PLC Unit-I PLC Basics PDF

Unit-I PLC Basics Dr D G Pradhan Professor EEE Department GRIET  Syllabus Unit-I PLC Basics Unit-II PLC Programming Unit-III Digital Logic Gates, PLC Registers Unit-IV PLC Functions, Data Handling Functions Uni...

Unit-I PLC Basics Dr D G Pradhan Professor EEE Department GRIET  Syllabus Unit-I PLC Basics Unit-II PLC Programming Unit-III Digital Logic Gates, PLC Registers Unit-IV PLC Functions, Data Handling Functions Unit-V Bit Pattern and Changing a Bit shift Register, Analog PLC Operation  Contents ❖ PLC Basics ❖ Programming equipment ❖ PLC System ❖ Programming formats ❖ I/O modules and interfacing ❖ Construction of PLC ladder diagrams ❖ CPU ❖ Devices connected to I/O modules. ❖ Processor  Definition of PLC ✓ A PLC is a user-friendly, microprocessor-based specialized computer that carries out control functions of many types and levels of complexity. ✓ Its purpose is to monitor crucial process parameters and adjust process operation accordingly. ✓ The PLC will operate any system that has output devices that go on and off (discrete or digital outputs) or with variable (analog) outputs ✓ The PLC can be operated on the input side by on-off devices (discrete or digital) or by variable (analog) input devices.  History of PLC ❑ PLC was introduced in late 1960’s ❑ First commercial & successful Programmable Logic Controllers was designed and developed by Modicon as a relay replacer for General Motors. ❑ Earlier, it was a machine with thousands of electronic parts. ❑ Later ,in late 1970’s,the microprocessor became reality & greatly enhanced the role of PLC permitting it to evolve form simply relay to the sophisticated system as it is today.  How does a PLC differ from a computer? ✓ A computer is optimized for calculation and display tasks ✓ A computer is programmed by specialists ✓ A PLC is designed for (logic) control and regulation tasks ✓ A PLC is programmed by non-specialists ✓ A PLC is well adapted to industrial environment Advantages of PLC  Disadvantages of PLC Newer Technology Fixed Program Applications Environmental Considerations Fail Safe operation Fixed circuit operation PLC System Layout and Connection  PLC System The major units of a PLC system are 1. Central Processing Unit (CPU) 2. Programmer/Monitor 3. I/O Modules 4. Racks and Chassis Central Processing Unit (CPU): CPU is a “brain” of the system, which has three subparts; ❖ Microprocessor: The computer center that carries out mathematic and logic operations. ❖ Memory: The area of the CPU in which data and information is stored and retrieved. Holds the system software and user program. ❖ Power Supply: It converts AC line voltages to various operational DC values. Programmer/Monitor (PM): ❑ The programmer/monitor is a device used to communicate with the circuits of the PLC. ❑ Hand-held terminals, industrial terminals and the personal computers exists as PM devices. I/O Modules: ❑ The input module has terminals into which outside process electrical signals, generated by sensors, transducers, are entered. ❑ The output module has terminals to which output signals are sent to activate relays, solenoids, various solid-state switching devices, motors, and displays. Racks and Chassis: ❑ The racks on which the PLC parts are mounted and the enclosures on which the CPU, PM, and I/O modules are mounted.  PLC as a Computer Data Processing Computer Process Control Computer Operational Section of a PLC CPU  PLC System The major units of a PLC system are 1. Central Processing Unit (CPU) 2. Programmer/Monitor 3. I/O Modules 4. Racks and Chassis  Central Processing Unit {CPU} Regardless of PLC Size, the processor and Memory are always in the same unit. This is called the Central Processing Unit (CPU). In larger PLCs, the CPU contains just processor and memory. In small PLCs, the CPU also consists of the I/O interfaces and Power supply. It is also possible for the CPU to contain the processor, memory and power supply, with the I/O interfaces placed in external modules. Fixed Memory: The fixed memory contains the program set by the manufacturer. This operating system program, which has the same function as a DOS program in PC, is set into special IC chips called Read-Only Memory (ROM). The fixed program in ROM cannot be altered or erased during the CPU’s operation. The program in this nonvolatile memory is retained when power is removed from the CPU. Alterable Memory: The alterable memory contains many sections. Its information is stored on an IC chip that can be programmed, altered and erased by the programmer/user. The alterable memory is stored mainly in Random Access Memory (RAM)chips. Information can be written into or read from a RAM chip. RAM is often called read/write memory. The typical RAM chip will lose any information it has stored when input power is lost. Processor: The processor section has computer flow connections to other subsections of the CPU and to outside devices. The processor is the controller that keeps information going from one place to another. It responds to programmed instructions stored in memory, causing output devices to be energized and deenergized in response to the on-off status of input devices.  Solid-State Memory The major types of solid-state memory chips used in PLC CPUs are PROM, EPROM, EEPROM, and NOVRAM. FIXED (F) OR CHIP APPLICATION ERASABLE BY ALTERABLE (A) ROM F Fixed operating memory No RAM A User Program No PROM F User Program No EPROM A User Program UV Light EEPROM A User Program Electrical Signals NOVRAM A User Program Electrical Signals  The Processor All computer processors are designed to carry out Arithmetic and Logic operations. Since the early 1970s, when Intel engineers were able to cram the complexity circuitry necessary to do these functions onto a single chip, processors have been known as Microprocessors. Microprocessor are the “brains” of every computer, have a unique characteristics. They are programmable, which means they are “told” what to do by a set of instruction, compiled to form a program. When the processor is to carry out a different task, a new program is written and fed to it.  Microprocessor are classified as to how powerful they are. Two factors determine power: ✓ Bit Size: the larger the bit, the more powerful the computer. ✓ Clock Speed: the faster the clock speed, the more powerful the computer. MICROPROCESSOR Bit Size Clock Speed 8085 8-bit 1 MHz 8086 16-bit 4.77 MHz 80186 16-bit 8 MHz 80286 16-bit 12.5 MHz 80386 32-bit 33 MHz 80486 32-bit 50 MHz Pentium 32-bit/64-bit 1.2 GHz  Input Modules The input module performs four tasks electronically, First: It senses the presence or absence of an input signal at each of its input terminals. The input signal tells what switch, sensor, or other signal is on or off in the process being controlled. Second: it converts the input signal for high, or on, to a DC level usable by the module’s electronic circuit. For a low, or off, input signal, no signal is converted, indicating off. Third: the input module carries out electronic isolation by electronically isolating the input module output from its input. Finally: its electronic circuit must produce an output, via output logic, to be sensed by the PLC CPU. PLC Input Module Layout ❑ All terminals in a given module have identical circuits. ❑ The first block receives the input signal from the switch, sensor, and etc. ❑ For AC voltage inputs, the direct current (DC) converter consists of rectifiers and a means to step the voltage down to a usable level, usually with a Zener diode. ❑ For input DC voltages, some type of DC-to-DC conversion within the converter block is required. ❑ The output of the converter is not directly connected to CPU. If it were, an input surge or circuit malfunction could reach the CPU. ❑ The isolation block protects the CPU from this type of damage. ❑ The isolation is usually accomplished by an Optoisolator. ❑ When its input is on, the isolator sends a signal to the CPU via the output logic block. ❑ When the isolator’s output is ON, it is sensed by a coded signal from the CPU.  Output Modules The output module operates in the opposite manner from the input module. ❑ A DC signal from the CPU is converted through each module section (terminal) to a usable output voltage, either AC or DC. ❑ A signal from the CPU is received by the output module logic, once for each scan. ❑ If the CPU signal code matches the assigned number of the module, the module section is turned ON. ❑ The identification numbers of the modules are again determined by the setting of the module SIP switches. ❑ If no matching signal is received by a terminal during the output scan, the module terminal is not energized. PLC Output Module Layout  Power Supplies The power available in most plants is 120 volts AC at 60Hz. Most PLCs operate on +5 and -5 volts DC. Therefore, the PLC CPU must contain circuitry to convert the 120-volt AC input to the required 5-volt DC values. The four parts of a diagram, plus a switching system for the backup system is shown in diagram. AC Conditioning Block Converter/Rectifier Filter Section Regulator PLC CPU Power Supply  Programming Equipment PLC Programming equipment exists to allow us to write, edit and monitor a program, as well as perform various diagnostic procedures. In most of the cases the programming device, the Programmer/Monitors (PM), must be connected to the CPU while programs are written. Other PMs, however, allow us to program offline and then download the program to the PLC CPU. The programs are written in ladder logic, although alternative programming languages are available.  Programming Equipment Three types of PMs, also referred as Program Loaders, are in common use. Hand-held, palm size units with dual function keypads and Liquid Crystal Display (LCD) or LED window. Full sized keyboard accompanied by a large Liquid Crystal Display (LCD) or Cathode ray Tube (CRT) screen. Software that allows programs to be developed on Personal Computers.  Programming Formats Some of the factors that vary between formats are Nomenclature, Numbering schemes, and screen appearance. A typical hand-held keypad sequence for a three-wire holding circuit is shown in below figure-a. In the circuit (figure-b), output Y0 can be turned ON or OFF through the operation of the two inputs X0 and X1. X0 and X1 are the two NO pushbuttons connected to the controller input.  The sequence is as follows: Clear RAM Memory 1. Turn PLC ON. 2. Clear RAM memory 3. Clear the screen. Programming can now begin. Program First Screen 4. Press contact device symbol (Normally Open). 5. Press function; X for input. 6. Assign contact number (0) by pressing numerical keys. 7. Press WRT to enter contact. 8. Press contact device symbol (Normally Open). 9. Press function; X for input. 10. Assign contact number (1) by pressing numerical keys. 11. Press WRT to enter contact.  12. Press coil device symbol (Normally Open). 13. Press function; Y for input. 14. Assign contact number (0) by pressing numerical keys. 15. Press WRT to enter contact. 16. Return to left of display, one line down 17. Press contact device symbol (Normally Open). 18. Press vertical connection symbol key (1) used to tie a device to the line above it on the ladder diagram. 19. Press function; Y for input. 20. Assign contact number (0) by pressing numerical keys. 21. Press WRT to enter contact. Write First Screen into RAM memory 22. Write first screen (program) into RAM by pressing SFT (shift) and PRG (program).  The PLC now is switched from HALT to RUN Mode. When the input X0 pushbutton is pressed; the Y0 output will energize and remain energized after the button is released. This is because Y0 contact is closed when Y0 coil is energized. The Y0 contact is said to latch the Y0 coil on because the now closed Y0 contact shunts the normally open X0 contact. Pressing input X1 pushbutton breaks the current path to the coil, causing Y0 to deenergize.  Proper Construction of PLC Ladder Diagrams 1. A contact must always be inserted in slot 1 in the upper left. 2. A coil must be inserted at the end of a rung. 3. All contacts must run horizontally. No vertically oriented contacts are allowed. 4. The number of contacts per matrix (network) is limited. 5. Only one output may be connected to a group of contacts. 6. Contacts must be “nested” properly or, in some PLCs, not at all. 7. Flow must be from left to right. 8. Contact progression should be straight across.  PLC Scanning Each PLC operational cycle is made up of three separate parts: INPUT SCAN PROGRAM SCAN OUTPUT SCAN The total time for one complete program scan is a function of processor speed and length of user program. ▪ During input scan, input terminals are read, and the input status table is updated. ▪ During the program scan, data in the input status table is applied to the user program, the program is executed, and the output status table is updated. ▪ During the output scan, data associated with the output status table is transferred to output terminals.  RUNG SCANNING COLUMN SCANNING Input ON/OFF Switching Devices Various types of ON/OFF switches which may be connected to PLC input modules are Toggle-type Switches (A-G) Push button Switches (H-K) Limit Switches SPST – Single Pole Single Throw SPDT – Single Pole Double Throw DPST – Double Pole Single Throw DPDT – Double Pole Double Throw  Input ON/OFF Switching Devices Some other common input ON/OFF devices used are Pressure Switches Level Switches Float (liquid level) Switches Photoelectric Systems Hall Devices Inductive sensitive devices Magnetic sensitive devices  Input Analog Devices Some input analog devices which produce a varying input electrical value which is sent to the appropriate PLC input module Potentiometers Linear Variable Differential Transformer (LVDT) Thermocouples