The Final design of the agricultural robot and the rimless wheel design of the wheel hub.

Executive Summary: https://docs.google.com/document/d/11QshnkDfgLp5oomu2FwTijRvsNPqIUAstlWqK1nnWUw/edit?usp=sharing

This project is an ongoing effort to develop a new type of robotic platform to support mobile sensing applications over rough terrain in agriculture and the perimeter security market. Our project sponsor Dr. James Burns has extensive experience and knowledge in the electric powered transportation industry. He recognizes the potential of a flexible robot that can serve in many different situations. The idea stemmed from a previous military robot carrying materials on and off the battlefield. This concept can be further utilized in the agricultural field to help carrying loads that humans are incapable of over rough terrains. This robot required a unique, rimless wheeled design, flexible-spoke wheel with differential steering allowing the robot to be capable of roaming over long distances over rough terrain with heavy payload.

There are various types of wheel designs, each having its own benefits to bring to the table in terms of mobility, speed and stability. The basic wheel which consists of a tire, hub and rims, is sufficient for high speed applications over a relatively flat surface. The most common type of off road vehicle utilizes a normal wheel. The method to overcome the rough terrain challenges comes from the suspension system. The spring damper system allows the ride to be smooth. The agricultural robot uses a more unconventional design. Since our environment is mostly considered to be that of an agricultural one with rough, hilly and dirt surfaces, it is in our best interest to discard speed for higher mobility and stability over this type of surface. 

Our rimless wheel design will sacrifice speed for more stability over rough terrains by having spokes that are not supported by tires but by feet. This removal of a rigid restriction by the tires allows the wheel to scale over uneven surfaces that is common among the rural areas. This report will look to provide its reader with the analysis and results of our design, and other future recommendations that may be worth exploring.

Design of Components

Spoke

The material of the spoke is Aluminum 6061, which is the most versatile aluminum alloy. Aluminum 6061 is known for its strength, workability, corrosion resistance and ease of manufacturing and assembling. The dimensions of the spoke such as its cross-sectional area and thickness were chosen to prevent any plastic deformation during buckling and any dynamic failures. These features ensure that our material is at least sufficient in handling the needs of an 250 lbs off-road vehicle.

Figure 2: The wheel hub consists of two covers and a cruciform. 

Feet

The feet of the robot are designed to have a curvy shape. This design helped reduce the impacts with the ground as well as increasing the robot’s stability. A metal tab is attached to one side toward the end of the foot to introduce an impact to the ground when the spoke is bending. This feature of the foot is still a concept to consider and test. The foot is a separate part that will be attached to the end of the spokes using bolts and nuts. The foot edges are round off to reduce stress concentration. The foot will be 3D printed using PETG for the prototype version. This design of the foot will ensure that our robot can operate over rough terrain.

Figure 3: The anchor shaped feet of the wheel

Result of Analysis

Our eccentric buckling analysis told us that there is a factor of safety of 46.5. Our fatigue has a factor of safety of 1.5. Our deflection analysis that our maximum deflection is about 9.7 inches. Based off our analysis, our design is sufficient in supporting the weight of the robot. The feet design above is also sufficient for our required loads and for navigating across our given terrain.