Drivetrains

Drivetrains are a type of Subsystem that control a robot's movement. In other words they move the robot. In this tutorial, we'll be going over the Romi's drivetrain, called "RomiDrivetrain" (no way!)

For this example, the Drivetrain should have two motor variables— one for each of Romi's motors. However, there are different ways teams like to set up their drivetrains. There are three general types: TankDrive, MecanumDrive, and SwerveDrive.

TankDrive

TankDrive is having both sides of the robot have fixed, ordinary wheels. As the name suggests, your robot gets to move like a tank. All the wheels on one side move at the same speed, letting your robot either move forward or rotate. This Drivetrain will be the main focus across all the curricula on this page.

Due to this nature, you can have one to two motors power one side of the robot. And since the wheels aren't particularly special, this drivetrain is the cheapest drivetrain you can use. This also means TankDrive drivetrains are significantly easier to code than the other drivetrains below.

However, the simplicity of TankDrive also proves to be its weakness. Holonomic movement— moving the robot in directions without having to rotate itself- is a perk other drivetrains have that TankDrive doesn't. This makes TankDrive much more clunkier than others. The wheels aren't particularly special either, so they're generally inferior to the wheels other drive trains use.

Here's an example of a TankDrive chassis (also the one you'll also be coding a bit after the Romi Curriculum!)

Pros:

• Easy to assemble, especially compared to other drive trains

• Easy to code, as you only have to handle the two sides

• Cheaper than other drivetrains

Cons:

• Can only drive forward, meaning the robot has to rotate before changing directions

• Wheels are ordinary, and don't have as good traction as others

• Autonomous movement will be a lot more clunky + complicated

MecanumDrive

Mecanum is like TankDrive, except that it has holonomic movement. This means it can do more interesting things, such as strafe.

MecanumDrive drive trains have specially designed wheels with 45° rollers on them. Each wheel has its own motor, coding MecanumDrive drivetrains slightly more complex than TankDrive (pretty low bar though).

However, MecanumDrive doesn't have perfect holonomic movement— it can only move in 8 directions in relation to the robot— forward, backward, left & right, and the four diagonals. This means your robot can't fine-tune its rotation, which can potentially be a problem. The rollers on each wheel could also make your robot slide a little bit when stopping suddenly.

MecanumDrive is popular in FTC due to its general cost-effectiveness and simplicity.

credits for image: https://www.studica.ca/en/build-drivetrain-mecanum

Pros:

• Holonomic movement (to some extent)

• Relatively cheap, somewhat easy to assemble

• Somewhat simple— code can be somewhat straightforward

Cons:

• Doesn't have true holonomic movement

• More expensive than TankDrive, and somewhat more complicated

• Wheels don't have perfect traction, could slip

SwerveDrive

SwerveDrive is arguably one of the most, if not the most, popular drive trains that FRC teams use. Think of it as the Swiss army knife of drivetrains.

Here is one of the wheel modules. You might be wondering. "Why are there two motors on the module? One of the motors is to change the wheel's direction. The other motor is to spin the wheel itself. Unlike MecanumDrive, SwerveDrive gives true holonomic movement.

The wheels on each of these modules also have special Velcro that grips onto the fields in FRC matches (it's generally carpeted). This stops the robot from rolling too far its intended position.

Here's a shot of our team's drivetrain, with the four wheel modules it has. If you couldn't tell already, a big problem with SwerveDrive is how stupidly expensive and complex a SwerveDrive drivetrain can be. The drivetrain here takes up 8 individual motors, each with their own encoders and whatnot. Maintenance as you could tell, is tedious. So, despite being arguably the best drivetrain, it is difficult to make it.

Due to its complexity, there are tons of resources online to help you. There are even some that write your SwerveDrive drivetrain for you!

image taken of FRC 9450's DriveTrain

Pros

• TRUE holonomic movement- can move in any direction at any time, unlike MecanumDrive

• Precise- velcro-lined wheels stop the robot from slipping

• Lots of tools to help you

Cons

• Incredibly expensive— requiring double the motors needed for MecanumDrive

• Very complex to assemble- lots of moving parts that could break

• More difficult to write code for SwerveDrive drivetrain than for others

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