Motor Board API

The kit can control multiple motors simultaneously. Each Motor Board can control two motors. See the Motor Board hardware page for more details.

Accessing the Motor Board

If there is exactly one Motor Board connected to your robot, it can be accessed using the motor_board property of the Robot object.

from sr.robot3 import Robot
robot = Robot()

my_motor_board = robot.motor_board

If you have more than one Motor Board attached, you need to specify which one you want to control. This is done using the serial number of the board.

The serial number is physically written on each board and the serial of any connected board will be printed to your log when the robot starts. It will look something like this:

sr.robot3.robot - INFO - Found MotorBoard, serial: srABC1
sr.robot3.robot - INFO - Found MotorBoard, serial: srXYZ1

You can then access the boards like this:

from sr.robot3 import Robot
robot = Robot()

my_motor_board = robot.motor_boards["srABC1"]
my_other_motor_board = robot.motor_boards["srXYZ1"]

When you have more than one Motor Board connected to your kit, you can’t use robot.motor_board. This is because the kit needs to know which Motor Board you want to control.

Setting motor power

Control of your motors is achieved by setting a power output from one of the channels on your Motor Boards. Valid values are between -1 and 1 inclusive. Fractional values (such as 0.42) can be used to specify less than 100% power. Negative values run the motor in the opposite direction.

The field to change the output power is power. As each Motor Board has two outputs you will need to specify which output you want to control:

# motor board srABC1, channel 0 to full power forward
robot.motor_boards["srABC1"].motors[0].power = 1

# motor board srXYZ1, channel 0 to full power reverse
robot.motor_boards["srXYZ1"].motors[0].power = -1

# motor board srABC1, channel 1 to half power forward
robot.motor_boards["srABC1"].motors[1].power = 0.5

The Motor Board will continue to output the requested power until it is told otherwise or until power to the board is removed (usually when your code ends and the robot turns off).

Therefore to stop your motors you must explicitly set the power output to zero:

# Put motor board srABC1, channel 1 at 25% power for 2.5 seconds:
robot.motor_boards["srABC1"].motors[1].power = 0.25
robot.sleep(2.5)      # wait for 2.5 seconds
robot.motor_boards["srABC1"].motors[1].power = 0

Since each output channel can be controlled separately, you can control several motors at once.

# Set one motor to full power in one direction and
# another to full power in the other:
robot.motor_boards["srABC1"].motors[0].power = 1
robot.motor_boards["srABC1"].motors[1].power = -1

# Wait a while for the robot to move
robot.sleep(3)

# Stop both motors
robot.motor_boards["srABC1"].motors[0].power = 0
robot.motor_boards["srABC1"].motors[1].power = 0

You will need to work out for your robot which values (positive or negative) result in it moving in each direction. If you want to swap the direction of a motor you can swap the wires connecting the motor to the Motor Board.

Getting the current motor power

You can read the current power value for a motor using the same field:

# Print the output power of Motor Board srABC1, channel 0
print(robot.motor_boards["srABC1"].motors[0].power)

Special Values

In addition to the numeric values, there are two special constants that can be used:

BRAKE
COAST

BRAKE will stop the motors from turning, and thus stop your robot as quick as possible.

BRAKE does the same thing as setting the power to 0.

from sr.robot3 import BRAKE, COAST

# Stop the motor as quick as possible
robot.motor_boards["srABC1"].motors[0].power = BRAKE

COAST will stop applying power to the motors. This will mean they continue moving under the momentum they had before and slowly come to a stop.

# Slowly coast to a stop
robot.motor_boards["srABC1"].motors[0].power = COAST

Motor currents

In the simulator, the current values always return 0.

The Motor Board can also measure the current being drawn by each of the ports on the board. This value is measured in amps.