Beginner guide (986746), страница 2
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Ittakes any form and varies in scale of implementation, from a power plant to a semi-conductormachine. As a result of rapid advancement of technology, complicated control tasks accomplishedwith a highly automated control system, which may be in the form of Programmable Controller(PLC) & possibly a host computer, etc. Besides signal interfacing to the field devices (such asoperator panel, motors, sensors, switches, solenoid valves and etc.), capabilities in networkcommunication enable a big scale implementation and process co-ordination besides providinggreater flexibility in realizing distributed control system. Every single component in a controlsystem plays an important role regardless of size.
For instance, as shown in Fig 1.1 the PLC wouldnot know the happenings around it without any sensing devices. And if necessary, an area hostcomputer has to be in place to co-ordinate the activities in a specific area at the shopfloor.Process ControlComputerArea HostComputerNetworkTouch PanelTouch PanelBZLimit switchSensorMotorSolenoidLightsBuzzerControl ComponentsFig. 1.1:It could also be an application as small as a single PLC controlling a single or some output devices.1-11.INTRODUCTIONMechanicalstructureRobot ArmPushbutton &Selector SwitchRevolvingLightIndicatorsFig.
1.2 : Gantry Robot Control System(Courtesy of Gintic)SwitchesPLCRelaysSensorqMagneticContactorTerminalBlockTypical Programmable Logic Controller-base Control SystemFig 1.2, is a typical application of a Gantry Robot Control Machine. It is used in a pick and placeoperation. The whole process sequence is controlled by a PLC. The various input devices such asselector switches, push buttons, togle switches, sensors are connected to the input of the PLC via theinput terminal block. The output devices such as the revolving light, indicators, relays, contactorsand solenoid valves are connected to the output terminals of the PLC. The whole process iscontrolled by a ladder program loaded into the PLC CPU memory.
The program will execute asequence automatically according to the pre-defined sequence of operations. Manual operation arealso provided to allow operator to activate the machine manually by the switches, emergency pushbutton for the purpose of safety in case you need to stop the operation abruptly. In this application,the control system operates as a stand-alone operation1-21.qINTRODUCTIONThe Role of the Programmable Controllers (PLC)In an automated system, the PLC is commonly regarded as the heart of the control system. With acontrol application program (stored within the PLC memory) in execution, the PLC constantlymonitors the state of the system through the field input devices’ feedback signal.
It will then basedon the program logic to determine the course of action to be carried out at the field output devices.The PLC may be used to control a simple and repetitive task, or a few of them may beinterconnected together with other host controllers or host computers through a sort ofcommunication network, in order to integrate the control of a complex process.Input DevicesIntelligence of an automated system is greatly depending on the ability of a PLC to read in thesignal from various types of automatic sensing and manual input field devices.Push-buttons, keypad and toggle switches, which form the basic man-machine interface, are types ofmanual input device.
On the other hand, for detection of workpiece, monitoring of movingmechanism, checking on pressure and or liquid level and many others, the PLC will have to tap thesignal from the specific automatic sensing devices like proximity switch, limit switch, photoelectricsensor, level sensor and so on. Types of input signal to the PLC would be of ON/OFF logic oranalogue. These input signals are interfaced to PLC through various types of PLC input module.INPUTLimit SwitchRotary EncoderTimerPhotoelectricSwitchProximity Switch1-31.INTRODUCTIONOutput DevicesAn automatic system is incomplete and the PLC system is virtually paralysed without means ofinterface to the field output devices.
Some of the most commonly controlled devices are motors,solenoids, relays indicators, buzzers and etc. Through activation of motors and solenoids the PLCcan control from a simple pick and place system to a much complex servo positioning system. Thesetype of output devices are the mechanism of an automated system and so its direct effect on thesystem performance.However, other output devices such as the pilot lamp, buzzers and alarms are merely meant fornotifying purpose. Like input signal interfacing, signal from output devices are interfaced to thePLC through the wide range of PLC output module.1-41.qINTRODUCTIONWhat is a Programmable Controller ?A PLC consists of a Central Processing Unit (CPU) containing an application program and Inputand Output Interface modules, which is directly connected to the field I/O devices.
The programcontrols the PLC so that when an input signal from an input device turns ON, the appropriateresponse is made. The response normally involves turning ON an output signal to some sort ofoutput devices.Power SupplySignalsfromSwitchesSensoretcInputInterfaceCentralProcessingUnit (CPU)OutputInterfaceMemorySignalstoSolenoidsMotorsetcFig. 1.3: Block Diagram of PLCCentral Processing UnitThe Central Processing Unit (CPU) is a microprocessor that co-ordinates the activities of the PLCsystem. It executes the program, processes I/O signals & communicates with external devices.MemoryThere are various types of memory unit.
It is the area that hold the operating system and usermemory. The operating system is actually a system software that co-ordinates the PLC. Ladderprogram, Timer and Counter Values are stored in the user memory. Depending on user’s need,various types of memory are available for choice:(a) Read –Only Memory (ROM)ROM is a non-volatile memory that can be programmed only once. It is therefore unsuitable.
It isleast popular as compared with others memory type.(b) Random Access Memory (RAM)RAM is commonly used memory type for storing the user program and data. The data in the volatileRAM would normally be lost if the power source is removed. However, this problem is solved bybacking up the RAM with a battery.1-51.INTRODUCTION(c) Erasable Programmable Read Only Memory (EPROM)EPROM holds data permanently just like ROM. It dose not require battery backup.
However, itscontent can be erased by exposing it to ultraviolet light. A prom writer is required to reprogram thememory.(d) Electrically Erasable Programmable Read Only Memory (EEPROM)EEPROM combines the access flexibility of RAM and the non-volatility of EPROM in one. Itscontents can be erased and reprogrammed electrically, however, to a limit number of times.1-61.INTRODUCTIONSCAN TIMEThe process of reading the inputs, executing the program and updating the outputs is known as scan.The scan time is normally a continuous and sequential process of reading the status of inputs,evaluating the control logic and updating outputs. Scan time specification indicates how fast thecontroller can react to the field inputs and correctly solve the control logic.PROGRAM011000I/O Update1000PROGRAM SCAN10002TIM0#50Factors influencing Scan TimeThe time required to make a single scan (scan time) varies from 0.1 ms to tens of ms depending onits CPU processing speed and the length of the user program.
The user of remote I/O subsystemsincreases the scan time as a result of having to transmit the I/O updates to remote subsystem.Monitoring of the control program also adds overhead time to the scan because the controller’s CPUhas to send the status of coils and contacts to the CRT or other monitoring device.1-71.qINTRODUCTIONConventional Control Panel and Its DifficultiesIn the beginning of the Industrial revolution, especially in the 1960 & 1970, automated machineswere controlled by electromechanical relays. These relays were all hardwired together inside thecontrol panel. In some cases, the control panel was so huge that it could cover the entire wall. Everyconnections in the relay logic must be connected.
Wiring is not always perfect, it takes time totroubleshoot the system. This is a very time consuming affair. On top of that, the relays have limitedcontacts. If modification is required, the machine has to be stopped, space may not available andwiring has to be traced to accommodate changes. The control panel can only be used for thatparticular process. It cannot be changed immediately to a new system. It has to be redone. In termsof maintenance, an electrician must be well trained and skillful in troubleshooting the controlsystem. In short, conventional relay control panel are very inflexible.A typical example of the conventional control panel is shown in Fig.
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