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Wheel Selection

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alexbarnett12 edited this page Nov 25, 2017 · 1 revision

Rover Wheels

It is important to carefully select your wheels so that your rover has the ability to handle tough terrain. For Mars, the most important aspect of the wheels is gaining traction. Martian sand (or BP-1) is very dense and loose, meaning any normal wheel will just slip on the surface. Lucky for us, there are many smart people who have extensively researched optimal rover wheels.

For wheel diameter and width, there is not one correct solution. Increasing wheel diameter means the robot can climb over larger obstacles, and increasing wheel width means the robot will have better traction in the sand. However, increasing these sizes will increase rover weight and motor power needed, so a careful balance is required. Ideal wheel diameter is equal to the diameter of the largest obstacle, but due to size constraints, it is currently best to aim for a wheel diameter of about 25% the height of the robot.

Below are some helpful links to research on grouser design.

Effect of Straight Grousers Parameters on Motion Performance of Small Rigid Wheel on Loose Sand

http://scialert.net/fulltext/?doi=itj.2008.1125.1132

Analysis of Grouser Performance to Develop Guidelines for Design for Planetary Rovers

https://www.ri.cmu.edu/pub_files/2014/6/i-SAIRAS2014_Hiroaki_grouser.pdf

Tl;dr Conclusions:

  • Increasing the number of grousers increases wheel traction; This is helpful for escaping steep slopes
  • Increasing grouser height increases wheel traction up to 10 mm, at the expense of a smooth ride
  • Grouser thickness does not affect performance, just keep it small (1.5 mm)
  • Chevron grousers (second link) are ideal for tractive efficiency, but are harder to manufacture

Wheel and Grouser Assembly

The wheels are made up of spokes, a rim, and grousers. To keep weight low, these spokes can be two thin plates along the outer edges of the rim. It is quite hard to machine these spokes, so it is best to look to a machine shop or to simplify the design if a machine shop is out of price range. The rim can be made out of thin aluminum sheet metal. 5052 Aluminum Alloy must be used over 6061-T6 since 6061-T6 cannot be bent. A metal bender (in the Wondry) can be used to bend the sheet metal into a circle.

Grousers can be made out of aluminum since it is very light and plenty strong for our applications. If possible, avoid machining them since it will take a long time due to the high number of grousers. L-brackets work great as grousers, since one of the sides can be bolted to the wheel. A spacer can be used as an attachment location to connect the grousers, wheel sides, and wheel rim together. It is just a rectangle with a curved side to match the curve of the rim. These can be 3D printed.

L-Brackets: https://us.misumi-ec.com/vona2/detail/110300466050/?Inch=0

  • These can be customized to a specific length and height

Machine Shops:

Sheet Metal:

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