1. PROGRAMMABLE LOGIC CONTROLLERS PLC’S

2. •Overview of Technology •PLC Configuration and Selection •Programming PLC’s

3. What is a PLC ?
•PLC (Programmable Logic Controller)
•A PLC works by looking at its inputs and depending on their state, and the user entered program, turns on/off outputs.
•A PLC can be thought of as:
Industrial Computers with specially
designed architecture in both their central units (the PLC itself) and their interfacing circuitry to field devices (input / output connections to the real world).

4. CPU •Power Supply •Memory •Input Blocks •Output Blocks •Communications •Expansion
Connections

5. CPU Module •The Central Processing Unit (CPU)
Module is the brain of the PLC.
•Primary role to read inputs, execute
the control program, update outputs.
•The CPU consists of the arithmetic logic unit (ALU), timing/control circuitry, accumulator, scratch pad memory, program counter, address stack and instruction register.
•A PLC works by continually scanning
a program
Scan
Inputs
Self
Check
PLC Program
SCAN
Update
Outputs
Execute
Code

7. The memory includes pre-programmed ROM memory containing the PLC’s operating system, driver programs and application programs and the RAM memory.
•PLC manufacturer offer various types of retentive memory to save user-programs and data while power is removed, so that the PLC can resume execution of the user-written control program as soon as power is restored.

8. Input and output (I/O) modules connect
the PLC to sensors and actuators.
•Provide isolation for the low-voltage, low- current signals that the PLC uses internally from the higher-power electrical circuits required by most sensors and actuators.
•Wide range of I/O modules available including: digital (logical) I/O modules and analog (continuous) I/O modules.

9. Inputs come from sensors that translate physical or
chemical phenomena into electrical signals. •The simplest form of inputs are digital/discrete in AC/DC. •In smaller PLCs the inputs are normally built in and are
specified when purchasing the PLC.
•For larger PLCs the inputs are purchased as modules, or cards, with 8,16, 32, 64, 96 inputs of the same type on each card.

10. The list below shows typical ranges for input
voltages.
5 Vdc
12 Vdc
24 Vdc
48 Vdc 12 Vac 24 Vac
120 Vac
240 Vac

12. Output modules rarely supply any power, but instead act
as switches.
•External power supplies are connected to the output card and the card will switch the power on or off for each output.
•A common choice when purchasing output cards is
relays, transistors or triacs.
•Relay are the most flexible output devices. They are capable of switching both AC and DC outputs. But, they are slower, cost more, and they will wear out after millions of cycles.

13. The most important consideration when selecting relays, or relay outputs on a PLC, is the rated current and voltage.
•For transistor outputs or higher density output cards relay terminal blocks are available.
–Advantage of individual standard
replaceable relays

14. Typical output voltages are listed below,
5 Vdc
12 Vdc
24 Vdc
48 Vdc
24 Vac
120 Vac
240 Vac

16. Typical Analogue Input
signals are:
–Flow sensors –Humidity sensors –Load Cells –Potentiometers –Pressure sensors –Temperature sensors –Vibration
•Analogue Output signals
control:
–Analogue Valves –Actuators –Chart Resorders –Variable Speed Drives –Analogue Meters
•Typical Analogue Signal
Levels
–4-20mA –1-5 Vdc –0-10 Vdc –-10 – 10Vdc

17. Analogue input cards convert continuous signals via a A/D
converter into discrete values for the PLC
•Analogue output cards convert digital values in then PLC to
continuous signals via a D/A converter. •Resolution can be important in choosing an applicable card •Example, for a temperature input of 0 to 100 degrees C
–For 8 bit resolution the value in the PLC is 0 to 255 –For 12 bit resolution the value in the PLC is 0 to 4095 –For 12.5 bit resolution the value in the PLC is 0 to 6000 –For 13 bit resolution the value in the PLC is 0 to 8192 –For 16 bit resolution the value in the PLC is 0 to 32768

18. RF ID •Voice •Gas Flow Calculation •Weigh Cell •Hydraulic Servo •ASCII •Fuzzy Logic •Temperature Sensor •Temperature Control •Heat/Cool Control •Field Bus Cards
–DeviceNet, Profibus etc
–Lonworks, BACNet
• Fast Response (Interrupt)
• PID • Loop Controller • BASIC Cards • RS232 Comm’s • Modbus ASCII/RTU • Ethernet Comm’s –
• High Speed Counters • Position Control Cards • Per to Per Comm’s
Controller Link – DH+ – Modbus Plus

19. •Sequence
–Input –Output –Control
•Logic •Timer and Counters •Comparison •Range Comparison •Data Movement •Data Shift •Step / Step Next •Serial Communications •Text String Processing •File Manipulation
–IF,THEN,ELSE,LOOP
Table Processing
–LIFO, FIFO
•Increment/Decrement
•Conversion
–ASCII
–Number Systems
•Math •Floating Point Math •Statistics •Scaling •PID •PID with Auto tune •Clock / Date •Block Processing

20. The configuration of PLC refers to the packaging of the
components.
•Typical configurations are listed below from largest to
smallest.
–Rack Type : A rack can often be as large as 18” by 30” by 10”
–Mini: These are similar in function to PLC racks, but about the
half size. Dedicated Backplanes can be used to support the cards OR DIN rail mountable with incorporated I/O bus in module.
–Shoebox: A compact, all-in-one unit that has limited expansion
capabilities. Lower cost and compactness make these ideal for
small applications. DIN rail mountable.
–Micro: These units can be as small as a deck of cards. They
tend to have fixed quantities of I/O and limited abilities, but costs
will be lowest. DIN rail mountable

21. Micro PLCs: I/O up to 32 points Small PLC: I/O up to 128 points Medium PLC: I/O up to 1024 points Large PLC: I/O up to 4096 points Very Large: I/O up to 8192 points

22. Criteria •Number of logical inputs and outputs •Memory •Number of special I/O modules •Expansion Capabilities •Scan Time •Communication •Software •Support •Dollars