Primarily intended for applications with high-cost sensitivity or limited GPS access (such as expendable smart munitions or unmanned underwater vehicles), a recently developed miniature accelerometer uses Fabry-Perot technology to provide navigation-grade accuracy at low cost. In addition to enabling precise navigation in these emerging applications, the technology promises to reduce the cost of navigation systems in several mature applications, such as general aviation. Omega Sensors, with support from The Center for Commercialization of Advanced Technology (CCAT) has advanced the refinement of this technology in order to reduce costs for eventual military and civilian applications.
The core technology, invented by Richard Waters while working at the U.S. Navy's Space and Naval Warfare Systems Center (SPAWAR), is based on the precise detection and compensation of movement within a displacement mass. The detection mechanism uses a vertical-cavity surface-emitting laser (VCSEL), and employs the principle of monochromatic interferometry to measure extremely small movements (to displacement accuracies better than 1 ppm of the wavelength of the VCSEL). The displacement mass and bias-force generator used for compensation are combined within an integrated MEMS structure.
The system incorporates two key control loops to enable high sensitivity. One control loop maintains stability in the wavelength and the intensity of the VCSEL, and the other maintains the bias force that keeps the displacement mass in the proper position. Stability in the VCSEL is critical to the precision of Fabry-Perot technology, because while the gain of the sensor is stable for a constant temperature, it is unable to distinguish between light intensity changes caused by shifts in the displacement mass, and changes caused by alterations within the laser itself. Therefore, a constant drive current and temperature are needed to stabilize the intensity and wavelength of the VCSEL.
Temperature compensation is achieved with a thermoelectric cooling module coupled to the package or a micro-resistive heater coupled to the VCSEL. In the case of the latter, a resistive heating element is used to maintain a bias temperature well above the maximum operating ambient temperature to ensure good thermal regulation. The power consumption for this configuration is low due to the small thermal mass of the VCSEL.
The principle of biasing is also used in the displacement-mass control loop. A constant bias force keeps the displacement mass in the best position to detect acceleration. Fluctuations in the signal controlling this force directly translate into acceleration experienced by the sensor along its monitored axis.
The combined result is navigation-grade sensitivity at low cost. This may also result in a slew of unforeseen applications, including enhancing location services for wireless handsets.