PROGRAMMABLE LOGIC CONTROLLERS

ANDLADDER LOGIC

Greg Zimmerman

Electrical Engineering Undergraduate

• Programmable Logic Controllers control most of the mechanical processes in many areas of production

• Very simple in operation, complex in design

Overview

1. PLC and Controls History

2. PLC Components

3. Ladder Logic

4. Programming of Ladder Logic

1. PLC and Controls History• Large amount of work required connecting wires• Difficulty with changes or replacements• Difficulty in finding errors; requiring

skillful/experienced work force• When a problem occurs, hold-up time is indefinite,

usually long• Too many moving parts

• Number of wires reduced by approximately 80%

• Fast and easy error detection.

• No change in wiring to change program

• Needs fewer spare parts• Cheaper when large number

of I/O instruments are needed

• Less moving parts• Compact• Cost effective for

installation/maintenance

Advantages of PLCs

2. PLC Components

• Definition• Components• PLC Operation

Definition

• A Programmable controller is a solid state user programmable control system with functions to control logic, sequencing, timing, arithmetic data manipulation and counting capabilities.

Components

• CPU• Memory Areas• Circuits to input or

output data

• Basically, a big box of math

Specific Components

• Input Relays (contacts)

• Output Relays (coils)

• Data Storage

• Internal Utility Relays

• Counters

• Timers

PLC Operation

• Continually scans ladder diagram

• Consists of 3 important steps

Rung Scanning

3. Ladder Logic

• Definition• Introduction• Comparison to Relay

Logic

Definition

• One form of drawing electrical logic schematics

• Very popular for PLCs• Originally invented for

use with relays

Comparison to Relay Logic

• First used for technicians, electricians & engineers

• Still first choice for most technicians, electricians, etc.

Relay Logic

• Ladder Logic

• Relay Logic

• “Jog” function added to previous relay circuit• 1 component added• 3 wires added

• Two status indicators added• 6 additional wires

• Most widely used program

• Shown here as a very small program

4. Ladder Logic Programming

• Introduction

• Basics – NO/NC Contacts/Coils

• AND & OR Gates

• Timers and Counters

• Building a PLC Ladder Logic Programming

Introduction

• First PLC programming system used• Borrowed heavily from relay diagrams plant

electricians already knew• Each rung solved left to right

Basics

• NO Contact

• NO Coil (Output)

• NC Contact

• NC Coil (Output)

AND Gate

OR Gate

Timers

• Very simple concept, it times

• 2 basic types, on-delay and off-delay

• Still sends logic as its output

Counters

• Counts number of times a lever is pulled, a button is pushed, etc.

• 3 types• Up Counter• Down Counter• Up-Down Counter

Building a PLC/Ladder Logic Program

• To illustrate, will start in relay logic, convert to ladder logic at end

• Will need to remove/replace some components

• Overload Device Removed• All components in relay diagram because wires

are run to them• Is not addressed in ladder logic• Motor relay is not a physical entity in ladder logic

as in relay logic

• ‘Jog Function’ added• Usually added for ease in troubleshooting

purposes only.• Now two ways to run motor – press start, or

press and hold Jog button

• Status Indicators added• Green for output on• Red for output off

• Relay Logic converted to Ladder Logic Diagram• Much fewer hard wired components• Double Pole Pushbutton for Jog switched to

Single Pole• Instead of motor relays, PLC just checks state of

motor output

Conclusion

1. Programmable Logic History

2. PLC Components

3. Ladder Logic

4. Ladder Logic Programming