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
|/|
CR3
CR3 M1
PB1 LS1 SOL2
PB2LS1
LS3
LS4
I/8
I/4 I/6 O/0
O/1
| | | | ( )
I/5I/7 B/0| | | | ( )
| |
|/|B/0
( )
Relay Logic vs. PLC & Ladder Logic
| |I/9
ProgrammableLogic
Controller
Inputs Outputs
CR
Terminal Block
1
2
3
4
5
6
7
8
9
Input Devices
L1
L1
L210
COM
PLC
IsolationBarrier
Input Wiring: PLC input is the load in the circuit, sensing if voltage is present
Input Devices
• Pushbuttons
• Selector Switches
• Limit Switches
• Level Switches
• Photoelectric Sensors
• Proximity Sensors
• Motor Starter Contacts
• Relay Contacts
• Thumbwheel Switches
• 120/230 VAC
• 24 VDC
– Sourcing
– Sinking
OUT 1
Output Devices
L2
L2
L1OUT 1
OUT 2
OUT 2
OUT 3
OUT 3
OUT 4
OUT 4
OUT 5
OUT 5
OUT 6
OUT 6
CR
L1PLC
Terminal Block
IsolationBarrier
Output Wiring: PLC output is the switch, controlling current flow to load
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
CR
IsolationBarrier
IsolationBarrier
MEMORY
program data
HighVoltage
HighVoltage
Low Voltage
AC Power Supply85-264 VAC,
50/60Hz
Output
Circuits
ExternalDC Power Supplyor
Communications
Input
Circuits
CentralProcessor
Input Scan
Program ScanOutput Scan
Housekeeping
START
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”).
The status of external inputs (terminal block voltage) is written to the Input image (“Input file”).
The Output Image data is transferred to the external output circuits, turning the output devices ON or OFF.
Internal checks on memory, speed and operation. Service any communication requests, etc.
PLC Operating Cycle
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
Programmable Controller
Inputs Outputs
CR
Modular Chassis Based PLC’s
The vast majority of PLC’s installed today are modular chassis based systems consisting of:
1. Processor Module (CPU)
2. Input & Output Modules
3. Chassis
4. 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
+VDC
SOURCING Pushbutton(PNP)
SINKING Pushbutton(NPN)
DC Power Supply
+
- DC COM
DC Power Supply
+
-
SOURCING vs. SINKING DC Inputs
DC Power Supply
Field Device
DC Input Module
+
- DC COM
IN1
DC Input Module
Field Device
DC Power Supply
+
-
+VDC
IN1
SOURCING
(PNP)
SINKING
(NPN)
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.
RULES
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
Motor Solenoid 1 Solenoid 2
Solenoid 3
Sensor 1
Sensor 2
Ingredient A Ingredient B
Typical PLC Application
Motor Solenoid 1 Solenoid 2
Solenoid 3
Sensor 1
Sensor 2
Ingredient A
Ingredient B
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.
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
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Read / Conditional Instructions
Write / Control Instructions
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| | |/|
| |
| | |/| ( )
| |
( )
( )
( )
( )
| |
Start (Rung #1)
End (Rung #5)
Ladder Logic Concepts
Read / Conditional Instructions
Write / Control Instructions
No Logical Continuity
|/| | |
T F F
|/| |/|
( )
( )
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
Logical Continuity
T T T
On
Logical OR Construction
IF input 4 OR input 5 have power
THEN energize output 0
| |I/4
| |I/5
( )O/0
Logical Continuity
F
T
On
| |I/4
| |I/5
( )O/0
Logical Continuity
T
F
On
Complex Construction
|/|I/11
| |I/5
|/|I/7
|/|I/1
| |I/3
| |I/2
| |I/4
|/|I/0
| |I/1
| |I/1
|/|I/8
| |I/9
( )O/0
| |I/10
Read Instructions
Unused I / 2I / 1I / 0COM I / 3 I / 6I / 5COMI / 4 I / 7 I / 9I / 8
Supply
Voltage
Unused
LS 1
False
True
Examine OFF
-|/|-XIO
False
The instruction is:The input
bit is
Logic 0
Logic 1 True
Examine ON
-| |-XIC
If the input
device is
Open (0)
Closed (1)
Write Instruction
RungState
OutputBit
OutputTerminal
De-energized
TRUE
FALSE
ON
OFF
OTEOutput Energize
-( )-
| | |/| ( )
T T T
ENERGIZED
GNDL 1 O / 0VACVDC
L 2 / N VACVDC
VACVDC
O / 2VACVDC
O / 1 O / 3 O / 5O / 4VACVDC
SupplyVoltage
Putting it Together
| | ( )I/8 O/0
GNDL 1 O / 0VAC
VDC
L 2 / N VAC
VDC
VAC
VDC
O / 2VAC
VDC
O / 1 O / 3 O / 5O / 4VAC
VDC
Supply
Voltage
Unused I / 2I / 1I / 0COM I / 3 I / 6I / 5COMI / 4 I / 7 I / 9I / 8
SupplyVoltage
Unused
PB1
Addressing Example
L1 L2
PB1 LS1 PS2 SOL6
DEVICE
PB1
LS1
PS2
SOL6
| | ( )| | | |I/5 I/6 O/0I/7
HHPI/5
I/6
I/7
O/0
LogixI:0/5
I:0/6
I:0/7
O:0/0
ADDRESS
INPUT Address Assignment:PB1- I/4 PB2- I/5LS1- I/6 LS2- I/7LS3- I/8 LS4- I/9
OUTPUT Address Assignment:SOL2- O/0 M1- O/1
|/|
CR3
CR3 M1
PB1 LS1 SOL2
PB2LS1
LS3
LS4
I/8
I/4 I/6 O/0
O/1
| | | | ( )
I/5I/7 B/0| | | | ( )
| |
|/|B/0
( )
Relay Logic to Ladder Logic
| |I/9
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.