|
Novel Portable Explosive Detection Systems |
|
|
|
|
Written by Azzam Moustapha
|
|
Saturday, 08 March 2008 |
|
Development of Novel Portable Explosive Detection Systems
Azzam I. Moustapha, Venkata Chivukla, Rastko R. Selmic *, and Frank Ji
Institute for Micromanufacturing, Louisiana Tech University
(* corresponding author, email:
This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
)
In this paper we present the recent research and developmental results on a novel wireless explosive detection system. An emerging sensing technology in micro/nanoelectromechanical system (MEMS/NEMS) and wireless information and communications technologies have been combined in developing a novel explosive detection device that can simultaneously monitor several explosive compounds in a real time.
The explosive detection device is based on an array of microbridges fabricated on a single chip
that includes mechanical and electronic components for smart actuation and sensing of
explosives. The design provides portable, transportable and movable monitoring device.
Explosive detector characteristics include real-time response, miniature size, low power and cost,
and high mobility and reliability.
A finite element simulation has been used for optimization of piezoresistive silicon covered SiO2
microbridges. Microbridges have been fabricated with a commercially available ROI wafer. The
dimensions of the designed microbridge are 400 μm in length, 50 μm in width, and 1 μm in
thickness. The thickness of the top boron Si and the following SiO2 insulator were 2 μm and 1μm,
respectively. The dimensions of the silicon piezoresistor on the bridge were 400 μm in length, 10
μm in width and 2 μm in thickness. The microbridge resistance variations due to the surface
stress changes were systematically investigated by varying microbridge geometries and the
boron doping concentrations in silicon. Simulation results have shown that the resistance change
of the microbridge is directly related to the actuation of the microbridge and that better sensitivity
can be obtained with thinner and narrower silicon piezoresistor and the SiO2 microbridge. Based
on these simulations, the piezoresistive microbridges have been fabricated. Fabricated
microbridges were found to be more stable than microcantilevers in terms of undesired
oscillations.
An electronic module for the explosive microbridge-based sensor actuation and sensing interface
has been developed. The presence of the explosive compounds is detected as a variation of the
voltage due to microbridge oscillations. The microbridge oscillations cause the oscillatory change
in the microbridge resistance and are detected by the interface circuit and the microcontroller.
Based on experimental results and fabrication data we have concluded that the nominal
microbridge resistance is 1 MΩ. Previous experiments in explosive detection have indicated that
the required current needed to heat the microbridge-based sensor is around 20 μA. This provides
a voltage drop of 20 V across the microbridge, creating an output reading source. Sensor
interface consists of constant current source that is controlled by the microcontroller. The
microcontroller initiates microbridge actuation, followed by the sensor readings that are read by
the microcontroller A/D converter.
Once sampled data have been collected by the microcontroller, a software algorithm processes
the data and decides if there are explosive molecules on the microbridge or not. The algorithm is
implemented in C for an easy integration with TinyOS-based wireless sensor nodes. In order to
reduce the false alarm occurrences NO2 sensor will be added and data base of false alarm
signatures will be created at the system base station. Upon microbridge oscillation has been
detected, the microcontroller will check the reading of NO2 sensor and, if detected, alert the base
station of possible explosive detection. The sensor node will also send the full signature data to
the base station. The base station will correlate newly received microbridge oscillation data with
existing data base vectors and decide about final user alert.
Future work includes an integration of microbridge-based sensor array and electronics module
into a single MEMS device.
|
|
Last Updated ( Sunday, 20 April 2008 )
|
Moustapha Home 
Moustapha News 
