[7] Basic PLC

Programmable Logic Controller (PLC)

a digitally operating electronic apparatus, as defined by NEMA

Programmable Logic Controller (PLC)

uses a programming memory for the internal storage of instructions

Programmable Logic Controller (PLC)

implement specific functions such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog modules, various types of machines or process

Programmable Logic Controller (PLC)

has a CPU to run one program that monitors a series of different inputs and logically manipulates the outputs for the desired control

1. Power Supply

2. CPU/Processor

3. Memory

4. Input Modules/Interface

5. Output Modules/Interface

6. Programming Device or Terminal

Major Components of a PLC

Power Supply

provides the power needed to run the primary PLC components

CPU/Processor

the “brain” of a PLC and governs everything in the system

Executive or Operating System (OS)

a program controlling the CPU/Processor

Executive

a collection of supervisory programs permanently stored in memory

CPU

reads input signal states, processes the control program, and controls the outputs; provides internal memory, timers, and counters

CPU

consists of a microprocessor for implementing the logic and controlling the communication among the modules

Memory

stores information, programs, and data in a PLC

1. EPROM

2. EEPROM

3. RAM

Types of Memory

Input Modules/Interface

interface directly to devices such as switches and temperature sensors and convert many different types of electrical signals such as 120VAC, 24VDC, or 4-20mA, to signals which the controller can understand

I/O Modules/Interface

can be a fixed I/O or modular card that forms the interface by which the field or “real world devices” are connected to the microcontroller

1. Laptop/Computer

2. Programming Console

Examples of Programming Device or Terminal

Programming Device or Terminal

used to enter the desired program that will determine the sequence of operation and control of process equipment or driven machine

1. Start scan

2. Internal checks

3. Scan inputs

4. Execute program logic

5. Update outputs

PLC Principles of Operation - CPU Operating Cycle

1. Fixed Configuration

2. Modular Configuration

PLC Configurations

Fixed Configuration

a type of PLC configuration wherein all PLC components are housed in one package including I/O interfaces, CPU, and power supply

Modular Configuration

a type of PLC configuration wherein each component part is a separate module

1. Allen-Bradley

2. GE Fanuc

3. Square D

4. Modicon

Common American PLC Brands

1. Siemens

2. Telemecanique

3. Klockner & Moeller

Common European PLC Brands

1. Mitsubishi

2. Omron

3. Toshiba

Common Japanese PLC Brands

1. Small Size

2. Medium Size

3. Large Size

PLC Sizes

Small Size

a PLC size covering units with up to 128 I/O’s and memories up to 2 Kbytes

Small Size

a PLC size capable of providing simple to advance levels or machine controls

Medium Size

a PLC size having up to 2048 I/O’s and memories up to 32 Kbytes

Large Size

the most sophisticated units of the PLC family that have up to 8192 I/O’s and memories up to 750 Kbytes

1. Switches and Pushbuttons

2. Sensing Devices

3. Condition Sensors

Categories of Input Devices

1. Limit Switches

2. Photoelectric Sensors

3. Proximity Sensors

Sensing Devices

1. Pressure Switches

2. Level Switches

3. Temperature Switches

4. Vacuum Switches

5. Float Switches

6. Encoders

Condition Sensors

1. Digital Input Devices

2. Analog Input Devices

Types of Input Devices

Digital Input Devices

binary devices which must be in one of only two states: on or off, 1 or 0

Digital Input Devices

type of input devices which examples include push button, limit switches, proximity, and photoelectric

Analog Input Devices

input devices that sense continuous parameters, providing information to the PLC with continuous range of values, not just an on or off indicator

Analog Input Devices

type of input devices which examples include RTD, pressure sensor, strain gauge

LVDT (Linear Variable Differential Transformer)

used for position and displacement sensing

Pressure Sensor

to detect change in pressure

Humidity Sensor

used to measure relative humidity

Magnetic Sensor

used primarily as detector of pistons in pneumatic and hydraulic cylinders

Float Switches

used primarily in liquid level sensing

1. Valves

2. Motor Starters

3. Solenoids

4. Actuators

5. Control Relays

6. Horns & Alarms

7. Stack Lights

8. Fans

9. Counter

10. Pumps

11. Printers

Output Devices

1. Digital Output Devices

2. Analog Output Devices

Types of Output Devices

Digital Output Devices

binary devices which may be either on or off, open or close

Digital Output Devices

a type of output devices which examples include relay, solenoid valves, and lamps

Analog Output Devices

respond to a range of output values from the controller

Analog Output Devices

a type of output device which examples include VFD, Stepper Motor, Servo Motor

Output Devices

also called as actuators

Output Devices

drive motion in mechanical systems; converts electrical energy into some form of mechanical motion

1. Electric motors

2. Solenoids

3. Pneumatic and hydraulic cylinders and motors

Logical Actuators