Depending on the controlled variable and how fast or slow it changes,  every control system must have a controller that provides a signal to  the final control element. Keep in mind that the controller is the  decision maker that sends a signal to a final control element based on  two things: the desired set-point and a current situation of a variable  to be controlled. 

 Discuss the four control modes (on-off, proportional,  proportional-integral, and proportional-integral-derivative) in terms of  advantages, disadvantages, and characteristics providing examples for  each scenario.