INTRODUCTION

In process control system, there are 3 main elements which consist of manipulated variables,

controlled variables and disturbance. Controlled variables are the variables which quantify the

performance or quality of the final product, which are also called output variables. It is

includes the temperature, level, pressure and flow. For each controlled variable, there is an

associated manipulated variable. The manipulated variables must be adjusted by the control

system so the desired value or “set point” of the controlled variable is maintained from any

disturbances.

Disturbances enter or affect the process and tend to drive the controlled variables away

from their desired value or set point condition. Typical disturbances include changes in

ambient temperature, in demand for product, or in the supply of feed material. The

manipulated quantity must be changed to adjust the controlled variable to its new desired

value, if the set point is changed.

A process control consists of four main elements which are process, measurement,

evaluation and control. A block diagram of these elements is shown in Figure 1. The diagram

also shows the disturbances that enter or affect the process. If there were no upsets to a

process, there would be no need for the control system. Figure 1 also shows the input and

output of the process and the set point used for control.

Figure 1: Four element of a control system

Set point

InputOutput

Disturbances

THEORY

The basic function of controller is to execute an algorithm (electronic controller) based

on the control engineer's input (tuning constants), the operators desired operating value (set

point) and the current plant process value. The PID control algorithm is used for the control of

almost all loops in the process industries, and is also the basis for many advanced control

algorithms and strategies. In order for control loops to work properly, the PID loop must be

properly tuned.

The PID controllers job is to maintain the output at a level so that there is no

difference (error) between the process variable (PV) and the set point (SP). What the PID

controller is looking at is the difference (or "error") between the PV and the SP, which are the

absolute error and the rate of change of error. When there is a "process upset", meaning, when

the process variable or the set point quickly changes- the PID controller has to quickly change

the output to get the process variable back equal to the set point.

Once the PID controller has the process variable equal to the set point, a good PID

controller will not vary the output, because the output have to be very steady not changing. If

the valves (motor or other control element) are constantly changing, instead of maintaining a

constant value, this could case more wear on the control element. Thus, there are these two

contradictory goals; fast response (fast change in output) when there is a "process upset", but

slow response (steady output) when the PV is close to the set point. The output often goes

past (over shoots) the steady-state output to get the process back to the set point.

Proportional term

The proportional term produces an output value that is proportional to the current error

value. The proportional response adjusted by multiplying the error by constant Kp, called the

proportional gain constant.

The proportional term is given by:

Integral Term

The integral term is proportional to both the magnitude of the error and the duration of

the error. The integral in a PID controller is the sum of the instantaneous error over time and

gives the accumulated offset that should have been corrected previously. The accumulated

error is then multiplied by the integral

( ) and added to the controller output.

The integral term is given by:

Derivative Term

The derivative of the process error is calculated by determining the slope of

the error over time and multiplying this rate of change by the derivative gain Kd.

The magnitude of the contribution of the derivative term to the overall control

action is termed the derivative gain,Kd.

The derivative term is given by:

OBJECTIVES

The objectives of this experiment are as follows:

Understand how level process control system works.

Study the method of setting PID parameter to achieve the optimum stability.

Study the method of setting PID controller to achieve the optimum recovery.

PROCEDURE

1. The gas oressure process control menu screen was selected and PIC 811 has been

clicked and the history chart was opened.

2. The set point value was set to 50 bar and click “AUTO”. Wait for a few seconds

until the manipulated variable and process variable is stable.

3. “MAN” was clicked and the manipulated variable was increased around 10%.

Wait for a few seconds until the process varible is stable.

4. By using the P controller, the gain values is set according the data from the table

4.0 and “AUTO” was clicked.

5. The respond was observed and have been written down.

6. Every time to change the gain value, make sure click “MAN” and increases the

manipulated variable around 10%. Wait for a few seconds until the process

variable is stable before change the gain value.

7. Proceed to PI and PID controller from step 3 by changing the gain, reset and rate

value such as in table 4.0, 4.1 and 4.2. the responds were observed and recorded.

Table 4.0: P controller

Gain Reset Rate Respond

0.5 0 0 Undershoot, then stable

1.07 0 0 Oscillate

3.15 0 0 Oscillate

Table 4.1: PI controller

Gain Reset Rate Respond

0.72 2.28 0 No oscillation

0.80 3.0 0 No oscillation

2.5 0.79 0 Oscillate vigorously

Table 4.2: PID controller

Gain Reset Rate Respond

0.8 3.0 0.67 No oscillation, stable

1.27 4.0 6.6 Oscillate vigorously, unstable

5.45 3.62 2.15 Oscillate vigorously, unstable

RESULTS AND DISCUSSION

P CONTROLLER

GAIN=0.5

GAIN=1.07

GAIN=3.15

PI CONTROLLER

GAIN=0.72, RESET=2.28

GAIN=0.80, RESET=3.0

GAIN=2.5, RESET=0.79

PID CONTROLLER

GAIN=0.8, RESET=3.0, RATE=0.67

GAIN=1.27, RESET=4.0, RATE=6.6

GAIN=5.45, RESET=3.62, RATE=2.15

DISCUSSION

In P controller mode, it can be seen that the loop has fast recovery. Based on the graph shown

above, the line increase as gain value increases. There are a few changes occur at lower gain

(0.5) and for highest gain (3.15), the line oscillates. The conclusion that we can made if the

gain value increase, the oscillatory also will increase. Although higher gain value can

eliminate offset, too high gain value can give an unstable system.

For PI controller, there is no oscillation occurs at lower gain (0.72 and 0.8) and reset

(2.28 and 3.0) and the system is stabilized. For high gain (2.5) and low reset (0.79), the graph

showed that too much oscillatory, have disturbance and not stable.

Lastly, for the PID controller mode, it shows that the oscillatory become higher

and unstable due to increasing of gain and rate. For gain (0.8), reset (3.0), rate (0.67), the

system is stable and no oscillation occur. Meanwhile, for gain (1.27, 5.45), reset (4.0,

3.62), and rate (6.6, 2.15), the oscillion, disturbance and unstable result is clearly seen

and it shows that it is not suitable for pressure process control.

CONCLUSION

As a conclusion, PID controller is the most common and widely used in industrial control

systems. The basic function of a controller is to execute an algorithm (electronic controller)

based on the control engineer's input (tuning constants), the operators desired operating value

(set point) and the current plant process value.  In most cases, the requirement is for the

controller to act so that the process value is as close to the set point as possible.  In a basic

process control loop, the control engineer utilises the PID algorithms to achieve this.

The PID control algorithm is used for the control of almost all loops in the

process industries, and is also the basis for many advanced control algorithms and

strategies. In order for control loops to work properly, the PID loop must be properly

tuned. Standard methods for tuning loops and criteria for judging the loop tuning have

been used for many years, but should be re-evaluated for use on modern digital control

systems.

Tuning of PID Controller Terms

The P, I and D terms need to be "tuned" to suit the dynamics of the process being

controlled. Any of the terms described above can cause the process to be unstable, or

very slow to control, if not correctly set. These days some controller using digital PID

controllers have automatic auto-tune functions. During the auto-tune period the PID

controller controls the power to the process and measures the rate of change, overshoot

and response time of the plant. This is often based on the Zeigler-Nichols method of

calculating controller term values. Once the auto-tune period is completed the P, I &D

values are stored and used by the PID controller.

REFERENCES

1. Lab Manual Process Control UiTM

2. http://www.facstaff.bucknell.edu/mastascu/econtrolhtml/intro/intro2.html

3. http://www.isa.org/Template.cfm?Section=Find_Books1&template=Ecommerce/

FileDisplay.cfm&ProductID=8879&file=ACFF375.pdf

4. http://en.wikipedia.org/wiki/PID_controller