UNIT 5 48 UNIT 3 INDUSTRIAL CONTROL SYSTEM CONTENTS I Overview II Industrial Control Classification III Element of Open and Closed Loop Systems IV Feed back Control V Practical Feedback Application VI.
Trang 1UNIT 3 INDUSTRIAL CONTROL SYSTEM CONTENTS
I Overview
II Industrial Control Classification
III Element of Open and Closed Loop Systems
IV Feed-back Control
V Practical Feedback Application
VI Dynamic response of a Closed Loop Systems
VII Feed-Forward Control
I OVERVIEW
I.1 READING
The industrial revolution began in England during the mid 1700s when it was discovered that productivity of spinning wheels and weaving machine could be dramatically increased by fitting them with steam powered engines Further inventions and new ideas in plant layouts during the 1850s enabled the United States to surpass England as the manufacturing leader of the world Around the turn of the twentieth century, the electric motor replaced steam and water wheel as a power source Factories became larger, machines were improved to allow closer tolerances, and the assembly line method of mass production was created
Between World Wars I and II, the feedback control system was developed, enabling manually operated machines to be replaced by automated equipment The feedback control system a key element in today’s manufacturing operations The term industrial controls is used to define this type of system, which automatically monitors manufacturing processes being executed and take appropriate corrective action if the operation is not performing properly
During World War II, significant advances in feedback technology occurred due to the sophisticated control systems required by military weapons After the war, the techniques used in military equipment were applied to industrial controls to further improve the quality
of products and to increase productivities
Because many modern factory machines are automated, the technicians who install, troubleshoot, and repair them need to be highly trained To perform effectively, these individual must understand the element, operational theory, terminology associated with industrial control systems
Industrial Control theory encompasses many fields but uses the same basic principles, whether controlling the position of an object the speed of a motor or the temperature and pressure of a manufacturing process
Trang 2Corrective (adj) [kә'rektiv]: Chỉnh định
Define (v) [di'fain] Định nghĩa
Dramatically (adv) [drә'mætikәli]: Đột ngột
Encompass (v) [in'kɑmpәs]: Bao gồm
Execute (v) ['eksikju:t]: Thực thi, chạy
Further (adj) ['fә:đә]: Tiếp theo
Improve (v) [im'pru:v]: Hoàn thiện, cải tiến
Individual (n) [,indi'vidjuәl]: Cá nhân, người
Industrial Control (n): Điều khiển công nghiệp
Mass production [mæs prә'dɑk∫n]: Xản xuất hàng loạt
Operational theory (n) [,ɑpә'rei∫әnl 'θiәri]: Nguyên lý hoạt động/vận hành Power source (n) ['pauә sɑ:s]: Nguồn động năng
Principle (n) ['prinsәpl]: Nguyên lý
Revolution (n) [,revә'lu:∫n]: Cách mạng
Significant (adj) [sig'nifikәnt]: Có nghĩa, đáng kể
Sophisticated (adj) [sә'fistikeitid]: Tinh vi
Spinning wheel (n) [spinniη wi:l]: Guồng se sợi
Steam wheel (n) [sti:m wi:l]: Guồng hơi nước
Technician (n) [tek'ni∫n]:: Kỹ thuật viên
Technique (n) [tek'ni:k]: Phương pháp, kỹ thuật
Trang 3Terminology (n) [,tә:mi'nɑlәdʒi]: Thuật ngữ
The turn of twentieth century: Đầu thế kỷ 20
Tolerance (n) ['tɑlәrәns]: Dung sai
Troubleshoot (v) ['trɑbl∫u:t]: Khắc phục sự cố
Water wheel ['wɑ:tә wi:l]: Guồng nước
I.3 READING COMPREHENSION
Answer the following questions:
1 Where and when did the industrial revolution begin?
2 What did enable the US to surpass England as the manufacturing leader?
3 In the beginning of the twentieth century, what replaced steam and water wheel as the power source?
4 What the consequences of the replacement?
5 Between World War I and World War II, what enabled manually operated machines to be replaced by automated equipment?
6 What is the role that the feedback control system plays in modern manufacturing systems?
7 What does the term “industrial controls” define?
8 When did significant advances in feedback control occur? And why did they occur?
9 After the war, how was the quality of products improved? And how was the productivities increased?
10 What technicians in modern factories have to do? And what they have to understand in order to do their jobs effectively?
11 What is the scope of industrial control theory though it uses the same basic principles?
II INDUSTRIAL CONTROL CLASSIFICATIONS
II.1 READING
Motion and process controls
Industrial control systems are often classified by what they control: either motion or process
Motion control
A motion control system is an automatic control system that controls the physical
motion or position of an object One example is the industrial robot arm which perform welding operation and assembly procedures
There are three characteristics that are common to all motion control systems First, motion control devices control the position, speed, acceleration, or deceleration of a mechanical object Second, the motion or position of the object being controlled is measured Third, motion devices typically respond to input commands within fractions of a
Trang 4second, rather than seconds or minutes, as in process control Hence, motion control systems are faster than process control systems
Motion control systems are also referred to as servos, or servomechanism Other examples of motion control applications are CNC machine tool equipment, printing presses, office copiers, packaging equipment, and electronics parts insertion machines that place components onto a printed circuit board
Process control
The other type of industrial control system is process control In process control, one
or more variables are regulated during the manufacturing of a product These variables may include temperature, pressure, flow rate, liquid and solid level, pH, or humidity This regulated process must compensate for any outside disturbance that changes the variable The response time of a process control system is typically slow, and can vary from a few seconds to several minutes Process control is the type of industrial control system most often used in manufacturing Process control systems are divided into two categories, batch and continuous
Batch Process or batch processing is a sequence of time operation executed on the
product being manufactured An example is an industrial machine that produces various types of cookies, as show in Figure 1-1 Suppose that chocolate-chip cookies are made in the first production run, first, the oven is turned on to the desired temperature Next the required ingredients in the proper quantities are dispensed into the sealed mixing chamber A large blender then begins to mix the contents
Trang 5After a few minutes, vanilla is added, and the mixing process continues After a prescribed period of time the dough is the proper consistency, the blender stops turning and the compressor turns on to force air into the mixing chamber When the air pressure reaches
a certain point, the conveyor belt turns on The pressurized air forces the dough through outlet jets onto the belt The dough balls become fully baked as the pass through the oven The cookies cool as the belt carries them to the packaging machine
After the packaging step is completed, the mixing vat become vat, blender, and conveyor belt are washed before a batch of raisin-oatmeal cookies is made Products from foods to petroleum to soap to medicines are made from a mixture of ingredients that undergo
a similar batch process operation
Batch process is also known as sequence or sequential process
Continuous Process
In the continuous process category, one or more operations are being performed as the product is being passed through a process Raw materials are continuously entering and leaving each process Producing paper, as shown in Figure 1-2, is an example of continuous process Water, temperature, and speed are constantly monitored and regulated as the pulp is placed on the screens, feed through rollers, and gradually transformed into a finished paper product The continuous process can last for hours, days, or even weeks without interruption Everything from wire to textiles to plastic bags is manufactured by using a continuous manufacturing process similar to the paper machine
Trang 6Other examples of continuous process control application are wastewater treatment, nuclear power production, oil refining, and natural gas distribution through pipe lines
Another term commonly used instead of process control is instrumentation
The primary difference between process and motion control is the control method that
is required In process control, the emphasis is placed on sustaining a constant condition of a parameter, such as level, pressure, or flow rate of a liquid In servo control, the input command is constantly changing The emphasis of the system is to follow the changes in the desired input signal as closely as possible Variations of the input signal are typically very rapid
Open and closed loop Systems
Trang 7The purpose of any industrial system is to maintain one or more variables in a production process at a desired value These variables include pressures, temperatures, fluid levels, flow rates, composition of materials, motor speeds, and position of a robotic arm
Open loop systems
An open loop system is the simplest way to control a system A tank that supplies water for an irrigation system can be used to illustrate an open loop (or manual control) system The diagram in Figure 1-3 show a system composed of a storage tank an inlet pipe with a manual control valve, and an outlet pipe A continuous flow of water from a natural spring enters the tank at the inlet, and water flows from the outlet pipe to the irrigation system The process variable that is maintained in the tank is the water level Ideally, the manual flow control valve setting and the size of the outlet pipe are exactly the same When this occurs, the water level in the tank remains the same Therefore, the process reaches a steady state condition, or is said to be balanced The problem with this design is that any change or disturbance will upset the balance For example, a substantial rainfall may occur, causing additional water to enter the storage tank form the top Since there is more water entering the tank than exiting, the level will rise If the situation is not corrected, the tank will eventually overflow Excessive evaporation will also upset the balance If it occurred over a prolonged period of time, the water level in the tank may become unacceptably low
A human operator who periodically inspects the tank can change the control valve setting to compensate for these disturbances
An example of a manually operated open loop system is the speed of a car being controlled by the driver The driver adjusts the throttle to maintain a highway speed when going uphill, down hill, or on level terrain
Closed loop systems
Trang 8There are many situations in industry where the open loop system is adequate However, some manufacturing applications require continuous monitoring and self correcting action of the operation over long periods of time without interruption The automatic closed loop configuration performs the self correcting function This automatic system employs a feedback loop to keep track of how closely the system is doing the job it was commanded to do
The reservoir system can also be used to illustrate a closed loop operation To perform automatic control, the system is modified by replacing the manually controlled valve with an adjustable valve connected to a float as shown in Figure 1-4 The valve , the float and the linkage mechanism provide the feedback loop
If the level of the water in tank go up the, the float is pushed upward; if the level goes down, the float moves downward The float is connected to the inlet valve by a mechanical linkage As the water level rises, the float moves upward, pushing on the lever and closing the valve, thus, reducing the water flow into the tank If the water level lowers, the float moves down ward, pulling on the lever and opening the valve, thus admitting more water into the tank To adjust for a desired level of water in the tank, the float is moved up or down
on the float rod A
Most automated manufacturing process use closed loop control These systems that have a self-regulation capability are designed to produce continuous balance
II.2 VOCABULARY
Acceleration (n) [әk'selәreit]: Tăng tốc
Adequate (adj) ['ædikwit]: Đủ, thích hợp
Assembly procedure [ә'sembli prә'si:dʒә]: Thao tác lắp ráp
Automatic (adj) [,ɑ:tә'mætik]: Tự động
Trang 9Bake (v) [beik]: Nung, nuớng
Balance (v) ['bælәns]: Cân bằng
Bleacher tower (n) [bli:t∫ bli:t∫]: Tháp tẩy
Category (n) ['kætigәri]: Loại, lớp
Characteristic (n) [,kæriktә'ristik]: Đặc tính
Classify (v) ['klæsifai]: Phân loại
Cleaning stage (n) ['kli:niη steidʒ]: Giàn rửa
Compensate (v) ['kɑmpenseit]: Bù
Compressor (n) [kәm'presә]: Máy nén
Consistency (n) [kәn'sistәnsi]: Độ quánh
Content (n) ['kɑntent]: Những thứ bên trong
Continuous (adj) [kәn'tinjuәs]: Liên tục
Conveyor belt (n) [kәn'veiә belt]: Băng tải, băng chuyền
Correct (v) [kә'rekt]: Sửa chữa, chỉnh định, uốn nắn Debarking drum (n) [debɑ:kiη drɑm]: Trống bóc vỏ cây
Deceleration (n) [,di:selә'rei∫n]: Giảm tốc
Digester (n) [dai'dʒestә]: Thùng thuỷ phân
Dispense (v) [dis'pens]: Phân phối, đưa đến
Disturbance (n) [dis'tә:bәns]: Nhiễu loạn
Downhill (adv) ['daunhil]: Xuống dốc
Egg white (n) [eg wait]: Lòng trắng trứng
Evaporation (n) [i,væpә'rei∫n]: Bốc hơi, bay hơi
Eventually (adv) [i'vent∫uәli]: Sau cùng, cuối cùng
Excessive (adj) [ik'sesivli]: Quá mức
Trang 10Finished product (n) ['fini∫t 'prɑdәkt]: Thành phẩm
Ideally (adj) [ai'diәli]: Một cách lý tưởng
Insertion machine (n) [in'sә:∫n mә'∫i:n]: Máy lắp ráp
Ingredient(n) [in'gri:djәnt]: Thành phần
Interruption [,intә'rɑp∫n]: Gián đoạn
Irrigation (n) [,iri'gei∫n]: Tưới tiêu, thuỷ lợi
Keep track (v) [ki:p træk]: Theo dõi, giám sát
Linkage (n) ['liηkidʒ]: Tay đòn
Liquid (adj) ['likwid]: Lỏng
Material (n) [mә'tiәriәl]: Vật liệu
Measure (v) ['meʒә]: Đo
Motion Control (n) ['mou∫n kәn'troul]: Điều khiển chuyển động Oil refining (n) [ɑil [ri'fainiη]: Lọc dầu
Outlet jet (n) ['autlet dʒet]: Vòi ra
Packaging (n) ['pækidʒiη]: Đóng gói
Plastic (adj) ['plæstik]: Chất dẻo,
Plastic bag (n) ['plæstik bæg]: Túi bóng, túi ni-lon
Periodically (adv) [,piәri'ɑdikli]: Định kỳ
Position (v) [pә'zi∫n]: Vị trí
Powdered milk (n) ['paudәd milk]: Sữa bột
Pressurize (v)['pre∫әraiz]: Tăng áp
Printing press (n) ['printiη'pres]: Máy in
Trang 11Process control (n) ['prouses kәn'troul]: Điều khiển quá trình
Regulate (v) ['regjuleit]: Điều chỉnh
Reservoir (n) ['rezәvwɑ:]: Hồ chứa, bồn chứa
Sealed (adj) ['si:ld]: Niêm phong, đóng kín
Sequence (n) ['si:kwәns]: Chuỗi, loạt
Shortening (n) ['∫ɑ:tniη]: Dầu làm bánh
Steady state (n) ['stedi steidʒ]: Trạng thái xác lập
Transform (n) [træns'fɑ:m]: Biến đổi
Treatment (n) ['tri:tmәnt]: Xử lý
Variation (n) [,veәri'ei∫n]: Thay đổi, biến động
Various (adj) ['veәriәs]: Nhiều, phong phú
Vat (n) [væt]: Thùng, bể
Trang 12Wastewater (n) [weist 'wɑ:tә]: Nước thải
II.4 READING COMPREHENSION
Answer the following questions:
1 How industrial control systems are often classified?
2 What is a motion control system? Give an example!
3 What are the characteristics that motion control systems have in common?
4 Is process control faster than motion control? Why?
5 What are the other names of motion control?
6 What can be the applications of motion control?
7 What characterizes process control?
8 What can be the regulated variables of process control?
9 What must process control compensate for?
10 What is the range of response time for process control?
11 What is the typical response time of motion control?
12 What are the two categories of process control?
13 How can we define batch process?
14 What can be “production run” for the described cookies production?
15 Draw a diagram of how a cookies batch production proceeds
16 What needs to be done between the productions of different cookies batch?
17 What can be classified as a batch process?
18 What characterizes continuous process?
19 Describe how the paper production proceeds
20 How long can a continuous process last?
21 What is the difference that distinguishes batch process from continuous process?
22 What can be classified as a continuous process?
23 What make motion control differ from process control?
24 What is the emphasis with process control and what is that with motion control?
25 What is the purpose of any industrial system?
26 What can be a variable in industrial processes?
27 What is the simplest way to control a system?
28 What is used in text to illustrate an open loop system?
29 What can be the other name for open loop control?
30 What are the components of the system in Figure 1-3?
31 What is the purpose of the described irrigation system?
32 What is the process variable?
Trang 1333 How the process variable relates to the purpose of the irrigation system?
34 What is the ideal case of the open loop irrigation system?
35 What is the problem with open loop irrigation system?
36 What can be disturbances in the open loop irrigation system?
37 How do the disturbances affect the open loop irrigation system?
38 How does a human operation do to offset the unwanted disturbances?
39 What does a driver need to do to maintain a constant speed over different types of terrain?
40 In what condition, is the use of open loop control systems adequate?
41 What makes open loop control inadequate for some industrial applications?
42 What do the automatic closed loop systems perform?
43 What is the purpose of the feedback loop? And what actually is the role that human operators play in open loop control systems?
44 What makes the system in Figure 1-4 differ from that in Figure 1-3?
45 In Figure 1-4, what together do provide the feedback loop?
46 For the reservoir system in Figure 1-4, what happens if the water level in the tank goes up or down?
47 How can the desired level of water in the tank be adjusted?
48 Do feedback loops form the essential parts of most of the automated manufacturing systems?
49What is the capability that the automated manufacturing systems need to have?
III ELEMENT OF OPEN AND CLOSED-LOOP SYSTEM
III.1 READING
A block diagram of a closed loop control system is shown in Figure 1-5 Each block shows an element of the system that performs a significant function in the operation The lines between the block show the input and output signal of each element, and the arrowheads indicate the directions in which they follow
Trang 14This section describes the function of the blocks, their signals and common terminology used in a typical closed loop network:
Controlled variable The control variable is the actual variable being
monitored and maintained at a desired value in the manufacturing process Examples in a process control system may include temperature, pressure, and flow rate Examples in a motion control system may be position or velocity In the water reservoir system (Figure 1-4), the water level is the controlled variable Another term used is the process variable
Measured variable To monitor the status of the controlled variable, it must be
measured Therefore, the condition of the controlled variable at a specific point in time is referred as the measured variable Various methods are used to make measurements One method of determining a controlled variable such as the level of water, for example, is to measure the pressure at the bottom of a tank The pressure that represents the controlled variable is taken at the instant of measurement
Measurement device The measurement device is the “eye” of the system It
senses the measured variable and produces an output signal that represents the status of the controlled variable Examples in a process control system may include a thermocouple to measure temperature or a humidity detector to measure moisture Examples in a motion control system may be an optical device to measure position or a tachometer to measure rotational speed In the water reservoir system, the float is the measurement device Other terms used are detectors, transducers, and sensors
Feedback Signal The feedback signal is the output of the measurement
device In the water reservoir system, the feedback signal is the vertical position of member
A in the linkage mechanism (see Figure 1-4) Other terms used are measured value,
Trang 15measurement signal, or position feedback if in a position loop, or velocity feedback in a velocity loop
Setpoint The set point is the prescribed input value applied to the loop that
indicates the desired condition of the controlled variable The set point may be manually set
by a human operator, automatically set by an electronic device or programmed into a computer In the water reservoir system, the set point is determined by the position at which the float is placed along the road A Other terms used are command or reference
Error detector The error detector compares the set point to the feedback
signal It then produces an output signal that is proportional to the difference between them
In the water reservoir system, the error detector is the entire linkage mechanism Other terms used are comparator or comparer and summing junction
Error signal The error signal is the output of the error detector If the set point
and the feedback signal are not equal, an error signal proportional to their difference develops When the feedback and set point signal are equal, the error signal goes to zero In the reservoir system (Figure 1-4), the error signal is the angular position of member B of the linkage mechanism Other terms used are difference signal and deviation
Controller The controller is the “brain” of the system It receives the error
signal (for the closed loop control) as its input, and develops an output signal that causes the controlled variable to become the value specified by the set point Most controllers are operated electronically, although some of the older process controls use air pressure in pneumatic devices The operation of an electronic controller is performed by hardwired circuitry or computer software The controller produces a small electrical signal that usually need to conditioned or modified before it is sent to the next element For example, it must be amplified if it is applied to an electric motor, or connected to a proportional air pressure if it
is applied to a pneumatic positioner or a control valve The control function is often realized
by the programmable logic controllers (PLCs) and panel-mounted microprocessor controllers
Actuator The actuator is the “muscle” of the system It is a device that
physically alters some type of energy or fuel supply, causing the controlled variable to match the desired set point Examples of energy or fuel are the flow of steam, water, air, gas, or electrical current A practical application is a commercial bakery where the objective is to keep the temperature in an oven at 375 degrees The temperature is the controlled variable The temperature is determined by how much gas is fed to the oven burner A valve is the gas line controls the flow by the amount it opens or closes The valve is the actuator in the system In the reservoir system, the actuator is the flow control valve, connected to the inlet pipe Other term used are the final control element, or final correcting device Common types of actuator are louvers, hydraulic cylinders, pump, and motors
Manipulated variable The amount of fuel or energy that is physically altered
by the actuator is referred to as the manipulated variable The amount at which the manipulated variable is changed by the actuator affects the condition of the controlled variable In the commercial oven example, the gas is the manipulated, and the temperature is the controlled variable In the reservoir system flow is the manipulated variable The flow rate is altered by control valve (actuator), which affects the condition of the controlled variable (level)