Mars Rover

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Introduction

The Canadian International Rover Challenge (CIRC) held in Drumheller, AB and the University Rover Challenge (URC) held in Hanksville, UT are two competitions where university student teams design and build a Mars Rover prototype. The teams compete in simulated tasks similar to tasks required of a rover on Mars. Such tasks are Autonomous Navigation, Soil Sampling, or Search & Rescue. In any given task, a rover chassis and suspension are required to house motor controllers, communication radios, and various microcontrollers. The rover needs to support auxiliary equipment such as a robotic arm, science soil collection drill, or a front loader for large surface soil samples.

Project Background

One common style of rover suspension is the tiple bogey suspension. Three independent, non-actuated, and free pivoting suspension arches are configured about the rover. The triple bogey allows for the independent arches to contort easily to the terrain while maintaining a level chassis. Steep and uneven terrain can be easily tackled. The center wheels, as seen from a top view on the right, are offset to the outside which results in a smoother skid steer (turning) operation.

Prototyping

The geometry of the suspension, the wheel distances and pivot height off the ground, was determined from various papers making it pretty theoretical. Building a prototype suspension was a great idea to validate the theoretical geometry quickly and cheaply. The prototype was made from laser-cut plywood sheets laminated together. The worst-case scenario for steering and turning would be when the rover is at its lightest configuration, around 30kg. The rear arch was free to pivot to further amplify issues with the front left and right arches. Testing revealed that on high friction surfaces (asphalt) and with a zero-turn radius, the arches would lift up and could cause potential damage to the rover. Given the terrain is loose dirt or rocks, and typical driving is slight turning, the geometry was deemed acceptable.

Manufacturing

Testing the manufacturing method for the suspension arches. Two "C-Channel" halves are made individually and combined by several carbon fiber strips bonded on the insides of the seams.

This jig was used to trim the two halves (C-Channel) of the suspension arches to their final height by holding a dremel with a cut-off disc.

Laser cut jigs were used to accurately drill the position of the pivot for the arches. The jig for the side arches is shown below.

Rivet nuts were inserted into the suspension arches to allow the motor housings to be secured to the suspension.

A simple rectangular chassis was made from purchased square carbon fiber tubes, gusset plates, rivets, and epoxy. The suspension arms have foam-cored carbon fiber forks to attach to the suspension arches. The suspension arches have internal wiring to be connected to two brushless DC motors and the wheels. The wheels have aluminum rims and spokes with a thick urethane tread. The tread has inlaid carbon fiber strips for shape retention and is stitched together with Kevlar thread. The wheels play a large role in passive shock absorption when dealing with rough terrain.

Below is an image of a composites lay-up using a vacuum bag setup. There are four parts in three separate bags. Here I am making a Kevlar plate for the electrical enclosure lid, a Kevlar battery box, and a carbon fiber front and side panel.

Lightweight carbon fiber panels were added to the front half of the rover to create an electrical enclosure. The panels will keep the elements, dust or water, out of from the electronics. The foam-cored Kevlar skid plate (yellow panel on the bottom of the chassis) can also be seen here.

The rear suspension connection is the only structural aluminum used on the frame. The CAD model and FEA for the rear suspension pivot can be seen below. The FEA showed failure at the worst-case scenario of a 1+ meter drop with a 50kg rover, however, this being a very unlikely scenario, the design was acceptable.

The full rover chassis, suspension, and electrical enclosure can be seen below at the 2019 CIRC Night Task. The rover can tackle large obstacles easily with the superior suspension design, high torque motors, and grippy wheels!

Front Loader

One of the tasks for the CIRC competition required taking larger surface soil samples of 4kg's for various locations. I designed, prototyped, and fabricated a front loader that quickly attached to the front of the rover. The CAD design can be seen below.

To ensure that the center of mass was acceptable, a prototype was quickly and cheaply built using aluminum square tube and laser-cut MDF. The two linear actuators were more than strong enough to lift a full load of soil. The amount of soil collected was measured using two calibrated load cells which were built into the frame of the front loader. The front loader and rover can be seen below at the 2019 CIRC competition.