03-07-2014, 10:11 AM
Basic PLC
[attachment=65570]
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.
Programmable Controller Development
1968 Programmable concept developed
1969 Hardware CPU controller, with logic
instructions, 1 K of memory and 128 I/O
points
1974 Use of several (multi) processors within a
PLC - timers and counters; arithmetic
operations; 12 K of memory
and 1024 I/O points
1976 Remote input/output systems introduced
1977 Microprocessors - based PLC introduced
rogrammable Logic Controllers
( Definition according to NEMA standard ICS3-1978)
A digitally operating electronic apparatus which uses a programming 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 modules, various types of machines or process.
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.
Major Components of a Common PLC
POWER SUPPLY
Provides the voltage needed to run the primary PLC components
I/O MODULES
Provides signal conversion and isolation between the internal logic- level signals inside the PLC and the field’s high level signal.
I/O Module
The I/O interface section of a PLC connects it to external 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.
I/O Circuits
2. General - Purpose Outputs
These are usually low- voltage and low-current and are used to drive indicating lights and other non-inductive loads. Noise suppression may or may not be included on this types of modules.
3. Discrete Inputs
Circuits of this type are used to sense the status of limit switches, push buttons, and other discrete sensors. Noise suppression is of great importance in preventing false indication of inputs turning on or off because of noise.
Analog Input
An analog input is an input signal that has a continuous
signal. Typical inputs may vary from 0 to 20mA, 4 to 20mA
or 0 to10V. Below, a level transmitter monitors the level of
liquid in the tank. Depending on the level Tx, the signal to the
PLC can either increase or decrease as the level increases
or decreases
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.
PLC Operation
Basic Function of a Typical PLC
Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.
This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.
PHASE 4 - Output Status Scan
An output status scan is then performed, whereby the stored output values are sent to actuators and other field output devices. The cycle ends by updating the outputs.
[attachment=65570]
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.
Programmable Controller Development
1968 Programmable concept developed
1969 Hardware CPU controller, with logic
instructions, 1 K of memory and 128 I/O
points
1974 Use of several (multi) processors within a
PLC - timers and counters; arithmetic
operations; 12 K of memory
and 1024 I/O points
1976 Remote input/output systems introduced
1977 Microprocessors - based PLC introduced
rogrammable Logic Controllers
( Definition according to NEMA standard ICS3-1978)
A digitally operating electronic apparatus which uses a programming 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 modules, various types of machines or process.
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.
Major Components of a Common PLC
POWER SUPPLY
Provides the voltage needed to run the primary PLC components
I/O MODULES
Provides signal conversion and isolation between the internal logic- level signals inside the PLC and the field’s high level signal.
I/O Module
The I/O interface section of a PLC connects it to external 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.
I/O Circuits
2. General - Purpose Outputs
These are usually low- voltage and low-current and are used to drive indicating lights and other non-inductive loads. Noise suppression may or may not be included on this types of modules.
3. Discrete Inputs
Circuits of this type are used to sense the status of limit switches, push buttons, and other discrete sensors. Noise suppression is of great importance in preventing false indication of inputs turning on or off because of noise.
Analog Input
An analog input is an input signal that has a continuous
signal. Typical inputs may vary from 0 to 20mA, 4 to 20mA
or 0 to10V. Below, a level transmitter monitors the level of
liquid in the tank. Depending on the level Tx, the signal to the
PLC can either increase or decrease as the level increases
or decreases
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.
PLC Operation
Basic Function of a Typical PLC
Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.
This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.
PHASE 4 - Output Status Scan
An output status scan is then performed, whereby the stored output values are sent to actuators and other field output devices. The cycle ends by updating the outputs.