How it works?

A 2-wheeled robot with a differential-drive controller typically has two independently driven wheels mounted on a common axis, with a caster wheel providing stability. The robot moves by varying the speed and direction of each wheel. Forward motion is achieved when both wheels rotate at the same speed in the same direction, while turning is accomplished by driving the wheels at different speeds or in opposite directions. The caster wheel, which is free to rotate and swivel, follows the robot’s motion without contributing to propulsion or steering. The controller calculates the desired wheel velocities based on the robot’s target linear and angular velocities, often using kinematic models to map these to individual wheel speeds.

Usages?

Differential-drive robots are widely used in applications requiring simplicity and agility, such as small autonomous robots, vacuum cleaners (e.g., Roomba), and educational platforms. They are ideal for indoor environments with flat surfaces, where precise navigation in tight spaces is needed. Their compact design and ability to pivot in place make them suitable for tasks like warehouse navigation, home automation, and research prototyping.

Pros & Cons?

• Pros: The differential-drive system is mechanically simple, requiring only two motors, which reduces cost and complexity. It offers excellent maneuverability, including zero-radius turning, making it ideal for confined spaces. The control algorithms are straightforward, relying on basic kinematics.

• Cons: Traction can be limited, especially on uneven terrain, as the caster wheel provides no propulsion. The robot may struggle with heavy loads or slopes due to the reliance on only two driven wheels. Additionally, precise straight-line motion requires accurate wheel synchronization, which can be affected by wheel slip or motor inconsistencies.

How it works?

In the tricycle-steering configuration, the front wheel handles steering while the rear wheels provide traction. The two rear wheels are typically driven together (like a single axle), and the front wheel is steered to control direction. The controller adjusts the rear wheels’ speed for propulsion and the front wheel’s angle for steering, using a kinematic model similar to the tricycle controller but with traction distributed across the rear wheels. This setup allows the robot to follow a path determined by the front wheel’s steering angle while maintaining stability through the powered rear wheels.

Usages?

This configuration is common in larger mobile robots, such as outdoor delivery robots, lawnmowers, or industrial transport vehicles. It is well-suited for applications requiring robust traction, such as navigating grassy fields or slightly uneven terrain, where the rear-wheel drive provides better grip than a single driven wheel.

Pros & Cons?

• Pros: The rear-wheel drive offers improved traction compared to the tricycle configuration, making it better suited for outdoor or uneven surfaces. The separation of steering and propulsion simplifies control for certain tasks. 

• Cons: The design is mechanically more complex than a standard tricycle, requiring multiple motors for rear-wheel drive. It still has a limited turning radius, reducing agility in tight spaces. The reliance on a single steering wheel can also make it less robust against obstacles that affect the front wheel’s alignment.

How it works?

In a tricycle configuration, both traction and steering are handled by the front wheel, with two passive rear wheels providing stability. The front wheel is powered by a motor for propulsion and controlled by a steering motor to set the direction. The tricycle controller adjusts the front wheel’s speed to control the robot’s linear velocity and its steering angle to determine the turning radius. The kinematic model relates the steering angle and wheel speed to the robot’s overall motion, typically using a bicycle-like model where the robot follows a circular path determined by the front wheel’s orientation.

Usages?

Tricycle robots are used in applications requiring simple steering mechanisms, such as automated guided vehicles (AGVs) in warehouses, small delivery robots, or agricultural robots. Their design suits environments with smooth surfaces and moderate navigation complexity, such as indoor logistics or campus delivery systems, where predictable steering is needed.

Pros & Cons?

• Pros: The tricycle design is mechanically straightforward, requiring only one drive motor and one steering motor, which simplifies construction and maintenance. It provides stable motion on flat surfaces and is relatively easy to control for basic navigation tasks.

• Cons: The reliance on a single driven wheel limits traction, making it less effective on uneven or slippery surfaces. Turning is less agile than differential-drive systems, as it requires a finite turning radius. The front wheel’s dual role in propulsion and steering can also introduce complexity in coordinating precise movements.

How it works?

A 4-wheeled robot with a differential-drive controller, often called skid-steering, has two pairs of wheels (left and right sides) driven independently. Each side’s wheels are coupled to move at the same speed, and the robot steers by varying the speed or direction of the left and right wheel sets. For example, turning left is achieved by slowing or reversing the left wheels while driving the right wheels forward. The controller uses a kinematic model to map desired linear and angular velocities to wheel speeds, but the wheels skid during turns, as they do not pivot independently.

Usages?

Skid-steering robots are prevalent in rugged applications, such as military robots, construction vehicles, and all-terrain robots (e.g., Mars rovers). They are ideal for outdoor environments with rough terrain, where durability and traction are critical, such as in search-and-rescue missions or agricultural automation.

Pros & Cons?

• Pros: Skid-steering provides excellent traction and stability due to four driven wheels, making it suitable for heavy loads and uneven terrain. The design is robust and mechanically simple, with no need for complex steering mechanisms.

• Cons: Skid-steering is less precise, as wheel slippage during turns can cause odometry errors. It is less efficient on smooth surfaces due to skidding, which increases wear on wheels and motors. The system also consumes more power during turns compared to other steering methods.

How it works?

The Ackermann-steering controller is modeled after car-like steering, where the front two wheels pivot to steer the robot while the rear wheels provide traction. The controller ensures that the inner and outer front wheels turn at different angles to follow a smooth circular path, minimizing wheel slip. This is achieved using the Ackermann geometry, which calculates the steering angles based on the robot’s desired turning radius. The rear wheels are driven at the same speed for propulsion, and the controller maps the desired linear and angular velocities to steering angles and wheel speeds.

Usages?

Ackermann-steering robots are used in applications mimicking automotive navigation, such as autonomous cars, delivery trucks, and large-scale AGVs. They are ideal for outdoor environments requiring smooth, efficient turns, such as urban delivery systems or campus shuttles.

Pros & Cons?

• Pros: Ackermann steering offers smooth, efficient turns with minimal wheel slip, improving odometry accuracy and energy efficiency. It is well-suited for high-speed navigation and large robots.

• Cons: The steering mechanism is mechanically complex, requiring precise coordination of front wheel angles, which increases cost and maintenance. The system has a larger turning radius, limiting maneuverability in tight spaces compared to differential-drive systems.

How it works?

A mecanum-drive robot uses four mecanum wheels, each with angled rollers that allow omnidirectional movement. The controller independently adjusts the speed and direction of each wheel to achieve translation (forward/backward, sideways) and rotation. By combining wheel velocities (e.g., rotating front-left and rear-right wheels forward while front-right and rear-left backward for sideways motion), the robot can move in any direction without changing its orientation. The kinematic model is more complex, requiring precise control to balance the contributions of each wheel’s rollers.

Usages?

Mecanum-drive robots are used in applications requiring high maneuverability, such as warehouse automation, robotics competitions, and industrial manipulators. They excel in confined spaces where omnidirectional movement is advantageous, like navigating crowded factories or performing precise positioning tasks.

Pros & Cons?

• Pros: Mecanum drives offer unparalleled maneuverability, allowing movement in any direction and rotation simultaneously. This makes them ideal for complex navigation tasks in tight spaces.

• Cons: The rollers reduce traction compared to standard wheels, limiting performance on uneven or slippery surfaces. The system is mechanically complex and expensive, requiring four independent motors and precise control. Wheel slip can also affect accuracy, and the rollers increase maintenance needs due to wear.