PROGRAMMABLE LOGIC CONTROLLERS AND LADDER LOGICGreg 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. 2. 3. 4.

PLC and Controls History PLC Components Ladder Logic 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

Advantages of PLCs 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

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) Internal Utility Relays

Output Relays (coils)

Counters

Data Storage

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, ondelay 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

Conclusion1. Programmable Logic History 3. PLC Components 5. Ladder Logic 7. Ladder Logic Programming