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Controle de Processos Automaticos
Tipologia: Notas de estudo
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Copyright (c) 1993-2003 Hugh Jack ([email protected]).
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".
This document is provided as-is with no warranty, implied or otherwise. There have been attempts to eliminate errors from this document, but there is no doubt that errors remain. As a result, the author does not assume any responsibility for errors and omissions, or damages resulting from the use of the information pro- vided.
Additional materials and updates for this work will be available at http://clay- more.engineer.gvsu.edu/~jackh/books.html
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PREFACE
Control systems apply artificial means to change the behavior of a system. The type of control problem often determines the type of control system that can be used. Each controller will be designed to meet a specific objective. The major types of control are shown in Figure 1.1.
Figure 1.1 Control Dichotomy
e.g. PID
e.g. MRAC
e.g. FUZZY LOGIC
e.g. TIMERS
e.g. COUNTERS
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tougher, and the problem becomes non-linear. e.g. We are driving the perfect car with no friction, with no drag, and can predict how it will work perfectly.
The difference between these control systems can be emphasized by considering a simple elevator. An elevator is a car that travels between floors, stopping at precise heights. There are certain logical constraints used for safety and convenience. The points below emphasize different types of control problems in the elevator.
Logical:
Logical and sequential control is preferred for system design. These systems are more stable, and often lower cost. Most continuous systems can be controlled logically. But, some times we will encounter a system that must be controlled continuously. When this occurs the control system design becomes more demanding. When improperly con- trolled, continuous systems may be unstable and become dangerous.
When a system is well behaved we say it is self regulating. These systems don’t need to be closely monitored, and we use open loop control. An open loop controller will set a desired position for a system, but no sensors are used to verify the position. When a system must be constantly monitored and the control output adjusted we say it is closed loop. A cruise control in a car is an excellent example. This will monitor the actual speed of a car, and adjust the speed to meet a set target speed.
Many control technologies are available for control. Early control systems relied upon mechanisms and electronics to build controlled. Most modern controllers use a com-
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This book supports Allen Bradley micrologix, PLC-5s, SLC500 series
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2. PROGRAMMABLE LOGIC CONTROLLERS
Control engineering has evolved over time. In the past humans were the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent rev- olution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.
PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer.
Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay
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