Digital Filtering for J211 Requirements using a Fast Fourier Transform Based Filter 2002-01-0796

The need for low pass filters stems from a need to eliminate high frequency noise from raw data (the output of the data acquisition system). As an example, consider the frame of a vehicle used in a crash test. The frame will exhibit high frequency vibrations, which do not affect the vehicles movement in space. The use of filters has since been expanded to include such things as the calculation of potential injury. Phaseless filters are now required for all FMVSS-208 injury calculations (see references). A single filter formula can not allow all test facilities to comply with the J211 CFC corridors. Even the SAE J211 recommended Butterworth filter may not comply with the J211 requirements. A new, universal, filtering system is required to harmonize the data processing at all testing facilities.

The use of Fourier series for filtering provides a very powerful, yet overlooked, solution to today's filtering problems. The focus of this article is with the SAE J211 CFC filter boundaries and the role of FFT filters to meet these requirements. The use of FFT filters provides a simple method to derive a set of custom filters for each data acquisition system. These custom filters will replace the SAE J211 recommended four-pole phaseless filter formula. This new filtering scheme will allow all testing facilities to meet the J211 corridors and improve compatibility between testing facilities.