PLC Architecture

In this session you will learn:
Programmable Logic Controller(PLC)
Types of PLC’s
PLC architecture
Scan cycles
Scan patterns
PLC programming
Ladder diagram programming
Latch and Unlatch
DCS architecture

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1. Control System Training PLC Architecture

2. Agenda: Programmable Logic Controller(PLC) Types of PLC’s PLC architecture Scan cycles Scan patterns PLC programming Ladder diagram programming Latch and Unlatch DCS architecture

3. Programmable Logic Controller The National Electrical Manufacturers Association (NEMA) defines a PLC as a "digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions, such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog I/O modules various types of machines or processes.’’ In simple terms PLC is a solid-state industrial control device which receives signals from user supplied controlled devices, such as sensors and switches, implements them in a precise pattern determined by ladder-diagram-based application progress stored in user memory, and provides outputs for control of processes or user-supplied devices, such as relays or motor starters. PLCs come in different types, and it is generally chosen for an application depending upon the number of inputs and outputs in a process.

4. TYPES OF PLCS The general classification of PLC based upon the number of inputs and outputs is Fixed type PLC Modular type PLC Rack type PLC Fixed type PLC: In this type of PLC all the components of the PLC are as a single unit. The number of I/O supported by the PLC is decided by the manufacturer and cannot be changed. This type of PLC can support a small number of I/ Os .

5. Programmable Logic Controller Modular Type PLC: In modular type PLC the number of I/ Os can be increased by the addition of modules to the existing PLC. In modular type PLC the number of I/O supported can be increased to few hundreds by adding I/O modules. Rack Type PLC: In rack type PLC all the components of the PLC are as separate modules and are assembled to form one unit by mounting the individual components on a rack. This PLC can support upto thousands of I/ Os .

6. BRANDS OF PLC There are lot of PLC manufacturers, some of the commonly known brands of PLC are, Allen Bradley(AB) Siemens ASEA Brown Boveri Omoron GE Fanuc DELTA Mitsubishi Messung Keyence In all the brands there may be the different types of PLCs available.

7. PLC ARCHITECTURE

8. PLC ARCHITECTURE MEMORY UNIT: The memory of the PLC is divided into two types Program memory : The Ladder Logic program downloaded will be stored in this memory. Data memory: The data from the I/O modules will be stored in this memory section

9. INPUT MODULE FUNCTIONING The input from the field device is received on the input module. The input signal could be digital input signal (0/1) or it could be analog input signal ( 4 to 20 mA, 0 to 10V) The PLC is a digital device and the CPU of the PLC can withstand voltage of 5V. The Input module receives the input signal from the field devices which passes through the isolation barrier in the module and is then convert to the required 5V and then sent to the PLC. The isolation barrier prevents the external events from interfering with the PLC and causing a module failure. In the PLC it is stored in the data memory as a binary value1 for 5V and binary value 0 for 0V.

10. OUTPUT MODULE FUNCTIONING The value of either 0 or 1 stored in the data memory after execution of the program is sent to the output module. The output module also contains the isolation barrier which is used to protect the PLC from external interference. The signal from the PLC is converted to corresponding analog or digital signal according to the module to which it is connected.

11. PLC ARCHITECTURE The input signal from the input module is stored in the data memory. This data is then used for execution of the program that is downloaded into the PLC. The output status which is there on the program output is stored in the data memory of the PLC. The data from this memory is sent to the output module which is used to control the field devices. This process of reading the input value , executing the program and writing the output value is called as scanning. ISOLATION BARRIER : Optical Isolator provides electrical isolation between field wiring and the PLC internal circuitry . It consists of Light-Emitting Diode & Photoelectric transistor . When Switch Close d =>Current flow through LED =>Generates Light => Triggers current flow through Photoelectric transistor => Sensed by PLC digital logic. Circuitry to the LHS of isolator powered from the field and Circuitry to the RHS is powered from internal PLC voltage source

12. PLC ARCHITECTURE The PLC scans the process continuously because the inputs can change at any time.

13. THE SCAN CYCLE PLCs operate by continually scanning programs and repeat this process many times per second. When a PLC starts, it runs checks on the hardware and software for faults, also called a self-test. If there are no problems, then the PLC will start the scan cycle. The scan cycle consists of three steps: input scan, executing program(s), and output scan. Input Scan: A simple way of looking at this is the PLC takes a snapshot of the inputs and solves the logic. The PLC looks at each input card to determine if it is ON or OFF and saves this information in a data table for use in the next step. This makes the process faster and avoids cases where an input changes from the start to the end of the program. Execute Program (or Logic Execution): The PLC executes a program one instruction at a time using only the memory copy of the inputs the ladder logic program. For example, the program has the first input as ON. Since the PLC knows which inputs are ON/OFF from the previous step, it will be able to decide whether the first output should be turned ON. Output Scan: When the ladder scan completes, the outputs are updated using the temporary values in memory. The PLC updates the status of the outputs based on which inputs were ON during the first step and the results of executing a program during the second step. The PLC now restarts the process by starting a self-check for faults .

14. SCAN PATTERNS

15. PLC PROGRAMMING There are 5 programming languages available , but the commonly used language is ladder diagram for PLC

16. PLC PROGRAMMING LADDER LOGIC Ladder logic is the main programming method used for PLC's. Ladder logic has been developed to mimic relay logic. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced. The first PLC was programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program - so this programming method has stuck and it is the most common technique for programming in today's PLC. INSTRUCTION LIST There are other methods to program PLCs. One of the earliest techniques involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal.

17. PLC PROGRAMMING SEQUENTIAL FUNCTION CHARTS (SFC) SFC have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. This method is much different from flowcharts because it does not have to follow a single path through the flowchart. STRUCTURED TEXT (ST) Programming has been developed as a more modern programming language. It is quite similar to languages such as BASIC and Pascal. Structured Text (ST) is a high level textual language that is a Pascal like language. It is very flexible and intuitive for writing control algorithms. FUNCTION BLOCK DIAGRAM (FBD ) FBD is another graphical programming language. The main concept is the data flow that start from inputs and passes in block(s) and generate the output.

18. LADDER DIAGRAM PROGRAMMING Introduction to Ladder Logic Ladder logic uses graphic symbols similar to relay schematic circuit diagrams. Ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal lines between these two verticals. Ladder diagram features Power flows from left to right. Output on right side can not be connected directly with left side. Contact can not be placed on the right of output. Each rung contains one output at least. Each output can be used only once in the program. A particular input a/o output can appear in more than one rung of a ladder. The inputs a/o outputs are all identified by their addresses, the notation used depending on the PLC manufacturer.

19. PLC PROGRAMMING

20. LADDER DIAGRAM PROGRAMMING

21. LATCH AND UNLATCH When input devices like Push Buttons are used the input contacts are not continuous but momentarily. With this momentary contacts devices like motor are run continuously. For this purpose the concept of latch is used. To break this continuous supply to output a NC switch is used to open the circuit condition. This is called as unlatch condition.

22. QUESTIONS question1 X0 – START pushbutton to Start Oil Pump Motor X1 – START pushbutton to Stop Main Motor X2 – STOP pushbutton to Stop Oil Pump Motor X3 – STOP pushbutton to Stop Main Motor question2 X0 – Start Switch. X1 – Low level float sensor. X1 = ON when the liquid level reaches X1. X2 – High level float sensor. X2 = ON when the liquid level reaches X2. X3 – Stop Switch. X10 - EMERGENCY STOP button. X10 = ON when the button is pressed.

23. DCS ARCHITECTURE

24. Topics to be covered in the next session: PLC programming languages Ladder logic Ladder diagram programming PLC programming

25. Thank you!