Impact of materials on performance of Vine Robot

Vine Robots are soft continuum robots designed with low-cost fabrication and for the navigation of difficult environments. Due to their movement patterns resembling those of natural vines, these robots also known as “everting vine robots” grow by pressure-driven eversion. This paper presents the impact of different materials on the performance of Vine Robot. Three of the most easily accessible materials namely Transparent LDPE (125 Microns), Black LDPE (150 Microns) and Tarpaulin HDPE (250 Microns) were acquired and utilized in fabricating the body of Vine Robot. Under the application of these materials, difference in various properties of the robot were studied. By systematic study, the suitability of one material over the other was determined in specific environmental scenarios.


Introduction
Vine Robot is a subset of Soft Robotics, these robots are called so because of their similarity to plants (vine) in their growth-trailing behaviour.Due to their growth-based movement, vine robots are well suited for navigation and exploration in cluttered environments [1].Non-destructive exploration of small areas remain difficult for existing robot designs in a number of possible robotic applications, including inspection, medicine, search and rescue, and archaeology.Vine robots can potentially fill this need for robots able to move in highly constrained environments [2].
The technology has gone through a lot of innovations and upgrades over the year and extensive research work is still being conducted in the field of vine robots based on the control systems, material, construction, modelling, and sensors being used.Researchers fabricated the robot using soft lithography whose simplicity allowed it to iterate the design rapidly [3].
Researchers fabricated a soft robot body driven by a pneumatic pump [4] and used it to evaluate the rate of lengthening by changing the internal pressure and derived a relationship between the two.Control chambers were used to selectively permit one side of the robot body to extend in comparison to the other side in order to accomplish active direction control.Researchers have also demonstrated advancements on control systems, material, modelling, and sensors [5].Researchers have further investigated the retraction of robot body along with a model predicting when controlled retraction is and is not possible, and present a design to aid in controlled retraction [6].
While unique designs could call for labor-intensive and complicated manufacturing procedures, the majority of everting vine robots were built in a very short amount of time.Everting vine robots were created in the most basic form by sealing one end of a tube and reversing this sealed end inside the body [7].Researchers have also been successful in developing a general model form, The Lockhart-Ortega equation, which relates the rate of volume change to the difference between the turgor pressure 1291 (2023) 012031 IOP Publishing doi:10.1088/1757-899X/1291/1/012031 2 and yield pressure.A modification of this viscoplastic model of plant cell extension described by Green relates pressure to a linear extension rate as shown in equation (1).
where r is the linear extension rate, Y is the yield pressure below which no extension occurs, P is the internal pressure, φ is the extensibility, and n is a power term close to unity.The model describes a monotonically increasing relationship between pressure and extension rate with an offset [8].This study investigates the use of various conventional materials in fabricating the body of Vine Robot.These materials are inextensible enough to produce eversion as opposed to radial expansion upon pressurization and that are both fluid impermeable and sealable, such that a closed pressure vessel can be developed.By making the use of materials like LDPE and HDPE sheets, the impact on the performance of Vine Robot was studied by plotting quantities such as Average Displacement in time, Diameter of body tube, Pressure input to the robot and Lengthening Rate.

Materials and Methods
For the effective functioning of the Vine Robot, a base station was fabricated to hold all the circuit components of the robot along with the confined body of the robot.The body was coiled around the spool connected to a DC Motor.Microcontroller system was used to manage Pressure input as well as the motor to cause eversion and inversion of the Vine Robot.In addition to this, series pouches were sealed along the sides of the body which allowed the steering of the Robot body due to inflation on one side with respect to the second side.

Figure 1. Fabricated Vine Robot
Initially, transparent LDPE (125 Microns) was utilized in making the body of the robot.The material was sealed using conventional plastic heat sealer, and a long 2m tube was created.An air compressor rated 1/3HP and 1440RPM was utilized along with a pressure regulator to specify a particular pressure level.By varying the pressure input to the robot, the distance travelled within a constant time was measured thus achieving the extension speed.The table was then plotted in order to relate the results to the General Model Form or Lockhart Ortega Equation.For Polyethylene Y=13.3kPa, φ = 0.75m/s-kPa −n , n= 1.1 and Extension speed(r) = φ{P − Y} n .As shown in 'figure 2' the results obtained in the experiment were found satisfying the Lockhart Ortega Equation as plotted by researchers in past.Same material was utilized with three different Diameters to measure Average Displacement travelled in each case.A similar experiment was conducted by changing the Pressure input to the robot and Average Displacement was noted down in each case.Lengthening rate was also plotted against Diameter of the body.The same experiment was repeated with two other materials, Black LDPE (150 Microns) and Tarpaulin HDPE (250 Microns).Using the obtained results, the performance of the Vine Robot was studied due to the influence of different materials.

Results and Discussions
The values measured in the experiment conducted are tabulated below, and are plotted on a graph to understand the difference in performance of the Vine Robot.
Three tubes with different diameters were sealed out of all the three materials.A constant pressure was supplied each time to observe the Average Displacement of the tip of the Robot at a constant time.Constant Pressure = 27.6 kPa (4 psi), Constant time = 5 Seconds and conversion of Diameter 3 in = 0.08 m.To study the change in lengthening rate of the robot, air pressure of 27.6 kPa (4 psi) was maintained and the Average Displacement was observed in each case of tube diameter for all three materials.Using the Average Displacement, lengthening rate was determined using the equation (2): From the 'figure 4', it was evident that a larger diameter of the robot body slowed down the displacement achieved by the robot because of the larger volume of air to cover thus slowing down the speed of travel.In contrast, with increase in Pressure, the speed was seen to be increased as seen in 'Figure 5'.Lengthening rate being directly proportional to the Average displacement, gave a similar plot to Average displacement v/s Diameter.However, by observing all the graphs, it can be noted that Tarpaulin HDPE material shows a very small change in its speed with change in diameter and pressure thus making it a suitable option in case of stable travel.Furthermore, the strength of the material was seen to be protecting the material in sharp and rough terrains.However, for the customised applications of the robot, transparent LDPE material was seen to be most useful as it was found to be the easiest to prototype among the three.

Conclusions
From the observations and results it is concluded that-1.In applications where higher displacement is required at a given pressure, the robot having smaller inflated diameter has to be used since the displacement decreases as the diameter increases.In such a situation, lighter material like transparent LDPE can also be prioritized.
2. In applications where the space of the robot is fixed that is the inflated diameter of the robot is fixed, in such a scenario to achieve higher displacement, either a higher internal pressure can be used or lighter material can be employed.
3. Lengthening rate is defined as the change in length per unit change in time.It is observed that at a given pressure as the diameter increases the lengthening rate decreases.
4. From the plots of lengthening rate v/s diameter it was observed that LDPE material delivers a higher lengthening rate in comparison to the other two materials.However, in rough terrains, LDPE got punctured due its thin and light structure.In rough terrains, HDPE material was seen to be performing well due to its thin yet strong structure.

Table 1 .
Pressure v/s Extension Speed

Table 2 .
Average Displacement v/s Diameter in Transparent LDPE

Table 3 .
Average Displacement v/s Diameter in Black LDPE

Table 4 .
Average Displacement v/s Diameter in Tarpaulin HDPE

Table 5 .
Average Displacement v/s Pressure in Transparent LDPE

Table 6 .
Average Displacement v/s Pressure in Black LDPE

Table 7 .
Average Displacement v/s Pressure in Tarpaulin HDPE