Semi-Rigid Helmet Rotation Measurement Using Linear Accelerometers

The number of sensors placed on warfighters’ personal protective equipment (PPE) continues to increase each year. It is important to be able to accurately measure the dynamic response of PPE in order to characterize new sensors that are meant to track warfighter movement. In an effort to help predict head motion, a method has been developed to accurately measure the angular and linear acceleration of a semi-rigid helmet using four triaxial linear accelerometers. This four-accelerometer array configuration is based on the 3-2-2-2 nine accelerometer package (NAP) method and was tailored to accurately measure the helmet response during impact and blast overpressure events. Method development and testing were performed using U.S. Army Advanced Combat Helmets. Since angular motion calculation using the NAP method requires orthogonal sensor placement, it was necessary to revise the standard NAP sensor configuration to account for the geometric constraints of a helmet. Modal analysis was performed to determine the locations of least vibration, and shock tube and drop tests were conducted to investigate helmet flex during impacts. Knowledge concerning the dominant vibration modes of the helmet guided accelerometer placement and helped mitigate the effects of sensor data oscillation on the calculated angular motion. Local helmet deformation strongly depends on the impact site; several accelerometer array configurations were developed to account for various impact directions. Linear accelerations were measured and angular accelerations were calculated for guided free drop and shock tube tests in the laboratory. In guided free drop tests, the helmet and headform were dropped onto an anvil at various velocities and were allowed to freely bounce after impact. In shock tube tests, the helmet and headform were allowed to swing freely when subjected to a high shock wave simulating an IED blast. The modified NAP method was able to accurately measure the linear and angular acceleration of the helmet for both types of tests. The angular motion calculation was validated using a high-speed video camera recording the helmet response at 10,000 frames per second. Results were also compared to angular rate sensors available on the market. It was determined that with a detailed understanding of a semi-rigid body’s vibration and proper placement of linear accelerometers, angular acceleration during high-shock impacts can be accurately calculated for semi-rigid, irregular shaped objects. This accelerometer placement method has been applied to several other military grade helmets and been used in models predicting head motion from helmet motion data.Copyright © 2013 by ASME