Six-Degree-of-Freedom Accelerations: Linear Arrays Compared with Angular Rate Sensors in Impact Events

SAE J211 provides no definitive specification as to the appropriate procedures for filtering angular rate sensor data prior to differentiation into angular acceleration data, especially for impact data. Accordingly, a 3-2-2-2 array (nine-accelerometer-package or NAP) of linear accelerometers and a triaxial angular rate sensor were mounted into a Hybrid III 50^th-percentile-male ATD headform and compared in a variety of impact events and multibody simulations. Appropriate low-pass digital filter cutoff frequencies for differentiating the angular rate sensor data into angular accelerations were sought via residual analysis in accordance with current SAE J211 guidelines. It was found that there was no specific cutoff frequency that consistently matched the angular accelerations determined by the 3-2-2-2 array, and the combination of a triaxial linear accelerometer and a triaxial rate sensor could not simultaneously match the 3-2-2-2 summed square and peak angular accelerations regardless of cutoff frequency. The differentiation step to convert angular velocity data into angular acceleration data was found to insert frequency-dependent noise which low-pass filtering could not adequately eliminate. This may be overcome in the future if the noise floor for angular rate sensors can be constructed to exhibit less noise than contemporary linear accelerometers. While angular rate sensors are essential for long-duration accelerometer recording and are effective for analyses requiring integration, care must be taken when they are used for impact analyses. Currently, for impact analyses, a 3-2-2-2 array or a 6DOF array paired with an angular rate sensor will generate results with lower noise, and should be used wherever possible.