Digital or analog data acqusition systems Discussion

August 7, 2013

Digital or analog data collection for NOCSAE certification testing? 

 There appears to be some confusion among laboratories that desire to use computer software driven digital data acquisition systems for NOCSAE standard testing.  The desirability of using a computerized digital system versus an analog system is unquestionable; this approach can sometimes streamline the reporting process.  Digital systems have been shown to be capable of accuracy and repeatability at least equal to that of analog systems in many applications.

 There is no question that digital acquisition systems can be designed for use in NOCSAE drop test standards that are demonstrably capable of providing Severity Index and Peak g results equivalent to the KME data acquisition system currently specified by NOCSAE across the range of critically important Severity Index results. 

 NOCSAE standards require that the user of equipment that has not been specifically called out in NOCSAE documents, including electronic data acquisition systems as well as mechanical systems, must demonstrate that the use of such equipment is equivalent and provides test results that are in agreement with the equipment that is currently specified in the NOCSAE standards, across the critical range. Currently the critical range is 300-2000 SI with particular emphasis at the 1200 SI pass/fail level.

 The central component of the  NOCSAE test systems are the NOCSAE headforms which must be calibrated in a very specific manner in order to verify its response when impacted on a verified 3” calibration MEP and when impacted with a projectile in accordance with NOCSAE docs ND001 and ND021. 

 In order to design a digital data acquisition system to correctly calibrate a NOCSAE headform to obtain the desired responses, careful consideration must be taken on how the analog system processes the accelerometer signals to accomplish the calibration.  It turns out that this is not a trivial task and requires development of sophisticated software programing along with quality digital hardware systems.

 NOCSAE has developed such a system that has been in use for many years with an update about to be released in the coming months. Of note this system is only for use in limited drop testing applications.  

 It has been and remains a desire of NOCSAE to implement a more automated digital data acquisition system that is capable of accurately replicating the results of the current analog system for all NOCSAE testing.  The current technical challenge is projectile impacts. In projectile impacts the analog system is able to collect all data from each channel in real time. While this is an antiquated approach in the eyes of some, in many ways the speed of this approach is critical to the results in short time duration events. It may be possible to create a digital system for these events, but it requires very high speed and high quality digital acquisition equipment capable of simultaneous signal sampling. Such systems are currently cost prohibitive.

 Towards this end, SIRC is working with developers on a digital system that we hope to validate in the near future. 

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