The conceptual design and development of novel low cost sensors for measuring the relative light emission in the pre-millisecond stages of detonating explosive charges

[摘要] ENGLISH ABSTRACT: During the course of the CSIR's research into the characterisation of explosive sourcesto devise methods of active intervention against threats, the need has arisen to researcha particular means of early identification of the threat, which is the intense light flashduring the threat detonation. For this purpose, a low cost rugged fast optical sensorwas sought, since the application thereof would imply possible destruction, especiallyif integrated into an active intervention system later on.Given the average time of about 1ms available for intervention, it is clear that theactive intervention system needs to operate within that period, hence the interest in thecharacteristic light emission of detonations in the pre-millisecond time frame. It wasthought that by characterising this emitted light in terms of wavelength (temperature)and amplitude (and maybe other unique phenomena), the size of the threat could bedetermined and logic decisions derived therefrom. Needless to say, the environment inwhich the detonation light emission sensor is to operate, is extremely hostile in termsof shock, dust, flying debris, fast rise time of the explosive event, and Electro-magneticInterference ( EMI) caused by the detonation itself. It must be noted that the lightsensor research was driven by the outcome of research tests performed in aid of thedevelopment of an active intervention system.During this research the possibility of using commercially available low cost opticaldetectors at room temperature in combination with cost effective narrow band pass op-tical filters for the relative measurement of the light emission at discrete wavelengthsduring explosive detonation events were investigated. In 2006, not much applicable lit-erature could be found on this subject, hence the educated 'shot-in-the-dark approachthen, which, by a systematic approach of explosive tests and continuous evaluation upto 2011, led to a surprisingly simple and robust low cost optical sensor. The researchcommenced with a range of optical detector elements selected for their responsivityand bandwidth in the optical spectrum of interest; the optical filtering by means of the recording of the emitted light signal during scaled down explosive tests at the BlastImpact Survivability Research Unit (BISRU) at the University of Cape Town. Thesetests were followed by full-scale tests at DBEL, and confirmed the findings at BISRUthat the light emissions at the longer wavelengths (>2 m) manifest themselves too latefor use within the intervention time frame. It was therefore decided to concentrate onthe ultra-violet (UV) to near infra-red (NIR) spectrum of the emitted light for furtherfull scale tests, since these discrete spectra showed the most promise for characterisa-tion of the emitted light. During this period a robust sensor housing with detector andfilter mounts was designed for protection against blast shock and EMI.During the following years, certain types of optical detectors that were used duringprevious tests were eliminated according to results obtained, and more discrete narrowband pass filters added in the visible to NIR spectrum. A dedicated fast instrumen-tation amplifier (bandwidth > 1MHz and selectable gain up to 40dB) was developedto amplify weak signals (mainly caused by the heavy load in the detector circuit toimprove rise times). However, the emission of light per wavelength in this region wasmeasured to be relatively strong, and actually not as fast as was anticipated. Thismeant that the load resistor value of the detector element could be increased withoutaffecting the signal negatively (bandwidth sufficient), thus adding to the amplitude ofthe signal to such a point that amplification in a 10m to 30 meter stand-off scenariowas no longer needed. This culminated in an unamplified universal detector elementbeing used with various narrow band pass filters up to 1 m, integrated as a very robustanalog sensor at a discrete wavelength, and facilitating the direct comparison of lightamplitude/relative intensity of the detonation at discrete spectral points.The sensor was employed in the field at various full scale explosive tests at DBEL,which led to the capture of a vast amount of light emitted data for different types ofexplosives, at various distances from the detonation, and of varying mass. Analysis ofthis data showed that the broadband light intensity of the emitted light scales to the explosive mass1/3 (as published by FJ Mostert and M Olivier in the Journal for AppliedPhysics, October 2011). Further analysis also confirmed the attenuation of the emittedlight intensity by the square of the distance. Besides the aforesaid, various other keyinputs to a possible active intervention algorithm have been identified. These findingsare inputs to the determination of i.a. the detonation threat size, a vital component inthe active intervention algorithm.The results of these experiments confirmed that the final low cost analog sensor canmeasure relative light emission at discrete wavelengths from detonation of explosives inthe very early stages of development, and that the sensor has many other applicationsin the detonics research fields as well.

[发布机构] Stellenbosch University