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PROCESS CONTROL (IT62)

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Introduction

In this chapter, we study the nature of controller action for systems with operations and

variables that range over continuous values. The controller inputs the results of

measurements of the controller variable and determines an appropriate output to the final

control element. Essentially, the controller is some form of computer – either analog or

digital, pneumatic or electronic. Using measurements, the controllers solve the certain

equations to calculate the proper output. The equations necessary to obtain the control are

independent of both process and controller function (i.e. analog or digital). The equations

describe the modes or action of controller operation. The nature of process and controlled

variable determine which mode of control to be used and the certain constants in the

mode equation.

Objectives

At the end of this chapter you will be able to:

• Define & understand the process characteristics

• Define & understand the process system parameters

• Describe the discontinuous and continuous controller modes

• Compare and differentiate discontinuous and continuous controller modes

• List the advantages and disadvantages of discontinuous and continuous controller

modes

• Describe the composite controller modes

• Advantages and disadvantages of composite controller modes

Process Characteristics

The selection of what controller modes to use in a process is a function of the

characteristics of the process. The following prominent characteristics of process are

helpful in understanding the controller modes and also in selection of appropriate

controller mode for implementation. To define and understand the various process

characteristics we will take an example of a process control loop as shown in Fig 1.1.

Process Load

The process equation provides the set of values of process parameters which results in the

controlled variable to reach setpoint. The process load refers to set of all process

parameters excluding the controlled variable and set of parameters is called nominal set.

When all the process parameters have their nominal value then the load on the system is

called nominal load.

Process Lag

Whenever a process load change or transient occurs, it causes a change in the controlled

variable. The process control loop responds to this change to ensure that, after some finite

time the controlled variable reaches the setpoint. The part of this time consumed by

process itself is called process lag.

In the example of Fig 1.1, if process load change occurs, then it will affect the controlled

variable (TL). The control loop responds immediately by adjusting the steam flow rate.

But there will be some time delay in opening the control valve and heating process which

contribute to the process lag. In most of the process control systems the loop reacts faster

than the process, and there is no advantage in designing control systems many times

faster than the process lag.

Cycling

Cycling is defined as the oscillations of the error about zero value or nominal value. This

means that the variable will be cycling above and below the setpoint value.

Steady-state cycling is one in which oscillations will continue indefinitely. In such

conditions peak amplitude of error and period of oscillations are important in

understanding the nature of process variable. Transient cycling is one in which

oscillations will decay to zero after some time. In such conditions initial error and period

of cyclic oscillations are important in understanding the nature of process variable.

Integral Control Mode

The integral control eliminates the offset error problem by allowing the controller to

adapt to changing external conditions by changing the zero-error output.

Integral action is provided by summing the error over time, multiplying that sum by a

gain, and adding the result to the present controller output. If the error makes random

excursions above and below zero, the net sum will be zero, so the integral action will not

contribute. But if the error becomes positive or negative for an extended period of time,

the integral action will begin to accumulate and make changes to the controller output.

Composite Control Modes

It is found from the discontinuous and continuous controller modes, that each mode has

its own advantages and disadvantages. In complex industrial processes most of these

control modes do not fit the control requirements. It is both possible and expedient to

combine several basic modes, thereby gaining the advantages of each mode. In some

cases, an added advantage is that the modes tend to eliminate some limitations they

individually posses. The most commonly used composite controller modes are:

Proportional-Integral (PI), Proportional-Derivative (PD) and Proportional-Integral-

Derivative (PID) control modes.

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