Sensing a Changing Chemical Mixture Using an Electronic Nose
- Wednesday, 02 January 2008
ASIC may enable continuous, high-speed monitoring.
A method of using an electronic nose to detect an airborne mixture of known chemical compounds and measure the temporally varying concentrations of the individual compounds is undergoing development. In a typical intended application, the method would be used to monitor the air in an inhabited space (e.g., the interior of a building) for the release of solvents, toxic fumes, and other compounds that are regarded as contaminants. At the present state of development, the method affords a capability for identifying and quantitating one or two compounds that are members of a set of some number (typically of the order of a dozen) known compounds. In principle, the method could be extended to enable monitoring of more than two compounds.
An electronic nose consists of an array of sensors, typically made from polymercarbon composites, the electrical resistances of which change upon exposure to a variety of chemicals. By design, each sensor is unique in its responses to these chemicals: some or all of the sensitivities of a given sensor to the various vapors differ from the corresponding sensitivities of other sensors. In general, the responses of the sensors are nonlinear functions of the concentrations of the chemicals. Hence, mathematically, the monitoring problem is to solve the set of time-dependent nonlinear equations for the sensor responses to obtain the timedependent concentrations of individual compounds.
In the present developmental method, successive approximations of the solution are generated by a learning algorithm based on independent-component analysis (ICA) — an established informationtheoretic approach for transforming a vector of observed interdependent signals into a set of signals that are as nearly statistically independent as possible. The algorithm can be characterized as being equivalent to a computational architecture known in the art as a space-invariant ICA architecture. In principle, this architecture is amenable to implementation in an application-specific integrated circuit (ASIC). The anticipated future development of such an ASIC would make it possible to construct a miniature, highspeed, low-power electronic-nose sensor system for continuous monitoring.
This work was done by Tuan Duong and Margaret Ryan of Caltech for NASA’s Jet Propulsion Laboratory.
Refer to NPO-42213, volume and number of this NASA Tech Briefs issue, and the page number.