Plc basics

PLC – The Basics

Programmable Logic Controllers.

PLC - Basics

1. What is a PLC

2. Before PLCs (Before PCs)

3. Advantages of PLC

4. Disadvantages of PLC

5. PLC Configurations

6. PLC versus PC

7. Parts of a PLC

8. PLC in Operation

PLC – The Basics

9. Ladder Logic

10.Programming the PLC

11.Siemens SIMATIC S7 300 – An Overview

12.Troubleshooting & Maintenance

13.Closing (Q/A)

14.Acknowledgements

1. What is a PLC ?

A Programmable Logic Controllers (PLC)

is a miniature industrial grade computer

that contains hardware and software –

capable of being programmed to perform

control functions

Image Source: SiemensImage Source: Koyo

2. Before the PLC.

The development of the PLC can be compared

analogously to the development of the Personal

Computer,

Before the PC what were the computing devices:

Abacus

Slide Rule

Table of Logarithms

Electronic Calculator

Personal Computer (Desktop, Laptop, Mobile

Devices

2. Before the PLC.

How were machines and industrial processes controlled before the advent of the PLC?

One of the means for controlling machines was through the use of

Power Relays and their associated

Control Relays

2. Before the PLC.

What are the disadvantages of relay based

control systems?

Complexity,

Costly

Hardwiring,

Logistical nightmare

Troubleshooting problems

Strict Maintenance routine

Not easy to modify

Etc, etc

2. Before the PLC.

Control devices:

• Rotary drum switch

• Limit switch

• Electromechanical Counter

• Fuses

• Control Transformers

• Motor Starter

• Solenoid Valves

• Pneumatic plunger timers

• etc

2. Before the PLC.

Historical Background

General Motors

Corporation specified the

design criteria for the first

programmable controller in

1968

The main goal:

To eliminate the high costs

associated with inflexible,

relay-controlled systems.

Main Criteria

• The controller had to be designed in modular

form, so that sub-assemblies could be

removed easily for replacement or repair.

• The control system needed the capability to

pass data collection to a central system.

• The system had to be reusable.

• The method used to program the controller

had to be simple, so that it could be easily

understood by plant personnel.

3. Advantages of the PLC.

A Programmable Logic

Controller (PLC) is a device that

was invented to replace the

necessary sequential relay circuits

for machine control. The PLC

works by looking at its inputs and

depending upon their state, turning

on/off its outputs. The user enters

a program, usually via software,

that gives the desired results.

PLCs are used in all industries.

Manufacturing

Process Plants & Systems

Machining,

Packaging,

Automated Plants

Etc


They are re-programmable

Solid state switches last much longer than relays

Complex logics can be easily represented

Multiple devices can be embedded in one unit

Can easily be scaled up or modified.

Smaller physical size than hard-wire solutions.

Easier and faster to make changes.

PLCs have integrated diagnostics and override functions.

Diagnostics are centrally available.

Applications can be immediately documented.

Applications can be duplicated faster and less expensively.


Connection between switches/output can be

modified through software easily.

Troubleshooting is Easier and Faster.

Ease of Maintenance – less downtime.

Easy to develop Programs by offline simulation

Less amount of Space Needed

Changes are easier and faster to implement,

Integrated diagnostics


Inputs and Outputs are easier to monitor by

HMI devices....and from PC's

Can withstand severe environmental

conditions.

Cost effective for controlling complex

systems.

Computational abilities make possible more

sophisticated controls

Reliable components make for long uptime

before failure.

4. Disadvantages of the PLC.

Most PLCs manufacturers offer only closed

architectures for their products .

PLC devices are proprietary, proprietary,

which means that parts and from one

manufacturer can’t easily be used in

combination with parts of another

manufacturer, which limits the design and cost

options.

Subject to the limitations imposed by

semiconductor based systems.

Setup and training costs could be high

5. PLC – ConfigurationsPLCs are of two main configurations.

• Modular Configuration

• Fixed Configuration .

Modular Configuration

5. PLC – ConfigurationsPLCs are of two main configurations.

• Modular Configuration

• Fixed Configuration

.

Fixed Configuration

6. PLC versus PC

PLC

• Designed for extreme industrial

environments

• Can operation in high

temperature and humidity

• High immunity to noise.

• Integrated Command interpreter

(proprietary)

• No secondary memory like HDD

• Optimized for a Single task

PC• Mainly for Data Processing & Calculation

• Optimized for Speed

• Not built for extreme enviroments

• Can be programmed in several languages

• Secondary Memory is Built in.

• Built for multitasks

7. Parts of a PLC.

Basic parts of a PLC

Power Supply

Processor Module

CPU

Memory

Communication Interface.

HMI – Status

HMI – Programming

I/O Modules

Discrete/Digital Inputs

Analog Inputs

Output Modules

Sections of a PLC module.

(Courtesy: Mitsubishi Automation)

7. Parts of a PLC.

(Courtesy: Hitachi)

7. Parts of a PLC.

Power Supply

PLC Power Supply (Courtesy: Allen Bradley)

The system power supply plays a major

role in the total system operation.

Its responsibility is not only to provide

internal DC voltages to the system

components (i.e., processor, memory, and

input/output interfaces), but also:

a) to monitor and regulate the

supplied voltages and warn the

CPU if something is wrong.

b) The power supply, then, has the

function of supplying well-

regulated power and protection

for other system components.

7. Parts of a PLC.

Power Supply

Usually, PLC power supplies require input

from an AC power source; however, some

PLCs will accept a DC power source. Those

that will accept

Most PLCs, however, require a 120 VAC or

220 VAC power source, while a few

controllers will accept 24 VDC.

Since industrial facilities normally

experience fluctuations in line voltage and

frequency, a PLC power supply must be

able to tolerate a 10 to 15% variation in

line voltage conditions.

The first step in estimating the load is to

determine how many modules are required

and then compute the total current

requirement of these modules.

The following table lists the module types,

current requirements for all inputs and

outputs ON at the same time, and the

available power supplies for our

programmable controller example.

7. Parts of a PLC. Power Supply The first step in estimating the load is to determine how many modules are

required and then compute the total current requirement of these modules.

The following table lists the module types, current requirements for all

inputs and outputs ON at the same time, and the available power supplies for

our programmable controller example.

4

7. Parts of a PLC. CPU (Controller/ Processor)

Memory

Typical Processor

Module

7. Parts of a PLC.

CPU (Controller/ Processor)

Memory

• Processors are either modular or built

into the PLC

• They vary in processing speed and

memory options.

• Processor is optimized for high speed

control and not general purpose

computing.

Allen Bradley SLC 500 CPU

(Courtesy: Allen Bradley)

7. Parts of a PLC.

CPU Functions:

• Executes the operating

system

• Manages memory,

• Monitors inputs,

• Evaluates the

• Means for connecting to

an external programming

device

• Provide system diagnostics

with status LED

indicators.

• It may have a switch for

selecting mode of

operation :

• RUN,

• PROG

• REM

RUN

• Places the processor in the Run mode

• Runs Ladder program and energizes output devices

• Prevents online program editing in this position

• Prevents use of programmer/operator interface device to change

the processor mode

PROG Position

• Sets the processor in the Program mode

• Prevents the processor from scanning or executing the ladder

program, and the controller outputs are de-energized

• Enables program entry and editing

• Prevents you from using a programmer/operator interface device

to change the processor mode

REM Position

• Places the processor in the Remote mode: either the REMote

Run, REMote Program, or REMote Test mode

• Allows you to change the processor mode from a

programmer/operator interface device

• Allows you to perform online program editing

7. Parts of a PLC.

I/O Module

Typical I/O Module

(Courtesy: Rockwell Automation)

7. Parts of a PLC.

Discrete devices are inputs and outputs that have

only two states: on and off.

Discrete I/O modules perform four tasks in the PLC:

• Sense when a signal is received from a field

device.

• Convert the input signal to the correct voltage

level for the particular PLC.

• Isolate the PLC from fluctuations in the input

signal’s voltage or current.

• Send a signal to the processor indicating which

sensor originated the signal.

Examples of discrete input

devices:

ON/SWITCHES

Limit switches.

Push buttons

Output can control ON OFF devices only

7. Parts of a PLC.

Analog devices represent physical

quantities that can have an infinite

number of values. Typical analog

inputs and outputs vary from

0 to 20 milliamps, 4 to 20 milliamps,

or 0 to 10 volts.

Analog I/O modules deals with signals that

are continuously changing. They are

needed for precise control of the process

under the control of the PLC.

Examples,

• Temperature

• Pressure

• Humidity

• Density

• Fluid Level

7. Parts of a PLC.

Communication ModulesUsed to establish point-to-point connections

with other intelligent devices for the exchange

of data.

Such connections are normally established with

computers, operator stations, process control

systems, and other PLCs.

Communication modules allow the user to

connect the PLC to high-speed local networks

that may be different from the network

communication provided with the PLC.

Serial Communication Module

(Courtesy: www.automationdirec.com

http://www.automationdirec.com/

7. Parts of a PLC.

Other types of output modules

Motion Control Modules

PID Modules

BCD/ASCII Modules

Stepper Motor Control

Encoder Counter Module

High Speed Counters

Motion & Position Control

8. The PLC in Operation.

Three Phase AC Motor Control

The opposite diagram

illustrates the use of a NC

and NO pushbutton switches

to control the 3 phase AC

motor.

The ON/OFF control of the 3

phase motor can also be

implemented with a PLC.

First, we need to understand

the use of Logic gates.

8. The PLC in Operation.Logic Gates A logic gate is a circuit with several inputs but only

one output that is activated by particular combinations of

input conditions.

Boolean algebra as related to AND, OR, and NOT functions.

8. The PLC in Operation.

The PLC, as used to control the operation of the AC

motor responds to the presence or absence of Logic

signals at its I/O module to control response the

output devices that receive signal from the output.

Examples of discrete inputs: Push Buttons, selector

switches, limit switches, proximity switches.

Example of discrete outputs devices: Indicator Lights,

Relays, Motor Starters.

8. The PLC in Operation. PLC Control of AC Motor

PLC Control: ( Phase Motor in OFF position)


PLC Control: (Phase Motor in START position)


PLC Control: (Phase Motor in RUNNING position)


PLC Control: (Phase Motor in switch OFF position)

8. The PLC in Operation.PLC Control – More Examples

Manufacturing,

Mining,

CNC

Assembly Line Processes

PLC Control: (Other Examples)

8. The PLC in Operation. PLC – Program Execution Cycle

The processor (CPU) is the “brain” of the

PLC.

What the CPU does:

Implements the logic and controlling the

communications among the modules.

Stores program information and logical operations

results in memory - EPROM or EEPROM plus RAM.

Controls all PLC activity.

Enables user to enter in the desired program in

relay ladder logic

Typical PLC CPU (Courtesy: Rockwell Automation)

8. The PLC in Operation. PLC – Program Execution Cycle

The PLC program is executed as

part of a repetitive process referred

to as a scan.

A typical PLC scan starts with the

CPU reading the status of inputs.

Next, the application program is

executed.

Next, the CPU performs internal

diagnostic and communication tasks.

Finally, the status of all outputs is

updated.

This process is repeated

continuously as long as the PLC is in

the run mode.

9. Ladder Logic

The devices that control the logic functions of

a control system are physical wired. This is

called hard wired logic.

Hard wired logic is done by using relay ladder

schematics.

Control scheme and the associated control

elements are represented between two power

lines.

All control element are placed in a ladder like

function between the two power lines.

9. Ladder Logic

The PLC logic function can be similary

represented in a ladder logic diagram.

Major difference is that the hard wired logic

can only be modified by rewiring and changing

element as needed. The PLC control function

depends on the logic states of the outputs and

these are very easy to change through the

software program.

9. Ladder Logic

Motor Ladder Logic

9. Ladder Logic• PLC express control logic in terms of contact

symbols symbols.

• Symbols are the same as those used for hard

wired relay control circuits.

• A rung is the contact symbolism required to

control an output.

• A complete ladder logic program has several

rungs of ladder, each of which controls an

output.

• In PLC logiclogic all mechanical switch contacts

are represented by a software contact symbol

and all electromagnetic coils are represented

by a software coil symbol.

• The PLC uses ladder logic diagrams, the

conversion from any existing relay logic to

programmed logic is therefore simplified.

• Each rung is a combination of input conditions

(symbols) connected from left to right, with the

symbol that represents the output at the far right.

• The symbols that represent the inputs are connected

in series, parallel, or some combination of the two

to obtain the desired logic.

CPU Scan Time

While in operation, the controller scans the logic

stored in the CPU memory continuously.

The completion of a cycle of the controller is

called a Scan.

The scan time needed to complete a full cycle by

the controller gives the measure of the speed of

execution for the PLC.

9. Ladder Logic

Two limit switches connected in series

and used to control a solenoid valve

9. Ladder Logic

Two limit switches connected in parallel

and used to control a solenoid valve

9. Ladder Logic

Two limit switches connected in parallel with each

other and in series with a pressure switch and used

to control a solenoid valve

9. Ladder Logic

A motor control circuit with two start/stop buttons. When

either start button is depressed, the motor runs. By use of

a seal-in contact, it continues to run when the start button

is released. Either stop button stops the motor when it is

depressed

10. Programming the PLC

The system to be controlled by

the PLC is first described in ladder

logic.

Next the ladder logic is compiled

and translated into basic

instructions that are uploaded into

the PLC memory by the

programmer.

The programming is done while

the PLC is set to TERMINAL OR

PROGRAMMING MODE.

Programming can be done through

a PLC Programmer or a PC that

has the programming software.

Totally Integrated

Automation

SIMATIC

S7-300

The universal, small

control system

supplemented by new,

compact CPUs

11. Siemens SIMATIC S7 300

SIMATIC S7-300

in the system familyUpper and midperformance range

S7-400

Lower and midperformance range

S7-300

Bottom performance range

S7-200

+ Programming devices

+ STEP 7 software

+ Communication

+ HMI

S7-300 - the universal, small control system for versatile applications in automation engineering

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


The new S7-300 compact CPUs

3 basic types:

With different memory sizes and performances

312C: 16 Kbyte

313C: 32 Kbyte

314C: 48 Kbyte

The versions differ with respect to

- I/Os

- Onboard interfaces

- Process functions

312C

313C

313C-2 DP313C-2 PtP

314C-2 DP314C-2 PtP

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


Dimensions and design

Onboard I/Os:

Integral interfaces:

Max. design:

DI/DO

MPI

1 tier

DI/DO + AI/AO

MPI

4 tiers

DI/DO

MPI + PtP/DP

4 tiers

DI/DO + AI/AO

MPI + PtP/DP

4 tiers

312C

80 mm

313C

120 mm

313C-2 PtP

313C-2 DP

120 mm 120 mm

314C-2 PtP

314C-2 DP

125 mm

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


Memory,

performance and

quantity breakdown

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC 312C 313C 314C

Main memory 16 kB 32 kB 48 kB

Statements 5 k 10 k 16 k

Loading memory plug-in 64k-4MB plug-in 64k-4MB plug-in 64k-4MB

Instruction runtime min. 0.2 µs min. 0.1 µs min. 0.1 µs

Alarm response time 800 µs 400 µs 400 µs

Bit memories 1024 2048 2048

Timers / counters 128 / 128 256 / 256 256 / 256

Address space I/O 1024 / 1024 byte 1024 / 1024 byte 1024 / 1024 byte

No. of digital channels 266 1000 1000

No. analog chann. I/O 64 / 64 248 / 248 248 / 248


Integral I/Os - summary Low-cost onboard I/O channels for universal use

Every digital input can be used as an alarm input

Analog inputs can also be used as digital inputs

* Additional input for resistance measurement

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC

312C 313C 314C

313C 313C-2 PtP / DP 314C-2 PtP / DP

Number of DIs 10 24 16 24

Number of DOs 6 16 16 16

Number of AIs -/- 4 + 1* -/- 4 + 1*

Number of AOs -/- 2 -/- 2


Integral digital I/OsOverview

Comparison

I/O‘s

Process Functions

Communication

MMC

Digital

inputs

Digital

outputs

Rated voltage DC 24 V DC 24 V

Permissible range DC 20.4 - 28.8 V DC 20.4 - 28.8 V

Current range --- 0.5 A

Input delay 0.1/0.5/3/15 ms ---

Switch-off delay --- 2 ms

Electrical isolation from backplane bus

yes yes

Groups of 16 8

Max. frequency --- 100 Hz


Integral analog I/OsOverview

Comparison

I/O‘s

Process Functions

Communication

MMC

Analog

inputs

Analog

outputs

Measuring ranges

Voltage ±10V; 0..10V ±10V; 0..10V

Current ±20mA; 0/4..20mA ±20mA; 0/4..20mA

Resolution 11 bits+sign 11 bits+sign

Filter (50/60 Hz) selectable ---

Input delay 5 ms ---

Output delay --- 1,2 ms

Electrical isolation from backplane bus

yes yes


Summary of process functionsOverview

Comparison

I/O‘s

Process Functions

Communication

MMC

312C 313C 314C

Counting

Connectable sources Incremental encoder,

pulse generator with

direction signal

Incremental encoder,


direction signal

Incremental encoder,


direction signal

Number of channels 2 3 4

Cut-off frequency 10 kHz 30 kHz 60 kHz

Frequency measurem. yes yes yes

PWM

Number of outputs 2 3 4

Cut-off frequency 2.5 kHz 2.5 kHz 2.5 kHz

Positioning no no 1 axis

Control - PID PID


Integral counters Integral counters in all compact CPUs

- Recording of pulse and incremental encoder

signals (DC 24V)

- Forward/reverse with reference values which

can be changed during operation

- 10 - 60 kHz (depending on CPU)

Various operating modes possible

- Single counting (e.g. filling, dosing)

- Periodic counting (e.g. recording of angle)

- Counting with gate control (e.g. length measurement)

Frequency measurement

- Counting with fixed time base

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


PWM outputs

Pulse outputs on all compact CPUs

- Direct control of valves, actuators, switchgear,

heaters etc. (DC 24 V/ 0.5 A)

- Period and pulse/pause ratio can be changed

during operation

- 2.5 kHz switching frequency, up to 4 outputs

(depending on CPU)

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


Simple motion control

without additional Components Low-price

Since no additional modules required

Optimum memory requirements and runtime

No additional programming requirements

since function is component of operating

system

Flexible

Parameters (delay, acceleration etc.) can be

changed for each travel

Various operating modes selectable: absolute

or relative positioning, inching etc.

Simple

Prepared functionality can be linked into

application program using standard blocks

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


Summary of communications

312C313C 314C-2 PtP

313C-2 PtP314C-2 DP313C-2 DP

MPI

Point-to-point

PROFIBUSDP

Interface present on all CPUs - networking of CPU, programming device and OPs

Low-cost communications without additional HW - extremely simple configuring

Communication with up to 7 OPs simultaneously (depending on type of CPU)

Serial onboard interface

Data exchange e.g. with devices from other vendors

Fast, cyclic data exchange

High data security

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


On every CPU:

multipoint interface MPI

Data exchange: 187.5 kbit/s

Up to 32 bus stations, up to 12

active connections per CPU

Communications functions:

- Programming device/operator

panel functions

- Global data communications

without programming input

- S7 basic communication up to 76

byte

- S7 communication (only server)

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC

Low-cost communication

without additional hardware


Point-to-point interface (RS422/485)

Connection of non-system components

CPU 313C-2 PtP / 314C-2 PtP

Transmission physics:

- RS 422/485 (X.27)

- Transmission rate: up to 19.2 / 38.4 kbit/s (half duplex/full duplex)

Protocols:

- ASCII

- 3964(R)

- RK 512

- (only 314C-2 PtP)

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


CPU 313C-2 DP,

314C-2 DP,

integral PROFIBUS-DP

Versatile use:

master or slave function

Data exchange at 12 Mbit/s

Up to 32 DP stations

to master interface

Max. distance 23 km using FO

Communications functions:

- All programming device/OP

functions

- PROFIBUS-DP

PG

PROCESS FIELD BUS

S I E M E N S

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC

No additional module or software required!


SIMATIC Micro Memory Card

Many functions - small format

Can be used in every S7-300 compact CPU

Functions as program memory, non-volatile and

resistant to overall reset; transportable for simple

program updating

Functions as loading memory - flexible as result

of selectable MMC sized between 64kB and 4MB

Permits project storage on CPU - save your

complete project on the MMC

Access to the MMC during RUN mode of CPU

- Load data into CPU (recipe)

- Write data onto MMC (archive)

MMC buffers your data in the main memory in

event of power failure

no backup battery required

MMC is required to operate the compact CPUs

Overview

Comparison

I/O‘s

Process Functions

Communication

MMC


12. Troubleshooting & Maintenance

• Ground yourself by touching a conductive surface before

handling static-sensitive components.

• Wear a wrist strap that provides a path to bleed off any charge

that may build up during work.

• Be careful not to touch the backplane connector or connector

pins of the PLC system (always handle the circuit cards by the

edge if possible).

• Be careful not to touch other circuit components in a module

when you configure or replace its internal components.

• When not in use, store modules in its static-shield bag.

• If available, use a static-safe work station.

13. Closing

14. Acknowledgements

• © 2011 Frank D Petruzella; Programmable Logic Controllers 4th Edition,

McGraw Hill

• PLC Hand Book; www.automationdirect.com

• http://www.plcs.net/chapters/history2.htm

• http://library.automationdirect.com/plc-software-features-you-want/

• http://advanceelectricaltraining.com/electrical-resources/

• Siemens Automation

• Rockwell Automation

• GE Fanuc

• Koyo

• OMRON

http://www.automationdirect.com/

http://www.plcs.net/chapters/history2.htm

http://library.automationdirect.com/plc-software-features-you-want/

http://advanceelectricaltraining.com/electrical-resources/

Engineering

Plc basics