Electronics > Mechanical & Automation Engineering

Micro Mouse Robot

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Mysion:
Recently I began working on the design of my second robot for the micro mouse competition. I designed V1 during my freshman year and now that I'm a Junior in college I figure it's time I take a second swing at it.

For those of you who don't know what micro mouse is it's a student robotics competition. Each run of the robot has two stages. In the first non-timed stage your robot maps the maze. Then in the second stage your bot it timed in how fast it can reach the center, now that is has mapped the maze.

Version 1 pictured bellow had a few major short comings. The biggest is the lack of encoder resolution. It only gets an encoder tick every 1.1 cm. Over time hardware was added to the bot to try and improve the situation. A I2C gryo was added underneath the robot and the IR distance sensors were replaced with ST time of flight sensors. I feel as if I've taken this robot as far as it can go and it's simply time for V2.




Originally the project did not include the raspberry pi strapped to the back of the robot. This was a late a addition. The idea here was that other team members would be able to code maze following code in python instead of C. While awkward this did work.


When planning V2 the most important part is the encoder selection. The robots that compete in the extremely competitive Japaneses compitition use expensive Swiss Faulhaber motors. Thankfully Pololu makes a motor with an encoder that will be almost as good.



These little guys will get me twelve tick per motor revolution which will then be multiplied by the gear ratio. With a 50:1 motor that's 600 ticks per wheel revolution! Far better than what I was getting before.

This time around I'm also planning to move away from AVR and give ARM a chance. I've got a STM32 dev board so that will probably be my pick.
Now that I know what motors I plan to use I can get started with CAD. I'm going to complete the mechanical design and then hopefully the circuit will fit in the area available.



This is what I have so far. There's a ball caster holding up the front. The distance sensors will be attached with vertical daughter boards similar to V1. This time I'll use connectors instead of soldering the boards together.

I'll post more updates as I work on the CAD model. Today is a snow day so time to make the best use of it!

Doctorandus_P:
Where do the yellow CAD wheels come from?
Are they real, or can you make them yourself?

These motors are relatively expensive from western "hobby" shops and a lot cheaper from Ali / Ebay / China direct.
Model is N20 and they come in different gear ratios and with or without encoder, or even with wheels.
https://www.aliexpress.com/wholesale?SearchText=N20+encoder

Also, for real competitive Micro Mouse acceleration becomes an important factor and a low center of gravity becomes important.
You also want to keep as much of the weight on the driven wheels during acceleration and braking, and not on the coaster wheels.
Your first version is not so good at this, try to factor this in in your next design.

Mysion:
The wheels are JSumo wheels available here https://www.robotshop.com/en/jsumo-slt20p-steel-silicone-wheel-set-33mm-20mm-pair.html

There nice since the overlap the gear box, this add width while keeping the robot small.

Thanks for the link. I didn't realize they were available so easy on aliexpress. I may still go with Pololu due to shipping times but I haven't crossed that bridge yet.

The placement of the wheels is a challenge. The only way to do away with the caster completely is to use a 4 wheel design.  I did play around with using gears on the Pololu motors for 4 wheels but then the bot ends up far too wide. I really want to keep the width at or bellow 80mm. I am concerned with the front pitching up during acceleration so the battery will be placed there to add weight.

The new one will be far lower to the ground with only 10mm of clearance this time around and far more compact.

alanb:
Great project, I would love to see more pictures.
What type of batteries do you use? are they located underneath to keep centre of gravity low?

Whats the resolution like for the time of flight sensor and what is the minimum distance that they can detect?

Good luck with version 2.

Mysion:
Good questions!

The TOF sensors have a minimum range of 3cm and go out to almost 200cm. When you are closer than 30 they have a nice failure mode and just keep reading 30. In practice this should be okay as 30mm is too close to a wall. I won't know how close, just too close but good enough.

The battery will be mounted in the front like in the image bellow.



All the red parts will be 3D printed. The bumper is in the rear to allow the robot to push up against rear walls to align it's self. It shouldn't need to but more options are good.

I've been thinking about my goals and Doctorandus_P is right this design will never be able to compete on an international level but that's okay. It's not my goal. (Nor do I have the funds)
This robot, if finished in time will compete in the IEEE east coast competition made up of college students. The previous robot was slightly above average so V2 should blow away it's expected competition.

In addition to the micro mouse competition it should serve as a fun little platform to work on my robotics skills and toy with Freertos.

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