Motors

Motor classes

The XRP has two drive motors connected to the ports Motor L and Motor R on the robot controller board. The board also supports two additional motors labeled Motor 3 and Motor 4. These motors can be used to create additional mechanisms for the XRP.

There are four classes related to motors:

Motor

The motor class handles a single motor with a single method for setting the effort between -1 and 1.

Encoder

The encoder class is responsible for measuring the current position of a motor. This is useful to derive the speed of the motor, or the distance traveled by the motor.

EncodedMotor

Encoded motors contain a motor and an encoder, and has higher level logic for functionality that incoporate both objects. This class supports features like resetting and getting the motor position, setting the effort and speed of the motor, and configuring what controller is used for closed-loop speed control.

MotorGroup

It is often desirable to treat several motors as if they were one. For example, in a four wheel drive robot, the left side motors usually get the same settings when driving the robot. A motor_group is created with multiple motors, and functionality like setting effort can be applied to all the motors in the motor group.

Since the XRP bot is built with an encoder on each motor, it usually is not necessary to directly deal with Motor or Encoder objects. Instead, use EncodedMotor or MotorGroup for higher level functionality.

Using EncodedMotor

Interacting with EncodedMotor objects is often the most convenient way to control motors on the XRP. The XRPLib.defaults module provides two ready-made EncodedMotor objects, left_motor and right_motor, which allow for fully independent control over the drive motors of the robot.

There are four motor controllers total on the XRP, numbered 1-4. 1 and 2 are the left and right motors, and 3 and 4 are labeled on the robot controller board. left_motor and right_motor objects are provided by default in the XRPLib.defaults module. Let’s take a look at how we can use the left_motor object to set the left motor to an effort of 0.75.

Alternatively, you may also construct your own EncodedMotor objects, which is needed to control motors 3 and 4. The following code sets Motor 3 to an effort of 0.75.

In Blockly, constructing motor objects is not necessary. Under the «Individual Motors» category, each block takes in a parameter to specify which motor to use. This is all that is needed to set Motor 3 to an effort of 0.75 in Blockly:

Methods for EncodedMotor objects

set_effort(effort_value)

As seen earlier, this method spins the motor at some raw effort. The effort value ranges from -1 to +1, where negative efforts spin the motor in reverse, and positive efforts spin the motor forwards. An effort of 0 stops the motor.

The programs shown below set Motor 3 to 80 percent effort for 5 seconds, then afterwards, back to 0 percent effort to stop the motor. This example uses motor 3, but any motor can be used in its place.

set_speed(speed_rpm)

Unlike the Drivetrain object which uses cm/second, the motor objects handle speed in rotations per minute. They reads from the encoder to determine the current speed, and adjust based on a closed-loop controller, which by default is a PID controller. Similarly to set_effort(), the sign of the speed determines the direction of the motor.

The example programs below set a speed of 60 rpm for the left motor:

set_speed_controller(controller)

The set_speed() function relies on a controller to determine how to vary the effort of the motor to maintain the specified speed. By default, the controller is a PID controller, but it can be changed to any object that implements the Controller abstract class.

The example below sets the speed controller with custom PID tunings. For more information on controllers, refer to the page under Miscellaneous Topics. Currently, there is no support for custom controllers in Blockly.

get_speed() -> float

Returns the current speed of the motor in rotations per minute, as measured by the encoder.

get_position() -> float

Returns the current position of the motor in rotations, as measured by the encoder.

get_count() -> integer

Returns the current position of the motor in encoder counts, as measured by the encoder.

reset_encoder_position()

Resets the encoder counts to 0. get_position() and get_count return the difference in distance since the last reset.