Control Valve Actuators 

Ya actually in the last part, it concept states that when the feedback signal from the stem which is connected to a motor will be in the form of a measured variable right. And one that coming out from the servo amp will be manipulated variable. So basically what does a control valve does, it adjusts the manipulated variable in adjust the controlled variable to the set point (Since this is the ultimate aim in the process control system). So the valve plug will move up and down in relation to when there is difference in the values of set point and controlled variable (One to be controlled). And when the deviation almost diminishes (I.e result or reading coming closer to the set point), there is no need for valve to move because the set point is reached. I hope you get it. This was the principle explained at the last.