Simplify the following text for a Grade 8 native English-speaking person.
Original: When one or more output variables of a system need to follow a certain reference over time , a controller manipulates the inputs to a system to obtain the desired effect on the output of the system .
Simplified: In a control system a controller manipulates the inputs to a system . In the control systems one or more output variables of a system need to follow a certain reference over time .