Explain the PID in control system.
PID In Control System
PID Controller –It stands for proportional–integral–derivative controller (PID controller). They are used in many industries as automatic process control. They can be used to regulate temperature, pressure, changing flow and many other processes in industries. The 3 coefficients of PID are different in different PIDs according to the need to get optimal response .
Fig 1: PID controller 
Example: Temperature Control using a PID controller
How PID works?
In manual control the controller will read the process variable time to time to change or control different variables. In case of automatic control adjustments and changes are made continuously to make continuous changes and maintain control over the variable . Industries use PID. This algorithm is basically for manipulating the error.
ERROR = PV – SP
PV - process variable,
SP - Set point .
Fig 2: Working of PID 
Different PID Equations
Each action (P, I, and D) occurs in separate equations, with the combined effect being a sum:
In the parallel equation, each action parameter (Kp, τi, τd) is independent. It is a drawback because adjustment made to the controller should only affect one aspect of the action and sometimes it is necessary that gain parameter should affect all the control actions (P, I, and D) .
To provide this very functionality another PID equation exist. This PID equation is called the Ideal or ISA equation:
In this equation, the gain constant (Kp) is distributed to all the terms inside the parentheses, affecting equally all control actions. Increasing gain constant of PID controller makes the individual actions P, I and D more aggressive .
Another third equation with origins in the unusual pressure and analog electronic circuit is there, and it is called the Series or Interacting equation:
Here, In this equation the gain constant (Kp) affects all three actions just as with the “ideal” equation. Here the integral and derivative cost effect proportional action means adjusting either τi or τd does not adjust those actions, but influences the aggressiveness of proportional action .
Limitations and Countermeasures of PID Controllers
- PID controllers show large time delay process.
- PID controllers show poor control performance for integrating process.
- PID controller cannot incorporate ramp type set point change/slow disturbance .
To resolve/remove above limitations several techniques have been proposed:
1. To avoid these poor control performances and incapability of controllers.
2. Control structure with 2 PID controllers is introduced to manipulate set point change process and the input disturbance rejection process.
3. To resolve ramp type set point and disturbance a feed forward and feedback loop are proposed. An additional feedback loop is also introduced to incorporate integrating process.
4. Tuning strategy by using model reduction is proposed to tune up the controller for high order processes.
5. To manipulate large time delay process a dead time compensator is used .
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“Limitations and Countermeasures of PID Controllers,” C&EN: WHAT'S THAT STUFF? JELL-O. [Online]. Available: https://pubs.acs.org/doi/abs/10.1021/ie960090 . [Accessed: 23-May-2018].