Unit V. Distributed control system

Distributed Control Systems (DCSs) are dedicated systems used to control manufacturing processes that are continuous or batch-oriented, such as oil refining, petrochemicals, central station power generation, pharmaceuticals, food & beverage manufacturing, cement production, steelmaking, and papermaking. DCSs are connected to sensors and actuators and use setpoint control to control the flow of material through the plant.

The most common example is a setpoint control loop consisting of a pressure sensor, controller, and control valve. Pressure or flow measurements are transmitted to the controller, usually through the aid of a signal conditioning Input/Output (I/O) device. When the measured variable reaches a certain point, the controller instructs a valve or actuation device to open or close until the fluidic flow process reaches the desired setpoint.

A typical DCS consists of functionally and/or geographically distributed digital controllers capable of executing from 1 to 256 or more regulatory control loops in one control box. The input/output devices (I/O) can be integral with the controller or located remotely via a field network. Today’s controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative (PID) control, can generally perform logic and sequential control.

Early minicomputers were used in the control of industrial processes since the beginning of the 1960s. The DCS was introduced in 1975. Both Honeywell and Japanese electrical engineering firm Yokogawa introduced their own independently produced DCSs at roughly the same time. The DCS brought distributed intelligence to the plant and established the presence of computers and microprocessors in process control, but it still did not provide the reach and openness necessary to unify plant resource requirements.

In many cases, the DCS was merely a digital replacement of the same functionality provided by analog controllers and a panelboard display. In the 1980s, users began to look at DCSs as more than just basic process control. A very early example of a Direct Digital Control DCS was completed by the Australian business Midac in 1981-1982 using R-Tec Australian designed hardware. The system installed at the University of Melbourne used a serial communications network, connecting campus buildings back to a control room "front end". Each remote unit ran 2 Z80 microprocessors whilst the front end ran 11 in a Parallel Processing configuration with paged common memory to share tasks and could run up to 20,000 concurrent controls objects.

It was believed that if openness could be achieved and greater amounts of data could be shared throughout the enterprise that even greater things could be achieved. The first attempts to increase the openness of DCSs resulted in the adoption of the predominant operating system of the day: UNIX. UNIX and its companion networking technology TCP-IP were developed by the Department of Defense for openness, which was precisely the issue the process industries were looking to resolve.

The drive toward openness in the 1980s gained momentum through the 1990s with the increased adoption of Commercial-Off-The-Shelf (COTS) components and IT standards. Probably the biggest transition undertaken during this time was the move from the UNIX operating system to the Windows environment. While the realm of the real time operating system (RTOS) for control applications remains dominated by real time commercial variants of UNIX or proprietary operating systems, everything above real-time control has made the transition to Windows.

 

Task I. Learn the following words by heart.

dedicated systems – специализированные системы

manufacturing process – производственный процесс

valve – клапан

distributed intelligence - распределённые средства искусственного интеллекта,

распределённые логические функции

panelboard – пульт управления

 

Task II. True or false?

1. DCSs control the flow of material through the plant.

2. Modern controllers have extensive computational capabilities.

3. DCS was first introduced in 1960.

4. In the 1980s DCSs functioned as basic process control systems.

5. The input/output devices (I/O) can be located remotely via a field network

6. Pressure or flow measurements are transmitted to the controller through the aid of a signal conditioning Input/Output (I/O) device

 

Task III. Answer the following questions

1. Where are DCSs used?

2. What does a typical DCSs consist of?

3. How does a typical DCS function?

4. When did the first DCSs appear?

5. What is the role of DCSs in process control?

6. What operating systems can run DCSs?

 

 

Task IV. Render the text into Russian

PID controller

A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism widely used in industrial control systems. A PID controller attempts to correct the error between a measured process variable and a desired setpoint by calculating and then outputting a corrective action that can adjust the process accordingly.

The PID controller calculation (algorithm) involves three separate parameters; the Proportional, the Integral and Derivative values. The Proportional value determines the reaction to the current error, the Integral value determines the reaction based on the sum of recent errors, and the Derivative value determines the reaction to the rate at which the error has been changing. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve or the power supply of a heating element.

 

Task V. Read the text, find key words, look them up in the dictionary, and render the text into English.

Диспетчерское управление и сбор данных.

Под термином SCADA (Supervisory Control And Data Acquisition) понимают инструментальную программу для разработки программного обеспечения систем управления технологическими процессами в реальном времени и сбора данных. Термин SCADA эволюционировал вместе с развитием технологий автоматизации и управления технологическими процессами. В 80-е годы под SCADA-системами понимали любые программно-аппаратные комплексы сбора данных реального времени. В 90-х годах термин SCADA больше используется для обозначения только программной части человеко-машинного интерфейса АСУ ТП.

SCADA-системы позволяют разрабатывать АСУ ТП в клиент-серверной или в распределенной архитектуре (DCS сокр. от англ. Distributed Control System — распределённая система управления). Иногда SCADA-системы комплектуются дополнительным ПО для программирования промышленных контроллеров. Такие SCADA-системы называются интегрированными и к ним добавляют термин SoftLogiс.

 

Unit VI. Controllers

 

In control theory, a controller is a device which monitors and affects the operational conditions of a given dynamical system. The operational conditions output variables of the system which can be affected by adjusting certain input variables. For example, the heating system of a house can be equipped with a thermostat (controller) for sensing air temperature (output variable) which can turn on or off a furnace or heater when the air temperature becomes too low or too high.

In this example, the thermostat is the controller and directs the activities of the heater. The heater is the processor that warms the air inside the house to the desired temperature (setpoint). The air temperature reading inside the house is the feedback. And finally, the house is the environment in which the heating system operates. In the natural world, individual organisms also appear to be equipped with controllers that assure the homeostasis necessary for survival of each individual.

In control theory there are two basic types of control. These are feedforward and feedback. The thermostat of a house is an example of a feedback controller. This controller relies on measuring the controlled variable, in this case the temperature of the house, and then adjusting the output, whether or not the heater is on. Feedforward control can avoid the slowness of feedback control. With feedforward control, the disturbances are measured and accounted for before they have time to affect the system. In the house example, a feedforward system may measure the fact that the door is opened and automatically turn on the heater before the house can get too cold.

Some examples of where feedback and feedforward control can be used together are dead-time compensation, and inverse response compensation. Dead-time compensation is used to control devices that take a long time to show any change to a change in input, for example, change in composition of flow through a long pipe. A dead-time compensation control uses an element (also called a Smith predictor) to predict how changes made now by the controller will affect the controlled variable in the future.

The controlled variable is also measured and used in feedback control. Inverse response compensation involves controlling systems where a change at first affects the measured variable one way but later affects it in the opposite way. An example would be eating candy. At first it will give you lots of energy, but later you will be very tired. As can be imagined, it is difficult to control this system with feedback alone, therefore a predictive feedforward element is necessary to predict the reverse effect that a change will have in the future.

Most control valve systems in the past were implemented using mechanical systems or solid state electronics. Pneumatics was often utilized to transmit information and control using pressure. However, most modern control systems in industrial settings now rely on computers for the controller. Obviously it is much easier to implement complex control algorithms on a computer than using a mechanical system. For feedback controllers there are a few simple types. The most simple is like the thermostat that just turns the heat on if the temperature falls below a certain value and off it exceeds a certain value (on-off control).

Another simple type of controller is a proportional controller. With this type of controller, the controller output (control action) is proportional to the error in the measured variable. Alternates to proportional control are proportional-integral (PI) control and proportional-integral-derivative (PID) control. PID control is commonly used to implement closed-loop control. Open-loop control can be used in systems sufficiently well-characterized as to predict what outputs will necessarily achieve the desired states. For example, the rotational velocity of an electric motor may be well enough characterized for the supplied voltage to make feedback unnecessary.

 

Task I. Learn the following words by heart.

operational conditions - рабочие условия, условия эксплуатации,

эксплуатационный режим, рабочий режим

output variable - выходная переменная

input variable - входная переменная

feedforward control - управление с прогнозированием

feedback control - управление с обратной связью

 

Task II. True or false?

1. Controller affects the operational conditions of a given dynamical system.

2. The thermostat of a house is an example of a feedforward controller.

3. Feedforward control can avoid the rapidity of feedback control

4. Most control valve systems in the past were implemented using computer systems.

5. Most modern control systems in industrial settings now rely on computers.

6. Open-loop control can be used to predict what outputs will necessarily achieve the desired states.

 

Task III. Answer the following questions

1. What is a controller?

2. What are basic types of control?

3. What is the difference between feedforward and feedback control?

4. Are organisms in natural world equipped with controllers?

5. Where was pneumatics utilized?

6. What kind of controllers do you know?

 

Task IV. Render the text into Russian

Open-loop controller

An open-loop controller, also called a non-feedback controller, is a type of controller which computes its input into a system using only the current state and its model of the system. For example, an irrigation sprinkler system, programmed to turn on at set times could be an example of an open-loop system if it does not measure soil moisture as a form of feedback. Even if rain is pouring down on the lawn, the sprinkler system would activate on schedule, wasting water.

Open-loop control is useful for well-defined systems where the relationship between input and the resultant state can be modeled by a mathematical formula. An open-loop controller is often used in simple processes because of its simplicity and low-cost, especially in systems where feedback is not critical. A typical example would be a conventional washing machine, for which the length of machine wash time is entirely dependent on the judgment and estimation of the human operator.

 

Task V. Read the text, find key words, look them up in the dictionary, and render the text into English.

Микроконтроллер

Микроконтроллер — микросхема, предназначенная для управления электронными устройствами. Типичный микроконтроллер сочетает в себе функции процессора и периферийных устройств, может содержать ОЗУ и ПЗУ. По сути, это однокристальный компьютер, способный выполнять простые задачи.. Микроконтроллеры являются основой для построения встраиваемых систем, их можно встретить во многих современных приборах, таких, как телефоны, стиральные машины и т. п. Бо́льшая часть выпускаемых в мире процессоров — микроконтроллеры.

Программирование микроконтроллеров обычно осуществляется на языке ассемблера или Си, хотя существуют компиляторы для других языков, например Форта используются также встроенные интерпретаторы Бейсика. Для отладки программ используются программные симуляторы (специальные программы для персональных компьютеров, имитирующие работу микроконтроллера), внутрисхемные эмуляторы (электронные устройства, имитирующие микроконтроллер, которые можно подключить вместо него к разрабатываемому встроенному устройству) и интерфейс JTAG.