Some vehicles use an internal measurement unit (IMU) system to determine the speed, acceleration, orientation, position, and/or direction of movement of the vehicle. Vehicles used for high-availability or life-critical systems may employ a fault-tolerant IMU design. Typically, such vehicles use three or more IMUs to detect the failure of an IMU and isolate the failing IMU from the other functional IMUs. A fault-tolerant system having multiple IMUs pays an associated mass, power, and volume penalty for each additional IMU. The mass/power/volume (M/P/V) of a fault-tolerant IMU system is the M/P/V of an individual IMU multiplied by the number of IMUs employed to do fault tolerance. Furthermore, each additional IMU adds to the cost of a fault-tolerant system.
This invention is an integrated rate isolation sensor (IRIS) system that, when used in a fault-tolerant IMU system, allows for fault detection and isolation using only two IMUs instead of the traditional three or more. Use of the IRIS system reduces the total M/P/V and cost of the overall system by reducing the number of IMUs required.
The IRIS system may include a microprocessor and a means for an independent rate measurement. In one embodiment, the IRIS system includes micro-electromechanical-system (MEMS) gyroscopic sensors for measuring the independent rate. The IRIS system may include three MEMS gyroscopes and associated electronics for operating the gyroscopes. The microprocessor may be disposed in the same chip or housing as the gyroscopes, or may be located remotely and communicate wirelessly with the gyroscopes and electronics of the chip.
The internal rate measurement of the IRIS system may be less precise than the rate measurement of the IMUs, but it consequently may require components that have less mass, power consumption, volume, and/or cost than the components in the higher-precision IMUs. Thus, the overall M/P/V of the fault-tolerant system is reduced.
Once a failure has been detected, the IRIS system may isolate the failing IMU. More specifically, the microprocessor may compare each rate stream to the independent rate measurement computed by, for example, MEMS gyroscopes X, Y, and Z. The comparison may proceed until a desired confidence level is reached or until a desired decision time expires. Each of these parameters may be given by user input or may be preset to typical values. Once the comparison is finished, the microprocessor may select the most likely failing IMU and assert the corresponding good/bad indicator output. This information, along with sampled statistical information, may then be used to actively switch to the non-failing IMU channel, or, in an alternative embodiment, the IRIS system automatically switches the overall fault-tolerant system to the non-failing IMU channel.
This work was done by Timothy Crain of Johnson Space Center and Tye Brady, Timothy Henderson, Richard Phillips, and Doug Zimpfer of Draper Laboratory. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-24564-1