When is a programmable logical controller the best choice? [duplicate]

Question

I am studying PLCs as a part of my Elements of Industrial Automation class. Trouble is, I can't imagine why would anyone use such a thing.

For complex tasks, microcontrollers are available. The vast majority of practicing programmers are well-versed in C, so creating a system with an ARM core (for example) is relative cheep. Furthermore, there are numerous free and tested RTOS for hard-realtime design to rest upon.

For massively parallelizable, hard realtime, low-latency tasks, there are FPGA-s. Those are a little harder to program. Yet there are many VHDL/Verilog guys out there in the job market.

Now to address the reliability of PLCs:

the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact

The same can IMHO be acheaved (and far surpassed) by extensive redundancy e.g. 3 identical systems operating simultaniously, upon detection of a problem in the primary system, another one, located at a different place, and programmed in a different way, kick in.

Where are PLCs used in modern industry? Why are they the best solution in those situations, instead of one of the solutions above?

Answer 1

In my world PLCs are used to control manufacturing machines.

PLCs are used instead of microcontrollers for many reasons:

1. They are modular. In trouble shooting a spare processor, I/O card or power supply can be swapped out.
2. Technicians can get online with troubleshooting software and view/troubleshoot the running program. This practice isn't really troubleshooting the program as much as checking to see what real world inputs are inhibiting operation.
3. PLCs can communicate with other vendors hardware through common protocols such as DeviceNet, Profibus or Firewire. This allows almost plug and play integration with drives, temperature controllers and other hardware.
4. In large factory floors with multiple production machines, made by multiple vendors around the world, it becomes advantageous to standardize on common PLC hardware.
5. Integration with an HMI product becomes an important factor. You must have some touch screen application to interface your controller to a person operating it. Again standardizing on an HMI product becomes a benefit. Wonderware, Intellution or WinCC are some common products.

The simple answer to your question is using a PLC standardizes the platform for Operators, Technicians and Engineers.