What is the difference between tank drive and mecanum drive?

A tank, or differential drive, consists of two sets of wheels on either side of the robot that are independently driven . These drivetrains commonly use traction wheels, which are designed for maximum grip .

A mecanum drive configuration typically uses four mecanum wheels oriented in an "X" shape, with the rollers angled towards the center when viewed from above . Mecanum wheels have rollers that rotate at 45 degrees to the rotation of the wheel, unlike omni wheels which have rollers rotated 90 degrees . This configuration allows for movement in any direction, though perfect movement is not possible due to friction, meaning forward/backward movement is slightly faster than other directions . Mecanum wheels with ball bearings strafe better than those with bushings due to less friction . Strafing, or moving sideways, is possible with mecanum wheels but not with traction wheels used in tank drivetrains .

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Holonomic Drivetrains > H-Drive > Advantages

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Combines tank and holonomic drivetrain advantages
Can be used with 3 or 5 motors
Good traction and top speed
Great maneuverability and agility

Strafing is slightly less effective than mecanum
Complex suspension occasionally needed, depending on design

(Figure: 9804 Bomb Squad, Relic Recovery)

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide

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One of the most important features of a mecanum wheel is the mechanism that allows the roller to spin---either bushings or ball bearings. Mecanums which use ball bearings strafe better than those which use bushings, as the rollers can spin with less friction.

Note: In the past, some teams have invested in 6 inch diameter mecanum wheels. These are generally much more expensive and generally do not provide significant performance benefits. We highly recommend sticking with wheels that are between 3 and 4 inches in diameter.

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Mecanum TeleOp

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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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Kinematics > Tank (Differential Drive)

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A tank, or differential drive, is a drivetrain consisting of two sets of wheels on either side of the robot that are independently driven. These are described under further detail in the /docs/common-mechanisms/drivetrains/tank section.

The following variables are used in this section.

vr denotes the linear velocity of the right wheel(s)
vl denotes the linear velocity of the left wheel(s)
vf denotes the forward velocity of the robot, relative to itself
omega denotes the rotational velocity of the robot in radians/second
rb denotes the base track radius, or the distance between the wheel and center of the robot (half of the distance between wheels)

Warning: These variables, with the exception of omega, represent linear velocities NOT rotational velocities. Wheel rotational velocity in radians/second can be converted to linear velocity by multiplying by the wheel's radius.

Positive rotational velocity (omega) will spin the robot COUNTERCLOCKWISE when viewed from above.

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Drivetrains > Drivetrain Options

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Omni Wheel — Omni(directional) wheels, sold by many different vendors, are a special type of wheel that prioritizes mobility and strafing (moving laterally) over traction or front-back movement. They are similar to mecanum wheels in that omni wheels have rubber rollers that rotate perpendicular to the plane of the wheel.

Thus, the robot can move sideways (although the robot is not powered in the sideways direction). It is also utilized as a low-friction wheel in 4 wheel, 6 wheel, and 8 wheel drivetrains instead of having corner traction wheels.

Furthermore, X-drive utilizes four omni wheels, though traction is at a minimum.

A mecanum wheel is technically an omnidirectional wheel, but when generally referred to, an "omni wheel" has rollers rotated 90 degrees to the rotation of the wheel, where a mecanum wheel is generally 45 degrees.

Traction wheel — A traction or grip wheel is a wheel designed for maximum grip. It has an outer ring made of rubber, and its wide track ensures a larger contact patch with the ground. Traction wheels are commonly found in tank drivetrains. They are sold in different sizes and thicknesses by different manufacturers.

Strafing — Strafing is the act of moving sideways or laterally (somewhat similar to drifting). It is possible with omni or mecanum wheels, and not possible with traction wheels.

Parallel Plate Drivetrain — A parallel plate drivetrain is a drivetrain that has drive pods that consist of 2 plates spread apart with wheels and drive transmission in between them.

These plates can be anywhere from 1" to 5" apart, depending on the space requirements of the wheels and drive system. Generally, a pod width of 3" or less is desired to maximize the space between the drive pods for mechanisms such as an intake.

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Holonomic Drivetrains > X-Drive

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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).

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