Clarify PID & FF slot usage in motor configuration

[SarperMakas]

Currently in Closed Loop Control the motor configuration sets PID values for both position and velocity as follows:

/*
 * Configure the closed loop controller. We want to make sure we set the
 * feedback sensor as the primary encoder.
 */
motorConfig.closedLoop
    .feedbackSensor(FeedbackSensor.kPrimaryEncoder)
    // Set PID values for position control. We don't need to pass a closed loop
    // slot, as it will default to slot 0.
    .p(0.1)
    .i(0)
    .d(0)
    .outputRange(-1, 1)
    // Set PID values for velocity control in slot 1
    .p(0.0001, ClosedLoopSlot.kSlot1)
    .i(0, ClosedLoopSlot.kSlot1)
    .d(0, ClosedLoopSlot.kSlot1)
    .velocityFF(1.0 / 5767, ClosedLoopSlot.kSlot1)
    .outputRange(-1, 1, ClosedLoopSlot.kSlot1);

It seems that slot 0 is intended only for position control and slot 1 only for velocity control. This could cause confusion for users who might think both slots can be used interchangeably for different control modes.

The same code snippet is also shown in the official documentation: REV Robotics Closed Loop Control Getting Started
.

[jfabellera]

No slot is tied to one control type. Any of the slots can be used interchangeably with all control types. In this example specifically, it is using slot 0 for position and slot 1 for velocity, but you could also use slot 0 for velocity gains and slot 1 for position gains. All that matters is the control type and slot passed to setReference, as seen in teleopPeriodic() of that example:

    if (SmartDashboard.getBoolean("Control Mode", false)) {
      /*
       * Get the target velocity from SmartDashboard and set it as the setpoint
       * for the closed loop controller.
       */
      double targetVelocity = SmartDashboard.getNumber("Target Velocity", 0);
      closedLoopController.setReference(targetVelocity, ControlType.kVelocity, ClosedLoopSlot.kSlot1);
    } else {
      /*
       * Get the target position from SmartDashboard and set it as the setpoint
       * for the closed loop controller.
       */
      double targetPosition = SmartDashboard.getNumber("Target Position", 0);
      closedLoopController.setReference(targetPosition, ControlType.kPosition, ClosedLoopSlot.kSlot0);
    }

If that is still confusing, we can expand the comment in the configuration portion to reiterate the above.

[SarperMakas]

Some members of our FRC team found this comment misleading and assumed that each slot is tied to a specific control mode. Clarifying the comment to explicitly state that slots are interchangeable across all control types would help prevent this confusion.
Thank you for your help in improving this.