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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 CH2-1 Normally closed Metal switch Normally open Normally closed switch Normally open switch Fig.2-1-1: Relay Chapter 2 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 CH2-2 CH2-3 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 CH2-4 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, CH2-5 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 CH2-6 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 CH2-7 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 CH2-8 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 CH2-9 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) CH2-10 2) Compact PLC: All units are integrated in one set - Simple and not expensive - Limited inputs and outputs CH2-11 3) Slot PLC: - Expensive because of special hardware - Real time control - With special power unit, slot plc can operate independent on PC CH2-12 4) Software PLC - Cheap hence only the program is needed - Direct monitoring on PC - No real time control - Reliability depends on PC CH2-13 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 CH2-14 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