Plc Basics

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Programmable Logic Controllers  (PLC) 

       

PLC Presentation Contents • • • • • • • •

Introduction What is a PLC PLC Types Choosing PLC Hardware PLC Applications Programming PLC’s Advantages of PLCs Questions

What is a PLC?

Nema Definition circa 1978 . The PLC, also known as programmable controller is defined by the National Electrical Manufacturers Association (NEMA) in 1978 as: "a digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions, such as logic, sequencing, timing, counting and arithmetic, to control through digital or analog input/output, various types of machines or process".

Traditional PLC Concept

• • • •

PLC performs relay equivalent functions PLC performs ON/OFF control Ladder diagram programming Designed for industrial environment

Relay Logic vs. PLC & Ladder Logic PB1

LS1

PB2

LS1

SOL2

Outputs

Inputs

Programmable Logic Controller

CR3

C R

LS3 I/4

I/6

||

CR3 |/|

LS4

M1

||

O/0

()

I/7

I/5

B/0

||

||

()

I/8

||

B/0

|/|

I/9

||

O/1

()

Input Wiring: PLC input is the load in the circuit, sensing if voltage is present Isolation Barrier

Terminal Block

Input Devices

1 2 3

L1 L1

4 5 6 7 8 9

L2

10 COM

P L C

Input Devices • Pushbuttons

• 120/230 VAC

• Selector Switches

• 24 VDC

• Limit Switches

– Sourcing

• Level Switches

– Sinking

• Photoelectric Sensors • Proximity Sensors • Motor Starter Contacts • Relay Contacts • Thumbwheel Switches

Output Wiring: PLC output is the switch, controlling current flow to load Isolation Barrier

CR

Terminal Block

OUT 1 OUT 1

Output Devices L1 L2

OUT 2

P L C

OUT 2 OUT 3 OUT 3 OUT 4 OUT 4 OUT 5 OUT 5 OUT 6 OUT 6

L1 L2

Output Devices • • • • • • • •

Valves Motor Starters Solenoids Control Relays Alarms Lights Fans Horns

• Relays – 120 VAC/VDC – 240 VAC – 24 VAC/VDC

• Triac – 120/230 VAC

• Transistor MOSFET – 24 VDC

Inside a PLC Communications

I n p u t

High Voltage Isolation Barrier

C i r c u i t s

Central Processor

MEMORY data

program Low Voltage AC Power Supply 85-264 VAC, 50/60Hz

or

External DC Power Supply

O u t p u t

C i r c u i t s Isolation Barrier

CR

High Voltage

PLC Operating Cycle START

Housekeeping Internal checks on memory, speed and operation. Service any communication requests, etc.

Output Scan The Output Image data is transferred to the external output circuits, turning the output devices ON or OFF.

Input Scan The status of external inputs (terminal block voltage) is written to the Input image (“Input file”).

Program Scan Each ladder rung is scanned using the data in the Input file. The resulting status (Logic being solved) is written to the Output file (“Output Image”).

PLC Hardware Types

Most Basic of PLC Systems In the most basic of PLC systems, a self contained (shoe box) PLC has 2 terminal blocks, one for Inputs and one for Outputs Today, most PLC’s in this category are know as Micros. Typically they provide front panel LED status indication of I/O and Processor states Outputs

Inputs

Programmable Controller C R

Modular Chassis Based PLC’s The vast majority of PLC’s installed today are modular chassis based systems consisting of: 2. Processor Module (CPU) 3. Input & Output Modules 4. Chassis 5. Power Supply

Modular Chassis-less PLC Systems Also available from many vendors are “Chassis less” but still Modular PLC systems. These systems still require a Processor, I/O Modules, and Power Supply, but in place of a chassis these components mount directly onto a panel, din rail, and many use a tongue and grove system to allow easy insertion and removal

Choosing PLC Hardware

PLC Application Considerations • Inputs/Outputs – Type, • AC, DC, sourcing, sinking, etc.

– Number of • 10, 16, 20, 32, 156

• Memory – Type • Flash or Battery backed

– Size • 1k, 6k, 12k, 16k, 64k

• Functions required – Instruction set • Messaging • PID • PTO, PWM

– Arithmetic – Communications • DeviceNet, Ethernet • Remote I/O, DH+

– Report generation

SOURCING vs. SINKING

SOURCING Pushbutton (PNP)

SINKING Pushbutton (NPN) +VDC

+

+ DC Power Suppl y

-

DC Power Suppl y

-

DC COM

SOURCING vs. SINKING DC Inputs

SINKING (NPN)

SOURCING (PNP) Field Device

+ DC Power Suppl y

-

+VDC IN1

DC

Input DC COM

Modul e

+ DC Power Suppl y

-

DC Input Field Device

Modul e

IN1

Rules

RULES • Field devices on the positive side (+VDC) of the field power supply are sourcing field devices. • Field devices on the negative side (DC COM) of the field power supply are sinking field devices. • Sourcing field devices must be connected to sinking I/O cards and vice versa. • Sinking field devices must be connected to sourcing I/O cards and vice versa.

PLC Applications

Installed and Running Systems in RI -

Conveyors Curtain rods Deodorants Donuts Duplex Receptacles Fibers Filters Forged Parts Glass Goggles Grinding and Polishing Heat Treating Metal Products Jails Lenses Nails Natural Gas Paper

-

Pharmaceuticals Plastics Plating Plating Machines Power Generation Power Supplies Product Assembly Machines Rubber products Seafood Processing Soda Staples Warehouse Automation Waste Water Systems Drinking Water Systems Water Heaters Web Handling (paper/plastic) Wire / Cable

Typical PLC Application

Solenoid 1

Motor

Solenoid 2

Ingredient A

Ingredient B

Sensor 1

Sensor 2

Solenoid 3

Operation of Mixer (Sequence of Control) • Solenoid 1 – On = Sol 3 is off, and Motor is off, and Sensor 2 is off, and Auto Switch is on – Off = Sol 3 is on, or Motor is on, or Sensor 2 is on

• Solenoid 2 – On = Sol 3 is off, and Motor is off, and Sensor 2 is on – Off = Sol 3 is on, or Motor is on, or Sensor 1 is on

• Motor – On = Sensor 1 is on, and Solenoid 2 is off, and Solenoid 1 is off – Off = Solenoid 3 on

• Solenoid 3 – On = Sol 1 is off, and Sol 2 is off, and Motor has run for 30 sec. – Off = Solenoid 3 has been on for 60 sec. Solenoid 1

Solenoid 2

Motor

Ingredient A

Ingredient B

Sensor 1

Sensor 2

Solenoid 3

PLC Applications: Packaging De-Palletizing, Washing, Filler, Capper, Case Packaging, Wrapping, Cartoning, Labeling, & Bagging Machines

PLC Application: SCADA & RTU Supervisory Control and Data Acquisition (SCADA)/Remote Terminal Unit (RTU) •

puts the remote site report data at your fingertips, but also provides affordable remote communications and control, minimizing the likelihood of remote site issues.

PLC Applications: Material Handling • Control of rollerbed conveyors or towveyors, and mobile equipment including transfer vehicles, elevated transfer vehicles, lifts and hoists, and stacker cranes.

PLC Applications: Distribution Start and stop conveyors, control traffic at merge points, redirect packages at divert points, and control a wide assortment of OEM machines such as palletizers and accumulators.

PLC Programming

Programming PLC’s The purpose of a PLC Program is to control the state of PLC outputs based on the current condition of PLC Inputs Different PLC’s support different languages, but the most popular PLC language is know as “Ladder Logic”. PLC Ladder Logic purposely resembles Relay Logic

Ladder Logic Concepts Read / Conditional Instructions Start (Rung #1)

||

()

|| ||

Write / Control Instructions

||

||

|/|

()

|/|

()

|/|

()

|/|

()

|| End (Rung #5)

||

Ladder Logic Concepts

Read / Conditional Instructions

Write / Control Instructions

|/|

||

()

T

F

F

No Logical Continuity

|/|

|/|

()

T

T

T

Logical Continuity

Logical AND Construction

IF input 4 AND input 5 have power THEN energize output 0 I/4

I/5

O/0

T

T

T

||

||

Logical Continuity

()

On

Logical OR Construction

IF input 4 OR input 5 have power THEN energize output 0 I/4

T

O/0

||

F

()

Logical Continuity

I/5

||

I/4

F

O/0

||

T

On

I/5

||

()

Logical Continuity

On

Complex Construction

I/4

I/0 I/1

||

|/|

||

I/5

I/1

I/7

I/2

I/3

||

|/|

||

|/|

I/9

||

I/8

|/|

||

I/11

|/|

I/1

||

I/10 O/0

|| ()

Read Instructions

Supply Voltage LS 1

Unused

Unused

COM

I/0

I/1

I/2

I/3

I/4

COM

I/5

I/6

I/7

The instruction is: If the input device is

The input bit is

Examine ON

Examine OFF

-| |-

-|/|-

XIC

XIO

Open (0)

Logic 0

False

True

Closed (1)

Logic 1

True

False

I/8

I/9

Write Instruction ||

|/|

()

T

T

T

OTE Output Energize

Rung State

Output Bit

Output Terminal

TRUE

ON

ENERGIZED

FALSE

OFF

De-energized

-( )-

L1

L2/N

GND

VAC VDC

Supply Voltage

O/0

VAC VDC

O/1

VAC VDC

O/2

VAC VDC

O/3

VAC VDC

O/4

O/5

Putting it Together

PB1

Supply Voltage

Unused

L1

Unused

COM

L2/N

GND

I/0

I/1

I/2

I/3

I/4

VAC

O/0

VAC

O/1

VAC

VDC

VDC

VDC

COM

O/2

I/5

I/6

I/7

I/8

I/9

VAC

O/3

VAC

O/4

O/5

VDC

Supply Voltage

I/8

||

O/0

()

VDC

Addressing Example

L1

L2 SOL6

PB1 LS1 PS2

I/5

||

DEVICE PB1

ADDRESS HHP Logix I/5 I:0/5

LS1

I/6

I:0/6

PS2

I/7

I:0/7

SOL6

O/0

O:0/0

I/6

I/7

O/0

||

||

()

Relay Logic to Ladder Logic PB1

LS1

LS1

SOL2

PB2

CR3

I/4

I/6

||

||

()

I/7

I/5

B/0

||

||

()

I/8

LS3

||

B/0 CR3 |/|

O/0

LS4

M1

|/|

INPUT Address Assignment: PB1- I/4 PB2- I/5 LS1- I/6 LS2- I/7 LS3- I/8 LS4- I/9 OUTPUT Address Assignment: SOL2- O/0 M1- O/1

I/9

||

O/1

()

Advanced Instructions

• SEQUENCERS • SHIFT REGISTERS • DATA HANDLING • HIGH SPEED COUNTER • SUBROUTINES

Advantages of a PLC

Basic PLC Advantages

• • • • •

Ease of programming Ease of maintenance Designed for industrial environment Quick installation Adaptable to change

Source: A-B’s Micro Solutions Presentation

Advantages over Relays • All the capabilities of the earlier systems • Dramatic performance increase over the relay logic systems • Greater reliability • Little maintenance due to no moving parts • No special programming skills required by maintenance personnel • Physical size of the PLC system is much smaller than the conventional relay based logic • And most importantly much lower cost Source:

[email protected]

Advantages over SBC’s (single board computers) • SBC’s have high design costs – Contract or Staff with overhead and maintenance issues associated with each

• SBC’s are Repair / Service nightmare for customers – Depending on markets served supplier must develop/support services

• SBC’s requires high level of technical expertise by technicians – Specialized circuit boards require specialized equipment and technical staff

• SBC’s not stocked through local distributors – Suppliers sell the controller imbedded, replacement parts are not readily available even in emergency situations

• SBC’s typically do not meet worldwide standards – Certifications cost money, typically a single controller does not warrant the investment. Volumes are not high enough and re-certification on each revision is unrealistic

• SBC’s typically a “Domestic” product – Because of these issues many manufacturers limit themselves to a single market. Source: A-B’s Micro Solutions Presentation

Advantages over Computer based Software • Maintenance personnel already experienced in PLC troubleshooting and servicing relay latter logic programming, not PC software • Better power failure response • Cost advantages for simple machine control • Higher reliability that minimizes the expense of shutdown, troubleshooting, repair, & startup • Industrially hardened packaging • Long availability and support for product models without the rapid obsolescence of PCs.

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