A PLC or programmable logic controller is a special computer that is primarily used by industry for control and regulation purposes. Again and again, you will hear the term PLC for the programmable logic controller. There are some large PLC manufacturers such as Beckhoff, B&R, Wago, etc even if the different manufacturers basically all work in a similar way, there are some specific peculiarities.
Siemens PLC:
Siemens is the market leader among PLC manufacturers. For this reason and because this hardware is required by most system operators anyway, we concentrate largely on this manufacturer.
When is a PLC needed?
You are currently sitting in front of a PC, a laptop, or a smartphone. The individual parts were certainly put together by a machine, the electronic components were tested by a test device and one or the other robot could also have been involved. But what does that have to do with a PLC? Well, the machine or system itself is just an amalgamation of a wide variety of components. So that these components do what they should, we breathe life into the machine with a PLC. Admittedly, this would also work (at least partially) with a pure contactor control, but the costs would far exceed the benefits and since the requirements also change in an ongoing project, such a solution would not be very efficient either, but more on that in a moment.
What else does a PLC need?
In addition to the power supply for the CPU, at least one input and one output module are required. External signals such as buttons or initiators (sensors) are connected to the input module. This 24V signal is converted by the input module into a binary signal consisting of 0s and 1s. Depending on how the PLC program was created, the CPU processes this information, and a binary signal is returned to the output module as a reaction.
However, since only PC-based devices can process 0 and 1 signals, this signal is converted back into a voltage signal by the output module.
A little example
We have a button that is connected to the input module, and when it is pressed we want to make a water pump run. The button was pressed, but the pump is not running.
As already mentioned above, the CPU also needs a user program (PLC program) so that it knows whether and how the signal is to be evaluated.
The PLC program has now been written, loaded into the PLC and the pump is running.
Advantages of a PLC control:
Space-saving and usually cheaper
Changes can be made very flexibly (believe me, changes have to be made all the time)
Faults can be quickly identified and remedied using diagnostic functions
A PLC can be networked with other IT systems (proverb Industry 4.0 and IoT)
Remote maintenance: Changes can be made from the office
Disadvantages of a PLC control:
Programming devices (laptop or similar) and licenses must be purchased
For very simple applications like in our example, a PLC is too expensive
High powers must be switched via relays or contactors
Highly qualified and expensive staff is required
