Drivetrain Designs

In this post, I will give an overview of the drivetrain designs I've come up with and reveal which ones I've decided to prototype. After browsing Pinterest and Instructables, I found six ideas for drivetrains to travel on sand.

1. Four Wheels. This is the most common type of drivetrain for RC vehicles, and there's no shortage of tutorials for how to make a small four-wheeled robot. This one uses four motors, while this one uses two. A four motor design is simple- each motor drives one wheel. The two-motor tutorial uses an RC truck as the base, and didn't really modify the drivetrain, so the tutorial doesn't provide much insight into how it works. However, in FRC we drove multiple wheels from one motor by chaining them together, and I assume the RC-truck-based drivetrain uses a similar principle:

This drivetrain has six wheels, but it uses the same principle. Image Credit: http://www.robotshop.com

However, since this robot will be driving on sand, I want to minimize the number of exposed parts that sand can get into and damage, so it would probably be better to use four motors. I would also want to use wide wheels, with large surface areas, to help keep them from sinking into the sand.

2. Two Wheels. A lot of Instructables (Obstacle Avoiding Arduino Robot, First Robot) for basic robots use two driven wheels, usually with some kind of passive roller in the back for balance. This would be simpler and cheaper to build, but I'm not sure it would have any advantage over a four-wheeled vehicle in its ability to move in sand.

Image Credit: http://www.instructables.com

 
3. Six C-Limbs. This one's really cool. I referenced it briefly in my last post.

Image Credit: http://blogs.scientificamerican.com

The robot, called Sandbot, was built by a team of researchers from Georgia Institute of Technology, Northwestern University, and the University of Pennsylvania. It has six comma-shaped limbs, called C-limbs. I wanted to see how the limbs move, so I looked it up on YouTube and found this video from NPR.



The limbs move three at a time- the outer two wheels on one side are synchronized with the middle wheel on the other side. The video also revealed that the timing of the c-limbs' rotation is critical. Each rotation has a slow phase (the part where the limbs are in the sand) and a fast phase (the part where the limbs are out of the sand). The timing of switching from fast phase to slow phase is the key to Sandbot's ability to move so quickly through sand.

4. Tank. Tanks are designed to be all-terrain vehicles, so I thought a tank-style drivetrain might do well on sand. This Instructable shows how to make cheap, customizable tank tread out of PVC and bike chain.

Image Credit: http://s356.photobucket.com

5. Simple Walker. The first four ideas were essentially variations of wheeled vehicles. The next two are variations of walking robots. This Instructable shows how to make a very simple walker with two motors, which drags its feet as it moves. Spatula feet would immediately sink into the sand, so it would need to have a different type of foot. I think something with a large surface area would be best (the same principle as snowshoes).



6. One-Motor Walker. This one's intriguing. It uses one motor to run its four legs, and it moves like a lizard. Lizards are evolved for deserts, so I thought maybe its gait would do well on sand.



Like the simple walker, this one would need some kind of wide feet to keep it from sinking in sand.

Next, I needed to choose two or three ideas to prototype. The two wheel design is interesting, but I was concerned that its balance issues, even resting on a rear roller, would make it difficult to install the metal detector's search coil in a way that would keep it parallel to the ground. I'm also not convinced that it would have any advantage over the four-wheel design, so I decided to put the two-wheel design aside for now. I love the idea of the one-motor walker, and I want to make one eventually just because it's cool. However, the gears have to mesh in the middle of the bot, which is where I want to put the search coil. So that's off the table for this project as well.

I knew I definitely want to try the six C-limb design, and while I'm prototyping that, it would be easy to prototype the basic four-wheeler as well- it would be the same motors and chassis, and all I would have to change is the attachments on the motors. I decided to prototype some version of the simple walker too, because it's so different from the wheels and C-limbs. I'm intrigued by the tank idea, but making the tread seems very time-consuming. I also know from FRC that dealing with chains can be a real pain, and having chains and sprockets would create another mechanism that would be vulnerable to sand. But if the four-wheel prototype doesn't go well, I'm going to try attaching PVC pieces to the wheels in a sort of chain-free version of the DIY tank tread. Since the main function of the chains is to help the tank go over obstacles, and I don't anticipate many obstacles on the beach, I think a chainless-tank-tread idea could potentially work well.

Having decided which designs to prototype, my next step was to figure out what parts I needed to order. Most online tutorials skip over this step, since their purpose is to teach you how to build what they built, so they just tell you what parts you need. However, if you are trying to get into designing projects like this as a hobby, learning the process is valuable, so I'll go over it in my next post.