Modifications have been made to upgrade the system described in "Optoelectronic Particle-Fallout Sensor" (KSC-11687), NASA Tech Briefs, Vol. 19, No. 4 (April 1995), page 17a. A description of the unmodified version of the system is necessary to place a description of the modifications in context:
This is a portable system that measures fallout of small airborne particles. The system includes a sensor module and a data-acquisition module, both of which are battery-powered and contain microcontrollers and other circuitry. The sensor module can operate either independently or under control by the data-acquisition module.
The sensor module includes a black acetal plastic housing with a top opening through which dust can fall onto a mirror. A portion of the mirror is illuminated by an infrared light-emitting diode (LED). When particles are not present on the mirror (and provided that the mirror is not scratched), the infrared light is reflected specularly by the mirror and absorbed by the black sides and top of the housing. When particles are present, the infrared light that they scatter is measured by a photodetector assembly. The optics are designed to minimize the detection of both ambient light and light scattered from surfaces other than that of the mirror. The photodetector output is amplified, digitized, and time-tagged as an indication of the particulate contamination of the mirror surface as a function of time.
To conserve battery energy, the sensor module is designed to operate for only a few seconds - just long enough to take a reading when it is commanded to do so. When separated from the data-acquisition module, the sensor module is turned on to take a reading by pressing a momentary-contact button switch. To increase sensitivity and facilitate discrimination against background signals, the infrared LED is turned off and on several thousand times during each sampling interval. The difference between the signals measured in the "on" and "off" states is averaged over the sampling interval to produce an output signal that contains relatively low noise.
To operate the two modules together, it is necessary to connect them via a ribbon cable. In this configuration, the data-acquisition module takes control of, and supplies power to, the sensor module. At time intervals selected by the operator, the data-acquisition module commands the sensor module to take readings and records both each reading and the time when it was taken. The module can later be connected to a computer to transfer the reading and time data to the computer for display and/or analysis and to program the data-acquisition module for subsequent readings. This completes the description of the unmodified version of the system.
The modifications were made primarily to overcome the following deficiencies of the unmodified version of the system:
- Sensitivity was not adequate for use in very clean environments;
- It was not possible to calibrate the system accurately by following commonly accepted practices;
- The system was not sufficiently thermally stable for use in non-temperature-controlled environments;
- The electrostatic and power-consumption characteristics did not satisfy requirements for use of the system in certain specific spacecraft-payload-processing clean rooms at Kennedy Space Center;
- It is now a one-piece instrument; and
- The instrument has RS-232 output and can be programmed remotely and used as part of a facility contamination monitor system.
The sensitivity of the system was increased by replacing a 12-bit analog-to-digital converter (ADC) with a new 20-bit ADC. The integrated-circuit chip of which the 20-bit ADC is a part affords additional digital filtering capabilities, which are exploited to increase the robustness of operation under adverse lighting situations. To accommodate the aforementioned circuitry, a circuit board was completely redesigned, with special attention paid to quality of signal and reliability. The redesigned circuit board supports additional features, including programming of operating parameters via a keypad, a current-loop or voltage analog output, and new power-utilization features. The overall number of integrated-circuit chips has been reduced, and the number of circuit boards has been reduced from 2 to 1.
The system now operates on modified software that provides, among other things, the ability to reprogram "on the fly" (without disrupting operation) to adjust such parameters as sensitivity ranges and data-taking intervals. (Previously, it was necessary to turn the system off, then back on again to put it in a programming mode.) The modified software makes it possible to reprogram or to download data from the system remotely by serial data communication via a cable.
A reference photodetector with an independent ADC was added. These components will make it possible to measure and hence correct for fluctuations in the output of the LED, thereby increasing the temperature stability of the system by an order of magnitude.
The mirror in the unmodified version of the system was replaced with a witness plate in the form of a silicon wafer bonded to an aluminum substrate. Holes in the aluminum substrate make it possible to scan the silicon wafer, by use of a semiconductor-wafer scanner, for extremely accurate analysis of the particulate contamination on the wafer; this makes it possible to perform acceptable calibration.
The optics have been simplified to reduce the number of surfaces through which light must pass, thereby increasing throughput of the light to the photodetectors. There is also now an option to use an LED with a wavelength of 935 nm in applications in which the shorter wavelength (880 nm) of the unmodified system would be problematic.
The electronic circuitry was modified to enable the system to operate on intrinsically safe power through an intrinsically safe barrier to enable use in hazardous environments. Finally, the electrostatic-charge issue was addressed by repackaging the system in an aluminum enclosure, which can be grounded.
This work was done by Paul A. Mogan of Kennedy Space Center and Christian J. Schwindt, Timothy R. Hodge, and Steven J. Klinko of Dynacs Engineering Co. Inc.
This invention has been patented by NASA (U.S. Patent No. 5,412,221). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Technology Programs and Commercialization Office
Kennedy Space Center
Refer to KSC-12105.