Write me a swerve drive code and explain to me why everyone tells me it's hard to code. Isnt it like mecanum drie, where the code/formulas are public?

Swerve drive is significantly more complex to program than mecanum drive because it requires coordinating both the steering angle and the drive velocity of independent modules, rather than just adjusting the speeds of fixed wheels. Game Manual 0 explains that kinematics applies geometry to control robot mechanisms, and while it provides the formulas for mecanum drive, it notes that swerve kinematics derivations are found in external resources like "Controls Engineering in the FIRST Robotics Competition" and "Mobile Robot Kinematics for FTC" .

For implementing swerve drive code, FTCLib provides dedicated libraries for WPILib-style kinematics and odometry, which are covered in the restricted sections FTCLib Swerve Drive Kinematics (https://docs.ftclib.org/kinematics/wpilib-kinematics/swerve-drive-kinematics) and FTCLib Swerve Drive Odometry (https://docs.ftclib.org/kinematics/wpilib-kinematics/swerve-drive-odometry) .

Additionally, hardware-specific programming and calibration are required. For example, REV Robotics provides guides on programming and calibrating MAXSwerve modules, which can be found in the restricted sections Programming MAXSwerve (https://docs.revrobotics.com/ion-build/motion/maxswerve/programming-maxswerve) and Swerve Calibration (https://docs.revrobotics.com/rev-hardware-client-2/guides/swerve-calibration) .

The provided documentation does not contain the actual swerve drive code because the relevant programming and kinematics sections are restricted.

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Sources16

WPILib Kinematics > Swerve Drive Kinematics

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WPILib Kinematics > Swerve Drive Odometry

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MAXSwerve Compatibility | FRC Kickoff Concepts

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Programming MAXSwerve | REV ION Build System

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MAXSwerve | REV ION Build System

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MAXSwerve Drivetrain Assembly | REV ION Build System

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Kinematics

Game Manual 0·CC BY-NC 4.0

Kinematics is the application of geometry to the control of various robot mechanisms. Kinematics equations are used to control mechanisms by providing specific inputs to achieve a desired output.

Many of the kinematics equations here were taken from Controls Engineering in the FIRST Robotics Competition (book) (https://file.tavsys.net/control/controls-engineering-in-frc.pdf) and Mobile Robot Kinematics for FTC (paper) (https://github.com/acmerobotics/road-runner/blob/master/doc/pdf/Mobile_Robot_Kinematics_for_FTC.pdf), which contain the relevant derivations. While only tank (differential drive) and mecanum kinematics equations are shown here, these sources also contain derivations for other mechanisms such as swerve and dead wheel odometry.

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Swerve Calibration | REV Hardware Client 2

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MAXSwerve Module Assembly | REV ION Build System

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WPILib Kinematics > Mecanum Drive Kinematics

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Holonomic Drivetrains > Mecanum Drive > Advantages

Game Manual 0·CC BY-NC 4.0

Fantastic maneuverability and agility due to strafing, can avoid defense very well
Good acceleration, can have high top speed
Very versatile drivetrain for nearly any game

Suffers in traction, as mecanum rollers have a lower coefficient of friction than traction wheels; cannot traverse terrain
Able to be pushed around on defense
Wheels must be powered independently, so there is no redundancy

CAD Examples of Mecanum Drive (Click to expand) goBILDA COTS Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/df9bcea72fcdd7e4aee4134b)

REV COTS Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/b8d4a6cfe4f26c5170f40a6c)

goBILDA Custom Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/cc2df2fbce0e9ca393696b54)

REV Custom Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/40c3266dab2e444011cce79c)

(Figure: Configuration for mecanum wheels, courtesy goBILDA)

Open Game Manual 0

Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide > Not Recommended

Game Manual 0·CC BY-NC 4.0

There is almost no reason to use these mecanum wheels---they perform very poorly and are not much cheaper than those in the Recommended section. We cannot recommend the use of these on an FTC robot, if at all possible replace them with mecanums from the Recommended section.

TETRIX Mecanum Wheels (https://www.pitsco.com/TETRIX-MAX-Mecanum-Wheels) ($149.00): These mecanums are designed with a bushing based, hard plastic roller which in turn creates poor strafing performance. The integrated hub is a set screw based design with a round bore, causing unreliability as well as taking up extra space.
Andymark Standard Duty Mecanum Wheels (https://andymark.com/products/4-in-sd-mecanum-wheel) ($77.00): These wheels barely strafe and are super fragile.

(Figure: 8103 Null Robotics, Rover Ruckus, using Nexus mecanum bearing)

(Figure: 731 Wannabee Strange, Rover Ruckus, using AndyMark HD mecanum wheels)

Open Game Manual 0

Mecanum TeleOp

Game Manual 0·CC BY-NC 4.0

Note: Some common COTS mecanum drivetrain kits are the goBILDA Strafer Chassis Kit (https://www.gobilda.com/strafer-chassis-kit-v5/) and the REV Mecanum Drivetrain Kit (https://www.revrobotics.com/rev-45-2470/).

A standard mecanum drive configuration possesses 4 mecanum wheels oriented in an "X" shape. This means that the rollers are angled towards the center when looking at it from above. This configuration allows one to add up the force vectors generated by the offset rollers and derive movement in any direction. It is important to note that because of friction, perfect movement isn't possible in every direction, so a mecanum drivetrain will be able to drive slightly faster forwards/backwards than any other directions. Combining translation and rotation will also result in slower movement.

In the image above, vectors 1, 2, 3, and 4 are the force vectors created by the mecanum wheels when the chassis is instructed to drive towards the top of the image. All motors are driving forward. The blue and red lines are their X and Y components, respectively. Here are a few examples of how the wheels must be driven to achieve different movements:

Attention: It is strongly advised to not hardcode these movements in; there is a much better way described below that allows for true holonomic movement and is much more elegant.

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WPILib Kinematics > Mecanum Drive Odometry

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Holonomic Drivetrains > X-Drive

Game Manual 0·CC BY-NC 4.0

X-Drive is a holonomic omni-wheel based drivetrain. This type of drive involves mounting 4 omni wheels at the corner of the robot at a 45 degree angle.

One notable difference between X-Drive and mecanum is strafe speed. While, as mentioned in the mecanum section, the ratio of strafe speed to forward speed is noticeably less than 1, the ratio on an X-Drive is exactly 1 due to the rotational symmetry of the wheel placement. This means that an X-Drive bot's strafe speed and forward speed are equivalent. The drivetrains are slower, however, when strafing at 45° (approximately frac{sqrt{2}}{2} of its forward speed).

Even though X-drive has good turning and acceleration, the main downside to the drive is packaging/form factor. Packaging refers to how easy/convenient the drivetrain fits into the overall design of the robot.

Ideally, the drivetrain should take up as little space as possible to make it easier to design mechanisms around. Because the omni wheels are offset, packaging a X-Drive is more difficult than other types of holonomic drive like mecanum or H-Drive. Also because of the strange packaging, it is relatively difficult to cleanly transfer power from the motors to wheels, meaning that most X-Drives end up being direct-driven, which is bad for the lifespan of the motor gearbox.

Note: When using X-Drive, the robot moves forwards/backwards/straight side-to-side sqrt{2} times faster than a drivetrain with wheels in the normal orientation (with the same gear ratio and wheel size).

For an explanation of why exactly this is, see this analysis (https://www.chiefdelphi.com/t/paper-mecanum-and-omni-kinematic-and-force-analysis/106153).

Open Game Manual 0

Kinematics > Mecanum Drive > Forward Kinematics

Game Manual 0·CC BY-NC 4.0

The forward kinematics of a mecanum drive relate the velocity of the wheels to the forward, strafe, and rotational velocities of the robot, relative to itself. These are as follows:

vf = frac{v_fr + v_fl + v_br + v_bl}{4}

vs = frac{v_bl + v_fr - v_fl - v_br}{4}

ω = frac{v_br + v_fr - v_fl - v_bl}{4*2r_b}

Open Game Manual 0