Definition – What is a PLC?
A programmable logic controller is a computer device used by industrial control systems. They can be used wherever there is a need for control devices, due to the fact that PLC offers a flexible way for components to be softwired together – meaning that the information gets stored electronically, as opposed to hardwiring components. The way a PLC works is by relying on internal procedures, meaning that in order to make a PLC control circuit, a program needs to be written in accordance to the need of the system. The program is then implanted into chips.
Origins and History
When talking about automation, we’d have to look back at the late 1960s and the importance programmable logic controller (PLC) has had on the industry, being an essential factor as far as automated manufacturing processes go. It was in 1968 when General Motors’ automatic transmission division had issued for proposals to have an electronic replacement for the hard-wired relay systems. To put it simply, relays have to be hardwired in a specific order to get a machine running properly. If there is an issue, the whole thing stops working. In addition, troubleshooting is not a fast process.
The solution to all of this came in 1968 and it was PLC programming. Computer control was starting to get some attention at that time, and the winning proposal for the General Motors’ issue came from Bedford Associates who had 2 on the team, a figure who would get to be considered the “father of PLC.”
PLC went on to evolve. The first PLCs were able to work with relay coil, timers, counters, input and output signals. With evolution came the addition of analog input signals, analog output signals, floating point math, enhanced timers and counters, mathematic functions, drum sequencers, and one-shots. Given that their functionality was increasing, programming devices grew alongside PLC. It also seems fitting that the automotive industry is still among the largest users of PLCs, given that said industry is where PLCs got their start. However, given their functionality, PLCs can be used in a number of industries. For example, PLC can be used when it comes to having an automated wastewater system.
As long as the procedure is rational, PLC can take care of a number of functions, being able to work on the whole circuit control, in accordance to the contents of the written program. Despite the fact that PLCs have been around for a while and there are other methods to get things done, PLCs have stood their ground throughout the decades and are still popular. Perhaps the reason they are still used is due to the fact that they can be very flexible when it comes to the written program, and they are compact, economical, and are very reliable – no mechanical failures or program crashes. Many devices are currently on the market, with the majority of them being produced by foreign companies.
Writing a program
Nowadays, dedicated pieces of software have been developed for the PC in order to help with PLC programming. Once the program is written, it is then downloaded from the computer to the PLC with a special cable. In the old days, up until the mid-1990s, PLCs were programmed by using either special purpose programming terminals or proprietary programming panels. Often times, they had function keys which represented the logical elements of PLC programs. As far as storing goes, programs would get put on cassette tape cartridges.
A popular form of programming is ladder logic, which is the most widely used one. It features symbols (as opposed to words) in order to emulate relay logic control, with the symbols being interconnected by lines, representing the flow of current. As the years went on, the number of symbols available has increased, thus increasing the level of functionality that PLCs have.
By using ladder logic, the completed program looks similar to a ladder (hence the name), but in reality, it’s meant to represent an electrical circuit – with the rails representing the positive and the ground of a power supply and the rungs being the wiring between components. Other methods of PLC programming are state logic or programming languages such as C and BASIC.
Photo credit: The feature image has been done by Jezael Melgoza. The photo “welding” was taken by Boostinjay. The picture “coding together” was prepared by NESA by Makers.
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