Pressure Control Lab Report

LABORATORY
REPORT
ON
PRESSURE
PROCESS
CONTROL

OBJECTIVE
To be able to understand, demonstrate and perform P, PI and PID (proportional–integral–derivative) control of a pressure process.

LEARNING OUTCOME OF THE LAB
It is practically learnt to perform various forms of control of a pressure process.

INTRODUCTION
The Pressure control system is a single loop pneumatic control system. It helps in studying the principles of both pressure regulation of a process and the control of flow in a pressurised system.
The system comprises low pressure air circuit supported on a bench-mounted panel, making it suitable for individual student work and for group demonstration.
PID controllers have three control modes:
•Proportional Control
•Integral

The controller output is given by Where,
• is the error or deviation of actual measured value (PV) from the setpoint (SP).

.
A PI controller can be modelled easily in various softwares involving Laplace operators:

Where,

= proportional gain = integral gain
Setting a value for is often a trade off between decreasing overshoot and increasing settling time.

• PID CONTROL

PID control is named after its three correcting terms, whose sum constitutes the manipulated variable (MV).
The proportional, integral, and derivative terms are summed to calculate the output of the PID controller.
Defining as the controller output, the final form of the PID algorithm can be written as:

Where,

: Proportional gain, a tuning parameter : Integral gain, a tuning parameter : Derivative gain, a tuning parameter : Error : Set Point : Process Variable : Time or instantaneous time (the present) : Variable of integration; takes on values from time 0 to the present .

The transfer function in the Laplace domain of the PID controller can be written as

Where, : complex number

By adjusting the Proportional Band in the controller constants and changing the set point, we observe that on decreasing the value of Proportional Band system moves from being stable to unstable.

(b) The effects of changing the integral time for the PI controller.
After selecting one PB and adjusting the integral time followed by changing set point and noting the process variables we observe that on decreasing the value of integral time, system moves from being stable to unstable. (c) The effects of changing the derivative time for the PID controller.
As we increase the derivative time and set point followed by noticing the process variables we observe that the system has moved from stable condition to unstable.

(d) Best controller
P Controller
Advantages:- simple, inexpensive, faster response
Disadvantages:-set point offset, delay if gain is lower hence gain adjustment is required.
PI Controller
Advantages:-no offset, have applications in many processes which are not integrating type.
Disadvantages:-more costly, little complex, slow response, oscillations and overshoot
PID