Jaakko Pasanen

Noise elimination is exactly what the low pass filter is doing in our case. I'm not talking about just white noise since that's not usually much of an issue, but eliminating higher unwanted frequency components like process induced vibrations might be critical for some applications. Of course usage of FFT isn't limited to low pass filtering, applications are numerous. However problem often is with PLCs' computing power: using FFT with as small as 256 elements might take tens if not hundreds of milliseconds. Whether that is a problem is completely dependent on the application and its requirements.

Hello World! I created PLC friendly code for Fast Fourier transform (FFT) on IEC-61131-3 Structured text called StFFT. Program code is freely available on SourceForge. Code is licenced under both open source LGPL and Commercial licence. I couldn't find FFT implementation on PLC anywhere, so I decided to write one myself. Actually I could hardly find any mention about FFT on PLC and certainly not anyone implementing one. StFFT is made with minimal system dependencied on mind, it doesn't use recursion and only needs sine and cosine functions to work. This means that it should be usable on just about any PLC available. For those who don't know what FFT is and where it is used, i recommend taking peek in the Wikipedia. FFT has many applications, we are using it in low-pass FIR filter for analog measurement on PLC without needing any external hardware. I hope somebody has use for this... - Jaakko Pasanen