DriveBase vs Move Steering

The DriveBase provided by Pybricks can be useful in some cases. Some benefits of the DriveBase includes...

• Move by distance
• Turn by degrees
• Gradual ramp up/down of speed when moving and turning

All of these can be replicated using just the normal Motor class, but the third one can take a bit more work to do.

When using the drive method in DriveBase, you specify the speed and rate of turn (deg/s). This causes a few problems...

• When turning, the outer wheel will speed up. This may bring it above the max speed of the motor, causing inconsistent behaviour.
• The turn radius is dependent on the speed. At higher speed, the turn radius will be larger, and the robot may not be able to make a sharp turn.
• To make the sharpest turn, the robot speed must be set to zero.
• Response may be slower due to internal speed control loop.

These can be compensated by slowing down the robot when making a turn, but to fully compensate for all the problems will require some rather complicated code.

Move Steering

The Move Steering function is available in EV3Dev_Python, EV3Lab, EV3Classroom, and Spike Prime (v3 only). Unlike the drive method in DriveBase, the Move Steering functions takes a speed (%) and a "steering" value (-100 to 100) as input.

The "steering" value control the ratio of speed between the left and right wheel, as illustrated below

The equation for the Move Steering function is...

def move_steering(speed, steer):
    if steer > 0:
        left_motor.dc(speed)
        right_motor.dc(speed - steer * speed / 50)
    else:
        left_motor.dc(speed + steer * speed / 50)
        right_motor.dc(speed)

Advantages of the Move Steering function includes...

• None of the wheels will speed up when turning. This avoids issues with exceeding the max motor speed and make for a smoother turn.
• Turn radius is not dependent on speed. As long as the steering value is the same, the robot will always make the same turn regardless of speed.
• By changing the steering value only, you can fully control the turning radius from the maximum to the minimum value.

Note that the above function changes the duty cycle (ie. power) of the motor, and not the speed. One advantage is that this avoid Pybrick's internal speed control loop, allowing a faster response.