41632553 Automation

Module 2Measurement SystemsVersion 2 EE IIT, Kharagpur 1

Lesson 6Displacement and Speed MeasurementVersion 2 EE IIT, Kharagpur 2

Instructional ObjectivesAt the end of this lesson, the student should be able to Name three methods of displacement measurement using passive electrical sensors. Sketch the construction and characteristics of LVDT. Explain the principles of operation of inductive and capacitive types of proximity sensors. Distinguish between variable distance and variable area type of capacitance displacement sensors. Sketch and explain the principle of operation of a optical type displacement sensor. Name two methods of noncontact type speed sensing and explain their principles of operation.

IntroductionDisplacement and speed are two important parameters whose measurements are important in many position and speed control schemes. Error free measurements of these two parameters are necessary in order to achieve good control performance. Displacement measurement can be of different types. The displacement may be in the range of few m to few cm. Moreover the measurement may be of contact type or noncontact type. Again displacement to be measured can be linear or angular (rotary). Similar is the case for speed measurement. Accordingly different measuring schemes are used for measurement of these two parameters. In this lesson, we shall discuss about few such schemes.

Displacement MeasurementBroadly speaking, displacement measurement can be of two types: contact and noncontact types. Besides the measurement principles can be classified into two categories: electrical sensing and optical sensing. In electrical sensing, passive electrical sensors are used variation of either inductance or capacitance with displacement is measured. On the other hand the optical method mainly works on the principle of intensity variation of light with distance. Interferometric technique is also used for measurement of very small displacement in order of nanometers. But this technique is more suitable for laboratory purpose, not very useful for industrial applications.

PotentiometerPotentiometers are simplest type of displacement sensors. They can be used for linear as well as angular displacement measurement, as shown in Fig. 1. They are the resistive type of transducers and the output voltage is proportional to the displacement and is given by: x eo = i E , xt where xi is the input displacement, xt is the total displacement and E is the supply voltage.

Version 2 EE IIT, Kharagpur 3

The major problem with potentiometers is the contact problem resulting out of wear and tear between the moving and the fixed parts. As a result, though simple, application of potentiometers is limited.

i E xt xi eo eo (a) Fig. 1 Potentiometer (a) Linear (b) Rotary E

(b)

Linear Variable Differential transformer (LVDT)LVDT works on the principle of variation of mutual inductance. It is one of the most popular types of displacement sensor. It has good linearity over a wide range of displacement. Moreover the mass of the moving body is small, and the moving body does not make any contact with the static part, thus minimizing the frictional resistance. Commercial LVDTs are available with full scale displacement range of 0.25mm to 25mm. Due to the low inertia of the core, the LVDT has a good dynamic characteristics and can be used for time varying displacement measurement range. The construction and principle of operation of LVDT can be explained with Fig. 2(a) and Fig. 2(b). It works on the principle of variation of the mutual inductance between two coils with displacement. It consists of a primary winding and two identical secondary windings of a transformer, wound over a tubular former, and a ferromagnetic core of annealed nickel-iron alloy moves through the former. The two secondary windings are connected in series opposition, so that the net output voltage is the difference between the two. The primary winding is excited by 1-10V r.m.s. A.C. voltage source, the frequency of excitation may be anywhere in the range of 50 Hz to 50 KHz. The output voltage is zero when the core is at central position (voltage induced in both the secondary windings are same, so the difference is zero), but increasing as the core moves away from the central position, in either direction. Thus, from the measurement of the output voltage only, one cannot predict, the direction of the core movement. A phase sensitive detector (PSD) is a useful circuit to make the measurement direction sensitive. It is connected at the output of the LVDT and compares the phase of the secondary output with the primary signal to judge the direction of movement. The output of the phase sensitive detector after low pass filtering becomes a d.c voltage for a steady deflection. The output voltage after PSD vs. displacement characteristics is shown in Fig. 2(c).


V0 (Measurement)

Primary Coil Insulating Form

Core Displacement X (Measurand) Ferromagnetic Core

Secondary Coil

Vref

Secondary Coil

Fig. 2(a) Construction of LVDT.

vo

Secondary Coil

Core

Primary Coil vref Fig. 2(b) Series opposition connection of secondary windings.


vo (d.c)Voltage Level

Displacement x

Linear Range

Fig. 2(c) Output voltage vs. displacement characteristics of LVDT after Phase sensitive detection.

Inductive type SensorsLVDT works on the principle of variation of mutual inductance. There are inductive sensors for measurement of displacement those are based on the principle of variation of self inductance. These sensors can be used for proximity detection also. Such a typical scheme is shown in Fig. 3. In this case the inductance of a coil changes as a ferromagnetic object moves close to the magnetic former, thus change the reluctance of the magnetic path. The measuring circuit is usually an a.c. bridge. AC Supply vref Inductance Measuring Circuit

Ferromagnetic Target Object

X (Measurand)

Fig. 3 Schematic diagram of a self inductance type proximity sensor. Version 2 EE IIT, Kharagpur 6

Rotary Variable Differential Transformer (RVDT)Its construction is similar to that of LVDT, except the core is designed in such a way that when it rotates the mutual inductance between the primary and each of the secondary coils changes linearly with the angular displacement. Schematic diagram of a typical RVDT is shown in Fig. 4. vo

Secondary Coil Core

Primary Coil

vref

Fig. 4 Rotary Variable Differential Transformer (RVDT)

ResolverResolvers also work on the principle of mutual inductance variation and are widely used for measurement of rotary motion. The basic construction is shown in Fig. 5. A resolver consists of a rotor containing a primary coil and two stator windings (with equal number of turns) placed perpendicular to each other. The rotor is directly attached to the object whose rotation is being measured. If a.c. excitation of r.m.s voltage Vr is applied, then the induced voltages at two stator coils are given by: v01 = KVr cos and

v02 = KVr sin ; where K is a constant.

By measuring these two voltages the angular position can be uniquely determined, provided (0 900 ) . Phase sensitive detection is needed if we want to measure for angles in all the four quadrants. Synchros work widely as error detectors in position control systems. The principle of operation of synchros is similar to that of resolvers. However it will not be discussed in the present lesson.


Output vo2 = Kvref sin

Rotor

AC Supply vref

Stator Output vo1 = Kvref cos

Stator

Fig. 5 Schematic diagram of the resolver.

Capacitance SensorsThe capacitance type sensor is a versatile one; it is available in different size and shape. It can also measure very small displacement in micrometer range. Often the whole sensor is fabricated in a silicon base and is integrated with the processing circuit to form a small chip. The basic principle of a capacitance sensor is well known. But to understand the various modes of operation, consider the capacitance formed by two parallel plates separated by a dielectric. The capacitance between the plates is given by: A (1) C= r 0 d where A=Area of the plates d= separation between the plates r = relative permittivity of the dielectric

0 = absolute permittivity in free space = 8.854 10 12 F / m .A capacitance sensor can be formed by either varying (i) the separation (d), or, (ii) the area (A), or (iii) the permittivity ( r ). A displacement type sensor is normally based on the first two (variable distance and variable area) principles, while the variable permittivity principle is used for measurement of humidity, level, etc. Fig.6 Shows the basic constructions of variable gap and variable area types of capacitance sensors mentioned above. Fig. 6(a) shows a variable distance type sensor, where the gap between the fixed and moving plates changes. On the other hand, the area of overlap between the fixed plate and moving plate changes in Fig. 6(b), maintaining the gap constant. The variable area type sensor gives rise to linear variations of capacitance with the input variable, while a variable separation type sensor follows inverse relationship. Version 2 EE IIT, Kharagpur 8

Fixed

(a)

(b)

Fig.6 Capacitive type displacement Sensors: a) variable separation type, b) variable area type. Capacitance sensors are also used for proximity detection. Such a typical scheme is shown in Fig. 4. Capacitive proximity detectors are small in size, noncontact type and can detect presence of metallic or insulating objects in the range of approximately 0-5cm. For detection of insulating objects, the dielectric constant of the insulating object should be much larger than unity. Fig. 7 shows the construction of a proximity detector. Its measuring head consists of two electrodes, one circular (B) and the other an annular shaped one (A); separated by a small dielectrical spacing. When the target comes in the closed vicinity of the sensor head, the capacitance between the plates A and B would change, which can be measured by comparing with a fixed reference capacitor. The measuring circuits for capacitance sensors are normally capacitive bridge type. But it should be noted that, the variation of capacitance in a capacitance type sensor is generally very small (few pF only, it can be even less than a pF in certain cases). These small changes in capacitor, in presence of large stray capacitance existing in different parts of

Target

x

A

B

Fig. 7 Capacitance Proximity Detector.


the circuit are difficult. So the output voltage would generally be noisy, unless the sensor is designed and shielded carefully, the measuring circuit should also be capable of reducing the effects of stray fields.

Optical SensorsOptical displacement sensors work on the basic principle that the intensity of light decreases with distance. So if the source and detector are fixed, the amount of light reflected from a moving surface will depend on the distance of the moving surface from the fixed ones. Measurement using this principle requires proper calibration since the amount of light received depends upon the reflectivity of the surface, intensity of the source etc. Yet it can provide a simple method for displacement measurement. Optical fibers are often used to transmit light to and from the measuring zone. Such a scheme with bundle fibers is shown in Fig. 8. It uses two bundle fibers, one for transmitting light from the source and the other to the detector. Light reflected on the receiving fiber bundle by the surface of the target object is carried to a photo detector. The light source could be Laser or LED; photodiodes or phototransistors are used for detection.Laser, LED, etc. Power Source Light Source Photodiode, Phototransistor, etc. Signal Processor Photodetector Receiving X Fibers (Measurand) Transmitting Fibers

Target Object

Position Measurement

Fig. 8 A Fiber optic position sensor.

Speed MeasurementThe simplest way for speed measurement of a rotating body is to mount a tachogenerator on the shaft and measure the voltage generated by it that is proportional to the speed. However this is a contact type measurement. There are other methods also for noncontact type measurements. The first method is an optical method shown in Fig. 9. An opaque disc with perforations or transparent windows at regular interval is mounted on the shaft whose speed is to be measured. A LED source is aligned on one side of the disc in such a way that its light can pass through the transparent windows of the disc. As the disc rotates the light will alternately passed through the transparent windows and blocked by the opaque sections. A photodetector fixed on the other side of the disc detects the variation of light and the output of the detector after signal conditioning would be a square wave (as shown) whose frequency is decided by the speed and the number of holes (transparent windows) on the disc.


voOutput

tLED Light Source

Transparent Windows Photocell, Phototransistor, or Photodiode (Light Sensor) Opaque Disk

Fig. 9 Schematic arrangement of optical speed sensing arrangement. Fig.10 shows another scheme for speed measurement. It is a variable reluctance type speed sensor. A wheel with projected teethes made of a ferromagnetic material is mounted on the shaft whose speed is to be measured. The static sensor consists of a permanent magnet and a search coil mounted on the same assembly and fixed at a closed distance from the wheel. The flux through the permanent magnet completes the path through the teeth of the wheel and cut the search coil. As the wheel rotates there would be change in flux cut and a voltage will be induced in the search coil. The variation of the flux can be expressed as: (t ) = o + m sin t (2) where is the angular speed of the wheel. Then the voltage induced in the coil is:

e = N

d = N m cos t dt

(3)

where N is the number of turns in the search coil. So both the amplitude and frequency of the induced voltage is dependent on the speed of rotation. This voltage is fed to a comparator circuit that gives a square wave type voltage whose amplitude is constant, but frequency is proportional to the speed. A frequency counter is used to count the number of square pulses during a fixed interval and displays the speed.d Yoke Permanent magnet

Search Coil Terminals Coil

Fig. 10 Variable reluctance type speed sensor. Version 2 EE IIT, Kharagpur 11

ConclusionFew techniques commonly used for displacement and speed measurements have been discussed in this lesson. The selection of the sensing scheme depends on the requirement, environment and accessibility. Displacement/position sensing can be done in two ways. One method is to convert the displacement signal into variation of inductance or capacitance and then use suitable measuring circuit to measure their variation. On the other hand, in optical method the intensity of the light reflected from a moving surface is measured and calibrated in terms of the distance. An important application of displacement measurement is proximity sensing. Few such schemes have been discussed in this lesson. However eddy current type proximity sensing has not been discussed in this lesson. The most popular type of speed sensor is the tachogenerator. The tachogenerator is mounted on the shaft and the voltage induced that is proportional to the speed is measured. But there are other noncontact methods also in which the speed signal is converted into frequency signal and the frequency is measured. Two such techniques have been discussed in this lesson.

References1. J.P. Bentley: Principles of Measurement Systems (3/e), Longman, U.K., 1995. 2. E.O. Doeblin: Measurement System Application and Design (4/e), Mcgraw-Hill, Singapore, 1990. 3. L.K.Baxter: Capacitance Sensors Design and Applications, IEEE Press, New Jersey, 1997. 4. D. Patranabis: Sensors and Transducers (2/e), PHI, New Delhi, 2003. 5. C.W. de Silva: Control Sensors and Actuators, Prentice Hall, New Jersey, 1989.

Review Questions1. What is the function of a Phase Sensitive Detector (PSD) in a LVDT circuit? 2. Discuss the construction and operating principle of a LVDT. 3. Distinguish between variable gap and variable area type capacitance displacement sensors. 4. What are the advantages and limitations of optical displacement (position) sensors? 5. Name two method of noncontact type speed measurement. Explain with a schematic diagram the principle of operation of any one of them. 6. An optical type speed sensor has a disc with 36 rectangular holes placed at regular intervals on the periphery of the disc. The frequency of the photodetector output is 360 Hz. Find the speed of the shaft in rpm on which the disc is mounted.

Answer6. 600 rpm.


Module 1IntroductionVersion 2 EE IIT, Kharagpur 1

Lesson 1Introduction to Industrial Automation and ControlVersion 2 EE IIT, Kharagpur 2

Lesson Objectives To define Automation and Control and explain the differences in the sense of the terms To explain the relation between Automation and Information Technology To underline the basic objectives of a manufacturing industry and explain how automation and control technologies relate to these To introduce the concept of a Product Life Cycle and explain how Automation and Control technologies relate to the various phases of the cycle To classify Manufacturing plants and categorise the different classes of Automation Systems that are appropriate for these

Understanding the Title of the CourseLet us first define the three key words in the title, namely,

IndustryIn a general sense the term Industry is defined as follows. Definition: Systematic Economic Activity that could be related to

Manufacture/Service/ Trade. In this course, we shall be concerned with Manufacturing Industries only.

AutomationThe word Automation is derived from greek words Auto(self) and Matos (moving). Automation therefore is the mechanism for systems that move by itself. However, apart from this original sense of the word, automated systems also achieve significantly superior performance than what is possible with manual systems, in terms of power, precision and speed of operation. Definition: Automation is a set of technologies that results in operation of machines and systems without significant human intervention and achieves performance superior to manual operation

A Definition from Encyclopaedia BritannicaThe application of machines to tasks once performed by human beings or, increasingly, to tasks that would otherwise be impossible. Although the term mechanization is often used to refer to the simple replacement of human labour by machines, automation generally implies the integration of machines into a selfgoverning system.


Point to Ponder: 1A. Why does an automated system achieve superior performance compared to a manual one? B. Can you give an example where this happens?

ControlIt is perhaps correct to expect that the learner for this course has already been exposed to a course on Control Systems, which is typically introduced in the final or pre-final year of an undergraduate course in Engineering in India. The word control is therefore expected to be familiar and defined as under. Definition: Control is a set of technologies that achieves desired patterns of variations of operational parameters and sequences for machines and systems by providing the input signals necessary.

Point to Ponder: 2A. Can you explain the above definition in the context of a common control system, such as temperature control in an oven? B. Is the definition applicable to open-loop as well as closed loop control? It is important at this stage to understand some of the differences in the senses that these two terms are generally interpreted in technical contexts and specifically in this course. These are given below. 1. Automation Systems may include Control Systems but the reverse is not true. Control Systems may be parts of Automation Systems. 2. The main function of control systems is to ensure that outputs follow the set points. However, Automation Systems may have much more functionality, such as computing set points for control systems, monitoring system performance, plant startup or shutdown, job and equipment scheduling etc. Automation Systems are essential for most modern industries. It is therefore important to understand why they are so, before we study these in detail in this course.

Point to Ponder: 3A. Can you give an example of an automated system, which contains a control system as a part of it? B. What are the other parts of the system?


Industrial Automation vs. Industrial Information TechnologyIndustrial Automation makes extensive use of Information Technology. Fig. 1.1 below shows some of the major IT areas that are used in the context of Industrial Automation.

Industrial ITControl and Signal Processing Simulation, Design, Analysis, Optimization Communication and Networking Real-time Computing

Database

Fig. 1.1 Major areas of IT which are used in the context of Industrial Automation.

Point to Ponder: 4A. Try to find an example automated system which uses at least one of the areas of Industrial IT mentioned in Fig. 1.1 (Hint: Try using the internet) However, Industrial Automation is distinct from IT in the following senses A. Industrial Automation also involves significant amount of hardware technologies, related to Instrumentation and Sensing, Actuation and Drives, Electronics for Signal Conditioning, Communication and Display, Embedded as well as Stand-alone Computing Systems etc. B. As Industrial Automation systems grow more sophisticated in terms of the knowledge and algorithms they use, as they encompass larger areas of operation comprising several units or the whole of a factory, or even several of them, and as they integrate manufacturing with other areas of business, such as, sales and customer care, finance and the entire supply chain of the business, the usage of IT increases dramatically. However, the lower level Automation Systems that only deal with individual or , at best, a group of machines, make less use of IT and more of hardware, electronics and embedded computing.

Point to Ponder: 5A. Can you give an example of an automated system, some of whose parts makes a significant application of Industrial IT?


B. Can you give an example of an automated system, none of whose parts makes a significant application of Industrial IT? Apart from the above, there are some other distinguishing features of IT for the factory that differentiate it with its more ubiquitous counterparts that are used in offices and other business. A. Industrial information systems are generally reactive in the sense that they receive stimuli from their universe of discourse and in turn produce responses that stimulate its environment. Naturally, a crucial component of an industrial information system is its interface to the world. B. Most of industrial information systems have to be real-time. By that we mean that the computation not only has to be correct, but also must be produced in time. An accurate result, which is not timely may be less preferable than a less accurate result produced in time. Therefore systems have to be designed with explicit considerations of meeting computing time deadlines. C. Many industrial information systems are considered mission-critical, in the sense that the malfunctioning can bring about catastrophic consequences in terms of loss of human life or property. Therefore extraordinary care must be exercised during their design to make them flawless. In spite of that, elaborate mechanisms are often deployed to ensure that any unforeseen circumstances can also be handled in a predictable manner. Fault-tolerance to emergencies due to hardware and software faults must often be built in.

Point to Ponder: 6A. Can you give an example of an automated system, which is reactive in the sense mentioned above? B. Can you give an example of an automated system, which is real-time in the sense mentioned above C. Can you give an example of an automated system, which is mission-critical in the sense mentioned above

Role of automation in industry9 Manufacturing processes, basically, produce finished product from raw/unfinished material using energy, manpower and equipment and infrastructure. 9 Since an industry is essentially a systematic economic activity, the fundamental objective of any industry is to make profit. 9 Roughly speaking, Profit = (Price/unit Cost/unit) x Production Volume (1) So profit can be maximised by producing good quality products, which may sell at higher price, in larger volumes with less production cost and time. Fig 1.2 shows the major parameters that affect the cost/unt of a mass-manufactured industrial product.


Cost/unit

Material

Energy

Manpower

Infrastructure

Fig. 1.2 The Components of per unit Manufacturing Cost Automation can achieve all these in the following ways, 9 Figure 1.4 shows how overall production time for a product is affected by various factors. Automation affects all of these factors. Firstly, automated machines have significantly lower production times. For example, in machine tools, manufacturing a variety of parts, significant setup times are needed for setting the operational configuration and parameters whenever a new part is loaded into the machine. This can lead to significant unproductive for expensive machines when a variety of products is manufactured. In Computer Numerically Controlled (CNC) Machining Centers set up time is reduced significantly with the help of Automated Tool Changers, Automatic Control of Machines from a Part Program loaded in the machine computer. Such a machine is shown in Figure 1.3. The consequent increase in actual metal cutting time results in reduced capital cost and an increased volume of production.

Point to Ponder: 7A. With reference to Eq. (1), explain how the following automation systems improve industrial profitability. a. Automated Welding Robots for Cars b. Automated PCB Assembly Machines c. Distributed Control Systems for Petroleum Refineries

Fig. 1.3 A CNC Machine with an Automated Tool Changer and the Operator Console with Display for Programming and Control of the Machine Version 2 EE IIT, Kharagpur 7

Production Volume

Production Time

Material Handling Time

Idle Time

Quality Assurance Time

Fig. 1.4 The major factors that contribute to Overall Production Time 9 Similarly, systems such as Automated Guided Vehicles, Industrial Robots, Automated Crane and Conveyor Systems reduce material handling time. 9 Automation also reduces cost of production significantly by efficient usage of energy, manpower and material. 9 The product quality that can be achieved with automated precision machines and processes cannot be achieved with manual operations. Moreover, since operation is automated, the same quality would be achieved for thousands of parts with little variation. 9 Industrial Products go through their life cycles, which consists of various stages. At first, a product is conceived based on Market feedbacks, as well as Research and Development Activities. Once conceived the product is designed. Prototype Manufacturing is generally needed to prove the design. Once the design is proved, Production Planning and Installation must be carried out to ensure that the necessary resources and strategies for mass manufacturing are in place. This is followed by the actual manufacture and quality control activities through which the product is mass-produced. This is followed by a number of commercial activities through which the product is actually sold in the market. Automation also reduces the over all product life cycle i.e., the time required to complete (i) Product conception and design (ii) Process planning and installation (iii) Various stages of the product life cycle are shown as in Figure 1.5.


Research and Development

Product Design

Market Feedbacks

Process Planning, Installation

Production, Quality Control

Fig. 1.5 A Typical Industrial Product Life Cycle

Economy of Scale and Economy of ScopeIn the context of Industrial Manufacturing Automation, Economy of Scale is defined as follows.

Economy of ScaleDefinition: Reduction in cost per unit resulting from increased production, realized through operational efficiencies. Economies of scale can be accomplished because as production increases, the cost of producing each additional unit falls. Obviously, Automation facilitates economy of scale, since, as explained above, it enables efficient large-scale production. In the modern industrial scenario however, another kind of economy, called the economy of scope assumes significance.

Economy of ScopeDefinition : The situation that arises when the cost of being able manufacture multiple products simultaneously proves more efficient than that of being able manufacture single product at a time. Economy of scope arises in several sectors of manufacturing, but perhaps the most predominantly in electronic product manufacturing where complete product life cycle, from conception to market, are executed in a matter of months, if not weeks. Therefore, to shrink the time to market drastically use of automated tools is mandated in all phases of the product life cycle. Additionally, since a wide variety of products need to be manufactured within the life period of a factory, rapid programmability and reconfigurability of machines and processes becomes a key requirement for commercial success. Such an automated production system also enables the industry to exploit a much larger market and also protects itself against fluctuations in demand for a given class of products. Indeed it is being driven by the economy of scope, and Version 2 EE IIT, Kharagpur 9

enabled by Industrial Automation Technology that Flexible Manufacturing (i.e. producing various products with the same machine) has been conceived to increase the scope of manufacturing. Next let us see the various major kinds of production systems, or factories, exist. This would be followed by a discussion on the various types of automation systems that are appropriate for each of these categories.

Point to Ponder: 8A. Can you give an example of an industry where economy of scope is more significant than the economy of scale? B. Can you give an example of an industry where economy of scale is more significant than the economy of scope? C. Can you give an example of an industry where both economy of scope, and economy of scale are significant?

Types of production systemsMajor industrial processes can be categorized as follows based on their scale and scope of production. Continuous flow process: Manufactured product is in continuous quantities i.e., the product is not a discrete object. Moreover, for such processes, the volume of production is generally very high, while the product variation is relatively low. Typical examples of such processes include Oil Refineries, Iron and Steel Plants, Cement and Chemical Plants. Mass Manufacturing of Discrete Products: Products are discrete objects and manufactured in large volumes. Product variation is very limited. Typical examples are Appliances, Automobiles etc. Batch Production: In a batch production process the product is either discrete or continuous. However, the variation in product types is larger than in continuous-flow processes. The same set of equipment is used to manufacture all the product types. However for each batch of a given product type a distinct set of operating parameters must be established. This set is often referred to as the recipe for the batch. Typical examples here would be Pharmaceuticals, Casting Foundries, Plastic moulding, Printing etc. Job shop Production: Typically designed for manufacturing small quantities of discrete products, which are custom built, generally according to drawings supplied by customers. Any variation in the product can be made. Examples include Machine Shops, Prototyping facilities etc. The above types of production systems are shown in Figure 1.6 categorized according to volumes of production and variability in product types. In general, if the quantity of product is more there is little variation in the product and more varieties of product is manufactured if the quantity of product is lesser. Version 2 EE IIT, Kharagpur 10

Oil Refinery Iron & Steel Chemical Appliances Bicycles Foundry Food Processing

Continuous Flow Process Mass Manufacturing Of Discrete Products

Quantity

Batch Production

Machine Tools Prototypes

Job shop Production

Variety

Fig. 1.6 Types of Production Systems

Types of Automation SystemsAutomation systems can be categorized based on the flexibility and level of integration in manufacturing process operations. Various automation systems can be classified as follows Fixed Automation: It is used in high volume production with dedicated equipment, which has a fixed set of operation and designed to be efficient for this set. Continuous flow and Discrete Mass Production systems use this automation. e.g. Distillation Process, Conveyors, Paint Shops, Transfer lines etc. A process using mechanized machinery to perform fixed and repetitive operations in order to produce a high volume of similar parts. Programmable Automation: It is used for a changeable sequence of operation and configuration of the machines using electronic controls. However, non-trivial programming effort may be needed to reprogram the machine or sequence of operations. Investment on programmable equipment is less, as production process is not changed frequently. It is typically used in Batch process where job variety is low and product volume is medium to high, and sometimes in mass production also. e.g. in Steel Rolling Mills, Paper Mills etc. Flexible Automation: It is used in Flexible Manufacturing Systems (FMS) which is invariably computer controlled. Human operators give high-level commands in the form of codes entered into computer identifying product and its location in the sequence and the lower level changes are done automatically. Each production machine receives settings/instructions from computer. These automatically loads/unloads required tools and carries out their processing instructions. After processing, products are automatically transferred to next machine. It is typically used in job shops and batch processes where Version 2 EE IIT, Kharagpur 11

product varieties are high and job volumes are medium to low. Such systems typically use Multi purpose CNC machines, Automated Guided Vehicles (AGV) etc. Integrated Automation: It denotes complete automation of a manufacturing plant, with all processes functioning under computer control and under coordination through digital information processing. It includes technologies such as computer-aided design and manufacturing, computer-aided process planning, computer numerical control machine tools, flexible machining systems, automated storage and retrieval systems, automated material handling systems such as robots and automated cranes and conveyors, computerized scheduling and production control. It may also integrate a business system through a common database. In other words, it symbolizes full integration of process and management operations using information and communication technologies. Typical examples of such technologies are seen in Advanced Process Automation Systems and Computer Integrated Manufacturing (CIM) As can be seen from above, from Fixed Automation to CIM the scope and complexity of automation systems are increasing. Degree of automation necessary for an individual manufacturing facility depends on manufacturing and assembly specifications, labor conditions and competitive pressure, labor cost and work requirements. One must remember that the investment on automation must be justified by the consequent increase in profitability. To exemplify, the appropriate contexts for Fixed and Flexible Automation are compared and contrasted. Fixed automation is appropriate in the following circumstances. A. Low variability in product type as also in size, shape, part count and material B. Predictable and stable demand for 2- to 5-year time period, so that manufacturing capacity requirement is also stable C. High production volume desired per unit time D. Significant cost pressures due to competitive market conditions. So automation systems should be tuned to perform optimally for the particular product. Flexible automation, on the other hand is used in the following situations. A. Significant variability in product type. Product mix requires a combination of different parts and products to be manufactured from the same production system B. Product life cycles are short. Frequent upgradation and design modifications alter production requirements C. Production volumes are moderate, and demand is not as predictable

Point to Ponder: 9A. During a technical visit to an industry how can you identify the type of automation prevailing there from among the above types? B. For what kind of a factory would you recommend computer integrated manufacturing and why? C. What kind of automation would you recommend for manufacturing Version 2 EE IIT, Kharagpur 12

a. b. c. d. e. f. g.

Light bulbs Garments Textile Cement Printing Pharmaceuticals Toys

Lesson SummaryIn this lesson we have dealt with the following topics: A. Definition of Automation and its relations with fields of Automatic Control and Information Technology: It is seen that both control and IT are used in automation systems to realize one or more of its functionalities. Also, while Control Technology is used for operation of the individual machines and equipment, IT is used for coordination, management and optimized operation of overall plants. B. The role played by Automation in realizing the basic goal of profitability of a manufacturing industry: It is seen that Automation can increase profitability in multiple ways by reducing labour, material and energy requirements, by improving quality as well as productivity. It is also seen that Automation is not only essential to achieve Economy of Scale, but also for Economy of Scope. C. Types of Factories and Automation Systems that are appropriate for them: Factories have been classified into four major categories based on the product volumes and product variety. Similarly Automation Systems are also categorized into four types and their appropriateness for the various categories of factories explained.

ExercisesA. Describe the role of Industrial Automation in ensuring overall profitability of a industrial production system. Be specific and answer point wise. Give examples as appropriate. B. State the main objectives of a modern industry (at least five) and explain the role of automation in helping achieve these. C. Explain with examples the terms economy of scale and economy of scope. How does industrial automation help in achieving these? Cite examples. D. Differentiate between a job shop and a flow shop with example what are their process plant analogues? Give examples. E. Run any internet search engine and type History of Automation to prepare a term paper on the subject. F. There are some aspects of automation that have not been treated in the lesson. Consult references and prepare term papers on the impact of automation on a. Environmental Appropriateness for Industries b. Industrial Standardisation Certification such as ISO 9001 c. Industrial Safety Version 2 EE IIT, Kharagpur 13

G. Locate the major texts on Manufacturing Automation H. From the internet find alternate definitions of the terms : Industry, Automation and Control


Answers, Remarks and Hints to Points to Ponder Point to Ponder: 1A. Why does an automated system achieve superior performance compared to a manual one? Ans: Because such systems can have more precision, more energy and more speed of operation than possible manually. Moreover using computing techniques, much more sophisticated and efficient operational solutions can be derived and applied in real-time. B. Can you give an example where this happens? Ans: This is the rule. Only few exceptions exist. How many of the millions of industrial products could be made manually?

Point to Ponder: 2A. Can you explain the above definition in the context of a common control system, such as temperature control in an oven? Ans: Consider a temperature-controlled oven as found in many kitchens. A careful examination of the dials would show that one could control the temperature in the oven. This is a closed loop control operation. One can also control the time for which the oven is kept on. Note that in both cases the input signal to the process is the applied voltage to the heater coils. This input signal is varied as required to hold the temperature, by the controller. B. Is the definition applicable to open-loop as well as closed loop control? Ans: Yes

Point to Ponder: 3C. Can you give an example of an automated system, which contains a control system as a part of it? Ans: Many examples can be given. One of these is the following: In an industrial CNC machine, the motion control of the spindle, the tool holder and the job table are controlled by a position and speed control system, which, in fact, uses a separate processor. Another processor is used to manage the other automation aspects. Another example is that of A pick and place automated robot is used in many industrial assembly shops. The robot motion can be programmed using a high level interface. The motion of the robot is controlled using position control systems driving the various joints in the robotic manipulator.


D. What are the other parts of the system? Ans: The other functional parts of the CNC System include: The operator interface, the discrete PLC controls of indicators, lubricant flow control, tool changing mechanisms.

Point to Ponder: 4Try to find an example automated system which uses at least one of the areas of Industrial IT mentioned in Fig. 1.2. (Hint: Try using the internet) Ans: Distributed Control Systems (DCS) used in many large Continuous-Flow processes such as Petroleum Refining and Integrated Steel Plants use almost all components of Industrial IT

Point to Ponder: 5A. Can you give an example of an automated system, some of whose parts makes a significant application of Industrial IT? Ans: Distributed Control Systems (DCS) used in many large Continuous-Flow processes such as Petroleum Refining and Integrated Steel Plants use almost all components of Industrial IT B. Can you give an example of an automated system, none of whose parts makes a significant application of Industrial IT? Ans: An automated conveyor system used in many large Discrete Manufacturing Plants such as bottled Beverage Plants use no components of Industrial IT.

Point to Ponder: 6A. Can you give an example of an automated system, which is reactive in the sense mentioned above? Ans: Any feedback controller, such as an industrial PID controller is reactive since it interacts with sensors and actuators. B. Can you give an example of an automated system, which is real-time in the sense mentioned above Ans: Any feedback controller, such as an industrial PID controller is real-time, since it has to compute its output within one sampling time. C. Can you give an example of an automated system, which is mission-critical in the sense mentioned above


Ans: An automation system for a Nuclear Power Plant is mission critical since a failure is unacceptable for such a system.

Point to Ponder: 7A. With reference to Eq. (1), explain how the following automation systems improve industrial profitability. d. Automated Welding Robots for Cars e. Automated PCB Assembly Machines f. Distributed Control Systems for Petroleum Refineries Ans: Some of the factors that lead to profitability in each case, are mentioned. a. Automated Welding Robots for Cars Increased production rate, Uniform and accurate welding, Operator safety. b. Automated PCB Assembly Machines Increased production rate, Uniform and accurate placement and soldering c. Distributed Control Systems for Petroleum Refineries Energy efficiency, Improved product quality

Point to Ponder: 8A. You give an example of an industry where economy of scope is more significant than the economy of scale? Ans: One such example would a job shop which manufactures custom machine parts by machining according to customer drawings. Another example would be a factory to manufacture Personal Computer components B. Can you give an example of an industry where economy of scale is more significant than the economy of scope? Ans: One such example would be a Power plant. Another one would be a Steel Plant.

Point to Ponder: 9A. During a technical visit to an industry how can you identify the type of automation prevailing there from among the above types? Ans: Check for the following. Whether automatic control exists for majority the equipment Whether supervisory control is manual, partially automated or largely automated Whether operator interfaces are computer integrated or not. Whether communication with individual control units can be done from supervisory interfaces through computers or not


Whether any information network exists, to which automation system and controllers are connected Product variety, product volumes, batch sizes etc. Whether the material handling systems are automated and if so to what extent. The type of automation system can be determined based on these information, as discussed in the lesson. B. For what kind of a factory would you recommend computer integrated manufacturing and why? Ans: For large systems producing sophisticated and expensive products in large volumes having many subunits to be integrated in complex ways. C. What kind of automation would you recommend for manufacturing a. Light bulbs Ans: Fixed b. Garments Ans: Flexible c. Textile Ans: Programmable d. Cement Ans: Programmable e. Printing Ans: Flexible f. Pharmaceuticals Ans: Flexible g. Toys Ans: Flexible


Module 1IntroductionVersion 2 EE IIT, Kharagpur 1

Lesson 2Architecture of Industrial Automation SystemsVersion 2 EE IIT, Kharagpur 2

Lesson Objectives To describe the various elements of an Industrial Automation Systems and how they are organized hierarchically in levels. To explain how these levels relate to each other in terms of their functions. To describe the nature of technologies involved in realizing these functional levels To describe the nature of information processing in these levels and the information flow among them

The Functional Elements of Industrial AutomationAn Industrial Automation System consists of numerous elements that perform a variety of functions related to Instrumentation, Control, Supervision and Operations Management related to the industrial process. These elements may also communicate with one another to exchange information necessary for overall coordination and optimized operation of the plant/factory/process. Below, we classify the major functional elements typically found in IA systems and also describe the nature of technologies that are employed to realize the functions.

Sensing and Actuation ElementsThese elements interface directly and physically to the process equipment and machines. The sensing elements translate the physical process signals such as temperature, pressure or displacement to convenient electrical or pneumatic forms of information, so that these signals can be used for analysis, decisions and finally, computation of control inputs. These computed control inputs, which again are in convenient electrical or pneumatic forms of information, need to be converted to physical process inputs such as, heat, force or flow-rate, before they can be applied to effect the desired changes in the process outputs. Such physical control inputs are provided by the actuation elements.

Industrial Sensors and Instrument SystemsScientific and engineering sensors and instrument systems of a spectacular variety of size, weight, cost, complexity and technology are used in the modern industry. However, a close look would reveal that all of them are composed of a set of typical functional elements connected in a specified way to provide signal in a form necessary. The various tasks involved in the automation systems. Fig 2.1 below shows the configuration of a typical sensor system.Sensing element Signal Conditioning element Signal Processing element Target Signal Handling element

Medium

Fig. 2.1 Functional configuration of a typical sensor system In Fig. 2.1 a sensor system is shown decomposed into three of its major functional components, along with the medium in which the measurement takes place. These are described below. Version 2 EE IIT, Kharagpur 3

A. The physical medium refers to the object where a physical phenomenon is taking place and we are interested in the measurement of some physical variable associated with the phenomenon. Thus, for example, the physical medium may stand for the hotga in a furnace in the case of temperature measurement or the fluid in a pipe section in the case of measurement of liquid flow rate. B. The sensing element is affected by the phenomenon in the physical medium either through direct or physical contact or through indirect interaction of the phenomenon in the medium with some component of the sensing element. Again, considering the case of temperature measurement, one may use a thermocouple probe as the sensing element that often comes in physical contact with the hot object such as the flue gas out of a boilerfurnace or an optical pyrometer which compares the brightness of a hot body in the furnace with that of a lamp from a distance through some window and does not come in direct contact with the furnace. In the more common case where the sensing element comes in contact with the medium, often some physical or chemical property of the sensor changes in response to the measurement variable. This change then becomes a measure of the physical variable of interest. A typical example is the change in resistivity due to heat in a resistance thermometer wire. Alternatively, in some other sensors a signal is directly generated in the sensing element, as is the case of a thermocouple that generates a voltage in response to a difference in temperature between its two ends. C. The signal-conditioning element serves the function of altering the nature of the signal generated by the sensing element. Since the method of converting the nature of the signal generated in the sensor to another suitable signal form (usually electrical) depends essentially on the sensor, individual signal conditioning modules are characteristic of a group of sensing elements. As an example consider a resistance Temperature Detector (RTD) whose output response is a change in its resistance due to change in temperature of its environment. This change in resistance can easily be converted to a voltage signal by incorporating the RTD in one arm of a Wheatstone's bridge. The bridge therefore serves as a signal-conditioning module. Signal conditioning modules are also used for special purpose functions relating to specific sensors but not related to variable conversion such as `ambient referencing' of thermocouples. These typically involve analog electronic circuits that finally produce electrical signals in the form of voltage or current in specific ranges. D. The signal processing element is used to process the signal generated by the first stage for a variety of purposes such as, filtering (to remove noise), diagnostics (to assess the health of the sensor), linearisation (to obtain an output which is linearly related with the physical measurand etc. Signal processing systems are therefore usually more general purpose in nature. E. The target signal-handling element may perform a variety of functions depending on the target application. It may therefore contain data/signal display modules, recording or/storage modules, or simply a feedback to a process control system. Examples include a temperature chart recorder, an instrumentation tape recorder, a digital display or an Analog to Digital Converter (ADC) followed by an interface to a process control computer. While the above description fits in most cases, it may be possible to discover some variations in some cases. The above separation into subsystems is not only from a functional point of view, Version 2 EE IIT, Kharagpur 4

more often than not, these subsystems are clearly distinguishable physically in a measurement system. Modern sensors often have the additional capability of digital communication using serial, parallel or network communication protocols. Such sensors are called smart and contain embedded digital electronic processing systems.

Point to Ponder: 1A. Draw the functional block diagram of a typical sensor system B. Consider a strain-gage weigh bridge. Explore and identify the subsystems of the bridge and categorise these subsystems into the above mentioned classes of elements mentioned above.

Industrial Actuator SystemsActuation systems convert the input signals computed by the control systems into forms that can be applied to the actual process and would produce the desired variations in the process physical variables. In the same way as in sensors but in a reverse sense, these systems convert the controller output, which is essentially information without the power, and in the form of electrical voltages (or at times pneumatic pressure) in two ways. Firstly it converts the form of the variable into the appropriate physical variable, such as torque, heat or flow. Secondly it amplifies the energy level of the signal manifold to be able to causes changes in the process variables. Thus, while both sensors and actuators cause variable conversions, actuators are high power devices while sensors are not. It turns out that in most cases, actuators are devices that first produce motion from electrical signal, which is then further converted to other forms. Based on the above requirement of energy and variable conversion most actuation systems are are structured as shown in Fig. 2.2.Signal Processing element Power Amplifying Element Variable Conversion Element Energy Conversion Element

Process

Fig. 2.2 Functional configuration of a typical actuator system In Fig.2.2 an actuator system is shown decomposed into its major functional components, The salient points about the structure are described below. A. The electronic signal-processing element accepts the command from the control system in electrical form. The command is processed in various ways. For example it may be filtered to avoid applying input signals of certain frequencies that may cause resonance. Many actuators are themselves closed feedback controlled units for precision of the actuation operation. Therefore the electronic signal-processing unit often contains the control system for the actuator itself.


B. The electronic power amplification element sometimes contains linear power amplification stages called servo-amplifiers. In other cases, it may comprise power electronic drive circuits such as for motor driven actuators. C. The variable conversion element serves the function of altering the nature of the signal generated by the electronic power amplification element from electrical to non-electrical form, generally in the form of motion. Examples include electrohydraulic servo valve, stepper/servo motors, Current to Pneumatic Pressure converters etc. D. The non-electrical power conversion elements are used to amplify power further, if necessary, typically using hydraulic or pneumatic mechanisms. E. The non-electrical variable conversion elements may be used further to tranform the actuated variable in desired forms, often in several stages. Typical examples include motion-to-flow rate conversion in flow-valves, rotary to linear motion converters using mechanisms, flow-rate to heat conversion using steam or other hot fluids etc. F. Other Miscellaneous Elements such as Auxiliaries for Lubrication/Cooling/Filtering, Reservoirs, Prime Movers etc., sensors for feedback, components for display, remote operations, as well as safety mechanisms since the power handling level is significantly high.

Point to Ponder: 2A. Draw the functional block diagram of an actuator system B. Consider an electro-hydraulic servo-valve actuator. Explore and identify the subsystems of the actuator and categorise these subsystems into the above-mentioned classes of elements mentioned above.

Industrial Control SystemsBy industrial control systems, we denote the sensors systems, actuator systems as a controller. Controllers are essentially (predominantly electronic, at times pneumatic/hydraulic) elements that accept command signals from human operators or Supervisory Systems, as well as feedback from the process sensors and produce or compute signals that are fed to the actuators. Control Systems can be classified into two kinds.

Continuous ControlThis is also often termed as Automatic Control, Process Control, Feedback Control etc. Here the controller objective is to provide such inputs to the plant such that the output y(t) follows the input r(t) as closely as possible, in value and over time. The structure of the common control loop with its constituent elements, namely the Controller, the Actuator, the Sensor and the Process itself is shown. In addition the signals that exist at various points of the


Disturbances di do uP

r (t)

+

e

Controller GC(s)

uA

Actuator GA(s)

Plant GP(s)

y(t) Output

Command/ reference/ Setpoint ds + +Sensor GS(s)

Fig. 2.3 Typical control loop system are also marked. These include the command (alternatively termed the set point or the reference signal), the exogenous inputs (disturbances, noise). The difficulties in achieving the performance objective is mainly due to the unavoidable disturbances due to load variation and other external factors, as well as sensor noise, the complexity, possible instability, uncertainty and variability in the plant dynamics, as well as limitations in actuator capabilities. Most industrial control loop command signals are piecewise constant signals that indicate desirable levels of process variables, such as temperature, pressure, flow, level etc., which ensure the quality of the product in Continuous Processes. In some cases, such as in case of motion control for machining, the command signal may be continuously varying according to the dimensions of the product. Therefore, here deviation of the output from the command signal results in degradation of product quality. It is for this reason that the choice of the feedback signals, that of the controller algorithm (such as, P, PI pr PID), the choice of the control loop structure (normal feedback loop, cascade loop or feedforward) as well as choice of the controller gains is extremely important for industrial machines and processes. Typically the control configurations are well known for a given class of process, however, the choice of controller gains have to be made from time to time, since the plant operating characteristics changes with time. This is generally called controller tuning. A single physical device may act as the controller for one or more control loops (singleloop/multi-loop controller). Today, many loop controllers supplement typical control laws such as PID control by offering adaptive control and fuzzy logic algorithms to enhance controller response and operation. PID and startup self-tuning are among the most important features. Among other desired and commonly found characteristics are, ability to communicate upward with supervisory systems, as well as on peer-to-peer networks (such as Fieldbus or DeviceNet), support for manual control in the event of a failure in the automation. Software is an important factor in loop controllers. Set-up, monitoring and auto-tuning and alarm software for loop controllers is now a common feature. The controllers also accept direct interfacing of process sensors and signals. Choice of inputs includes various types of thermocouples, RTDs, voltage to 10 V dc, or current to 20 mA. While most sophisticated controllers today are electronic, Version 2 EE IIT, Kharagpur 7

pneumatic controllers are still being used. Pneumatic controllers are easy to use, easy to maintain, and virtually indestructible.

Point to Ponder: 3A. Draw the block diagram of a typical industrial control system B. Consider a motor driven position control system, as commonly found in CNC Machine drives. Identify the main feedback sensors in the system. Identify the major sources of disturbance. How is such a drive different from that of an automated conveyor system?

Sequence / Logic ControlMany control applications do not involve analog process variables, that is, the ones which can assume a continuous range of values, but instead variables that are set valued, that is they only assume values belonging to a finite set. The simplest examples of such variables are binary variables, that can have either of two possible values, (such as 1 or 0, on or off, open or closed etc.). These control systems operate by turning on and off switches, motors, valves, and other devices in response to operating conditions and as a function of time. Such systems are referred to as sequence/logic control systems. For example, in the operation of transfer lines and automated assembly machines, sequence control is used to coordinate the various actions of the production system (e.g., transfer of parts, changing of the tool, feeding of the metal cutting tool, etc.). There are many industrial actuators which have set of command inputs. The control inputs to these devices only belong to a specific discrete set. For example in the control of a conveyor system, analog motor control is not applied. Simple on-off control is adequate. Therefore for this application, the motor-starter actuation system may be considered as discrete having three modes, namely, start, stop and run. Other examples of such actuators are solenoid valves, discussed in a subsequent lesson. Similarly, there are many industrial sensors (such as, Limit Switch / Pressure Switch/ Photo Switch etc.) which provide discrete outputs which may be interpreted as the presence/absence of an object in close proximity, passing of parts on a conveyor, or a given pressure value being higher or lower than a set value. These sensors thus indicate, not the value of a process variable, but the particular range of values to which the process variable belongs. A modern controller device used extensively for sequence control today in transfer lines, robotics, process control, and many other automated systems is the Programmable Logic Controller (PLC). In essence, a PLC is a special purpose industrial microprocessor based realtime computing system, which performs the following functions in the context of industrial operations

Point to Ponder: 4A. State the major aspect in which sequence/logic control systems differ from analog control systems B. Describe an industrial system that employs discrete sensors and discrete actuators.


Supervisory ControlSupervisory control performs at a hierarchically higher level over the automatic controllers, which controls smaller subsystems. Supervisory control systems perform, typically the following functions: Set point computation: Set points for important process variables are computed depending on factors such as nature of the product, production volume, mode of processing. This function has a lot of impact on production volume, energy and quality and efficiency. Performance Monitoring / Diagnostics: Process variables are monitored to check for possible system component failure, control loop detuning, actuator saturation, process parameter change etc. The results are displayed and possibly archived for subsequent analysis. Start up / Shut down / Emergency Operations : Special discrete and continuous control modes are initiated to carry out the intended operation, either in response to operator commands or in response to diagnostic events such as detected failure modes. Control Reconfiguration / Tuning: Structural or Parametric redesign of control loops are carried out, either in response to operator commands or in response to diagnostic events such as detected failure modes. Control reconfigurations may also be necessary to accommodate variation of feedback or energy input e.g. gas fired to oil fired. Operator Interface: Graphical interfaces for supervisory operators are provided, for manual supervision and intervention. Naturally, these systems are dependent on specific application processes, in contrast with automatic control algorithms, which are usually generic (e.g. PID). Computationally these are a mixture of hard and soft real time algorithms. These are also often very expensive and based on proprietary knowledge of automating specific classes of industrial plants.

Point to Ponder: 5A. State three major functions of a Supervisory Control System B. Consider the motor driven automatic position control system, as commonly found in CNC Machine drives. Explore and find out from where such systems get their set points during machining. Identify some of the other functionalities

Level 3: Production ControlProduction control performs at a hierarchically higher level over the supervisory controllers. Typical functions they perform are: Process Scheduling: Depending on the sequence of operations to be carried on the existing batches of products, processing resource availability for optimal resource utilization. Version 2 EE IIT, Kharagpur 9

Maintenance Management: Decision processes related to detection and deployment of maintenance operations. Inventory Management: Decision processes related to monitoring of inventory status of raw material, finished goods etc. and deployment of operations related to their management. Quality Management : Assessment, Documentation and Management of Quality Typically, the algorithms make use of Resource Optimisation Technology and are non-real-time although they may be using production data on-line.

Point to Ponder: 6A. State three major functions of a Production Control System B. Explore and find out concrete activities for production control under at least two of the above major functions in any typical factory such as a Power Plans or a Steel Plant.

The Architecture of Elements: The Automation PyramidIndustrial automation systems are very complex having large number of devices with confluence of technologies working in synchronization. In order to know the performance of the system we need to understand the various parts of the system. Industrial automation systems are organized hierarchially as shown in the following figure.

Level 4 Industrial IT Level 3 Enterprise Productio n Control Supervisory Control Automati c Control Sensors Actuator s Process / Fig. 2.4 Automation pyramid Version 2 EE IIT, Kharagpur 10

Level 2

Level 1 Industrial Auto Level 0

Various components in an industrial automation system can be explained using the automation pyramid as shown above. Here, various layers represent the wideness ( in the sense of no. of devices ), and fastness of components on the time-scale. Sensors and Acuators Layer: This layer is closest to the proceses and machines, used to translate signals sothat signals can be derived from processes for analysis and decisions and hence control signals can be applied to the processes. This forms the base layer of the pyramid also called level 0 layer. Automatic Control Layer: This layer consists of automatic control and monitoring systems, which drive the actuators using the process information given by sensors. This is called as level 1 layer. Supervisory Control Layer: This layer drives the automatic control system by setting target/goal to the controller. Supervisory Control looks after the equipment, which may consis of several control loops. This is called as level 2 layer. Production Control Layer: This solves the decision problems like production targets, resource allocation, task allocation to machines, maintenance management etc. This is called level 3 layer. Enterprise control layer: This deals less technical and more commercial activities like supply, demand, cash flow, product marketing etc. This is called as the level 4 layer. The spatial scale increases as the level is increased e.g. at lowest level a sensor works in a single loop, but there exists many sensors in an automation system which will be visible as the level is increased. The lowest level is faster in the time scale and the higher levels are slower. The aggregation of information over some time interval is taken at higher levels. All the above layers are connected by various types of communication systems. For example the sensors and actuators may be connected to the automatic controllers using a point-to-point digital communication, while the automatic controllers themselves may be connected with the supervisory and production control systems using computer networks. Some of these networks may be proprietary. Over the last decade, with emergence of embedded electronics and computing, standards for low level network standards (CANBus, Fieldbus etc.) for communication with low level devices, such as sensors and actuators are also emerging.

Point to Ponder: 7A. Draw the Automation Pyramid and identify the Layers B. Give examples of the above major functional layers in any typical factor. . A concrete example of the Automation Functionality in a large manufacturing plant is presented in the appendix below. The appendix reveals the nature of functionality expected in modern automation systems, the elements that are used to realise them, and the figures of merit for such systems. The learner is encouraged to study it.


Appendix: An Example Industrial Specification for Automatic and Supervisory Level Automation SystemsThis appendix contains the specification of a section of a Cold Rolling Mill complex, referred to here as PL-TCM which stands for Pickling Line and Tandem Control Mill. Such specification documents are prepared when automation systems for industrial plants are procured and installed. The document captures the visualisation of automation functionality of the customer. Here basic level refers to the automatic control supervisory control levels, while process control level refers to a level. Some of the terms and concepts described below have been discussed in subsequent lessons. Platforms: The above levels of controls shall be achieved through programmable controllers PLCs, micro-processor based systems as well as PCs / Work stations, as required. Each of the automation systems of the PL-TCM shall be subdivided in accordance with the functional requirements and shall cover the open loop and closed loop control functions of the different sections of the line and the mill. Modes of Operation: The systems shall basically have two modes of operation. In the semiautomatic mode the set point values shall be entered manually for different sections of the line through VDU and the processors shall transmit these values to the controls in proper time sequence. In fully automatic mode the process control system shall calculate all set point values through mathematical models and transfer the same to the subordinate systems over data link. The functions to be performed by the basic level automation shall cover but not be limited to the following. Functionality at Basic Level: The Basic Level shall cover control of all equipment, sequencing, interlocking micro-tracking of strip for specific functions, dedicated technological functions, storage of rolling schedules and look-up tables, fault and event logging etc. Some of these are mentioned below. All interlocking and sequencing control of the machinery such as for entry and exit handling of strips, shear control etc. Interlocking, sequencing, switching controls of the machines. This shall also cover automatic coil handling at the entry and exit sides, automatic sequencial operation of welding/rewelding machine and strip threading sequence control as well as for acid regeneration plant. Calculation of coil diameter and width at the entry pay-off reels. Position control of coil ears for centrally placing of coils on the mandrels. Generation of master speed references for the line depending on operator's input and line conditions and down loading to drive control systems. Speed synchronising control of the drives, as required. Strip tenstion, position and catenary control through control of related drives and machinery.


Initiation of centre position control for Power Operated Rolls, steering/dancer rolls; Looper car position control. Automatic pre-setting control, measurement and control of tension and elongation for tension leveller. Auto edge position control at tension reels if required. Control of entry shear for auto-cutting of off-gauge strip. Control of pickling parameters for correct pickling with varying speed of strip in the pickling section. Side trimmer automatic setting contro. Interlockings, sequencing and control of scrap baller, if provided. Auto calibration for position control/precision positioning shall be provided as necessary. Manual/Auto slowdown/stoppage of strip at weld point at tension leveller, side trimmer, mill and exit shear. Control of technological functions for tandem mill such as : o Automatic gauge control along with interst and tension control. o Shape control o Roll force control Storage of tandem mill rolling schedules, for the entire product mix and all possible variations. Suitable look-up tables as operators guidance for line/equipment setting. Automatic roll changing along with automatic spindle positioning. Constant pass line control based on roll wear as well as after roll change. Automatic control of rotary shear before tension rells. Automatic sequence control of inspection reel. Provision of manual slow down/stoppage of strip as well as chearing for `run' for inspection of defects at tension leveller, side trimmer entry and exit of the Tandem Mill throuth push button stations. Micro-tracking of strip and flying gauge change (set point change) for continuous operation with varying strip sizes. Setting up the mill either from the stored rollings schedule with facility for modification by the operator of down-loading from process control level system. Automatic control of in-line coil weighing, marking and circumferential banding after delivery tension reels. Supervisory Functions at Basic Level: Centralised supervisory and monitoring control system shall be provided under basic level automation with dedicated processors and MMI. All necessary signals shall be acquired through drive control system as well as directly from the sensors/instruments as, required. The system shall be capable of carrying out the following fuctions.


Centralised switching and start up of various line drives and auxiliary systems through mimic displays. Status of plant drives and electrical equipment for displaying maintenance information. Monitoring and display of measured values for tandem mill main drives and other large capacity drives such as winding temperature, for alarm and trip conditions. Centralised switching and status indication of 33 kV and 6.6 kV switchboards. Display of single line diagram of 33 kV and 6.6 kV switchboards, main drives, in-line auxiliary drives etc. Acquisition of fault signals from various sections of the plant with facility for display and print-out of the fault messages in clear text.

Comprehensive diagnostic functionsFunctionality at Process Control Level: The Process Control Level shall be responsible for computation and control for optimization of operation. Functions like set point generation using mathematical models, learning control, material tracking within the process line/unit including primary data input, real time control of process functions through basic level automation, generation of reports etc. shall be implemented through this level of automation. Some of the specific functions to be performed by the process control level automation are the following. Coil strip tracking inside the process line/unit by sensing punched holes at weld seams. Primary Data Input (PDI) of coils at entry to PL-TCM with provision for down loading of data from production control level. Generation of all operating set points for the mill using PDI data, mill model, roll force model, power model, strip thickness control model, shape/profile control model with thermal strip flatness control as well as for other sections of the line. Learning (Adaptive) control using actual data and the mathematical model for set-up calculations. Storage of position setting values of levellers, side trimmer. Input of strip flaw data manually through inspection panel at the inline inspection facility after side trimmer. Processing of actual data on rolling operation, generation of reports logs and sending data to production control level. Information System Functions: The information system shall generally comply with the following features. Data of importance shall be available with the concerned personnel in the form of logs and reports. Output of logs and reports at preset times or on occurance of certain events. It shall be possible to change the data items and log formats without undue interference to the system. Version 2 EE IIT, Kharagpur 14

Logged information shall be stored for adequate time period ensuring the availability of historical data record. Data captured by the system shall be checked for integrity with respect to their validity and plausibility with annunciation. Man Machine Interface: The visualisation system for both the automation levels shall be through man-machine interface (MMI) for the control and operation of the complete line. The system shall display the following screens, with facilities for hard copy print out. Process mimics for the complete line using various screens with status information of all important in-line drives as well as the references and actual values of important parameters. Dynamic informations in form of bar graph for indication of reference and actual values of important parameters. Screens providing trends of the important process variables. Acquisition of actual parameters (averaging/maximum/minimum) for the complete line, on coil to coil basis through weld seam tracking or TCM exit shear cut for the generation of logs on process/parameters and production. Standards: The programmable controllers and other micro processor based equipment offered shall generally be designed/structured, manufactured and tested in accordance with the guidelines laid down in IEC-1131 (Part 2) apart from the industry standards being adopted by the respective manufactures. Hardware: The hardware of each basic controller/equipment of a system will generally comprise main processing unit, memory units, stabilised power supply unit, necessary communication interface modules, auxiliary storage where required. I/O modules in the main equipment, remote I/O stations where required and the programming and debugging tool (PADT). The hardware and software structure shall be modular to meet wide range of technological requirements. I/Os shall be freely configurable depending on the requirement. The programming units shall preferable be lap-top type. Networking: The networking would conform to the following specifications. In each of the two automation levels, all the controllers of a system shall be connected as a node over suitable data bus forming a LAN system using standardised hardware and software. The LAN system shall be in line with ISO-Open system Interconnect. All drive level automation equipment shall be suitably linked with the basic level for effective data/signal exchange between the two levels. However, all the emergency and safety signals shall be directly hardwired to the respective controllers. Similarly, the LAN systems for the basic level and process control level shall be suitably linked through suitable bridge/interface for effective data/signal exchange. Provision shall also be made for interfacing suitably the process control level with the production level automation system specified in item . Version 2 EE IIT, Kharagpur 15

The data highways shall be designed to be optimally loaded and the same shall be clearly indicated in the offer. The remote I/Os, the microprocessor based measuring instruments and the microprocessor based special machines like coil weighing, marking and circumferantial banding machines shall be connected over serial links with the respective controllers. The personal computers and work stations shall be connected as a LAN system of the corresponding level. Data and Visualisation: The following specifications would apply in respect of data security, validity and its proper visualisation. All the operator interfaces comprising colour VDU and keyboard as MMI for interacting with the respective system and located at strategic locations, shall be connected to the corresponding LAN system. Keylock/password shall be provided to prevent unauthorised entry. Entry validity and plansibility check shall also be incorporated. An Engineer's console comprising of necessary processor, color VDU, keyboard/mouse and a printer unit shall be provided for the automation systems. The console shall have necessary hardware and software of communicating with the LAN and shall have access to the complete system. Basic functions of this console shall be off-line data base configuration, programme development, documentation etc. Application Software: The application software shall be through functional block type software modules as well as high level language based software modules. The software shall be user friendly and provided with help functions etc. Only one type of programming language shall be used for the complete system. However, ladder type programming language may be used for simple logical functions. Only industrially debugged and tested software shall be provided.

Basis of System SelectionFuture Expandibility: The selection of equipment, standard software and networking shall be such as to offer optimum flexibility for future expansion without affecting the system reliability. Fault Tolerance: The system shall be designed to operate in automatic or semi automatic mode of operation under failure conditions. Spare Capacity: The system shall have sufficient capacity to perform all functions as required. A minimum of 30 per cent of the total memory shall be kept unallocated for future use. Loading: The data highway shall be designed to be optimally loaded and the same shall be clearly indicated in the offer. Software Structure and Quality Programs: shall be in high level language that is effective and economical for the proposed system in respect of Modularisation, rate of coding, store usage and running time. The software structure of the system shall besuitably distributed/centralised for supervision and control of the related process areas following the state of the art architecture. Version 2 EE IIT, Kharagpur 16

Integration: The communication software shall be such that the systems shall be able to communicate independently among themselves as well as with the lower level Basic Control/Process control automation system, as required. Provision shall be made for interfacing the production control system with the higher level Business Computer system to be provided for the entire steel plant in future. Programmability: The information system shall generally be designed such that it shall be possible to change the data items and log formats without undue interference to the system. Data Integrity and Protection: Logged information shall be stored for adequate time period ensuring the availability of historical data record. Data captured by the system shall be checked for integrity with respect to their validity and plausibility with annunciation. Storing of essential data to be protected against corruption when the system loses power supply or during failure.

Point to Ponder: 8A. State three major functions of Supervisory Control mentioned in the lesson that have also been mentioned in the Automation System for the PL-TCM. B. State three major figures of merit for an Automation System mentioned in the appendix PL-TCM.


Answers, Remarks and Hints to Points to Ponder Point to Ponder: 1Draw the functional block diagram of a typical sensor system Ans: The diagram is given in Fig. 2.1.in the lesson. Consider a strain-gage weigh bridge. Explore and identify the subsystems of the bridge and categorise these subsystems into the above mentioned classes of elements mentioned above Ans: A strain gage weighbridge contains the weighing platform and pillar, which senses the weight and produces a proportional strain (sensing element 1). This strain is sensed by a strain-gage which produces a proportional change in resistance (sensing element 2). The gage is incorporated into a Wheatstones bridge circuit (Signal conditioning) which generates a proportional unbalanced voltage.

Point to Ponder: 2A. Draw the functional block diagram of an actuator system Ans: The diagram is given in Fig. 2.1.in the lesson. B. Consider an electro-hydraulic servo-valve actuator. Explore and identify the subsystems of the actuator and categorise these subsystems into the above-mentioned classes of elements mentioned above. Ans: An electrohydraulic servo valve is driven by current through a solenoid, which moves the spool of the valve, by applying a voltage across it. The voltage is derived by an electronic controller (electronic signal processing element), which gives a voltage input that is amplified by a servo amplifier (electronic power amplification element). The force due to the current produces motion of the spool (variable conversion element), which is converted to pressure (non electrical power conversion element) within the servo valve and applied to the final control element. Miscellaneous elements, such hydraulic system auxiliaries, indicators etc. are also present

Point to Ponder: 3A. Draw the block diagram of a typical industrial control system Ans: The diagram is given in Fig. 2.3 in the lesson. B. Consider a motor driven position control system, as commonly found in CNC Machine drives. Identify the main feedback sensors in the system. Identify the major sources of disturbance. How is such a drive different from that of an automated conveyor system? Ans: Many examples can be given. One of these is the following:


In an industrial CNC machine, the motion control of the spindle, the tool holder and the job table are controlled by a position and speed control system, which, in fact, uses a position sensor such as shaft angle encoder or resolver, speed sensors such analog or digital tachometers and current sensors such Hall-effect sensors. The major sources of disturbances are changes in load torque arising in the machine due to material inhomogenity, tool wear etc. While both drives use motors for creating displacements, conveyor drives have very little demand on position and speed accuracy requirements. On the contrary there are very stringent requirement on these in the case of the CNC Machine.

Point to Ponder: 4State the major aspect in which sequence/logic control systems differ from analog control systems Ans: The two major aspects in which they differ is in the nature of sensor inputs and the actuator outputs. These are discrete elements in the case of logic Control (on-off, low-high-medium etc.) and continuous valued in case of analog control. Similarly for actuator output (motor start/stop). The controller outputs are generally functions of inputs and feedback. Describe an industrial system that employs discrete sensors and discrete actuators. Ans: There are many such systems. For examp

41632553 Automation

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