Various Attitude and Heading Reference Systems (“AHRS”) have been developed for the monitoring and estimation of flight parameters thereby assisting a pilot in the control and operation of an aircraft. These systems typically sense and transmit the body angles (pitch, roll, and heading), body rates and body accelerations of an aircraft. The transmitted data is typically used by cockpit displays or autopilot systems.
Older AHRS systems use spinning-mass inertial gyroscopes from which body angles can be sensed directly. Recently, AHRS system designers have replaced spinning mass inertial gyroscopes with solid-state rate gyroscopes. Various technologies are used for the rate gyroscopes including Fiber-Optic Gyros (“FOGs”), piezoelectric-based devices, and Micro Electro-Mechanical Sensor (MEMS) based devices. These modem AHRS systems have the advantage of lighter weight and reliability, but the disadvantage of not being able to sense body angles (pitch, roll and heading) directly. Instead, the modem AHRS systems with rate gyroscopes must integrate the sensed body rates over time to determine the body angles. Therefore, accurate determination of a rate gyroscope's bias (or zero rate position) is of critical importance to prevent accumulation of gross body angle errors.
Most modem AHRS employ three orthogonal rate gyroscopes, three orthogonal accelerometers, and three orthogonal magnetometers in a package with advanced digital signal processing. In some embodiments, the three orthogonal magnetometers are replaced by a traditional magnetic flux valve. Either type of magnetic sensor is often mounted remotely in areas where aircraft systems or structure do not cause magnetic disturbances. The orthogonal devices are typically aligned with respect to the aircraft so that the rate gyros sense roll rate, pitch rate, and yaw rate, and the accelerometers and magnetometers sense accelerations and magnetic flux with respect to the aircraft's longitudinal, lateral and vertical axes.
During periods of high dynamics, rate gyro integrations are used directly to compute body angles. During periods of low dynamics (i.e., near straight and level flight), which is the majority of a typical flight profile, parameters from other sensors are compared to the integrated body angles in order to determine proper bias values for each of the three rate gyroscopes. This comparison is done via a filtering algorithm, with the Kalman filter being the most widely used method. Bias corrections for the pitch and yaw rate gyros are straightforward. For the pitch rate gyro, using the accelerometers to sense the pull of gravity provides a reliable and accurate method for bias determination. Likewise, for the yaw rate gyro, the Earth's magnetic flux provides a reliable and accurate method for bias determination. However, the determination of the roll rate gyro bias is not nearly so straightforward. Various methods have been used in the prior art, each with deficiencies.
One method is to minimize the amount of bias correction required. This is the typical approach, and requires the use of rate gyros using exotic technologies such as ring laser gyros or fiber optic gyros. These types of gyros typically have bias drift rates on the order of 2° to 5° per second per hour, which means that their rate outputs can be integrated into position readings with relatively little outside correction. What little outside correction is needed is then provided, as with the pitch rate gyro, by long term correction to perceived gravity. However, with such precision comes a very high cost. Implementing an AHRS in an aircraft with an integrated aerospace rate gyro is extremely expensive.
In contrast, less-costly MEMS gyros have drift rates an order of magnitude higher (up to 20-30° per second per minute). Such drift rates require heavy bias corrections from other sources. Erecting to perceived gravity does not work in this case, as the correction cannot be applied on a long enough term and aircraft are typically flown “balanced” which means that, in any attitude (even inverted), the perceived pull of gravity remains through the vertical axis of the aircraft. Thus, strongly erecting to perceived gravity generally causes more problems than it solves.
To solve this problem, various methods are taught by the prior art. One is to integrate the AHRS inertial platform closely with GPS derived positions. However, close coupling with GPS leads to safety concerns as the GPS system does not have sufficient integrity for driving systems with catastrophic failure modes such as an AHRS.
Another method is to use the vertical component of the Earth's magnetic field as a reference. This method is taught in U.S. Pat. No. 4,608,641 to Snell, titled Navigation Aid. Snell teaches an aircraft operating in a gravitation field and having conventional sensors for measuring true air speed, angles of incidence and yaw, rotation about x, y and z axes and acceleration therealong is provided with means for calculating the inertial component of the acceleration from data concerning the true air speed, heading and rotation of the aircraft obtained from the sensors, and means for comparing the inertial component with the total acceleration sensed, thereby to deduce the orientation of the gravitation component and hence obtain an estimate of the pitch and bank angles of the aircraft. Unlike using perceived gravity, this method is not susceptible to errors caused by aircraft accelerations. However, it requires accurate positioning information and a method for determining variations in the Earth's magnetic field.
The disclosure of each above-referenced patent and application is incorporated herein and constitute a part of the specification.
A need exists for a highly reliable and simple method and/or process to correct roll rate gyro bias in an AHRS that uses rate gyros having relatively high drift rates.
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