1. Industrial Electronics
DAE 32003
Industrial Electronics
Jabatan Kejuruteraan Elektrik, PPD

2. Chapter 2: LADDER DIAGRAM
3.1 Schematic symbols and drawing procedures
3.2 Two-wire and Three-wire control circuit
3.3 AC & DC Motor control circuits
3.3.1 Start-Stop
3.3.2 Hiccup
3.4 Overload protection scheme

3. Introduction:
What is Ladder diagram?
Ladder diagrams are specialized schematics commonly used to document industrial control logic systems.
It is a type of graphic language for automatic control systems.
It is a drawing program of a switching circuit.
It is a language or 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.
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.
*PLC = programmable logic controller

4. Basic concept of ladder diagram is similar to electrical wiring.

6. Ladder diagram operation principle
Input
Interface
Output
+24V
-0V
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.

7. Ladder diagram symbols
Ladder diagram uses standard symbols to represent the circuit components and functions found in a control system.

8. INPUT symbols
Normally Open
(NO)
Normally Close
(NC)

9. OUTPUT symbols

10. Ladder diagram application Conveyor control using Allen-Bradley PLC.

11. How do we know which relay contact is actuated by which relay coil?
Activity
How do we know which relay contact is actuated by which relay coil?

12. How do you think the system works?1 2
How do you think the system works?

13. Draw a simple ladder diagram for the circuit below.
Activity
Draw a simple ladder diagram for the circuit below.

14. Activity
Predict how the operation of this motor control circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):
"Stop" pushbutton switch fails open.
Relay contact CR1-1 fails open.
Relay contact CR1-2 fails open.
Relay coil CR1 fails open.

15. "Stop" pushbutton switch fails open: Motor cannot start, lamp never energizes.
Relay contact CR1-1 fails open: Motor starts and lamp energizes when "Start" button is pressed, but both immediately de-energize when it is released.
Relay contact CR1-2 fails open: "Motor run" lamp turns on and off as expected, but the motor itself never runs.
Relay coil CR1 fails open: Motor cannot start.

16. Digital Logic Functions
Ladder diagrams are capable of performing logic functions similar to the logic gates used in digital electronics. We can construct simple logic functions for our electrical wiring diagram, using multiple contacts. There are 6 logic functions that can be developed using combination of switches:
AND OR
NOT
NOR
NAND
EX-OR (XOR)

17. Digital Logic Functions: AND Logic
Solenoid coil is not energized unless both switch A & B are closed.

18. Digital Logic Functions: OR Logic
Solenoid coil is not energized until either switch A & B is closed.

19. Digital Logic Functions: NOT Logic
The logical inversion, or NOT, function can be performed on a contact input simply by using a normally-closed contact instead of a normally-open contact.

20. Digital Logic Functions: NAND Logic
There is no output when both A and B have an input.

21. Digital Logic Functions: NOR Logic
There’s no output when either A & B or both are opened.

22. Digital Logic Functions: XOR Logic
There’s no output when both 1 & 2 have input.

23. Activity
What type of logic function behavior (AND, OR, NAND, or NOR) does the circuit exhibit? 
AND Logic

24. Activity
What type of logic function behavior (AND, OR, NAND, or NOR) does the circuit exhibit? 
AND Logic

25. Activity
What type of logic function behavior (AND, OR, NAND, or NOR) does the circuit exhibit? 
OR Logic

26. Process Sequence
There are several method to describe the process sequence of a ladder diagram:
Grafset
Motion diagram
Displacement diagram
Simple narrative
Boolean

27. Process sequence: Grafset
Initial step / Reset = Double box
Actions
Consequence of actions

28. Process sequence: Grafset
Example
Solution
1st: Identify input & output 
LA – level A
LB – level B
ES – empty switch
TIM – timer contact (on for 5 mins)
SB – start button
INPUT
VA – valve A
VB – valve B
VC – valve C
H – heater
ST – stirrer
OUTPUT
The tank system has the narrative description below:
Fill the tank to level A from valve A.
Fill the tank to level B from valve B.
Start a timer, heater and stirrer for 5 minutes.
Open output valve C until the empty switch engages.

29. Process sequence: Grafset
Solution

30. Process sequence: Motion Diagram
The input and the output are separated with the input at the upper side and the output is at the bottom side.

31. Process sequence: Displacement Diagram
Presented in form of graph indicates the position of the cylinders and the displacement.
Step 1 : Cyl 1 and Cyl 2 extend.
Step 2 : Cyl 1 retracts while Cyl 2 remains extend for 3 seconds.
Step 3 : Cyl 1 remains retracts for 3 seconds and Cyl 2 retracts.
Step 4 - Step 6 : The position of Cyl 1 and Cyl 2 similar as Step 1 – Step 3.

32. Process sequence: Simple Narrative
Example
A push button is used to turn on the conveyor belt A that brings the bottles to the filing line. Sensor 1 that is placed at the center of the conveyor A will stop the conveyor belt A upon detecting the bottle and open the valve for filing for 2 second. After that the conveyor belt will be on again until the bottle is sensed by sensor 2 at the end of conveyor belt A. Conveyor belt A once again stop and the solenoid is energized for 3 seconds in order for the piston to push the bottle to conveyor belt B. The process repeats until stop button is pressed.

33. Process sequence: Simple Narrative
Solution
1st: Identify input & output 
2nd: Simplify statement 
Start button
Sensor 1
2 sec timer contact
Sensor 2
3 sec timer contact
Stop button
INPUT
Conveyor belt
Valve
2 sec timer coil
3 sec timer coil
Solenoid (piston)
OUTPUT
SB turns on conveyor A.
Sensor 1 stops conveyor A, opens valve. Timer 1 on for 2 sec.
After timer 1 times up (after 2 sec), valve closed and conveyor A on.
Sensor 2 stops the conveyor, energizes solenoid (piston). Timer 2 on for 3 sec.
Timer 2 times up (after 3 sec), solenoid de-energized and conveyor A on again.
Stop button reset all.

34. Process sequence: Boolean
Another method to describe process sequence.
Is a logic expression using gates.
Solution
Example
D = Door
D = Lock
D’ = Unlock
S1 = Sensor 1
S1 = Sense
S1’ = Not sense
S2 = Sensor 2
S2 = Sense
S2’= Not sense
S2 = Sensor 2
S2 = Sense
S2’= Not sense
SB = Start button
SB = Pressed ,
SB’ = Not pressed
CNT = Counter
RB = Reset button
RB = Reset
RB’ = Not reset
The door of the automatic train will be locked if start button is pushed and will unlocked when sensor 1 detects it’s arrival at station 1. Sensor 2 that is placed at the entrance door of the train will count the number of train passengers on that day.
D = SB.S1’
D’ = SB’.S CNT = S2.S1.RB’

35. Draw a Grafset and Motion diagram for the circuit below.
Activity
Draw a Grafset and Motion diagram for the circuit below.

36. AC & DC Motor control circuit
START – STOP circuit
When PB is pressed, relay coil will energized and relay contact will close.
When PB is depressed, relay coil is still energized since current can go through the relay contact unless disturbed by stop button.
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Ladder diagram

37. VIBRATING / JOG circuit
AC & DC Motor control circuit
VIBRATING / JOG circuit
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When PB is pressed, output IR is activated.
After several microsecond, the normally closed IR is open then it will stop the output IR.
This set-up is applicable to move an object which needs to be precisely located.
Ladder diagram