1. Control Relays Functionally, relay is a solenoid operated switch.
It consists of a coil of wire, a soft iron armature and one or more contacts.
Figure below show the relay principle.

2. Control Relays Cont.
Relays are used in many modern control systems, as it is an electrical switch with a high current rating, that is indirectly operated by low control current.
Many industrial applications use relays to act as interfaces between the low signal levels (5-12V) from controllers to high-current devices.

3. Control Relays Cont.
Since all relays contain moving parts and/or electrical contacts, they are therefore limited in terms of operating speed, reliability and lifespan.
They are also very big in size, requiring large mounting racks in most applications.
A typical relay control system can consists of several hundred or thousand switching contacts, which makes designing becomes a considerably difficult task.
It is very difficult to change the control function of a relay system once it has been connected up, and usually a complete re-wiring is necessary.
Relay system is also expensive, slow and unreliable (for prolong use).
Therefore an alternative based on electronic and microprocessor is used to replace relay.

4. PLC PLC is based on a hardware CPU and programmable memory.
PLCs provide ease and flexibility of control using software and executing simple logic instructions.

5. PLC PLCs have internal functions such as timers, counters and relays.
A PLC operates by reading the input signals from a process and carrying out logic instructions on these input signals, producing output signals to drive output actuators.
Through the use of PLCs, it became possible to modify a control system without having to disconnect or redirect a single wire; it is only necessary to change the control program.
PLCs are similar to computers which also consist of processing unit, memory and input/output.
Input signals to the PLC are read and then stored in memory, where the processor performs the programmed logic instructions on these input signals.

6. PLC and Relay Comparison of PLC and RELAY systems: Characteristic PLC
Price per function
Low
Physical size
Very compact
Bulky
Operating speed
Fast
Slow
Elec. noise immunity
Good
Excellent
Construction
Easy to program in ladder using a PC
Time consuming, wiring coils and contacts
Capable of complicated operations
Yes No
Ease of changing the control sequence
Very simple
Very difficult
Maintenance
Excellent, PLCs rarely fail
Poor, if large number of mechanical contacts and coils have to be maintained

7. Control System Using PLC
In an automated system, the PLC is commonly regarded as the heart of the control system.
With a control application program in execution, the PLC constantly monitors the state of the system through input devices.
It will be based on the program logic to determine the course action to be carried out.
Intelligence of an automated system is greatly depending on the ability of a PLC to read in the signal from various types of automatic sensing and manual input devices.

8. Control System Using PLC
An automatic system is incomplete and the PLC system is virtually paralyzed without means of output devices.
Some of the most commonly controlled devices are motors, solenoids, relays indicators, buzzers etc.
Through activation of motors and solenoids, the PLC system can control from a simple PnP system to a much complex servo positioning system.

9. Control System Using PLC
A block diagram of a PLC control system is shown below

10. PLC Interfacing
PLC Wiring

11. PLC Interfacing
PLC Wiring

12. PLC Interfacing
Basic ladder diagram symbols

13. PLC Interfacing
Example of a simple ladder diagram of the PLC control system is shown below

14. PLC Interfacing
Example: Sorting machine – Wiring diagram

15. PLC Interfacing
Example: Sorting machine – Ladder diagram

16. PLC Interfacing
PLC interfacing with a computer/programmable terminal.

17. PLC Interfacing
OMRON Sysmac CQM1 Programming Console

18. PLC Interfacing OMRON Sysmac CQM1 Programming Console
There are 3 sets mode that can be selected using the key on the console:
PROGRAM-for programming or altering the program and at this mode the PLC is not running the program.
MONITOR-is used when changing the setting value of the counter and timer while PLC is operating.
RUN-the mode is used during executing the program that has been load in the PLC

19. Interfacing PLC/PC to Robot
Basic Terms
Each condition in a ladder diagram is either ON or OFF depending on the status of the operand bit that has been assigned to it.
A normally open condition is ON if the operand bit is ON; OFF if the operand bit is OFF.
A normally closed condition is ON if the operand bit is OFF; OFF if the operand bit is ON.
In other words, you use a normally open condition when you want something to happen when a bit is ON, and a normally closed condition when you want something to happen when a bit is OFF.

20. Interfacing PLC/PC to Robot
Mnemonic code
The ladder diagram cannot be directly input into the PC via a Programming Console; the CX-Programmer is required.
To input from a Programming Console, it is necessary to convert the ladder diagram to mnemonic code.
The mnemonic code provides exactly the same information as the ladder diagram, but in a form that can be typed directly into the PC.
Also, regardless of the Programming Device used, the program is stored in memory in mnemonic form, making it important to understand mnemonic code.
Because of the importance of the Programming Console as a peripheral device and because of the importance of mnemonic code in complete understanding of a program, it will introduced and described along with the ladder diagram.

21. Interfacing PLC/PC to Robot
Mnemonic code
An example of mnemonic code is shown below.
When programming, addresses are automatically displayed and do not have to be input unless for some reason a different location is desired for the instruction.
When converting to mnemonic code, it is best to start at Program Memory address unless there is a specific reason for starting elsewhere.

22. Interfacing PLC/PC to Robot
Basic programming using the Console
Clear the memory: CLR->SET->RESET->MNTR->CLR
To start inserting the program: For example(Figure Below):
LD 00000<WRITE>
AND NOT 00001<WRITE>
OUT 10002<WRITE>
FUN(01)<WRITE>

23. Interfacing PLC/PC to Robot
Basic programming using the Console
LOAD and LOAD NOT
AND and AND NOT

24. Interfacing PLC/PC to Robot
Combining AND and OR
OUTPUT and OUTPUT NOT

25. Interfacing PLC/PC to Robot
OUTPUT and OUTPUT NOT

26. Interfacing PLC/PC to Robot
The END Instruction

27. Start active students

28. Interfacing PLC/PC to Robot
AND LOAD

29. Interfacing PLC/PC to Robot
OR LOAD

30. 3.9 Interfacing PLC/PC to Robot
Logic Block Instructions in Series

31. Interfacing PLC/PC to Robot
Combining AND LOAD and OR LOAD

32. Interfacing PLC/PC to Robot
Complicated Diagrams

33. Interfacing PLC/PC to Robot
Complicated Diagrams

34. Interfacing PLC/PC to Robot
Complicated Diagrams

35. 3.9 Interfacing PLC/PC to Robot
TIMER
For CQM1H the timer is ON-delay type. The address is from TIM and the Timing setting is from 0000 – 9999 that is equivalent to sec – seconds respectively.

36. 3.9 Interfacing PLC/PC to Robot
COUNTER
For CQM1H the timer is ON-delay type. The address is from TIM and the Timing setting is from 0000 – 9999 that is equivalent to sec – seconds respectively.

37. End