PROPORTIONAL PLUS RATE CONTROL SYSTEMS
Figure 25 Derivative Output for a Constant Rate of Change Input
Figure 26 Rate Control Output
Derivative cannot be used alone as a control mode. This is because a steady-state input produces
a zero output in a differentiator. If the differentiator were used as a controller, the input signal
it would receive is the error signal. As just described, a steady-state error signal corresponds to
any number of necessary output signals for the positioning of the final control element.
Therefore, derivative action is combined with proportional action in a manner such that the
proportional section output serves as the derivative section input.