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PLC - Introduction What does PLC stand for? PLC - programmable logic controller PLC implements logic control functions by means of a program.

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Presentation on theme: "PLC - Introduction What does PLC stand for? PLC - programmable logic controller PLC implements logic control functions by means of a program."— Presentation transcript:

1 PLC - Introduction What does PLC stand for? PLC - programmable logic controller PLC implements logic control functions by means of a program

2 PLC - Introduction An application example 1: Gate Control PLC can sense a vehicle at the entrance or exit, and open and close the gate automatically The current vehicle count is easily determined by programming a simple counter

3 An application example 2: Conveyor System PLC can be used to start/stop latching logic for motor control Counters can be used for monitoring product amounts PLC - Introduction

4 Comparing traditional and programmable control systems - 1 PLC - Introduction

5 In traditional control, the switches S1, S2 and S3 must close for K1 to be turned on - the wiring makes the rule In 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 PLC - Introduction

6 How does a PLC differ from a computer? A computer is optimized for calculation and display tasks A computer is programmed by specialists A PLC is designed for (logic) control and regulation tasks A PLC is programmed by non-specialists A PLC is well adapted to industrial environment PLC - Introduction

7 Why are PLCs so common? They are cost-effective They are flexible, reliable and compact They have significant advantages over traditional control systems based on relay or pneumatics PLC - Introduction

8 Where are PLCs used? In every industry where automation is involved, from individual machines to whole processes PLC - Introduction

9 lecture note 9 PLC9 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 tasks

10 lecture note 9 PLC10 Inputs Outputs & Power Supply Communication Ports (RS-485)

11 lecture note 9 PLC11 Structure of a PLC Level of Liquid in Tank

12 The PLC processor

13 13 PLC Input/Output Devices

14 lecture note 9 PLC PLC Input Devices nPush buttons nSwitches (limit switches, level switches, etc.) nSensors n... PLC Output Devices nRelay contacts nSolenoid valves nSignal devices (such as lamps, alarms, etc.) nMotors n...

15 PLC (Programmable Logic Controller) Inputs Output Devices CRCR A solid state (electronic) device that controls output devices based on input signals and a user developed program. Originally developed to directly replace relays used for discrete control.

16 16 Understanding the PLC Operating Cycle House Keeping START Input Scan Program Scan Output Scan PLC OPERATING CYCLE The status of external inputs (terminal block voltage) is written to the Input image (“Input file”). Each 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”). The Output Image data is transferred to the external output circuits, turning the output devices ON or OFF. Internal checks on memory, speed and operation. Service any communication requests, etc.

17 17 What you must consider when selecting a PLC Inputs/Outputs – How many Inputs/Outputs? including embedded, local expansion, and networked I/O 10, 16, 20, 32, 138, 156, >256 – How many Discrete vs. Analog Type of I/O AC, DC, Analog, Thermocouple, sourcing, sinking, etc. Communications Networks DF1 Full Duplex, DF1 Half Duplex, DF1 Radio Modem, DH485, ModBus Master / Slave DeviceNet, Ethernet Functions required – PID – PTO/PWM ( Pulse Train Output/Pulse Width Modulated ) – Data Logging – Messaging between PLC’s – Math Calculations Memory Size 1k, 6k, 8k 12k, 14k,

18 Ladder diagrams are specialized schematics commonly used to document industrial control logic systems They 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,

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20 Digital Logic Functions OR AND

21 NOT XOR

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 start e.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, including high 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 contact is wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened

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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

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28 Can you explain following ladder diagrams ?

29 Product packaging is one of the most frequent cases for automation in industry Industry Application:

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32 Cxxx is the name of the counter yyyyy is the number of pulses we want to count before doing something Counters

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34 :. Timers

35 An accumulating timer

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 M 1 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 M 2 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 system will work fine, so long as no one pushes both buttons at the same time BUT If someone were to do that ???? To prevent this occurrence from happening, use interlocking…. Try………………

39 when M 1 is energized, the normally-closed auxiliary contact on the second rung will be open, thus preventing M 2 from being energized, even if the "Reverse" pushbutton is actuated. Likewise, M 1 's energization is prevented when M 2 is energized

40 Explain this…. When the "Forward" pushbutton is actuated, M 1 will energize, closing the normally- open auxiliary contact in parallel with that switch. When the pushbutton is released, the closed M 1 auxiliary contact will maintain current to the coil of M 1, thus latching the "Forward" circuit in the "on" state. The same sort of thing will happen when the "Reverse" pushbutton is pressed.

41 Exercise: Explain the following ladder diagram

42 Answer: If the motor has been running in the forward direction, both M 1 and TD 1 will have been energized. This being the case, the normally-closed, timed-closed contact of T D1 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 rev e rse pushbutton circuit open for the duration so M2 can't be e nergized. 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, T D 2 will prevent the "Forward" p u shbutton from energizing M1 until the prescribed time delay aft e r M2 (and TD2) have been de-energized.

43 43 Leading Brands Of PLC AMERICAN1. Allen Bradley 2. Gould Modicon 3. Texas Instruments 4. General Electric 5. Westinghouse 6. Cutter Hammer 7. Square D EUROPEAN1. Siemens 2. Klockner & Mouller 3. Festo 4. Telemechanique JAPANESE1. Toshiba 2. Omron 3. Fanuc 4. Mitsubishi

44 44 PLC Size 1. 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. 2.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.

45 45 Discrete Input A 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 Pushbutton Normally Closed Pushbutton Normally Open switch Normally Closed switch Normally Open contact Normally closed contact


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