From Team 449 Wiki
A robot's "drive" comprises the set of components that enable it to move around the field. There are a variety of different types of common drive systems in FRC, each with their own benefits and drawbacks. In most FRC games, the drive is arguably the most important robot subsystem, as an immobile robot is generally incapable of play.
Essentially all FRC drives depend on either wheels or treads to move the robot around the field. These are powered by motors (usually CIMs or mini-CIMs). Between the motors and the wheels or tread, there is some sort of gearing, and usually some form of motion transfer to transmit the motor power from the gears to the wheels.
Different types of FRC drives generally differ in their specific choices for these components, and their arrangement.
Types of drives
At the broadest level, FRC drives are divided into two categories: unidirectional, and omnidirectional.
A "unidirectional" drive is only capable of linear motion along a single axis - forwards and backwards. In order to move in a different direction, a unidirectional drive must therefore reorient itself by turning. The majority of drives in FRC are unidirectional, as they tend to be simpler and more robust than omnidirectional drives.
A tank drive is the simplest drive used in FRC. Contrary to the name, most tank drives do not actually use tank treads. Rather, like a tank, the wheels in a tank drive are not capable of turning, but rather the robot turns by running each side of the drive at different speeds. Our team usually implements this in the form of a West Coast drive.
An "omnidirectional" drive is capable of linear motion along any axis, meaning it may move in any direction without first turning to face that direction. Omnidirectional drives are very useful for games that require fine positioning or maneuvering, but are either less powerful or far more complex than unidirectional drives.
A mecanum drive uses mecanum wheels to achieve omnidirectional motion. These wheels have rollers along their perimeter oriented 45 degrees to the wheel axis, and a properly-oriented set of four can produce motion in any direction. However, in exchange for the increased mobility, mecanum drives have less pushing power and are slightly more complicated than tank drives.
Omni holonomic drive
An omni holonomic drive uses omni wheels arranged in a square or diamond to produce omnidirectional motion. Omni wheels have rollers along their perimeter oriented at a 90 degree angle to the wheel axis, and thus can roll freely sideways. Omni holonomic drives have extremely low pushing power and are roughly similar in complexity to mecanum drives, and are consequently not particularly popular in FRC. A variant of this is kiwi drive, in which the wheels are mounted in a triangle.
H-drive has 5 omni wheels, or doubled omni wheels, arranged in an "H" shape. The 4 corner wheels allow for tank-like (unidirectional) driving, and the center wheel allows for sideways motion. This is fairly simple to build and control, but it is reeeeeally easy to push.
A swerve drive achieves omnidirectional motion by independently turning and powering each drive wheel. Swerve drives are capable of the greatest range of motions of any omnidirectional drive, and push comparably well to a tank drive. However, the extra motors and sensors required to individually turn every wheel and the complexity of correctly handling the kinematics in programming makes swerve drives extremely complex. It is generally advised that a team should not attempt to field a swerve drive in competition until they have worked with them successfully for two consecutive offseasons. To date, team 449 has not yet successfully built and programmed a swerve drive.
There are also some drivetrains that can use actuators to swap between omnidirectional and unidirectional drives. The two most common are butterfly and octocanum. In both of these, each corner has a pair of wheels, one traction and one not. They can be actuated, usually with a pivot, to change which one is touching the ground. In this way, the drivetrain is effectively changed. The difference between them is that octocanum toggles between tank and mecanum, and butterfly toggles between tank and H-drive.
Drive design is, as with all robot design, driven by the needs of the year's game and the chosen strategy. Some games and strategies require large amounts of pushing power, while others require nearly none. Some benefit greatly from omnidirectional motion, while others do not.
Above all else, however, all FRC drives must be robust. A drive failure in match will nearly always totally compromise a robot's ability to compete in that match, and usually results in a loss. FRC drives are subject to multiple failure points in competition: common ones include chain and belt (derailment, breakage), the main breaker (tripping due to over-current), gearboxes (gear failure, wear, jamming due to foreign objects), bearings (breakage due to shock loading), and the wheels themselves. Each failure mode has separate causes and design considerations, and a well-designed drive will not only be robust to these failures, but allow for easy maintenance and replacement of components.
Power, speed and gearing
Main article: Drive gearing
Most FRC drives are powered by 4 to 6 CIMs. The number of motors used on a drive depends on the drive constraints - a mecanum drive will often use 4 motors (one for each wheel), while a tank drive that needs to push or accelerate quickly may use 6. In general, while the specifics depend on gearing, a high-traction drive with only 4 motors will struggle to push without tripping the individual 40-amp breakers, while a drive with 6 motors may trip the main breaker or brown out the control system unless care is taken to ensure that the drive is traction-limited. 6 motor drives are also much harder on the battery.
The gearing of a drive determines its top speed and maximum torque. Note that the gearing of a drive depends not only of the effective gear ratio, but also the wheel size - two robots with identical gearing but different wheel sizes will have different top speeds and effective torques. In general, pushing ability requires a slower gearing, while a faster gearing will allow faster traversing of the field. However, faster gearings are also slower to accelerate, and thus on a field with many obstructions and little open space, a fast gearing can be wasteful as a robot will rarely reach top speed.