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Impact Sensors: Brain Sentry



The Brain Sentry Impact Sensor is one of the first commercially available product to integrate a new generation of nano, low power, digital output high-g microelectromechanical (MEMS) 3-axis linear accelerometers.

When an acceleration force is applied to the sensor a proof mass displaces from its nominal position, causing an imbalance in a capacitive half bridge. This imbalance is measured using charge integration in response to a voltage pulse applied to the capacitor. This innovative sensor, combined with Brain Sentry's proprietary firmware, is capable of measuring up to 690g of peak linear acceleration and it is capable of measuring accelerations with output data rates from 0.5 Hz to 1 kHz. The high-g capabilities are necessary to achieve Brain Sentry's 

Brain Sentry sensor on lacrosse player's helmet

usability goal of mounting the sensor on the helmet shell, where peak linear acceleration approaches ten-fold what is experienced at the head CG.

A significant part of the development effort has been focused on usability, battery life, and packaging (small size). The microprocessor is a16-bit RISC CPU that has been architected with five low-power modes -- optimized to achieve extended battery life. The digitally controlled oscillator (DCO) allows wake-up from low- power modes to active mode in less than 1 μs.

Several innovative screening algorithms have been developed by Brain Sentry to determine if an acceleration event should trigger an alert.

Spatial Transfer FunctionTM (STFTM): An impact to the helmet shell results in an acceleration of the shell that is significantly shorter in duration and larger in peak linear acceleration then the force experienced at the head CG. The Brain Sentry STF is designed to account for both spatial and force scale differences from the rear of the helmet sensor mounting location and the head CG.

Directionally Adaptive SensingTM (DASTM): Energy absorption varies based on the location of impact to the helmet. A volumetrically tuned impact correction factor was established for each sport to more accurately map
the transference of energy transferred to the head CG. The result of this effort has been a unique calibration for the Brain Sentry Impact Sensor for each sport (football, hockey and lacrosse). As new helmets enter the market, Brain Sentry will continually test and modify the DAS algorithms as necessary.

Impact Duration Culling: The duration of the impact plays a significant role in what is clinically relevant. A peak acceleration of 100g applied for a 1 μs pulse is not likely to cause an injury because the duration of the acceleration is so short that there is not enough time for the head to move relative to the brain. Brain Sentry's software filters impacts that are too short to be clinically significant.

Helmet Testing and Helmet Warranty

Brain Sentry sensors have been tested at an independent test lab under the direction of NOCSAE's Technical Director (NOCSAE is the helmet safety standards organization). The results: no adverse affect on helmet performance - even when an impact is directly on the sensor. If the helmet manufacturer refuses to honor their warranty because the sensor has been added to the outside of the helmet, Brain Sentry will cover the warranty.