Arrays of multiple, nominally identical sensors with sensor-output-processing electronic hardware and software are being developed in order to obtain accuracy, reliability, and lifetime greater than those of single sensors. The conceptual basis of this development lies in the statistical behavior of multiple sensors and a multisensor-array (MSA) algorithm that exploits that behavior. In addition, advances in microelectromechanical systems (MEMS) and integrated circuits are exploited. A typical sensor unit according to this concept includes multiple MEMS sensors and sensor-readout circuitry fabricated together on a single chip and packaged compactly with a microprocessor that performs several functions, including execution of the MSA algorithm.
In the MSA algorithm, the readings from all the sensors in an array at a given instant of time are compared and the reliability of each sensor is quantified. This comparison of readings and quantification of reliabilities involves the calculation of the ratio between every sensor reading and every other sensor reading, plus calculation of the sum of all such ratios. Then one output reading for the given instant of time is computed as a weighted average of the readings of all the sensors. In this computation, the weight for each sensor is the aforementioned value used to quantify its reliability.
In an optional variant of the MSA algorithm that can be implemented easily, a running sum of the reliability value for each sensor at previous time steps as well as at the present time step is used as the weight of the sensor in calculating the weighted average at the present time step. In this variant, the weight of a sensor that continually fails gradually decreases, so that eventually, its influence over the output reading becomes minimal: In effect, the sensor system “learns” which sensors to trust and which not to trust.
The MSA algorithm incorporates a criterion for deciding whether there remain enough sensor readings that approximate each other sufficiently closely to constitute a majority for the purpose of quantifying reliability. This criterion is, simply, that if there do not exist at least three sensors having weights greater than a prescribed minimum acceptable value, then the array as a whole is deemed to have failed.
Monte Carlo simulations of the MSA algorithm on a computational model of a representative multisensor array have demonstrated that a sensor package equipped to implement the MSA algorithm can monitor its own health and estimate its remaining lifetime. In addition, the simulations showed that the array can have a lifetime up to three times that of a single sensor and the errors in the readings delivered by the MSA algorithm are characterized by error bands smaller than those of a single sensor. As a consequential further benefit, calibrations and replacements are needed less frequently than they are in the cases of single sensors.
The figure shows a prototype sensor MSA unit that includes eight surfacemount pressure transducers and an eight-channel multiplexer circuit on a circular circuit board potted with epoxy in a chamber in a sealed aluminum housing. The housing is fitted with a threaded port that gives access to the chamber. A microprocessor and its supporting electronic circuitry are on a separate board that is plugged into the sensor board. The supporting circuitry comprises almost all of the peripheral circuitry needed to complete the functionality of the sensor package, including a self-calibrating 16-bit analog-to-digital converter, a bandgap voltage reference, ample program flash memory, a nonvolatile data memory, and a serial port for communications.
Upon receiving a command to take a measurement, the microprocessor cycles through the multiplexer to measure the voltage from each pressure transducer. It then converts each transducer voltage to a pressure reading via a linear calibration, using unique calibration coefficients for each transducer. The calibration coefficients are stored in the nonvolatile memory and can be easily updated by means of a simple download routine. The pressure readings are entered into the MSA algorithm.
This work was done by Christopher Immer, Anthony Eckhoff, John Lane, Jose Perotti, John Randazzo, Norman Blalock, and Jeff Rees of Dynacs, Inc. for Kennedy Space Center. This invention is owned by NASA, and a patent application has been filed. 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-12221/359.