It should be no surprise to many that one can use a Raspbery Pi SBC as an industrial controller, but is it any good at that? That was the question which [Dough Reneker] and [William Shaffer] built a test rig to see how a Raspberry Pi performs in head to head tests. They compared a Python-based control loop on a Raspberry Pi 3B against an C0-12DD1E-2-D AutomationDirect CLICK Programmable Logic Controller (PLC) using a simple water heating example.
A major snag with using the Raspberry Pi as a PLC is the lack of industrial I/O capacity. This requires additional hardware, in this case adding a four-channel ADC board as well as a custom board to condition the signals. The Raspberry Pi looks for 0-3 V inputs where industrial control applications are usually in the -10 to 10 V range and often use a 4-20 mA current loop.
Using a PLC leverages so-called ladder logic, where each action depends on conditions. With each update scan, the PLC ensures that all input conditions are translated into the appropriate output conditions in real-time. It’s only job is to monitor the process at hand and it does this very well.
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