In this tutorial, we are going to learn about Programmable Logic Controller (PLCs).
PLC – It is full form is Programmable Logic Controller and nowadays basically a small computer with a highly specialized operating system. The operating system is capable of handling dynamic incoming events in real time basis. The input lines of a PLC have sensors connected to them. The sensors respond to temperature, liquid level, pressure and the like process the data and send signal to the output lines of the PLC to act according to changes in the input data. For example, a rise in temperature of a process sensed by an input device may cause fans to start operating. A change in the liquid level or pressure on the input side may result in opening of valves on the output side. In this respect, the PLC acts as a computer. However, in PLCs the program may be written once and used repeatedly as needed.
History of PLC –
In the 1990s when earlier PLCs were programmed, the language used was called Relay Ladder logic. The control logic was built on the operation of the relays, i.e. on-off control was being simulated. This was cumbersome as often relays tend to fail, there is a delay in turning on and turning off of relays and the hardware consisting of a large number of relays is difficult to design and wire. Nowadays, PLCs are getting to be more intelligent. They are integrated into electrical networks that are hierarchically organized. The supervision of the PLCs is by a control center. A widely known proprietary type of network is SCADA is a centralized system which monitors and controls the systems spread out over large areas of an industrial plant. Most of the control actions are performed by PLCs.
Use of PLC –
A modern day PLC connected to a network or more PLCs connected to a central computer are common in industrial environment. Such a system could look like the one pictured below-
PLCs are designed for multiple input and output lines with ability to withstand harsh environmental conditions of temperature, humidity, vibration, impact electrical noise, etc.
Response time of PLC-
The total response time of the PLC is a fact we have to consider when shopping for a PLC. Just like our brains, the PLC takes a certain amount of time to react to changes. In many applications speed is not a concern, in others though, it is.
INPUT RESPONSE TIME ————–
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PROGRAM EXECUTION TIME——– = TOLAT RESPONSE TIME
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OUTPUT RESPONSE———————
A typical response time is of the order of 40 – 50 ml seconds