Modelos Hedónicos

Ground locomotion is important within a nuclear cave as it provides the most likely form of entry. This is because the entry hole can only be up to 15cm in diameter and ground locomotion methods are most easily miniaturised.

Having reviewed the literature covering the various locomotion strategies, it was considered prudent to compare the most promising ground locomotion methods through experimentation. The locomotion strategies compared are wheels, tracks, whegs and spherical locomotion.

This section aims to outline the experiments undertaken to compare wheeled, tracked, whegged and spherical locomotion strategies and thus make suggestions on the best strategies for a heterogeneous swarm traversing a nuclear cave environment. Thus, this section will detail: the

Figure 4.7: Shows the layout of each robot used; all units are in mm. The front of the robot is on the left for side views and upwards for birds eye view diagrams. a) & b) show the top and side views of the whegged robot respectively. c) & e) show the side and top view of the tracked robot respectively. d) & f) show the side and top view of the wheeled/ speherical robot respectively.

robotic platform and design of the comparative robot; the methods and metrics for comparison; and the results from the experiments.

4.5.1 Robotic Platform and Robot Design

In order to compare the different locomotion strategies, first a suitable robotic platform was required. It was important that the robot is reconfigurable, so that it could be used to test the various locomotion strategies. In addition, it was desirable for the robot to be low cost and readily available.

The LEGO Mindstorm EV3 platform fulfilled all these requirements. This robot has been widely used in education and is capable of an array of configurations and control modes [246] [131]. The Mindstorm kit includes two stepper motors for propelling robot designs, a control brick, gears and support structures; allowing rapid reconfiguration.

The layout of each design is shown in figure 4.7. The wheeled and spherical robots utilised the same underlying design. To adapt the wheeled design to spherical locomotion, the robot was placed inside a spherical ball of diameter 32cm. The wheeled robot used two front wheels driven by motors, with a passive caster wheel at the rear. The whegged robot used two pairs of

whegs, the front of each pair was driven by a motor. The rear whegs were connected to the driven whegs via gears, giving four-wheel drive. The whegs themselves were designed in SolidWorks and printed using a 3D printer. Finally, the tracked robot used two driven wheels at the front, with 4 passive wheels to ensure the track remains in place and holds its shape.

4.5.2 Methods for Comparison

The comparison was made through adapting criteria used by the National Institute for Standards and Technology (NIST). The selected criteria for comparison were gap width able to overcome, obstacle clearance and climbable incline. The test document states: "Test trials consists of 30 repetitions to demonstrate statistical significance to at least 80% reliability with 80% confidence. During the first trial within a particular apparatus setting, the test administrator may stipulate that the robot was dominating the apparatus at that setting after demonstrating the first 10 successful repetitions with no failures. However, if there are any failed repetitions, a second set of 10 repetitions is required. For a trial to be noted as statistically significant, no more than 1 failure in 20 repetitions, or 3 failures in 30 repetitions are allowed." [115]. These test methods were implemented for the locomotion comparison tests. The selected criteria were gap width, obstacle clearance and incline as these are the most important aspects of traversing a nuclear cave; due to the sumps and channels on the floor, the obstacles caused by debris and incline used to collect leakages in the centre of the cave.

In order to alter the obstacle height 10cm x 20cm pieces of acrylic were used, with thicknesses of 2mm, 3mm and 5mm. These varying thicknesses were stacked until the robot was no longer able to surmount them. The incline test was implemented using a 100cm x 60cm x 1cm piece of track. The start point and end point of the test were marked 65cm apart, 17.5cm from each end of the piece. One end of the track was attached to an arm whose height could be varied, the other laid on the ground. The angle of this slope was then varied in increments of 3 degrees until the locomotion method being tested could no longer climb the required 65cm portion. At this point variations of 1 degree were made to find the angle of incline, to the nearest degree, that the robot was able to climb. Finally, the gap width test was made using two 30x30cm platforms, of height 6cm. These platforms were moved apart 1cm at a time until the robot was no longer able to surpass the gap.

4.5.3 Results

The results are shown in figure 4.8. The whegs and spherical locomotion strategies were repeated with increased traction. The traction was increased using rubber on the outside edge of the whegs, and in a ring around the outside of the sphere. The extra traction was used because the performance of the lower traction was considerably less than expected; traction was determined to be the reason. This was due to the fact that when conducting the experiments, it was observed that the robot would not adequately grip the surface. An additional obstacle clearance test was

Figure 4.8: Shows the results from the comparative test. The (2) denotes addition traction tested on spherical and whegged robots. The (3) denotes an additional obstacle clearance test carried out with whegged robot with increased traction; in this test only the front whegs were required to surmount the obstacles.

carried out with the whegged robot with increased traction. This test focussed on the front whegs climbing the obstacle, rather than the entire robot overcoming it. This was implemented because the literature suggested that whegs are able to climb obstacles up to 25% larger than their diameter [175]. Despite this extra test, the whegs were still only able to climb obstacles of 51mm, compared to their 56mm diameter.

It was found that overall tracks perform best. This was due to their ability to overcome the largest obstacle and incline. This result is expected, as tracks offer the most traction when compared to the other locomotion strategies due to their contact area. However, the spherical robot was found to be capable of crossing the largest gaps, likely due to its large diameter. Interestingly, the additional traction added to the spherical and whegged robot only slightly increased performance.

Overall these experiments suggest that a combination of spherical and tracked locomotion strategies would give the best results when traversing the floor of a nuclear cave. If both types of locomotion were used together in a heterogeneous swarm then the inclines, sumps, channels and debris could be more easily surmounted in order to give the best area coverage for mapping of the nuclear cave environment.