# Building with EXP Motor Groups

When building a custom VEX EXP Robot, sometimes you just need more power. One easy way to do this is to add another motor. These two motors working together are known as a motor group.

## How motor groups are mechanically tied together

In order for two motors to work together they need to be mechanically connected in some manner.

Some methods of connecting motors together mechanically include:

Both motors share the same drive shaft.

Both motors share the same gear set.

Both motors share the same chain and sprocket system.

Both motors have wheels on the same side of the drivetrain.

## The importance of motor spin direction

When two motors are working together it is very important that the direction each motor is spinning do not fight with one another. The orientation of the motors to one another will determine which direction each will need to spin. A typical robot arm with two motors working together to lift the arm is an example of how this works.

In this case, the driven gear attached to the right side of the arm will need to rotate counterclockwise for the arm to lift. Since the driving gear needs to rotate in the opposite direction of the driven gear on the arm, the right motor of the arm will need to spin the smaller driving gear in a clockwise direction.

However, on the left side of the arm the driven gear will need to rotate in the opposite direction or clockwise. This also means the left motor will need to spin in the opposite counterclockwise direction.

As a general rule, if the two motors in a motor group are facing each other as in the application with the arm above, the spin of one motor in the motor group will need to be reversed so the motors do not fight against one another.

If the motors are facing in the same direction, then both motors in the motor group will need to spin in the same direction.

When using VEXcode EXP, it is very easy to reverse a motor within a motor group. This can be done when you are adding the motor group as a device.

## Applications in which motor groups will be helpful

The principles of mechanical advantage tell us whenever:

• More weight needs to be lifted.
• More distance needs to be traveled.
• More speed is needed.
• More force will be needed.

These principles can be seen with robot arm as well as drivetrains.

### Robot arms

A single swing arm may be able to lift light things with a single motor. However, if the arm needs to lift a heavy object, a second motor may be necessary.

When designing advanced arms such as a six-bar, two motors will be required. This is because these arms are capable of lifting objects higher and faster.

### Drivetrains

When designing a drivetrain you may want to go faster, climb steeper, or push more with your robot. A four motor drivetrain will allow you to accomplish this.

VEXcode EXP has a DRIVETRAIN 4-motor device which will allow you to program your drivetrain.

However, a 4-Motor Drivetrain device limits your robot turns to pivot turns. If your robot navigation requires different turns, motor groups can allow these.

## Using Motor Groups for different types of turns

A skid-steer robot is a robot which turns by adjusting the speed and direction of the drive wheels on each side of the robot. The types of turns are:

Pivot turns: this type of turn pivots on a center point between the drive wheels. This happens when the drive wheel/wheels on one side of the robot move in reverse to the drive wheel/wheels on the other side of the robot. This type of turn is helpful when the robot needs to turn in place.

Drag turns: this type of turn has the pivot point on the side of the robot. This happens when the drive wheel/wheels on one side of the robot move forward or reverse and the drive wheel/wheels on the other side of the robot do not move. This type of turn can be helpful when lining up with a game piece.

Arc turns: this type of turn has the pivot point located outside of the drivetrain of the robot. This happens when the drive wheel/wheels on one side of the robot spin at a faster or slower speed than the drive wheel/wheels on the other side of the robot. This type of turn allows for a shorter travel distance when navigating around obstacles.

For more information, help, and tips, check out the many resources at VEX Professional Development Plus

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