Final Design

Off-Road Tank Design:

CAD of the prototype

In order to reliably navigate rough terrain, provide adequate soil thrust, and distribute the vehicle's weight more efficiently, a tracked vehicle design was chosen for this project. The limitations to the ability to manufacture or assemble  a prototype due to COVID19 disruptions restricted our team to theoretical analysis, and isolated component testing. Thus, we purchased a test tank (Tamiya tank) and performed tests to quantify its performance in soil conditions. As going inside a burrow necessitates a safety tether that may double as a communication line, we carefully analyzed the tension that may arise in a tether inside the burrow and evaluated the Tamiya tank's ability to pull the tether. We also researched and tested ways to improve traction of the tank treads with soil to increase its ability to pull the tether.

Another component we analyzed is an on-board spool. By having it on the tank, we theorized the ability to bring down the tether's pull on the tank to zero. Thus, negating the risk of the tether and allowing for improved locomotion inside the burrow. 

These three components: tether, treads, and spool are discussed below in more detail.

The final performance of the tank 

The first video is of the Tamiya tank without modifications traversing through the realistic burrow simulation while the second video shows the final prototype pulling a 20 gauge power cable through the entire length of the burrow that is displayed in the first video.

1. Tether 

A previous team working with Engineers for Exploration that developed a tunnel exploring vehicle reported frequent failure due to the tether. High tension in the tether due to frictional forces with the ground and walls were in some situations higher than their tank's pulling capability. Therefore, we devoted a considerable amount of effort into finding a model that can accurately predict tether tension for any burrow configuration. Using this model, we hoped, would help us determine the conditions needed to avoid high tether tension failure. 

The work here was split into two parts: quantifying the drawbar pull (maximum pulling force) of the tank, and the tension in the tether for a simple one 90 degree turn burrow configuration.The table below shows the results of various drawbar pull tests that were performed on various surfaces. The graph shows the drawbar pull values found for varying the weight of the tank.

Table of Pullbar Test Results for Varying Terrain Types

Trench used to test tether tension

Types of tethers tested

All of these tests showed that the tank in general would be able to pull the lighter and smaller tethers. However, a more complex burrow with multiple turns will mostly create too high of tension in the tether for the tank to pull. Additionally, sensors that would be added could require heavier cables with possibly larger coefficient of frictions. This is why a spool design was considered as a main component in this project.

2. Spool

Spool CAD

To minimize the pull of the tether on the tank, we proposed using an on-board spool. This would help slowly release the tether as the vehicle moves forward in the burrow. The spool was designed to "trail" the vehicle such that its weight and size do not restrict the tank, and its contact with the ground helps roll it for passive dropping and retrieval of the tether depending on the direction of motion. 

However, when using the tether as a communication line with the vehicle, the tether connecting to the vehicle can easily get entangled as the spool unravels. To solve this, two hollow shafts were incorporated. By inserting the first layer of tether that connects to the tank through the shafts, the spool can freely rotate in operation with no entanglement.

We haven't been able to test this design due to difficulties in manufacturing it, but believe it sufficient enough to be included in the final prototype. In future work, proper testing of this design could be really useful.

3. Treads

To increase the traction of the treads and as a consequence increase the pulling capability of the tank, "grouser" attachments were explored. Grousers are protrusions used on tracks and wheels to increase soil thrust. We explored two approaches: metal spiked "cleats" and rubber flaps to characterize how they change the Tamiya tank's traction in different kinds of off-road terrains.

Both the metal spikes and rubber grousers were found to improve traction; with the latter being more efficient. This can be attributed to the increased surface area in contact with soil granules causing more soil shearing. This helped put us in the right track to safely recommend the rubber grousers in the final design.

The following images depict the differing grousers types that were tested:

Tamiya Tank with Metal Grousers

Parallel Rubber Grouser Design

Offset V-Shaped Grouser Design