Transcript الشريحة 1

Chapter 2
Structure and functional principle of Programmable Logic Control (PLC)
Chapter 2
2-1 Realization of a control program
There are mainly two methods to realize a control program:
1- Hard –wired relay logic controller: In these control concepts a connection of relays and switches are used to
realize the control program.
A relay is a device that uses a magnetic field to control a switch. When a voltage is applied to the input coil,
the resulting current creates a magnetic field which pulls a metal switch toward at and the contact touches.
The contact that closes when the coil is energized is called normally open (NO) (see Fig.2-1-a). The normally
closed (NC) contacts touch when the input coil is not energized (see Fig.2-1-b)
a) Normally closed
b) Normally open (NO)
(NC)
Fig.2-1: Control relay with NO and NC contacts
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Normally closed
Metal switch
Normally open
Normally closed switch
Normally open switch
Fig.2-1-1: Relay
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Disadvantage of hard-wired relay logic:
* Functionality of the circuit cannot be tested until all wiring has been
completed.
* The application of wired circuit is limited to specific control task. Any •
modification of the circuit is time consuming and difficult.
*It requires frequent maintenance because of the short time of the devices •
* Required place
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Chapter 2
Example of Relay logic
Realize the following logic function using relay logic:
C=not A and B
(C = A ∩ B)
A
A
B
C
0
0
0
0
1
1
1
0
0
1
1
0
B
C
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Chapter 2
2- Programmable Logic Controllers (PLC): PLC is an electronic system based on digital processing of
information for industrial application. It uses a programmable memory for instructions to control processes or
machines according to digital or analog outputs.
Advantages of PLC
* Flexibility
*Since the control process of programmable logic controllers is created with programs, control elements •
can be changed in the programs to use in another control duty.
*PLCs also perform arithmetic operations and control high-speed lines, which cannot be performed •
using a combination of relays,
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Program Monitor
CPU
Processor
Timer
counter
Data image
memory
Flags
valves, etc…
Program Memory
Actautors (motors,
output module
Output signal condition circuit
Input signal condition circuit
Sensors
Input module
Power supply
Bus system
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Fig.2.2-1: Condition circuit for DC input signals
Fig.2.2-1: Condition circuit for AC input signals
2.2 PLC Hardware
The minimal equipment of PLC contains of the following parts (see Fig 2-2-5):
1) Input Module: The input module contains of
- Input Terminals where the signals coming from the sensors are connected to. The
input voltage can reach values up to 60 VDC or 200VAC
- Input signal condition circuit: All input signals must be conditioned to drive an
optocoupler which delivers the required TTL signal to the CPU. Typical condition
circuits for DC and AC inputs are shown in Fig.2.2-1 and 2.2-2 respectively. The
optocoupler isolates also the internal circuitary of the PLC from the input circuit
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2. Central Processing Unit (CPU): The CPU is the brain of the PLC. It has the
following subparts:
- Processor : The computer center that carries out mathematic and logic
operations
- Timer, counters and flags
- Data Image Memory: In the data memory a copy of the states of the Input and
output signals is saved. This copy is actualized at the begin of each processing
cycle.
- Program memory where the instructions of the control program are saved
3. Program Monitor (PM): PM is a device used to communicate with the circuit of
the PLC.
4. Power supply (usually 24 V DC)
5. Output module: The output module contains of:
- Output signal condition circuit: The TTL signal coming from the CPU must be
conditioned to switch the output terminal to the desired output voltage level. Typical
condition circuits for DC and AC output signals are shown in Fig.2-2-3 and 2-2-4
respectively
- Output terminals where the conditioned signals are connected to the
actuators
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Fig.2.2-3: Condition circuit for DC
output signals
Fig.2.2-4: Condition circuit for AC
output signals
Input module
output
module
Power
supply
CPU
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Batteries in case of interruption of the main power
supply
2-3 Construction types of PLC
1) Modular PLC (Rack): The units (modules) are delivered separately and
connected with each others via information bus as shown in Fig. 2-3-1
-Rack PLC are used for
large control tasks
- Further units can be joined
(extendable)
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2) Compact PLC: All units are integrated in one set
- Simple and not expensive
- Limited inputs and outputs
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3) Slot PLC:
- Expensive because of special hardware
- Real time control
- With special power unit, slot plc can operate independent on PC
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4) Software PLC
- Cheap hence only the program is needed
- Direct monitoring on PC
- No real time control
- Reliability depends on PC
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2.4 Operation Principle of PLC
The processing of the control program follows by the CPU.
-The program is processed cyclically and instruction by instruction.
- At the begin of each cycle, the states of the inputs are checked and copied to
the Data Image Memory (Input process image).
- Any changes in the inputs during one cycle has no effect.-
- Any changes in the outputs is copied to the Data Memory (output process
image)
- When the last instruction is processed, the states of outputs are copied in the output process image are transferred to the output module; the states of the
inputs are actualized and saved in the Data Memory (Input process image) and
the CPU starts anew with the first instruction
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Check and copying of the
inputs states
Instruction 1
Cyclic processing
Instruction 2
Instruction 3
Last instruction
Transfer of the out signals
from data memory to the
output module
Fig.2.4-1: Operation principle of PLC