PLC’s are used in wide range of industries like petrochemical,biomedical,distillery,beverages,food industries and any other industry that seek less human help to produce the product.With the help of PLC some of the process we can do is listed below 1.Continuous bottle filling system 2.Batch mixing system 3.stage air conditioning system 4.Traffic control etc
What is a PLC ?
Control engineering has evolved over time. In the past, humans were the main method to control a system. More recently, electricity has been used for control and electrical control is based on relays. These relays allow you to turn the power on and off without a mechanical switch. It is common to use relays to make simple decisions of logical control. The development of low-cost computers has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s and has become the most common option for manufacturing controls.
PLCs have been gaining popularity in the factory and will probably remain predominant for some time to come. Most of this is due to the advantages they offer.
Advantages of PLC
• Cost effective to control complex systems. • Flexible and can be reapplied to control other systems quickly and easily. • Computer skills allow more sophisticated control. • Troubleshooting accessories make programming easier and reduce downtime. • Reliable components make them work for years before failure. • Less manpower for design • Downsizing and standardization • Improved maintainability
Internal Architecture of PLC
The internal structure of the CPU depends on the microprocessor concerned. In general, CPUs have the following:
An arithmetic and logic unit (ALU) that is responsible for data manipulation and carrying out arithmetic operations of addition and subtraction and logic operations of AND, OR, NOT, and EXCLUSIVE-OR. Memory, termed registers, located within the microprocessor and used to store information involved in program execution. A control unit that is used to control the timing of operations.
The buses are the paths used for communication within the PLC. The information is transmitted in binary form, that is, as a group of bits, with a bit being a binary digit of 1 or 0, indicating on/off states.
To operate the PLC system there is a need for it to access the data to be processed and instructions, that is, the program, which informs it how the data is to be processed. Both are stored in the PLC memory for access during processing.
The input/output unit provides the interface between the system and the outside world, allowing for connections to be made through input/output channels to input devices such as sensors and output devices such as motors and solenoids.
Types of PLCs
- Integrated or Compact PLC
- Modular PLC
- Small PLC
- Medium PLC
- Large PLc
A programmable logic controller (PLC) is an electronic device used in many industries to monitor and control construction systems and production processes. Unlike PCs and smartphones, which are designed to perform any number of roles, a PLC is designed to perform a single set of tasks, except in the case of limitations in real time and with superior reliability and performance.
To meet the demands of rigorous industrial environments, PLCs are designed to be extremely robust, often capable of withstanding extreme temperatures, humidity, vibration and electrical noise. The logical controllers are commonly responsible for monitoring and controlling a large number of sensors and actuators, and therefore are different from other computer systems in their extensive input / output (I / O) arrangements. PLC 's are used in several industries like petrochemical ,biomedical,cement manufacturing,oil and gas sector etc
PLC selection criteria consists of:
- SCAN time
Scan time of PLC is duration of: Read Input Excute program Update the output Therefore, the larger the program is, the longer the Scan Time will be.
- Number of I/O s
The majority of PLC systems today adhere to the IEC 61131/3 control systems programming standard that defines 5 languages: Ladder Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), Instruction List (IL) and Sequential Flow Chart (SFC).
To know more about check the link below https://forumautomation.com/t/types-of-programming-languages-in-plc/2564
where two liquids are mixed in required proportion to form a batch .Rate of the flow is already fixed. We only control the time of the flow. Level of the liquids in the tank are sensed by the level sensor switches
We try a simple blending of water and acid in a container where we only have three level sensors(L1,L2, and L3) and two liquids flowing in through two solenoid valves, solenoid a(water control) and solenoid b(acid control)and draining out through solenoid c(blend outflow).The batch is to be controlled by timer. After required level of blend is sensed (by L1)the mixer runs for 3 mins. by the motor. They are mixed in ratio of 3:2. The process initiates with the drain valve open, water and acid valves closed, mixer motor is off, and the tank is empty
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