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PROCESS CONTROL (IT62)
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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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