As has been pointed out the problem with PID controlling velocity directly is that as the motor speed reaches the setpoint, the motor speed goes to zero.
New for 2013 is the addition of a feedforward term (4th parameter on the constructors) that is a baseline value that is added to the output value. This term is multiplied by the setpoint so that it scales with differing speeds. The feedforward term will prevent the motor speed oscillation as the motor speed approaches the setpoint.
Here's the code that actually computes the final value in the PID controller class:
Code:
m_result = m_P * m_error
+ m_I * m_totalError
+ m_D * (m_error - m_prevError)
+ m_setpoint * m_F;
where m_P, m_I, m_D, and m_F are the P, I, D, and F constants. The m_error is the difference between the current value and the setpoint, and the m_prevError is the value of the error from the previous iteration.
Brad