My current "guess" at what you did was ----- you set the 'set point' to zero, while somewhere else in your software code, you added your own input value to the 'error' signal (ie. summed your input value with the error signal)..... which would basically be the SAME thing as changing your setpoint. In other words, your system is still a basic PID system. You're still applying a 'set point', or the total equivalent of a set point.
