Chapter 2Chapter 2

FUNDAMENTALS OF PLC FUNDAMENTALS OF PLC PROGRAMMINGPROGRAMMING

Industrial Electronics

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Ladder Diagram

Ladder diagrams are specialized schematics commonly used to document industrial control logic systems.

It is a language or a method to create a program. The first PLCs were programmed with a technique that was based on relay logic wiring schematics.

Basic concept of this diagram is similar to the electrical wiring. However in ladder logic, the symbol has changed and been standardized.

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Ladder Diagram cont..

Type of graphic language automatic control systems.

A drawing program of a switching circuit.

Called "ladder" diagrams because they resemble a ladder, with two vertical lines representing the power trails (supply power) and circuits are connected in horizontal lines called rungs. Connection between the element in the rung called link.

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Ladder Diagram Operational Principle

Referring to figure below, between these two rails, a horizontal straight line was drawn with two symbols. These two symbols refer to the input and output devices, which are used in the actual process/system.

On the left, we put all kinds of input. While on the right, we place all types of the outputs.

Once we complete one line of the program it seems like a ladder. This horizontal line which places the input and output make one rung.

Input

Interface Output

+24V -0V

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Ladder Diagram Symbols Ladder diagram uses standard symbols to

represent the circuit components and functions found in a control system.

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Normally Open Schematics (NO)-Input Schematics

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Normally Closed Schematics (NC)-Input Schematics

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Output Schematics

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Logic Function

There are 5 logic functions can be developed by combination of switches

1. AND2. OR3. NOR4. NAND5. EX-OR (XOR)

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Logic Function…cont

1. AND logiccoils is not energized unless both switch A and B closed. Fig. (b) represent for PLC input.

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Logic Function…cont - AND

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Logic Function…cont

2. OR logicCoils is not energized until either A or B closed

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Logic Function…cont - OR DeMorgan's Theorem would predict:

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Logic Function…cont

3. NOR logicSince there has to be an output when neither A nor B have an input, and when there is an input either A or B.

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Logic Function…cont - NOR

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Logic Function…cont

4. NAND logicThere is no output when both A and B have an input

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Logic Function…cont - NAND

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Logic Function…cont - NOT

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Logic Function…cont

5. XOR logicThere is no output when both input 1 and 2 given.

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Combination of AND, OR, and inverter (NOT) gates

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Control CircuitStart stop circuit

-PB is pressed, output IR is activated-IR contact will closed-When PB is depressed, the output IR is still activated since current can go through contact IR unless is disturbed by stop button.

-PB is pressed, output IR is activated-IR contact will closed-When PB is depressed, the output IR is still activated since current can go through contact IR unless is disturbed by stop button.

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Vibrating/Jog circuit

-When, PB is pressed, output IR is activated.

-But after several microsecond, the normally closed IR is open then will stop the output IR.

-This applicable to move an object which need to locate at precise location.

-When, PB is pressed, output IR is activated.

-But after several microsecond, the normally closed IR is open then will stop the output IR.

-This applicable to move an object which need to locate at precise location.

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Overload protection device

There are several overload protection device:1) TOR: Thermal Overload Relay -excess heat2) CB: Circuit Breaker -excess current3) Fuse -excess current

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Control circuit has Overload Protection Device

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Control circuit in ladder diagram.

-When PB is pushed(forward/reverse),motor will move according to the

command.

-But, when there is an overcurrent,CB will stop the current flow by blow the fuse.

-Means, NC of CB will open and stop the current flow to the circuit.

-The circuit can be recovered by change the fuse.

-If there is thermal overload, TOR will cut the current flow.

-Lamp,L3 will turn on and TOR need to be reset.

-When PB is pushed(forward/reverse),motor will move according to the

command.

-But, when there is an overcurrent,CB will stop the current flow by blow the fuse.

-Means, NC of CB will open and stop the current flow to the circuit.

-The circuit can be recovered by change the fuse.

-If there is thermal overload, TOR will cut the current flow.

-Lamp,L3 will turn on and TOR need to be reset.

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Two wire control

-only has one contact for switching device either ON or OFF state.-When ON state, M will energize and activate three phase motor.-Heater will heat if there is over current and open the circuit.

-only has one contact for switching device either ON or OFF state.-When ON state, M will energize and activate three phase motor.-Heater will heat if there is over current and open the circuit.

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Three wire control

-the connection almost the same as two wire circuit except has an extra set of contact connected in parallel as the pilot switch.

-this extra contact provide extra third wire.

-this auxiliary contact keep coil M energize even after start push button release.

-the connection almost the same as two wire circuit except has an extra set of contact connected in parallel as the pilot switch.

-this extra contact provide extra third wire.

-this auxiliary contact keep coil M energize even after start push button release.

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Two-Wire Control Circuit

Used in application of automatic system.

Two wire used to provide voltage to load.

Applications: pump, heater or compressor.

Typically closed a disconnect switch or circuit breaker to energize the circuit.

An overload coils is located to protect the circuit against over current.

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