling robot. The performance of bidirectional motion mainly includes three aspects: heating performance, friction analysis, and the influence of power supply amplitude and frequency.
Design and manufacture of crawler crawler robots
As can be seen from Figure 3, the conductive nanosilver wire consists of two symmetrical parts, each containing two sections. As shown in Figure 3A, the first section evenly covers a serpentine conductive wire with a width of 0.65 mm, and the second section consists of two parts, each group contains a serpentine line with a width of 0.65 mm and a parallel thick straight line of 2.4 mm.
The circuit model is shown in Figure 3B, and in this study, R1, R21, and R22 in Figure 3B have resistance components of 115.4, 38.3, and 3.2 ohms, respectively. Therefore, Channel 1 has a resistance of 193 ohms and Channel 2 has a resistance of 121.8 ohms. In order to demonstrate the heating and driving performance of the crawling robot, the researchers parametrically studied the double thickness ratio of electric current and nanosilver wire, polydimethylsiloxane, carbon black film and liquid crystal elastomer tape.
Figure 3D shows the curvature of a sample as a function of time at different currents of 10-30mA. With the increase of current, the heating time decreased significantly from 80s to 12s. When the sample is bent to a round shape, the power stops.
Figure 3E shows the change in curvature over time at different thickness ratios, with the maximum curvature at a thickness ratio of 0.239 at the same applied current and the same heating time.
Heating performance of soft crawling robots
In Figure 4A, the left side is a snapshot of the forward movement of the crawler robot. In snapshot 2, when channel 1 of the actuator is activated, actuator A begins to arch and cause friction at the left and right ends (fA and fB). Due to the asymmetry of the arc, assuming that fA increases before fB and reaches the sliding friction criterion, the left end slides to the right while the left end remains stationary. When the power is turned off, the relaxation of the asymmetrical circular arc causes the friction force fA and fB to switch directions at the same time, this time fB first reaches the sliding friction criterion and begins to move to the right, while the left end is anchored until the robot returns to its original flat state.
In Figure 4B, the left side shows the reverse mode of the actuator when channel 2 of actuator A is activated. In reverse mode, the middle part of the robot is lifted so that the contact surface between the right end and the ground is smaller. This difference in contact area with the ground leads to the opposite friction result.
To test the above hypothesis, the researchers performed a finite element analysis using a full Abaqus environment. The flexible crawler model is a three-dimensional formable structure, and the ground model is a rigid surface. The researchers measured the coefficient of friction by dragging the deformed crawling robot, and found that the delimited heating area was the same as observed in the infrared image of the experiment, and the simulation results and experimental results were displaced in the direction of motion and relative out-of-plane displacement.
Figure 4C shows that fA
Two crawling modes for tracked robots
Figure 5 shows the speed of motion of a crawling robot in relation to the applied current and driving frequency. In general, the speed of both positive and negative modes increases with the increase of the applied current. However, from the perspective of the relationship between speed and frequency, the motion speed first increases and then decreases with the increase of driving frequency. When the maximum is reached, a further increase in frequency significantly reduces the speed of movement, due to the minimum heating and cooling times required for each drive cycle.
The speed of movement of the crawling robot
Finally, the researchers hope to verify the robot's ability to pass through small, confined spaces. Specifically, the researchers set up a closed tunnel with a height of only 3mm and a length of 30mm, and under unconstrained conditions, the crawling robot can reach a forward height of 8.9mm and a reverse height of 14.5mm.
Demonstration of a crawling robot through a gap
The study found that heat-driven soft crawling robots have multi-gait capabilities under different drives, which is different from most previously reported soft crawling robots. Previous soft crawling robots could neither change their gait through confined spaces nor move in both directions. The ability of heat-driven soft crawling robots to pass through confined spaces in forward and backward motion has great potential in many applications such as search and rescue in the future.
Conclusion: Bionic robots are developing rapidly, and there are many future application scenarios
In fact, like soft crawling robots, there are many robots that work on biological characteristics by imitating the external shape, movement principle and behavior of organisms in nature. With the development of artificial intelligence technology, bionic robots will perform more diverse tasks, complete more humane actions, and control levels will be more precise.
When the use scenarios are more clear, bionic robots will develop in a more bionic and miniaturized direction to meet diversified use scenarios, just as these soft crawling robots may be used in search and rescue missions in the future, and because they are small enough to explore places that search and rescue dogs and humans cannot enter.
Source: Science Advances