24-01-2013, 02:14 PM
Basic PLC
Basic PLC.ppt (Size: 1.18 MB / Downloads: 175)
Description
This training introduces the basic hardware and software components of a Programmable Controller (PLC). It details the architecture and basic instruction set common to all PLC’s. Basic programming techniques and logic designs are covered. This training describes the operating features of the PLC, the advantages of the PLC over hard-wired control systems, practical applications, troubleshooting and maintenance of PLC’s.
Objectives
At the end of the training the participants should be able to:
Describe the major components of a common PLC.
Interpret PLC specifications.
Apply troubleshooting techniques.
Convert conventional relay logic to a PLC language.
Operate and program a PLC for a given application.
INTRODUCTION TO PLCS
Advantages of PLCs
Less wiring.
Wiring between devices and relay contacts are done in the PLC program.
Easier and faster to make changes.
Trouble shooting aids make programming easier and reduce downtime.
Reliable components make these likely to operate for years before failure.
PLC Origin
- Developed to replace relays in the late 1960s
- Costs dropped and became popular by 1980s
- Now used in many industrial designs
Historical Background
The Hydramatic Division of the General Motors Corporation specified the design criteria for the first programmable controller in 1968
Their primary goal
To eliminate the high costs associated with inflexible, relay-controlled systems.
Tank Used to Mix Two Liquids
A tank is used to mix two liquids. The control circuit operates as follows:
1. When the start button is pressed, solenoids A and B energize. This permits the two liquids to begin filling the tank.
2. When the tank is filled, the float switch trips. This de-energizes solenoids A and B and starts the motor used to mix the liquids together.
3. The motor is permitted to run for one minute. After one minute has elapsed, the motor turns off and solenoid C energizes to drain the tank.
I/O Module
The I/O interface section of a PLC connects it toexternal field devices.
The main purpose of the I/O interface is to condition the various signals received from or sent to the external input and output devices.
Input modules converts signals from discrete or analog input devices to logic levels acceptable to PLC’s processor.
Output modules converts signal from the processor to levels capable of driving the connected discrete or analog output devices.
Processor
The processor module contains the PLC’s microprocessor, its supporting circuitry, and its memory system.
The main function of the microprocessor is to analyze data coming from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through output modules to the field devices. Field sensors: switches, flow, level, pressure, temp. transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.
The memory system in the processor module has two parts: a system memory and an application memory.
Memory Designs
The words random access refer to the ability of any location (address) in the memory to be accessed or used. Ram memory is used for both the user memory (ladder diagrams) and storage memory in many PLC’s.
RAM memory must have battery backup to retain or protect the stored program.