Chassis and Drive Trains
What is a Chassis?
A chassis is the base frame of a motor vehicle or other wheeled conveyance. The chassis of your robot will not only have the wheels and motors mounted to it, but also will serve as the central skeleton that holds everything else on the robot.
Tank Drive Chassis
A tank drive chassis moves similar to its namesake, a tank. A tank has tank treads on each side of the frame that controls its motion. When the treads are both moving in the same direction the tank will move forward or backward. When the treads are moving in opposite directions, the tank will spin left or right in place.
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How Tank Drive Works
Just like how a tank moves, you are controlling each side of the chassis with its corresponding motor.
Forward: both motors move forward at an equal speed, the robot will move forward.
Backwards: both motors move backwards at an equal speed, the robot will move backward.
Left Turn: the right motor moves forward and the left motor moves backwards.
Right Turn: the left motor moves forward and the right motor moves backwards.
If using an analog controller where each joystick controls a corresponding motor, you can control speed and the degree of rotation of turns by varying the intensity between 0 and 1 on the y-axis of each joystick. See our sample code for Tank Drive here.
Holonomic Drive: Omni Drive Chassis
An Omni Drive chassis consists of a frame with four independently powered Omni wheels mounted at a 45° angle to the corners of the frame. This assembly and orientation allows for the wheels to all move at different directions and speeds from each other. The combination of the forces depending on the direction and speed of each wheel will allow an Omni Drive Chassis to move or spin in any direction.
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How Omni Drive Works
The motion of an Omni Drive Chassis is built on the principle of force vectors. Vectors allow us to see the combination of force and direction of any single force. Look to the picture on the left as an example. All dark blue vectors are the actual direction of rotation of an omni wheel. Breaking the main force vectors down to their x and y component forces will allow us to see direction an Omni Drive chassis will actually move. The orange vectors are the x-direction forces and the light blue vectors are the y-direction forces. If you look at the x-direction, orang vectors you will see that the end up being in equal and opposite directions and therefore cancelling the left and right motion out to zero. If you look at the y-direction, light blue vectors, you will see they are all pointing in the same direction with the same force which will provide a forward movement motion. To sum this up, in a scenario where all speeds are the same and the front wheels are spinning inwards and the back wheels are spinning outwards, the robot will move forwards.
You can use these vector diagrams to figure out the direction the robot will move in any combination of speeds and directions of each individual wheel. In the GIF to the right, the vector diagram examples are for the four cardinal directions Forward, Back, Right, and Lef as well as Right Spin, and Left Spin. Also depending on the speed of each motor, you can move in directions in between the cardinal directions.
Note: Because of the orientation of the wheels mounted at 45° to the chassis, you will never be able to get 100% power of the motors translated to 100% speed in motion. At max, for any combination of vectors that amplify or negate each other, you will have a max power of 50% of total possible power of the motors.
See our sample code for Omni Drive here.
Holonomic Drive: Mecanum Drive Chassis
Mecanum drive, similar to an Omni Drive, is a method of driving using specially designed wheels that allow the robot to drive in any direction without changing the orientation of the robot. Unlike the Omni Drive, the Mecanum Wheels are mounted parallel to the robot chassis similar to a tank drive chassis. This is because the rollers on a Mecanum wheel are mounted at a skewed 45° angle so they can be mounted perpendicular to the frame to achieve the same effect as an Omni wheel mounted at a 45° angle to the frame. Mecanum Drive also has a few more degrees of freedom than an Omni Drive chassis such as pivoting on one wheel or center edge of the chassis- Omni drive can only spin along its center. Another key note is the the Mecanum Wheels must be aligned in a way that the rollers form an 'X' pattern with the wheels.
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How Mecanum Drive Works
By varying the rotational speed and direction of each wheel, the summation of the force vectors from each of the wheels will create both linear motions and/or rotations of the vehicle, allowing it to maneuver around with minimal need for space. For example:
Running all four wheels in the same direction at the same speed will result in a forward/backward movement, as the longitudinal force vectors add up but the transverse vectors cancel each other out.
Running (all at the same speed) both wheels on one side in one direction while the other side in the opposite direction, will result in a stationary rotation of the vehicle, as the transverse vectors cancel out but the longitudinal vectors couple to generate a torque around the central vertical axis of the vehicle.
Running (all at the same speed) the diagonal wheels in one direction while the other diagonal in the opposite direction will result in a sideways movement, as the transverse vectors add up but the longitudinal vectors cancel out.
A mix of differential wheel motions will allow for vehicle motion in almost any direction with any rotation. This mix includes varying center points of rotation and degrees diagonally.
Moving Forwards
Moving Backwards
Moving Right
Moving Left
Moving 45° Diagonally
Rotating around the Center Point
Turning around the Center Point of the Back Left Wheel
Rotating around the Center Point of one Axle
See our sample code for Mecanum Drive here.
Slide Drive Chassis
A slide Drive Chassis is unique because it combines the advantages of a tank drive chassis with on with a holonomic drive- that said, it does neither better than either of the individual chassis, but it does provide more flexibility in movement than a tank drive with less motors of a holonomic drive. A slide drive has two
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How Slide Drive Works
Just like Tank Drive, you are controlling each side of the chassis with its corresponding motor.
Forward: both side motors spin forward at an equal speed, the robot will move forward.
Backwards: both side motors spin backwards at an equal speed, the robot will move backward.
Left Turn: the right motor spins forward, the left motor spins backwards, and the center motor will spin to the left.
Right Turn: the left motor moves forward, the right motor moves backwards, and the center motor will spin to the right.
Slide Left: the center motor will spin to the left and the side motors will not spin at all.
Slide Right: the center motor will spin to the right and the side motors will not spin at all.
Move Diagonally: The side motors will spin in the direction that to want to move forwards or backwards while the center motor will spin the direction that you want to move left or right. the combined forces will push you in a slanted motion combining the directions of all the spinning motors.
See our sample code for Slide Drive here.