ion desired for the load, scale that value appropriately and add it to the output of the PID velocity loop controller. This means that whenever the load is being accelerated or decelerated, a proportional amount of force manded from the prime mover regardless of the feedback value. The PID loop in this situation uses the feedback information to effect any increase or decrease of bined output in order to reduce the remaining difference between the process setpoint and the feedback value. Working together, bined open-loop feed-forward controller and closed-loop PID controller can provide a more responsive, stable and reliable control system. Another problem faced with PID controllers is that they are linear. Thus, performance of PID controllers in non-linear systems (such as HVAC systems) is
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