Automated Manufacturing System (SRD 35403) Introduction to PLC _____________________________________________________________________________________________________ CHAPTER 1 INTRODUCTION TO PROGRAMMABLE LOGIC CONTROLLER 1.1 Introduction Automation was developed as a way of placing the burden of certain tedious, repetitive and dangerous tasks upon machines, and, as a consequence of technological development in electronics and microprocessors PLCs appeared. A PLC is a system based upon a microprocessor, generally of a modular configuration, which can be programmed in a non-computer language and which is designed for controlling sequential processes in real time in the industrial environment. A PLC monitors inputs, make decisions based on its program and controls outputs to automate a process or machine. PLCs provide a great deal of flexibility to the system and occupy minimal space. Furthermore, they require very low labour costs for installation, maintenance is inexpensive and they may be reused on other machines. 1 ______________________________________________________________ _____________ Prepared by: Haris & Izwan Unikl Malaysian Spanish Institute Section Of Electrical, Electronics & Automation Jan 2009
Transcript
Automated Manufacturing System (SRD 35403)Introduction to PLC
1.1 IntroductionAutomation was developed as a way of placing the burden of certain tedious, repetitive and dangerous tasks upon machines, and, as a consequence of technological development in electronics and microprocessors PLCs appeared.
A PLC is a system based upon a microprocessor, generally of a modular configuration, which can be programmed in a non-computer language and which is designed for controlling sequential processes in real time in the industrial environment. A PLC monitors inputs, make decisions based on its program and controls outputs to automate a process or machine.
PLCs provide a great deal of flexibility to the system and occupy minimal space. Furthermore, they require very low labour costs for installation, maintenance is inexpensive and they may be reused on other machines.
1___________________________________________________________________________Prepared by: Haris & Izwan Unikl Malaysian Spanish Institute
Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
1.2.2 PLC’s Structure & OperationProgrammable Logic Control (PLC) is made up basically of three parts:
Input section.
Central processing unit (CPU).
Output section.
a) Input section . Accepts a variety of digital or analog signals from various field devices (pushbuttons, limit switches, transducers, sensors) and converts them into a logic signal that can be used by the CPU.
b) The CPU (central processing unit). This is the thinking part of the system. The CPU makes decisions and executes control instructions based on program instructions in memory. It interprets the instructions from the user program and according to input values, enables the desired outputs.
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Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
c) Output section . Its function is to convert control instructions from the CPU into a digital or analog signal that can be used to control various field devices (actuators - lamps, relays, contacts, solenoids).
The simple example: Pushbuttons is connected to PLC inputs can be used to start and stop a motor connected to a PLC through a motor starter (actuator).
Furthermore, for the PLC to be operative, the following should be added (show examples):
Power supply.
Programming console.
Peripheral devices.
Interfaces.
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Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
f) Peripherals. Peripherals are external auxiliary equipment with a specific function, which are
connected to the PLC, extending its field of application or facilitating its use. Normally they are not directly involved in the creation and execution of the program (keyboards, monitors, displays, etc.).
g) Interfaces. Interfaces are circuits or devices that allow the CPU to be connected to the external
elements (peripherals).
1.3 Architecture of a PLC
(*) - The program is in ROM and data memory in RAM. Some manufacturers use a single EPROM.
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Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
1.3.1 Memory typesThese are devices where information is stored.
Types:
a) RAM (Random Access Memory), memory which can be read from and written to. Reading and writing are done electronically. Information is lost if the power supply is not maintained.
b) ROM (Read Only Memory) Its contents may be read, but it may not be written to. Data or instructions are recorded by the manufacturer and they may not be altered. Information is not lost when power is switched off.
Other types of memory are as follows:
Programming Erasing If power is lost
PROMProgrammable ROM
Electronic not possible Data are not lost
EPROMElectrically
programmable PROM
Electronic Ultraviolet light (UV) Data are not lost
EEPROMElectrically erasable
PROM
Electronic Electronic Data are not lost
Use of memory in a PLC:
a) User memory:
The user program is normally recorded in RAM since it will be read by the microprocessor and changes made by the user must be permitted. Since RAM loses data if power is lost, a backup battery is installed to keep power flowing to the circuitry, thus preventing it from being erased. In some PLCs a EPROM or EEPROM is used instead.
b) Data table:
This is the area where the I/O image, internal variables such as counter values, etc. are stored. They are stored in RAM since they are data which are constantly changing.
c) System memory:
On one hand we have:
ROM or EPROM which contains CPU boot data.
On the other hand:
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Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
May replace the RAM once a definitive program has been developed.
1.3.2 CPU.The CPU is made up of the following elements: processor, memory, and associated auxiliary circuits.
ROM memory is what tells the microprocessor what it should do when booting up, during processing and at system shut-down (firmware or system software).
Its general functions are:
Supervision and control of cycle times (watchdog).
Self-test at power-on and during program execution.
Start-up of the program and system configuration scan cycle.
Generating the cycle time base.
External communications.
Etc.
Basic CPU work cycle.
1) Creating the input image, which is stored in internal registers which do not vary during program execution.
2) Access and execution of program instructions, updating registers as indicated by the program.
3) Updating outputs according to the state of the registers.
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Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
1.3.3 Input/Output (I/O) UnitsThese are the basic devices which take information from the sensors and pass it on to the CPU in the case of the inputs, or pass information from the CPU to the actuators in the case of the outputs.
Their main functions are:
Adapting voltages and working output of the sensors and actuators to those of the PLC’s electronic circuits.
Electrically insulating the logic circuits from the power circuits.
Assigning specific processor addresses to the sensors and actuators.
1.3.4 Types of inputs.Inputs (Input, Entrada, Eingabe) may be grouped:
According to the type of power:
No power.
Direct current.
Alternating current.
According to the type of signal:
Analogue.
Digital (zero or one).
Digital inputs.These are the most commonly used and their input signal is of the all or nothing type, that is, at a certain voltage level or at zero voltage. An example of this type of device are limit switches, push-buttons, toggle switches, etc.
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Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
Analogue inputs.There are some magnitudes whose input values are not all or nothing, but are a range of continuous values. For these cases it is necessary to have an analogue input module. Analogue inputs give variable voltage or current values. Typical values are:
Voltage: 0 to 10 Volts or -10 to 10 V.Intensity: 4 to 20 mA.
The most typical cases are electrical devices for measuring temperature, level, position, the level of a liquid, the speed of a motor (tachometer), etc.
Example of a liquid level measurement:
+10V
0V
Value in volts proportional to the level
In order for the analogue value to be adapted to the CPU there should be an Analogue/Digital converter. The precision of the Analogue Input unit depends on the precision of this converter (number of bits).
Example:If we have a two-bit converter, we have two combinations of bit values, where the combination 00 corresponds to an input level of 0V and the combination 11 corresponds to 10V (maximum signal), the intermediate values will be:
00 0V
01 3.33V
10 6.66V
11 10V
If instead of a two-bit converter we had a four-bit converter, the intermediate values would be as follows:
0000 0V
0001 0.66
0010 1.33
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interval
10
2 110
2 1103
3332
Vn .
interval
10
2 110
2 11015
0 664
VVn .
Automated Manufacturing System (SRD 35403)Introduction to PLC
When applications require high precision, a resolution of 12 bits is generally used.
interval
10
2 12 4412
VmV.
Special inputs.Many PLCs, in addition to the inputs discussed so far, have other types of inputs which may be categorised as special inputs.
Fast counting input:These are inputs which are connected directly to an internal counter; their operation is completely independent of the PLC’s normal cycle.
+
encodersignal-
Fast inp.
Alarm or error inputs:These inputs are generally used to interrupt normal operation of the PLC cycle in order to execute special tasks.
1.3.5 Types of outputsAs in the case of the inputs, we can divide the outputs into:
Analogue.
All or nothing (digital).
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Section Of Electrical, Electronics & Automation Jan 2009
Automated Manufacturing System (SRD 35403)Introduction to PLC
Analogue outputs give a variable voltage or intensity value. Digital outputs are used to actuate elements which are either enabled or disabled (all or nothing). Under the heading of digital outputs there are three types:
Relay output.
Transistor output.
TRIAC output.
Relay outputs.These are used with actuators that can operate on direct current or alternating current. They are used when actuating speed does not need to be fast, at average power consumption (around 2 amperes).
Transistor outputs.Currently the most common outputs are transistor outputs.
These cards are only used for DC current at low power (0.5 amperes); they feature quick response and long MTBF as they last longer than relays.
TRIAC outputs.The elements to be placed on a TRIAC output may operate on AC or DC current, and they are used in applications requiring fast switching where a relay would be too slow. They last longer than a relay and can handle similar power levels (2 amperes).
Analogue outputs.The inverse of analogue inputs is applied here, as a digital/analogue converter is required:
binary value
to externalelements
Oearth
D
Aconvertor
The external elements to be connected are usually:
Motor speed regulators.
Proportional solenoids.
As in the case of analogue inputs, the higher the precision of the D/A converter, the greater the analogue values that can be obtained.
Analogue outputs can be on the order of:
Voltage: 0-10V or -10 to 10 V
Current 4-20 mA.
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Automated Manufacturing System (SRD 35403)Introduction to PLC
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