Introduction to Process Control and PID controllers

1. Module 2
Process Control and various
controllers

2. Contents
• Introduction to process control
• PID controller and tuning
• Various control configurations such as cascade
control, feedforward control, split range
control, ratio control, override control,
selective control

3. What is Feedback Control System?
• The process: It is the system to be controlled
• The process variable: It is the quantity to be measured and
controlled
• Sensor or transmitter: It is a device that measures process
variable
• The Controller: It decides the control variable in order to
bring the process variable as close as to the set point.
• Final Control Element: It is a device that directly
manipulates the manipulating variable to control over the
process.
• Manipulating Variable: It is the quantity which can be
directly altered to control over the process variable.

4. Concept of Feedback Control
(a) Process and (b) Corresponding feedback loop

5. Concept of Feedforward Control
Structure of (a) Feedforward and (b) Feedback control schemes

6. Feedback temperature control of tank heater

7. Feedforward temperature control of tank
heater

9. Introduction to PID Controllers
• PID controllers are most widely used automatic industrial controllers. In
process industries, most of the control loops (typically 90-95 percent) are of
PID type.
• These controllers receive inputs from sensors, meters, etc. and depending
on PID control function they deliver output control signals to the controlled
or manipulating devices such as relays, actuators, etc.
• These are the most common form of feedback systems and become a
standard tool for precise control applications.

11. Step response of second order system

16. PID Tuning Method
• The determination of corresponding PID parameter
values for getting the optimum performance from
the process is called tuning. This is obviously a crucial
part in case of all closed loop control systems.

17. General Tips for Designing a PID Controller
• Obtain an open-loop response and determine what needs
to be improved.
• Add a proportional control to improve the rise time.
• Add a derivative control to improve the overshoot.
• Add an integral control to eliminate the steady-state error.
• Adjust each of Kp, Ki, and Kd until you obtain a desired
overall response.

18. Trial and Error Method
• Set integral and derivative terms to zero first and then increase the
proportional gain until the output of the control loop oscillates at a
constant rate. This increase of proportional gain should be such that
response the system becomes faster but should not become
unstable.
• Once the P-response is fast enough, set the integral term, so that
the oscillations will be gradually reduced. Change this I-value until
the steady state error is reduced, but it may increase overshoot.

19. Trial and Error Method
• Once P and I parameters have been set to a desired
values with minimal steady state error, increase the
derivative gain until the system reacts quickly to its set
point. Increasing derivative term decreases the
overshoot of the controller response.

20. Zeigler-Nichols Method
First Method
• Process reaction curve method (open loop
method)

21. • The S-shaped reaction curve can be characterized by two constants,
delay time L and time constant T, which are determined by drawing a
tangent line at the inflection point of the curve and finding the
intersections of the tangent line with the time axis and the steady-state
level line.

22. Process reaction curve method
Advantages
• Quick and easier to use than other methods
• It is a robust and popular method
• Of these two techniques, the Process Reaction Method is the easiest and least
disruptive to implement
Disadvantages
• It depends upon purely proportional measurement to estimate I and D controllers.
• Approximations for the Kc, Ti, and Td values might not be entirely accurate for
different systems.
• It does not hold for I, D and PD controllers.

23. Zeigler-Nichols Method
Second Method
• Ultimate gain method (Closed loop method)

24. • Start with Closed-loop system with a proportional controller.
• Begin with a low value of gain, Kp. Increase until a steady-state
oscillation occurs, note this gain as Kcr.

25. Ultimate gain method
Advantages
• Used for systems with time delay.
• Quicker closed loop response time.
Disadvantages and Limitations
• Unstable closed loop systems.
• Can only be used for first order models including large process delays.
• Offline method.
• Approximations for the Kc, τi, and τd values might not be entirely accurate for
different systems.

26. • Ziegler-Nichols tuning method to determine an
initial/estimated set of working PID parameters for
an unknown system
• Usually included with industrial process controllers
and motor controllers as part of the set-up utilities
• Some controllers have additional autotune routines