PLC 1 Intro PDF

SM 3011 Introduction to IA Industrial Control Computer  Monitoring subsystems These system display the process state to the operator and draw attention to abnormal or fault conditions that need attention. The plant condition is measured by suitable sensors. Digital sensors me...

SM 3011 Introduction to IA Industrial Control Computer  Monitoring subsystems These system display the process state to the operator and draw attention to abnormal or fault conditions that need attention. The plant condition is measured by suitable sensors. Digital sensors measure conditions with distinct states. Analog sensors measure conditions which have a continuous range such as temperature, pressure, flow or liquid level. The results of these measurements are displayed to the operator via indicators (for digital signals) or by meters and bargraphs for analog signals.  Monitoring subsystems The signals can also be checked for alarm conditions. An over-travel limit switch or an automatic trip of an overloaded motor are typical digital alarm conditions. A high temperature or a low liquid level could be typical analog alarm conditions. The operator could be informed of these via warning lamps and an audible alarm.  Points of Distinction – PLCs are specifically designed to survive the harsh conditions of the industrial environment. – A well-designed PLC can be placed in an area with substantial amounts of electrical noise, electromagnetic interference, mechanical vibration, and noncondensing humidity. – Hardware and software are designed for easy use by plant electricians and technicians. – The hardware interfaces for connecting field devices are actually part of the PLC itself and are easily connected.  Points of Distinction – The modular and self-diagnosing interface circuits are able to pinpoint malfunctions and, moreover, are easily removed and replaced. – The software programming uses conventional relay ladder symbols, or other easily learned languages, which are familiar to plant personnel. – It provides reliability, repeatability and programmability to control solutions.  Advantages of PLCs Cost effective for complex systems Flexible (easy to add new timers/counters, etc) Computational abilities Reliable Easy to add new components  Objectives – Describe the structure of a PLC Program. – Describe the execution of a PLC Program. – Describe the typical elements of ladder diagram. – Design ladder diagrams for simple industrial logic control problems.  The programmable logic controller (PLC) is a microprocessor-based system that – accepts input data from switches and sensors, – processes the data by making decisions in accordance with the logic of the stored program, and – generates output signal to devices that perform a particular function. The purpose of a PLC was to directly replace electromechanical relays as logic elements, by a solid- state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks. A typical PLC  PLC Architecture Mainly consists of I/Os – Digital – Analog – Remote – Function modules – Communication processor – Network communication – Serial port (RS232 kind) – Parallel port (printer, etc) Schematic  Functional interaction of a PLC  Input Cards – Converts process level signals to Processor levels (operating at 5 V DC) – It can take 24 V DC signal, RTD signal such as Pt 100, 4-20 mA current or contact. – Status of the inputs is stored in the memory in process- image input table (PII). – Status of input devices like switches, sensors.  Processor – It processes the program stored in the program memory. – It consists of a list of logic functions and instructions that are successively processed. – The results are written into a process-image output table (PIQ). – Also other storage areas for counters, timers and memory bits will be accessed during program processing by the processor if necessary.  Output Cards – Converts back the signal so that power may be enhanced at the output. – The status from the PIQ will be transferred to the outputs and then be switched on and/or off. Motor actuation Solenoid Control Valves Lamp For switching purposes.  The program Scan The time required by MuP to complete one cycle is called Scan Time. Scan time depends on size of the program, its complexity and processor speed. A PLC program can be considered to behave as a permanent running loop. The user’s instructions are obeyed sequentially, and when the last instruction has been obeyed the operation starts again at the first instruction. A PLC therefore does not communicate continuously with the outside world, but acts intermittently.  The program scan The PLC does not read inputs as needed. At the start of the scan it reads the state of all the connected inputs and stores their state in its memory (PII). When the PLC program accesses an input, it reads the input state as it was at the start of the current program scan (from PII). Similarly, it does not change outputs instantly (goes to PIQ). All the outputs are updated simultaneously at the end of the scan. This method enables faster program execution because access to the process image is executed much faster than access to the I/O – modules.  Scan Time It can be between 10-20 ms. A scan time has four main tasks – Read input: The physical values of the inputs are written in the input variables memory (PII). – Execute the logic: Prog execution. – Write the output: The output values are updated in the output variables memory (PIQ). – Work on the remaining interval with other low priority tasks, system processing, etc.  Program Scan PLC program is always scanned from left to right, top to bottom in a single section. Suppose there are six sections written (modules), then the first section numbered will be executed from left to right in a rung; then from top rung to the bottom rung and so on, till the sixth section. Then, the cycle repeats again. The program scan  Sequential Processing A typical Signal Scan A typical Signal Scan A typical Signal Scan  Effect of Scan Time Response of PLC – In the extreme case, whole blocks of pulses are totally ignored. – In general, any input signal that a PLC reads must be present for longer than the scan time. – Shorter pulses may be read if they happen to be present at the right time but this cannot be guaranteed.  So, if input has arrived just after the input scan is over, it will not be read. If it goes off before the next scan, it will not be read at all. If it remains till the next scan, it will be read. But the whole one scan time is elapsed to read that input. Output will be observed after 2 scan time.  There are five types of languages that can be used to program a PLC device: – Ladder Logic (LAD) – Functional Block Diagram (FBD) – Sequential Function Chart (SFC) – Structured Text (ST) – Instruction List (IL)  At first, you have to learn the LAD language very well as it is the basic one. – Helps in debugging For advanced programming, one has to choose another language to learn. For the rest, one has to just be aware of the basic instruction for them.