Ladder Logic

1

Line (Ladder) Diagram rules

Universal set of graphic symbols and rules (standards) on how line (ladder) diagrams are laid out.

By applying these standards we establish a working practice understood by all technical personnel.

2

Ladder-Line Diagram rules

Load devices (lights, solenoid, relay coil, etc) must be powered by full (rated) voltage to operate correctly.

Multiple loads must be placed in parallel.

If loads are placed in series a voltage divider effect occurs and neither load operates correctly.

3

Ladder-Line Diagram rules

Light and Solenoid each require 120V to operate properly.

How much voltage will each receive if they are the same impedance?

What voltage will each receive if three loads are placed in series

4

Ladder/Line Diagram rules

Multiple loads must be placed in parallel to achieve full-voltage.

One side of a load is always connected to L2 (0V).

One side of Motor starter coils are connected indirectly to L2 through NC overload contacts.

5

Ladder/Line diagram rules

Control devices are connected between L1 and the load device (operating coil).

Two or more control devices may be connected in series or parallel to control a load (operating coil). Why? Contacts are either open or closed

6

Line Number Reference

Each horizontal rung is assigned a line number. The Line number is located on the left side of each horizontal rung. Three line numbers shown.

7

Numerical Cross-Reference

Numerical cross references are located to the right of a coil.

They identify the line number that their contacts are located on.

8

Documentation details

A numeric cross reference number indicates the location of a N.O. contact.

A numeric cross reference number with an underscore indicates the location of a N.C. contact.

9


Wire Number Reference

Adhesive number placed on the end of each wire where it is connected.

Easier to wire and troubleshoot circuit. Consider 50 red wires, which one is which?

Rule: When the ladder/line diagram goes through a graphic symbol (contact, coil, etc.) the wire number must change.

All wires connected to the same point in a circuit must have the same wire number.

10


Manufacturer terminal numbers are included on the terminal connection points of a device.

These numbers are used to identify the different parts of the device (coil, NO & NC contacts

CR Coil is 2 & 10

CR contacts are 1&3, 6&5, 11&9

11


Mechanically Connected

Two methods are used to indicate the location of other mechanically operated contacts from the same device: Dashed line method and Numerical Cross reference method.

Dashed line indicates multiple contacts from the same device.

Numerical cross reference indicates line number of other contacts from the same device.

12

Signals, Decisions and Actions All control circuits contain

three major sections known as the Signal, Decision and Action sections.

Signal section: Starts or stops current flow to the circuit. Allows circuit to operate. (I.e. on/off switch).

Decision section: Uses logic from multiple devices to determine what work is to be done. Decision is used to control a load device.

Action section: Work to be performed based upon decision. (I.e. turn a load device on or off.)

13

Logic Functions

Logic functions include AND, OR, NOT, NOR, NAND, XOR and MEMORY. (Same as Digital Logic)

AND Logic: Two or more NO contacts in series.

OR Logic: Two or more NO contacts in parallel.

Student: Write the Boolean algebra expression and truth table both diagrams shown here.

14

Logic Functions

Circuits can contain a combination of AND/OR logic.

The circuit can contain multiple different logic functions to make a decision

15

Logic Functions

NOT logic is created by a NC contact.

NOR logic is created by two or more NC contacts in series. Figure 5-18

Student: Write the Boolean Algebra expression for each diagram.

16

Logic Functions

NAND logic is created by two or more NC contacts in parallel.

Student: Write the Boolean algebra expression for the diagrams

17

Logic Functions

Memory logic is created by adding a NO contact in parallel with a pushbutton.

Student: Write the Boolean algebra expression for the diagrams shown.

A common start/stop memory circuit consists of NOT logic and OR logic.

18

Common Control Circuits

Numerous common control circuits are used in industry.

This diagram shows a start/stop memory circuit.

This diagram shows a start/stop memory circuit with two stop buttons and two start buttons. This circuit is used when a motor must be started and stopped from two different locations.

19


This diagram shows two separate start/stop stations and an emergency stop for two motors. (Hint for Jeopardy problem in lab 2)

This diagram shows how one motor starter can be used to start a 2nd motor starter. Note that two separate contacts are used from M1

20


Pilot lights are available in a variety of shapes, colours and sizes.

Used to indicate to humans the present state of a machine or system.

The diagram below shows a pilot light & starter coil (M1) that are energized when the selector switch is in the ON position, and the pressure is low.

21


Here a pilot light is used to indicate when a motor starter is activated in a memory circuit.

Here NOT logic is used to indicate when a motor starter is not activated

Pilot light PL1 is energized when M1 is off.

22


Here a sequence control circuit ensures that conveyor # 2 (M1) is running, before conveyor # 1 (M2) can operate.

23


Here a selector switch is added to a start/stop memory circuit to provide jog control.

When the selector switch is in the run position, the circuit works with memory

When the selector switch is in the jog position, the motor stops when the start button is released.

24

AutoCAD Symbols

The symbols shown are provided to students in an AutoCAD Drawing

Students are required to use AutoCAD and the Symbols.dwg to make Wblocks for Labs and assignments.

File is required for Lab # 3

25

AutoCAD Symbols Contd Students are also provided

with an electronic copy of a title block and border. You MUST use it!

Students use the symbol file and title block file to create drawings as required in this course.

Symbols and title block are in Imperial units (inches).

Every drawing is graded for: Orientation

Paper space

Readability

Accuracy (scaling and units w/a)

Proper edits (Name, title, etc.)

Professionalism

26 I/

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NL1 120 VACFROM LINE CORD

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NOTES:

1. > SYMBOL REPRESENTS A FEMALE

BANANA JACK.

2. LABEL THE ENDS OF EACH WIRE AS

PER THE WIRE NUMBERS INDICATED

ON THE SCHEMATIC.

3. INPUT AND OUTPUT WIRE COLOURS

SHALL BE RED.

4. 120 VAC POWER SHALL USE BLACK WIRE

5. NEUTRAL SHALL USE WHITE WIRE.

1 PBLT

2 PBLT

3 PBLT

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MAN

I/7

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START

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120 VAC

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6. BANANA JACK COLOURS:

120 VAC = RED

INPUTS = YELLOW

OUTPUTS = BLACK

NEUTRAL = WHITE

MICROLOGIX TRAINER

WIRING DIAGRAM

LAB - H230

INDUSTRIAL CONTROLS

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YELLOW

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I/11

Wiring Diagrams

Wiring Diagrams show the connection of all components on a piece of equipment.

The location of components is generally as close as possible to the actual circuit configuration

A wiring diagram is similar to a pictorial drawing except the components are shown as graphic symbols.

27

Schematic Diagrams

Shows connection and function of a circuit with graphic symbols.

Has no relationship to the physical configuration of the circuit.

Only intended to show the logical operation of the circuit.

28

Line (Ladder) Diagrams Ladder/line diagrams are the primary means of

communicating the control language of Industrial Control Systems. (Lab 1 TYK answer)

Appearance is similar to a ladder.

Left vertical rail (L1) is power or hot, right vertical rail (L2) is neutral (0V) (or Ground).

Horizontal Rungs represent logical operation of circuit.

29

One-Line Diagrams

Uses single lines and graphic symbols to indicate the path and components of an electrical system.

Typically only used for 3-phase power.

Does not provide a complete circuit diagram.

30

Line (Ladder) Diagrams

Incorrect symbols can change circuit operation and cause hazardous situations.

Here we see four limit switch symbols.

Each symbol is unique and provides a different control function

31

Electrical Circuits Five basic components

1. Load that converts electrical energy to another form (I.e. light)

2. Power Source

3. Conductors/wires (circuit connections

4. Control method (switch)

5. Protection device (fuse/circuit breaker)

32

Normally Open (N.O.) & Normally Closed (N.C.)

Contacts are described as N.O. & N.C.

Normal refers to the de-energized/de-activated state of the electrical contact.

I.e. the normal state for a pushbutton is when its not pressed.

Normal is also referred to as the shelf-state by some professionals.

Contacts from devices are always drawn in their normal state for this course.

33

N.O. & N.C. continued

A N.O. contact is an open circuit (infinite ) when the device it is not activated/energized.

An N.O. contact closes (low ) when the device is activated/energized.

A N.C. contact is a closed circuit (low ) when the device is not activated/energized.

A N.C. contact is a open circuit when the device is activated/energized.

34

25

Pushbuttons

Pushbutton have

internal contacts that

can be either N.O.

and/or N.C.

Here we see a

cutaway portion of

the terminal block

for a pushbutton.

35

Digital Logic Comparison Same concepts as digital logic can be applied to ladder/line

diagrams.

1=on/closed, 0=off/open, gate logic, Boolean Algebraremember?

Physically - N.O. PB = 0 not pressed and a 1 when pressed.

Physically - N.C. PB = 0 when pressed and a 1 when not pressed.

36

Manual Control Circuits

Manual control requires a person (man) to turn devices on and off. (I.e. wall switch)

Pushbutton is a manual device

37

Automatic Control Circuits Automatic controls turn

devices on/off without a human operator.

Here we illustrate the use of a common float switch to create some simple automatic circuits.

Can you think of a common automatically controlled device?

Here we have answers to two TYK questions in Lab 1

38

Automatic Control Circuits

Here we automatically maintain the water level of a livestock water tank.

Float switch FS1 is the automatic control element.

It opens and closes its contacts based on the position of the float that is in contact with the water level.

Documents

Ladder Logic