Drivetrain

This is the location for all things drivetrain. Drivetrain refers to the system that is responsible for moving the chassis (main body of the robot) around an environment. On this page, you will find information regarding some of the different types of drivetrain as well as wheel types.

The two wheel drive (aka “casterbot”) is undeniably the easiest one to make. A two-wheel drive consists of having two powered wheels at one end of the robot and a unpowered surface that slides on the other end of the robot.

The two wheel drive offers high maneuverability while maintaining a very low level of complexity. It can also be the most difficult to use. A casterbot turns with such ease because there is virtually no side friction working against the wheels of the robot as it turns. However, that same ease of turning can be a nightmare - with no friction regulating the bot’s speed of turning, its inertia will always want to continue turning even after the motors have stopped. This results in a robot that is extremely difficult to control using basic control methods - it is a rare casterbot that can drive in a straight line without the use of internal sensors or gyros. Furthermore, the unpowered surfaces in contact with the floor detract from available pushing power, and significantly lower a robot’s ability to maintain its position when hit.

The design, for all its shortcomings, is nevertheless viable if correctly put to use. When designing a two-wheel drivetrain, the powered wheels should be in the center. This allows the robot’s pivot point to remain close to its center of mass, minimizing the area through which it must travel in order to turn. Placing the powered wheels along the 30” sides of the robot will further slow its rate of turn, making it more controllable (helpful, but not necessary).

The four wheel drive system is probably the most common drivetrain used in FRC. It offers a number of advantages and disadvantages over the more basic two wheel drive. With the addition of two extra driven wheels, a four wheel drive robot has more traction and control over a two wheel drive robot. The trade-off is the increased wheel base can cause problems turning (when the wheel base exceeds the wheel width).

The six wheel drive is a moderately common type of drivetrain. It offers a good compromise between traction and maneuverability. Most teams choose to lower the center wheels by approximately 1/8 - 3/16”, allowing the robot to turn more easily because normally it has a shortened wheel base but at the same time, can tip over slightly, and enjoy the benefits of a full length wheel base.

Some six wheel drive robots are in fact four wheel drive robots with an unpowered pair of extra wheels that just act to increase the wheel base for stability. When tipped to the unpowered set, some of its weight is diverted to unpowered wheels and traction is therefore reduced.

A car drive (aka Ackerman Steering) robot has a steering system much like what would be found on a standard automotive vehicle with front wheel steering.

Usually, the drivetrain system has four wheels, with the two wheels in the back providing power, and the two wheels in the front providing steering, though there are designs with power and steering to all wheels. While the design gives increased speed and pushing power and reduces the learning curve for the driver, it is not a very common choice due to its lack of maneuverability. Generally, the large turning radius makes it very difficult to maneuver in a corner or tight space.

A holonomic drive (aka “omni drive”) also allows a robot to travel with three degrees of freedom rather than two. The difference is that a holonomic drive allows a robot to instantly change direction without having to turn the wheels to a different position. The major benefit of using a holonomic drive is a great increase in maneuverability without having to add an entirely new mechanism to turn the wheels. The major trade-off is that the robot isn’t very good when it comes to a pushing and shoving. Holonomic wheels have poor traction, as they can’t be made inflatable or with treads. They also demand individual, speed controlled motors for each wheel.

Here is a video of team 1418’s 2007 holonomic drive in action. Note the slight listing when the chassis is supposed to be driving straight; this was caused by asymmetries in the power outputs of the Victor speed controllers. This problem was successfully fixed by using a lookup table to force individual joystick positions to map to a linear set of power outputs.

A swerve drive robot has the ability to rotate its wheels, allowing the robot to travel with three degrees of freedom.

Several types exist, such as the crab drive, in which all four wheels are linked such that they always have the same angle as each other (generally requires one drive motor per wheel plus two angling motors). A 2+2 configuration has two pairs of wheels which share angles (typically requiring four drive motors plus two angle motors), and a full omni system allows each wheel to be independently angled (requires four drive motors and four angle motors.

The main advantage of a swerve drive is a great increase in maneuverability. The trade-offs are that swerve drives are much more complex to build and consume much more resources (time, money, weight, space) than most other drivetrains. Some forms of swerve drive are also known to have less power for pushing other robots around on the field. However, increased maneuverability is gained to make up for it.

A mecanum drive is another omnidirectional drive system. It consist of wheels with their rollers angled in a conventional four wheel drive layout. With an independent motor/transmission on each wheel, omni driving can be achieved by varying speeds.

At Windward, there are three main types of wheels that we typically use on our robots: mecanum, omni, and traditional. Each type of wheel has its own unique advantages and disadvantages, and the best choice for a given robot will depend on the specific requirements and goals of the team.

Mecanum wheels are a type of wheel that consists of a series of rollers arranged at 45-degree angles to the surface of the wheel. This allows the wheel to move in any direction, including sideways, without changing its orientation. This makes mecanum wheels very versatile, as they can be used to navigate complex environments and perform a wide range of tasks.

One of the main advantages of mecanum wheels is their ability to move in any direction. This allows robots equipped with mecanum wheels to move quickly and easily in any direction, which can be useful for tasks such as collecting game pieces or navigating around obstacles. In addition, mecanum wheels are relatively small and lightweight, which can be helpful for robots that need to be fast and agile.

However, mecanum wheels also have some disadvantages. For one, they can be difficult to control, as the rollers can sometimes slip or spin out of control. In addition, mecanum wheels are also harder to code and clean, making for a lengthier building and upkeep process.

Omni wheels are another type of wheel that is commonly used in FRC robotics. Unlike traditional wheels, which have a single circular rolling surface, omni wheels have a series of small rollers arranged around the circumference of the wheel. This allows the wheel to move in any direction, just like mecanum wheels, but with a different mechanism.

One of the main advantages of omni wheels is their simplicity. Unlike mecanum wheels, which require complex mechanisms to control the rollers, omni wheels are relatively simple and easy to control. This can make them a good choice for teams that want a versatile wheel but don't have the resources or expertise to build and control a mecanum wheel.

However, omni wheels also have some disadvantages. For one, they are not as efficient as traditional wheels, which means that robots equipped with omni wheels may not be able to move as quickly or smoothly. In addition, omni wheels can be prone to slipping, especially on surfaces that are not smooth or flat.

Traditional wheels are the most common type of wheel used in FRC robotics. They consist of a single circular rolling surface, which allows the wheel to move in a straight line. Traditional wheels are simple and reliable, and they are the most efficient type of wheel for moving in a straight line.

One of the main advantages of traditional wheels is their simplicity and reliability. They are easy to control, and they are less likely to slip or spin out of control than other types of wheels. In addition, traditional wheels are relatively inexpensive, which can be a big advantage for teams with limited budgets.

However, traditional wheels also have some disadvantages. For one, they can only move in a straight line, which means that robots equipped with traditional wheels may not be able to navigate complex environments as easily as robots with mecanum or omni wheels. In addition, traditional wheels are not as versatile as other types of wheels, which can limit the tasks that a robot equipped with traditional wheels can perform.