robotics – Robotics

Robotics module for the Pybricks API.

class DriveBase(left_motor, right_motor, wheel_diameter, axle_track)

A robotic vehicle with two powered wheels and an optional support wheel or caster.

By specifying the dimensions of your robot, this class makes it easy to drive a given distance in millimeters or turn by a given number of degrees.

Positive distances and drive speeds mean driving forward. Negative means backward.

Positive angles and turn rates mean turning right. Negative means left. So when viewed from the top, positive means clockwise and negative means counterclockwise.

Parameters:
  • left_motor (Motor) – The motor that drives the left wheel.
  • right_motor (Motor) – The motor that drives the right wheel.
  • wheel_diameter (dimension: mm) – Diameter of the wheels.
  • axle_track (dimension: mm) – Distance between the points where both wheels touch the ground.

Driving for a given distance or by an angle

Use the following commands to drive a given distance, or turn by a given angle.

This is measured using the internal rotation sensors. Because wheels may slip while moving, the traveled distance and angle are only estimates.

straight(distance)

Drives straight for a given distance and then stops.

Parameters:distance (distance: mm) – Distance to travel.
turn(angle)

Turns in place by a given angle and then stops.

Parameters:angle (angle: deg) – Angle of the turn.
settings(straight_speed, straight_acceleration, turn_rate, turn_acceleration)

Configures the speed and acceleration used by straight() and turn().

If you give no arguments, this returns the current values as a tuple.

You can only change the settings while the robot is stopped. This is either before you begin driving or after you call stop().

Parameters:

Drive forever

Use drive() to begin driving at a desired speed and steering.

It keeps going until you use stop() or change course by using drive() again. For example, you can drive until a sensor is triggered and then stop or turn around.

drive(speed, turn_rate)

Starts driving at the specified speed and turn rate. Both values are measured at the center point between the wheels of the robot.

Parameters:
stop()

Stops the robot by letting the motors spin freely.

Measuring

distance()

Gets the estimated driven distance.

Returns:Driven distance since last reset.
Return type:distance: mm
angle()

Gets the estimated rotation angle of the drive base.

Returns:Accumulated angle since last reset.
Return type:angle: deg
state()

Gets the state of the robot.

This returns the current distance(), the drive speed, the angle(), and the turn rate.

Returns:Distance, drive speed, angle, turn rate
Return type:(distance: mm, speed: mm/s, angle: deg, rotational speed: deg/s)
reset()

Resets the estimated driven distance and angle to 0.

Measuring and validating the robot dimensions

As a first estimate, you can measure the wheel_diameter and the axle_track with a ruler. Because it is hard to see where the wheels effectively touch the ground, you can estimate the axle_track as the distance between the midpoint of the wheels.

In practice, most wheels compress slightly under the weight of your robot. To verify, make your robot drive 1000 mm using my_robot.straight(1000) and measure how far it really traveled. Compensate as follows:

  • If your robot drives not far enough, decrease the wheel_diameter value slightly.
  • If your robot drives too far, increase the wheel_diameter value slightly.

Motor shafts and axles bend slightly under the load of the robot, causing the ground contact point of the wheels to be closer to the midpoint of your robot. To verify, make your robot turn 360 degrees using my_robot.turn(360) and check that it is back in the same place:

  • If your robot turns not far enough, increase the axle_track value slightly.
  • If your robot turns too far, decrease the axle_track value slightly.

When making these adjustments, always adjust the wheel_diameter first, as done above. Be sure to test both turning and driving straight after you are done.

Using the DriveBase motors individually

Suppose you make a DriveBase object using two Motor objects called left_motor and right_motor. You cannot use these motors individually while the DriveBase is active.

The DriveBase is active if it is driving, but also when it is actively holding the wheels in place after a straight() or turn() command. To deactivate the DriveBase, call stop().

Advanced Settings

The settings() method is used to adjust commonly used settings like the default speed and acceleration for straight maneuvers and turns. Use the following attributes to adjust more advanced control setttings.

You can only change the settings while the robot is stopped. This is either before you begin driving or after you call stop().

distance_control

The traveled distance and drive speed are controlled by a PID controller. You can use this attribute to change its settings. See The Control Class for an overview of available methods.

heading_control

The robot turn angle and turn rate are controlled by a PID controller. You can use this attribute to change its settings. See The Control Class for an overview of available methods.