We describe an optical flow processing system that works as a virtual motion sensor. It is based on an FPGA device; this enables the easy change of configuring parameters to adapt the sensor to different motion speeds, light conditions and other environment factors. We call it virtual sensor because it consists on a conventional camera as front-end and a processing FPGA device which embeds the frame grabber, the optical flow algorithm implementation, the output module and some configuring and storage circuitry. To the best of our knowledge this paper represents the first description of a fully working optical flow processing system that includes accuracy and processing speed measurements to evaluate the platform performance.