eee3420 lecture02 rev2011
TRANSCRIPT
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│ Lecture 2 │
Programmable Logic Controller (PLC) Hardware
EEC3420 Industrial ControlDepartment of Electrical Engineering
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EEE3420 Industrial Control
Learning Objectives
Understand the basic hardware components.
Understand the operating principles of PLC.
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Background of PLC
low cost, compact, versatile units based on the standard microprocessor architecture used in the control of machines or processes
designed for ease of programming and maintenance
replace the old relay logic control systems in automated manufacturing
provide an easy and efficient replacements for the bulky relay logic controllers
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Definition of PLC
also known as programmable controller (PC) defined by the National Electrical Manufacturers Association (NEMA) in 1978 as:
"a digitally operating electronic apparatus which uses a programmable 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 input/output, various types of machines or process".
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Advantages of PLC
Cost effective
Flexible
Computational abilities allow more sophisticated control
Trouble shooting aids make programming easier
Reliable
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Typical applications of PLC
Embedded control in standard or custom-designed machines
Control of conveyor systems
Networked to manage large complex distributed control systems
Security alarm monitoring system
Elevator control
Air-conditioning control panel
Water treatment plant
Warning system on-board of ocean vessels.
Smoke candle control on-board aircraft for air-show.
Underground coal-mining machines.
Central building vacuum system.
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Electromagnetic relays
a voltage applied to the input coil, the resulting current creates a magnetic field
magnetic field pulls a metal switch towards it and the contacts touch, closing the switch
contact that closes when the coil is energized is called Normally Open (NO) contact
contact that is closed when the input coil is not energized is called Normally Closed (NC ) contact
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Relay circuit and ladder diagram
The first relay on the left is used as NC, and will allow current to flow until a voltage is applied to the input A.
The second relay is NO and will not allow current to flow until a voltage is applied to the input B.
If current is flowing through the second relay coil then current will flow through the coil in the third relay, and close the switch for output C.
This circuit would normally be drawn in the ladder diagram as shown.
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Simulated model of a PLC there are two inputs from
push buttons
imagine the inputs as activating 24V DC relay coils in the PLC
in turn drives an output relay that switches 220 VAC, that will turn on a light
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A self-hold circuit
input B will only be on when the output B is on
if B is off, and A is energized, then B will turn on
if B turns on then the input B will turn on, and keep output B on even if input A goes off
after B is turned on the output B will not turn off
Note: When A is pushed, the output B will turn on, and the input B will also turn on and keep B on permanently - until power is removed.
A
B
B
Note: The line on the right is being left off intentionally and is implied in these diagrams.
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PLC standards - IEC 61131
Developed with the input of vendors, end-users and academics, IEC 61131 consists of five parts:
• General information
• Equipment and test requirements
• PLC programming languages
• User guidelines
• Communications
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PLC Programming Languages Standard
IEC 61131-3 is the international standard for programmable controller programming languages.
As such, it specifies the syntax, semantics and display for the following suite of PLC programming languages:
• Ladder diagram (LD)
• Sequential Function Charts (SFC)
• Function Block Diagram (FBD)
• Structured Text (ST)
• Instruction List (IL)
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Ladder diagram and instruction list
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Sequential Function Charts (SFCs)
SFCs have been developed to accommodate the programming of more advanced systems
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PLC simulation using Personal Computer
With TRiLOGI’s simulator anyone could write ladder programs and immediately test them using just the keyboard and see the result of the action on screen.
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PLC scan cycle
• the control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs
• like any computer this does not happen instantly
Read inputs
PLC program changes outputsby examining inputs Set new outputs
Process changes and PLC pauseswhile it checks its own operation
THECONTROLLOOP
Power turned on
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The PLC internal working cycle• PLC accepts the inputs from the
Input Port and stores the data to the Input Image Register
• PLC then executes the ladder logic program with the input image content in a sequential manner, that is, from the left to the right and from the top to the bottom
• upon the execution of a rung in the ladder, the contents in the Component Image Register are updated and at the end of a cycle, the outputs are updated
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PLC Hardware
Power Supply - This can be built into the PLC or be an external unit. Common voltage levels required by the PLC (with and without the power supply) are 24Vdc, 120Vac, 220Vac.
CPU (Central Processing Unit) - This is a computer where ladder logic is stored and processed.
I/O (Input/Output) - A number of input/output terminals must be provided so that the PLC can monitor the process and initiate actions.
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PLC Hardware
Indicator lights - These indicate the status of the PLC including power on, program running, and a fault. These are essential when diagnosing problems.
Software - A software based PLC requires a computer with an interface card
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INPUTS and OUTPUTS of PLC• Inputs to, and outputs from, a PLC are necessary
to monitor and control a process. Both inputs and outputs can be categorized into two basic types: digital or continuous.
• Outputs to actuators allow a PLC to cause something to happen in a process. A short list of popular actuators is given below in order of relative popularity
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INPUTS and OUTPUTS of PLC• Inputs come from sensors that translate physical
phenomena into electrical signals. Typical examples of sensors are listed below in relative order of popularity.
• Proximity Switches - use inductance, capacitance or light to detect an object logically.
• Switches - mechanical mechanisms will open or close electrical contacts for a logical signal.
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INPUTS and OUTPUTS of PLC• Potentiometer - measures angular positions
continuously, using resistance.
• LVDT (linear variable differential transformer) - measures linear displacement continuously using magnetic coupling.
• Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs.
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INPUTS and OUTPUTS of PLC• Outputs to actuators allow a PLC to cause
something to happen in a process. A short list of popular actuators is given below in order of relative popularity.
• Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow.
• Lights - logical outputs that can often be powered directly from PLC output boards.
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INPUTS and OUTPUTS of PLC
• Motor Starters - motors often draw a large amount of current when started, so they require motor starters, which are basically large relays.
• Servo Motors - a continuous output from the PLC can command a variable speed or position.
• Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs.
• Continuous outputs require special output cards with digital to analog converters.
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Input connection• some PLCs
require external DC supply
• some PLCs have built-in 24 VDC supply
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INPUTS of PLC• PLC inputs must
converts a variety of logic levels to the TTL +5V dc logic levels used on the data bus
• can be done with circuits similar to those shown.
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PLC Input Trade-offs• DC voltages are usually lower, and therefore safer (i.e., 12-24V)
• DC inputs are very fast, AC inputs require a longer on-time. For example, a 60Hz wave may require up to 1/60s for reasonable recognition
• DC voltages can be connected to larger variety of electrical systems
• AC signals are more immune to noise than DC, so they are suited to long distances, and noisy (magnetic) environments
• AC power is easier and less expensive to supply to equipment
• AC signals are very common in many existing automation devices
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OUTPUTS of PLC• PLC must convert the
+5V dc dc logic levels on the PLC data bus to external voltage levels
• can be done with circuits similar to those shown, where it may be transistor output, or solid state relay (triac) output, or simply relay output
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PLC output with sinking current• the output card
shown in Figure on the right is an example of a 24Vdc output card that has a shared common
• this type of output card would typically use transistors for the outputs
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PLC output with sourcing current• the circuit shown in
Figure on the right has the sequence of power supply, then device, then PLC outputs, then power supply
• this requires that the outputs have a common
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PLC with relay output • in Figure shown on
the right the 24VDC supply is connected directly to both relays
• when an output is activated the output switches on and power is delivered to the output devices
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PLC Wiring
• Wiring Diagram for a Car-washing Machine
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Double coil designation
• double or dual coiling is not a recommended practice.
• using multiple output coils of the same device can cause the program operation to become unreliable
• the coil operation designated last is the effective coil
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Input durations
• the ON or OFF duration of the PLC inputs must be longer than the operation cycle time of the PLC
• taking a 10 ms (standard input filter) response delay into account, the ON/OFF duration must be longer than 20 ms if the operation cycle (scan time) is 10 ms
• in the example above, input pulses of more than 25 Hz (1/(20ms ON+20ms OFF) = 1/40ms) cannot be sensed
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Summary of PLC Hardware
• PLC input modules convert the AC or DC inputs to the suitable format to be detected by the logic of the PLC.
• Outputs of PLC are transistors (DC), triacs (AC) or relays (AC and DC).
• Input and output addresses are a function of the card location and input bit number.
• The input module usually shares a common ground and so does the output module of a PLC
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PLC Hardware
End of Lecture 2
RevisionMitsubishi PLC FX series hardware handbook.Allen Bradley PLC handbook