自控简单极其入门PPT.ppt

Introduction to Control Systems,Elements of a Control System,Timothy J. Maloney Modern Industrial Electronics, Fourth Edition,Copyright 2001 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.,Elements of a control system,Information Gathering: input devices such as PBs, SSWs, limit switches, proximity switches, etc that provide information about a machine or process to the logic section. Decision Making: the logic section of a control system; originally this was relay control and is now the domain of programmable logic controllers (PLCs) Activating Devices: output devices such as pilot lights, solenoids, motors, contactors, etc that allow the control system to control or command a machine or process.,Discrete Versus Analog Devices,Manually Operated Switches,Manually operated switches are controlled by hand. Pushbutton switches are the most common form of manual control found in industry.,Normally Open (NO) pushbutton makes a circuit when it is pressed and returns to its open position when the button is released.,Manually Operated Switches,Normally Closed (NC) pushbutton opens the circuit when it is pressed and returns to the closed position when the button is released. The abbreviations NO and NC represent the state of the switch when it is not actuated.,Selector Switch,Selector switch positions are made by turning the operator knob not pushing it.,Off,Limit Switch Operation,Limit switches take the place of a human operator.,They are often used in the control of machine processes to govern the starting, stopping, or reversal of motors.,Temperature Switch,The temperature switch or thermostat is used to sense temperature changes and is actuated by some specific environmental temperature change.,Pressure Switch,Pressure switches are used to control the pressure of liquids and gases and are activated when a specific pressure is reached.,Level Switch,Level or float switches are used to sense the height of a liquid.,Opens or closes an electric circuit in response to a change in liquid level.,Switch,Float,Two-wire level switch control of starter.,Proximity Sensor,Proximity sensors or switches detect the presence of an object without making physical contact with it.,Proximity Sensor Connections,The method of connecting and exciting a proximity sensor varies with the type of sensor and its application. Proximity sensor must be compatible with the input module.,Proximity Sensor Connections,Load is connected between the sensor and ground,Current-sourcing output (PNP),Load,Main circuit,Sensor,Control output,Proximity Sensor Connections,Load is connected between the positive supply and sensor,Current-sinking output (NPN),Load,Main circuit,Sensor,Control output,Reflective Photoelectric Sensor,Emits a light beam (visible, infrared, or laser) from its light emitting element and detects the light being reflected.,Through-Beam Type Photoelectric Sensor,A through-beam photoelectric sensor is used to measure the change in light quantity caused by the targets crossing the optical axis.,Strain /Weight Sensors,A strain gauge transducer converts a mechanical strain into an electric signal.,Temperature Sensors,Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor, and IC sensor.,Thermocouple pair of dissimilar conductors fused together to form a measuring junction. Thermistor resistance varies with changes in temperature. Resistance Temperature Detector resistance varies with changes in temperature. Integrated Circuit Sensor change in temperature produces in changes in current or voltage.,Flow Measurement,The usual approach used in measuring fluid flow is to convert the kinetic energy that the fluid has into some other measurable form.,Flow Measurement,The usual approach used in measuring fluid flow is to convert the kinetic energy that the fluid has into some other measurable form.,Flow Measurement,The usual approach used in measuring fluid flow is to convert the kinetic energy that the fluid has into some other measurable form.,Velocity/RPM Sensors,A tachometer is a small permanent magnet DC generator which when rotated produces a voltage that is directly proportional to the speed at which it is driven.,Velocity/RPM Sensors,The rotating speed of a shaft is often measured using a magnetic (inductive) pickup sensor.,Output Control Devices,Actuators,An actuator is any device that converts an electrical signal into mechanical movement. The principle types of actuators are relays, solenoids, and motors.,The solenoid converts electric current into linear motion.,Solenoid Valve,A solenoid valve is a combination of: a solenoid with its core or plunger a valve body containing an orifice in which a disc or plug is positioned to restrict or allow flow,Electromagnetic Relay Operation,An electromagnetic relay is a magnetic switch. It uses electromagnetism to switch contacts.,A relay will usually have only one coil but may have any number of different contacts.,Electromagnetic Relay Operation,With no current flow through the coil (coil de-energized), the armature is held away from the core by spring tension.,When the coil is energized, the electromagnetic field moves the armature causing the contact points of the relay to open or close.,Contacts are open when no current flows through the coil but close as soon as the coil is energized.,Contacts are closed when no current flows through the coil but open as soon as the coil is energized.,Magnetic Contactor,A contactor is a special type of relay designed to handle heavy power loads that are beyond the capability of control relays.,Contactors are designed to operate such loads as lights, heaters, transformers, capacitors, and electric motors for which overload protection is provided separately or not required.,Seal-In Circuits,A seal-in circuit is a method of maintaining current flow after a momentary switch has been pressed and released.,Stop,Start,Motor starter coil,M,M,Seal-in contact,Hardwired Seal-In Circuit,The seal-in auxiliary contact of the starter is connected in parallel with the Start button to keep the starter coil energized when the Start button is released.,Control System Documentation - Hardware,Electrical prints line drawings showing the complete functional wiring of a machine or process from bus drop through field devices; includes control panel layout diagrams and interfaces among devices (networks),Example of Electrical Drawing from SD-107743 Welder,Elements of a control system,Bus drop 380 V, 3 from the plant electrical bus Disc