As a digital filter, a 4-pole Butterworth low pass filter is used with a linear phase and specific starting conditions.
The following difference equation describes the filter sequence:
The filter coefficients are calculated in accordance with ISO 6487:
When performing a calculating based on SAE J211, the following applies for the variable wd:
With
X[t] |
Input data sequence |
Y[t] |
filtered output data sequence |
a0, a1, a2, b1, b2 |
Filter constants dependent on CFC |
T |
Sampling rate in seconds |
The difference equation describes a two-pole filter. To implement a four-pole filter, the data must pass through the two-pole filter twice, once forwards and once backwards. This prevents phase shifts.
References
•ISO 6487:2002 (2002). Road Vehicles - Measurement Techniques in Impact Tests - Instrumentation.
•SAE J211/1 MAR95. Instrumentation for Impact Test—Part 1—Electronic Instrumentation.
•Arbeitskreis Messdatenverarbeitung Fahrzeugsicherheit (Study Group for Measurement Data Processing for Vehicle Safety, April 2005). Crash-Analyse, Beschreibung der Kriterien (Crash Analysis, Description of Criteria). Version 1.6.2.