Presentation on theme: "PLC - Introduction What does PLC stand for?"— Presentation transcript:
1 PLC - Introduction What does PLC stand for? PLC - programmable logic controllerPLC implements logic control functions by means of a program
2 PLC - IntroductionAn application example 1: Gate ControlPLC can sense a vehicle at the entrance or exit, and open and close the gate automaticallyThe current vehicle count is easily determined by programming a simple counter
3 PLC - IntroductionAn application example 2: Conveyor SystemPLC can be used to start/stop latching logic for motor controlCounters can be used for monitoring product amounts
4 PLC - IntroductionComparing traditional and programmable control systems - 1
5 PLC - IntroductionIn traditional control, the switches S1, S2 and S3 must close for K1 to be turned on - the wiring makes the ruleIn PLC systems, the program is written to perform the logic “when S1 is closed AND S2 is closed AND S3 is closed, THEN turn on K1” - the program makes the rule
6 How does a PLC differ from a computer? PLC - IntroductionHow does a PLC differ from a computer?A computer is optimized for calculation and display tasksA computer is programmed by specialistsA PLC is designed for (logic) control and regulation tasksA PLC is programmed by non-specialistsA PLC is well adapted to industrial environment
7 PLC - IntroductionWhy are PLCs so common?They are cost-effectiveThey are flexible, reliable and compactThey have significant advantages over traditional control systems based on relay or pneumatics
8 PLC - IntroductionWhere are PLCs used?In every industry where automation is involved, from individual machines to whole processes
9 What tasks do PLCs perform? The logic control tasks such as interlocking, sequencing, timing and counting (previously undertaken with relays or pneumatics)In addition, PLCs can perform a variety of calculation, communication and monitoring taskslecture note 9 PLC
10 Outputs & Power Supply Communication Ports (RS-485) Inputs lecture note 9 PLC
11 Structure of a PLCLevel of Liquidin Tanklecture note 9 PLC
15 A solid state (electronic) device that controls output devices based on input signalsand a user developed program.Originally developed to directly replace relays used for discrete control.InputsOutput DevicesCRPLC(Programmable Logic Controller)What is a PLC? A solid state device that controls output devices based on input signals and a user developed program.Output Device: Any device or equipment that I want to control, that I want to turn on or off, that I want to open or close, that I want to show indication of a situation.Input Device: any equipment that gives the PLC information about the current state of the system (ie. Temperature, speed, position). Input device is also the equipment that an operator uses to tell the PLC what to do.PLC Basics - Automation Fair 2006
16 Understanding the PLC Operating Cycle Input ScanSTARTThe status of external inputs (terminal block voltage) is written to the Input image (“Input file”).House KeepingInternal checks on memory, speed and operation. Service any communication requests, etc.PLC OPERATINGCYCLEProgram ScanEach ladder rung is scanned using the data in the Input file. The resulting status (Logic being solved) is written to the Output file (“Output Image”).Output ScanThe Output Image data is transferred to the external output circuits, turning the output devices ON or OFF.PLC Basics - Automation Fair 2006
17 What you must consider when selecting a PLC Inputs/OutputsHow many Inputs/Outputs? including embedded, local expansion, and networked I/O10, 16, 20, 32, 138, 156, >256How many Discrete vs. AnalogType of I/OAC, DC, Analog, Thermocouple, sourcing, sinking, etc.Communications NetworksDF1 Full Duplex, DF1 Half Duplex, DF1 Radio Modem, DH485, ModBus Master / SlaveDeviceNet, EthernetFunctions requiredPIDPTO/PWM (Pulse Train Output/Pulse Width Modulated)Data LoggingMessaging between PLC’sMath CalculationsMemory Size1k, 6k, 8k 12k, 14k,PLC Basics - Automation Fair 2006
18 Ladder diagrams are specialized schematics commonly used to document industrial control logic systemsThey are called "ladder" diagrams because they resemble a ladder, with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are control circuits to represent.The "L1" and "L2" designations refer to the two poles of a 120 VAC supply,
22 Permissive and interlock circuits A practical application of switch and relay logic is in control systems where several process conditions have to be met before a piece of equipment is allowed to starte.g. 01.A burner control for large combustion furnaces.In order for the burners in a large furnace to be started safely, the control system requests "permission" from several process switches, includinghigh and low fuel pressure,air fan flow check,exhaust stack damper position, access door position, etc.Each process condition is called a permissive, and each permissive switch contactis wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened
24 e.g. 02.Application of relay logic is in control systems where we want to ensure two incompatible events cannot occur at the same time.An example of this is in reversible motor control, where two motor contactors are wired to switch polarity (or phase sequence) to an electric motor, and we don't want the forward and reverse
25 Take note of the normally-closed "OL" contact, which is the thermal overload contact activated by the "heater" elements wired in series with each phase of the AC motor. If the heaters get too hot, the contact will change from its normal (closed) state to being open, which will prevent either contactor from energizing.This control system will work fine, so long as no one pushes both buttons at the same time. If someone were to do that, phases A and B would be short-circuited together by virtue of the fact that contactor M1 sends phases A and B straight to the motor and contactor M2 reverses them; phase A would be shorted to phase B and visa-versa.Obviously, this is a bad control system design!
26 To prevent this occurrence from happening, we can design the circuit so that the energization of one contactor prevents the energization of the other. This is called interlocking, and it is accomplished through the use of auxiliary contacts on each contactor
36 Case1:Ensure two incompatible events cannot occur at the same time.An example of this is in reversible motor control, where two motor contactors are wired to switch polarity (or phase sequence) to an electric motor, and we don't want the forward and reverse contactors energized simultaneously
37 When contactor M1 is energized, the 3 phases (A, B, and C) are connected directly to terminals 1, 2, and 3 of the motor, respectively.However, when contactor M2 is energized, phases A and B are reversed, A going to motor terminal 2 and B going to motor terminal 1.This reversal of phase wires results in the motor spinning the opposite direction. Let's examine the control circuit for these two contactors:Take note of the normally-closed "OL" contact, which is the thermal overload contact activated by the "heater" elements wired in series with each phase of the AC motor. If the heaters get too hot, the contact will change from its normal (closed) state to being open, which will prevent either contactor from energizing.
38 BUT If someone were to do that ???? system will work fine, so long as no one pushes both buttons at the same timeBUT If someone were to do that ????To prevent this occurrence from happening, use interlocking….Try………………
39 when M1 is energized, the normally-closed auxiliary contact on the second rung will be open, thus preventing M2 from being energized, even if the "Reverse" pushbutton is actuated. Likewise, M1's energization is prevented when M2 is energized
40 Explain this….When the "Forward" pushbutton is actuated, M1 will energize, closing the normally-open auxiliary contact in parallel with that switch.When the pushbutton is released, the closed M1 auxiliary contact will maintain current to the coil of M1, thus latching the "Forward" circuit in the "on" state. The same sort of thing will happen when the "Reverse" pushbutton is pressed.
42 Answer:If the motor has been running in the forward direction, both M1 and TD1 will have been energized.This being the case, the normally-closed, timed-closed contact of TD1 between wires 8 and 5 will have immediately opened the moment TD1 was energized.When the stop button is pressed, contact TD1 waits for the specified amount of time before returning to its normally-closed state, thus holding the reverse pushbutton circuit open for the duration so M2 can't be energized.When TD1 times out, the contact will close and the circuit will allow M2 to be energized, if the reverse pushbutton is pressed. In like manner, TD2 will prevent the "Forward" pushbutton from energizing M1 until the prescribed time delay after M2 (and TD2) have been de-energized.
43 Leading Brands Of PLC AMERICAN 1. Allen Bradley 2. Gould Modicon 3. Texas Instruments4. General Electric5. Westinghouse6. Cutter Hammer7. Square DEUROPEAN 1. Siemens2. Klockner & Mouller3. Festo4. TelemechaniqueJAPANESE 1. Toshiba2. Omron3. Fanuc4. Mitsubishi43
44 PLC Size1. SMALL - it covers units with up to 128 I/O’s and memories up to 2 Kbytes.- these PLC’s are capable of providing simple to advance levels or machine controls.MEDIUM - have up to 2048 I/O’s and memories up to 32 Kbytes.3. LARGE - the most sophisticated units of the PLC family. They have up to 8192 I/O’s and memories up to 750 Kbytes.- can control individual production processes or entire plant.44
45 Normally Open Pushbutton Discrete InputA discrete input also referred as digital input is an input that is either ON or OFF are connected to the PLC digital input. In the ON condition it is referred to as logic 1 or a logic high and in the OFF condition maybe referred to as logic 0 or logic low.Normally Open PushbuttonNormally Closed PushbuttonNormally Open switchNormally Closed switchNormally Open contactNormally closed contact