Module 6. Process control and automation

Lesson 14

PROCESS CONTROL SYSTEM

14.1 Introduction

This lesson will discuss the need and importance of process control system and Programmable Logic Controller (PLC) in dairy industry. Various topics related to the concept of process control system such as types of process control systems; open loop; closed loop; closed loop with feed back; cascade types; and concept of PLCs; system scale; user interface; communication and programming are elaborated here. The topics discussed in this lesson and next lesson will be useful to the students for understanding the state of the art technology being used in dairy plants for automation.

Process control system facilitate mass production of continuous process such pasteurization of milk, milk powder, ghee production, oil refining, cleaning in place (CIP), etc. On detection of an out-of-control situation by process monitoring, the person responsible for the process makes a change to bring the process back into control. Process control enables automation with which a small staff of operating personnel can operate a complex process from a central control room. Therefore it is being used extensively in many industries including dairy plants. Process control is a synergy of statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range.

For example, heating up the temperature in pasteurization of milk a process that has the specific, desired outcome to reach and maintain constant over time at 70°C. Here, the temperature is the controlled variable. At the same time, it is the input variable since it is measured by a thermometer and used to decide whether to heat or not to heat. The desired temperature (70°C) is the set point. The state of the heater (e.g. the setting of the valve allowing hot water to flow through it) is called the manipulated variable since it is subject to control actions.

Two of the most important signals used in process control are called:

  1. Process variable or PV
  2. Manipulated variable or MV

At this time it is necessary to define some terms used in the field of automatic process control. The first term is controlled variable, which is the variable that must be maintained, or controlled, at some desired value. If the process outlet temperature has to be maintained at some constant value, then T(t), is the controlled variable. Sometimes the term process variable and/or measurement are also used to refer to the controlled variable. In industrial process control, the PV is measured by an instrument in the field, and acts as an input to an automatic controller which takes action based on the value of it. Alternatively, the PV can be an input to a data display so that the operator can use the reading to adjust the process through manual control and supervision.

The set point is the desired value of the controlled variable. Thus the job of a control system is to maintain the controlled variable at its set point. The ideal value of the PV is often called the target value, and in the case of an automatic control, the term set point (SP) value is preferred.

Following are the process control equipments.

1.      Transmitter: It senses the parameter being controlled and transmits the corresponding signal

2.      Controller: It is the device that serves to maintain the process variable value at the set point. It receives the signal of measured variable and compares with that of preset value.

3.      Actuator: It physically responds to the signal received from controller. Controller sends a signal to actuator if the difference between measured value and desired value is higher than the limit.

General working principle of process control equipment involving the functioning of process control loops is shown in the fig. 14.1 given below

Fig. 14.1 Diagram shows the working principle of process control loop

14.2 Types of Control Systems

Control system will be designed based on the process. Most of the process will have dynamic variables, pneumatic, electrical or electronic type signals are being used to measure the process variables. The controller compares the measured signal with that of preset value and initiates the corrective actions if it is deviating. The control element exerts a direct influence on the process by accepting the input signal from controller and transforms into some action to regulate the process.

There are four major process control loops, they are as follows.

1.      Feed-back control loop

2.      Feed-Forward control loop

3.      Sequence control loop

4.      Cascade Control

14.2.1 Feed-back control loop

This is very commonly used control loop. Here the sensor measures the actual value of controlled variable and compares with reference value (set point). The difference between desired and measured value is used as input to the feedback controller to regulate the controlled variable. This loop corrects the error between the controlled output and its corresponding reference value. It tries to maintain the controlled variable to set point. Schematic diagram of feedback control loop is shown in diagram (fig.14.2) given below.

Fig. 14.2 Feed-back control loop

14.2.2 Feed –Forward control loop

This type of control is different from the feedback control. This has more advance technology in it, unlike the feedback control, which aims at eliminating the errors, feed-forward control aims at minimizing the errors. Feed-forward control acts before the output is disturbed and the instrumentation will take corrective action before a deviation in the process variable. The objective of feed-forward control is to measure the disturbances and compensate for them before the controlled variable deviates from the set point. If applied correctly, the controlled variable deviation would be minimal.

A concrete example of feed-forward control is the heat exchanger. Suppose, “major” disturbances are the inlet temperature and the process flow. To implement feed-forward control these two disturbances must first be measured and then a decision made as to how to manipulate the steam valve to compensate for them. The feed-forward controller makes the decision about how to manipulate the steam valve to maintain the controlled variable at set point, depending on the inlet temperature and process flow.

14.2.3 Sequence control loop

This loop performs the step by step execution of timely ordered events. Each step may be simple actions such as opening of valve, starting a pump or stopping a motor drive.  Here combinations of feed-forward and feed-back control can be made to avail the best suited solution to control the process variable.

14.2.4  Cascade control

In this loop, two or more control loops are nested. Example, feedback control loops is a secondary control loop located inside a primary control loop (Feed-forward control loop). The primary loop controller is used to calculate the set point for the inner (secondary) control loop.

14.3 Programmable Logic Controller

A programmable logic controller or programmable controller is a microprocessor-based device used to control industrial processes or machines used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines. They provide advanced functions, such as analog monitoring, high speed motion control as well as share data over communication networks. Unlike general-purpose computers, PLC is designed to work for multiple inputs and output arrangements. PLCs are robust devices which are not affected by extreme temperature ranges, electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a hard real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result. In simple terms PLCs are digital computer to control the functioning of machines automatically.

14.3.1 Components of PLC

Main components of PLCs are described below and shown in fig. 14.3.

1.      Input Module: Main function is to converts incoming signals into signals which can be processed by PLC, and to pass these to the central control unit. This module is connected/embedded with hardware/software drivers for the industrial process sensors such as switch status sensors, proximity detectors, interlock settings and so on.

2.      Output Module:  Main function is to convert the PLC signal into signals suitable for the actuators. This module is connected with hardware/software drivers for the industrial process actuators, such as solenoid switches, motors and valves which perform some action.

3.      Central Control Unit: Actual processing of input signals is performed in this unit in accordance with the logic of program stored in memory.

4.      Data Memory: Status of switches, interlocks, past values of data and other working data is stored here

5.      Program Memory: Instructions for the logical control sequence are stored here in program memory.

Fig. 14.3 Schematic diagram of PLC

14.3.2 Advantages of PLC

•         Cost effective for controlling complex systems.

•         Flexible and can be reapplied to control other systems quickly and easily.

•         Computational abilities allow more sophisticated control.

•         Trouble shooting aids make programming easier and reduce downtime.

•         Reliable components make these likely to operate for years before failure.

14.3.3 System scale

Normally a small PLC will have a fixed number of connections built in for inputs and outputs but expansions are available if the base model has insufficient I/O. Modular PLCs have a chassis (also called a rack) in which different modules are placed with different functions. The processor and selection of I/O modules may be customized for the particular application.

14.3.4 User interface

As discussed in previous chapters, user interface are required for human and machine interface. PLCs are interactive systems therefore needs to interact with users for the purpose of configuration, alarm reporting or everyday control. A human-machine interface is used for this purpose. These are also referred as graphical user interface. A simple system may use radio button, text box, check box, lights, command buttons etc. to interact with the user. Advanced GUI with text displays as well as graphical touch screens is also available.

14.3.5 Communication

PLCs need to communicate with input device to receive data; GUI to display inputs/ processed data and output devices to act finally based on results. PLCs have built in communications ports for this purpose, usually 9-pin RS-232 is used for communication, but optionally EIA-485 or Ethernet ports are also available. Commonly used communication protocols are Modbus, BACnet or DF1. Other options include various fieldbuses such as DeviceNet or Profibus. Modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control and Data Acquisition) system or web browser.

14.3.6 Programming

PLCs are programmed to perform certain predefined tasks as per input signals received from input devices. PLC programs are typically written in a special application on a personal computer and then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. At basic level, PLCs are programmed in a simple form of assembly code.   PLCs can also be programmed using standards-based programming languages, including the IEC 61131-3 standard, Function Block Diagrams etc.  A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. However, a well established standard programming language for programmable logic, called ‘Ladder Logic’, sis universally understood by PLC programmers. Fundamental concepts of PLC programming are common to all manufacturers. However some differences exist in I/O addressing, memory organization and instruction sets. It means that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.