CONTROLLER PRINCIPLE 3

PID Controllers

•A particular control structure that has become almost universally used in industrial 
control.

•It is based on a particular fixed structure controller family, the so-called PID
controller family.

•These controllers have proven to be robust and extremely beneficial in the control of
many important applications.

•PID stands for:

–P (Proportional) 
–I (Integral) 
–D (Derivative)
Proportional Controllers (P)

•Each mode of control has characteristic advantages and limitations. 

•The modes of control are discussed in this and the next several sections of this 
module.

•In the proportional (throttling) mode, there is a continuous linear relation between 
value of the controlled variable and position of the final control element.

•In other words, amount of valve movement is proportional to amount of deviation.

•Three terms commonly used to describe the proportional mode of control are  
proportional band gain and offset. 

•Proportional band, (also called throttling range), is the change in value of the 
controlled variable that causes full travel of the final control element. 

•Gain, also called sensitivity, compares the ratio of amount of change in the final 
control element to amount of change in the controlled variable. 

•Mathematically, gain and sensitivity are reciprocal to proportional band.

•Offset, also called droop, is deviation that remains after a process has stabilized. 
Offset is an inherent characteristic of the proportional mode of control. In other 
words, the proportional mode of control will not necessarily return a controlled 
variable to its set point. 

•Proportional control is also referred to as throttling control.