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Advantages of Plcs Over Relay Logic

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Advantages of Plcs Over Relay Logic PLC • In old days, electrical control was mostly based on relays (Contactors) (i.e. Relay logic). Since from 1970 PLCs (Programmable Logic Controllers) have become most common choice for manufacturing / process control. • Advantages of PLCs over Relay logic: – PLCs are programmbale – Many control relays can be replaced by software, which means less hardware failure, – It is easier to make changes in software than in hardware. – Special functions such as time delay actions, counters are easy to produce in software. – Reliability is more. – There is almost no task limit to tasks for which PLCs can be used. – It is cost effective control system 1 • Disadvantages of PLCs: – PLC were Designed for Relay Logic Ladder and have Difficulty with some Smart Devices. – To maximize PLC performance and Flexibility, a number of Optional Modules must be added 2 Dr.Mahesh S Narkhede 1 PLC Vs PC • PLCs are designed to be operated in industrial environment with wide range of ambient temperature, vibration and humidity conditions. It is not effected by the electrical noise present in the industrial environment. • In computer, the inputs are floppy drives and CD Roms and output is a printer, but in PLC the inputs are signals from control elements like push-buttons, limit switches, temperature switches, pressure switch and transducer etc., installed on the machines to be controlled. Also the output are final control elements like contactors, solenoids, positioning valves, indication lights, an so forth. • The PLC is not a disc based system like PC. With a disc based system there is a continuous checking of what to do next. With a PLC the answer of what to do next is inherent, there is no consulting. The PLC program is stored in battery backed RAM or EPROM. 3 • Another important difference between PLC and PC is that the PLC contains the operating systems and application programs in ROM memory. The operating system does not have to load an application program, as it is already in ROM. It is difficult to differentiate between PLCs, ‘BIOS’, operating system and application program. • Another major difference between PLC and computer is the programming language. PLC is not programmed in one of the high language used for programming a computer. As PLC is designed to be operated by plant engineers and maintenance personnel with limited knowledge of computers, it is designed to be programmed using RELAY LADDER LOGIC. However some PLCs are available which can be programmed using FORTRAN and BASIC, but relay ladder logic programming is the most popular. Some PLCs are also programmed using, Boolean Algebra, Statement lists and Control system flow chart languages. 4 Dr.Mahesh S Narkhede 2 • Another major difference is that troubleshooting is simplified in most PLCs because they include fault indicators, blown-fuse indicators, input and output status indicator, and written fault information that can be displayed on the programmer. 5 Block Diagram of PLC 6 Dr.Mahesh S Narkhede 3 • A typical PLC can be divided into five components. These components consist of the processor unit, memory, power supply, input/output section (interface) and the programming device. Some manufacturers refer to the processor as a C.P.U. or central processing unit. The components are shown in previous Fig. • The power supply is required to convert 240 volts AC voltage to the low voltage DC required for the logic circuits of the processor and the internal circuits of the input and output modules. DC power for the input and output devices, if required is generally provided from a separate source. • The input module and output modules are referred to a I/O section (I for input and O for output). The real word input devices like push buttons, limit switches, analog sensors are wired to input module and real word output devices like contactors, solenoid valves indicator lights, positioning valves etc., are wired to the output modules. Real word input and output devices are of two types: discrete and analog. Discrete I/O devices are either ON or OFF (open or closed), while analog devices have infinite number of 7 possible values. • Examples of discrete input devices are pushbutton and limit switch while analog input device are temperature probes, Pressure transducers etc. which gives varying voltage and current. The input from the analog input device is converted by the analog module called Analog- to-Digital Convener (ADC) into a proportional binary number and stored in memory of the PLC for further use by the processor • Discrete output devices like contactors, solenoid valve coils, indication lamps are either energised or de- energized, but Analog output devices require varying current or voltage to control the output. Example of analog output device is the positioning valve, which gives variable opening of the valve depending upon the variable voltages/current applied to the valve which is obtained using the Digital-to Analog Converter (DCS) as the output module. 8 Dr.Mahesh S Narkhede 4 • The processor unit operates on low DC voltage of 5 volts. Input modules thus contains circuitry that converts input voltages of 120-240 V AC or 0-24 DC, etc., from discrete input devices to low level DC voltage typically 5 V DC. Analog input modules converts the 4 -20 milli ampere signals from sensors to low-level DC voltages for the processor unit. Similarly, the output modules change low level DC signals from the processor to 120- 240 AC or low level DC voltages or give output currents in range 4-20 milli ampere. • The programming device for a PLC can be a Hand-held programmer, Dedicated programmer or Personal computer. The program is entered using Relay Ladder Logic, Statement Lists or Control System flow charts but the most popular method of programming is the Relay Ladder logic. 9 Control Panel with PLC 10 Dr.Mahesh S Narkhede 5 Components Central Processing Unit (CPU) Input Output Modules , Remote Input Output Modules Power Supply Bus system 11 Central Processing Unit It is a micro-controller based circuitry. The CPU consists of following blocks : • Arithmetic Logic Unit (ALU), • Program memory • Process image memory (Internal memory of CPU) • Internal timers and counters • Flags CPU performs the task necessary to fulfill the PLC functions. These tasks include Scanning, I/O bus traffic control, Program execution, Peripheral and External device communication, special functions or data handling execution and self diagnostics. 12 Dr.Mahesh S Narkhede 6 Input module These modules act as interface between real-time status of process variable and the CPU. Analog input module : Typical input to these modules is 4-20 mA, 0-10 V Ex : Pressure, Flow, Level Tx, RTD (Ohm), Thermocouple (mV) Digital input module : Typical input to these modules is 24 V DC, 115 V AC, 230 V AC Ex. : Switches, Pushbuttons, Relays, pump valve on off status 13 Output module These modules act as link between the CPU and the output devices in the field. Analog output module : Typical output from these modules is 4-20 mA, 0-10 V Ex : Control Valve, Speed, Vibration Digital output module : Typical output from these modules is 24 V DC, 115 V AC, 230 V AC Ex. : Solenoid Valves, lamps, Actuators, dampers, Pump valve on off control Remote I/O module These modules are attached to computers which are connected to network 14 Dr.Mahesh S Narkhede 7 Power Supply The power supply gives the voltage required for electronics module (I/O Logic signals, CPU, memory unit and peripheral devices) of the PLC from the line supply. The power supply provides isolation necessary to protect the solid state devices from most high voltage line spikes. As I/O is expanded, some PLC may require additional power supplies in order to maintain proper power levels. 15 Bus System It is path for the transmission of the signal . Bus system is responsible for the signal exchange between processor and I/O modules The bus system comprise of several single line i.e. wires / tracks 16 Dr.Mahesh S Narkhede 8 Different PLCs available in market • Siemens – S5 -110U, 115U, 135U – S7 - 200, 300, 400 • Allen Bradley – Micrologix 1000, 1200, 1500 – SLC 5/01, 5/02, 5/03 – PLC 5/10, 5/25 and 5/40 • Modicon – Nano – Micro – Premium – Quantum 17 Origins of Ladder Diagram • The Ladder Diagram (LD) programming language originated from the graphical representation used to design an electrical control system – Control decisions were made using relays • After a while Relays were replaced by logic circuits OR – Logic gates used to make control AND decisions • Finally CPUs were added to take over the function of the logic circuits – I/O Devices wired to buffer transistors – Control decisions accomplished through CPU programming • Relay Logic representation (or LD) was developed to make program creation and maintenance easier – Computer based graphical representation of wiring diagrams that was easy to understand 18 – Reduced training and support cost Dr.Mahesh S Narkhede 9 What is a Rung? • A rung of ladder diagram code can contain both input and output instructions – Input instructions perform a comparison or test and set the rung state based on the outcome • Normally left justified on the rung – Output instructions examine the rung state and execute some operation or function • In some cases output instructions can set the rung state • Normally right justified on the rung Input Instruction Output Instruction 19 Series Vs Parallel Operations • Ladder Diagram input instructions perform logical AND and OR operations in and easy to understand format – If all Input Instructions
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