PLC PROGRAMMABLE LOGICAL CONTROLLER PLC Programmable Logic Controller
PLC (PROGRAMMABLE LOGICAL CONTROLLER)
PLC (Programmable Logic Controller) It is a digital electronic device that uses programmable memory to store instruction and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes and has been specifically designed to make program easy. � E. g. Input device like switches and output device like motors being controlled are connected to the PLC and controller control the machine according to the input, output and program stored in PLC by operator.
PLC replaces hard wired relays PLCs are like computers but different in certain manner � Are rugged and can withstand vibration, temperature, humidity and noise � Interfacing for input and output are inside the controller � Easily programmable � Programming is concerned with logic and switching operations
Basic Structure
It consist of a central processing unit (CPU), memory and input, output interfaces. CPU controls and process all the operations within the PLC It has a clock with a frequency 1 and 8 MHz to determine operating speed of PLC and provide the timing and synchronise all elements in the system Bus system carries data to and from the CPU, memory and input/ output units.
ROM provide permanent storage for the operating system and fixed data RAM is used for user’s program (changeable) and temporary buffer stores for input and output channels Battery is used to maintain the RAM content for a period of time, when power is switched off
Input/output
Input/output processing The basic form of programming commonly used with PLCs is ladder programming � 1) Scan the inputs associated with one rung of ladder program � 2) Solve the logic operation involving those inputs � 3) Set/reset the outputs for that rung � 4) Move on to the next rung and repeat operations 1, 2, 3 and so on till the end of program
(Read this topic from book) There are two methods that can be used for input/output processing: � Continuous updating � Mass input/ output copying � Latching, Sequencing, � Timers, internal relay and counters
Selection of PLC There are some criteria need to be considered while selecting type and size of PLC � 1) What input/output capacity is required, i. e no. Of input output require � Capability of expansion for future needs ? � 2) What type of input/ output require i. e. Isolation on board power supply for input/outputs, signal conditioning � 3) What size of memory required? This depend upon no. of inputs/outputs and the complexity of program used
� 4)What speed of power is required of the CPU? This depend upon no. Of types of instruction handled by a PLC
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