Attitude Heading Reference System
Parts that make up an AHRS
Attitude Heading and Reference Systems better known as AHRS is a 3-axis Inertial Measurement Unit (IMU) combined with a 3-axis magnetic sensor, and an onboard processor that creates a virtual 3-axis sensor capable of measuring heading (yaw), pitch, and roll angles of an object moving in 3D space.
AHRS sensors were originally designed to replace the large traditional mechanical gyroscopic aircraft flight instruments and provide better reliability and accuracy. Typically an AHRS will consists of either a fiber optic (FOG) or MEMS 3-axis angular rate gyro triad, a 3-axis MEMS accelerometer, and a 3-axis magnetic sensor known as a magnetometer. A onboard Kalman filter is used to compute the orientation solution using these various measurements. Some AHRS sensors will also use GPS to help stabilize the gyro drift and provide a more accurate estimate of the inertial acceleration vector.
Figure 1 - EFIS with internal AHRS
Examples of High-End AHRS Systems
An AHRS starts with a calibrated Inertial Measurement Unit. In some cases this is provided as a single drop in part, in other cases it is constructed using separate single-axis accelerometers and gyroscopes. A magnetometer is added to the sensor package to measure the magnetic field vector. A 32-bit processor or DSP is added to provide a platform to run a Kalman filter attitude estimation algorithm.
Many high-end tactical grade AHRS sensors are built around the Northrop Grumman LN-200. The LISA 200 AHRS manufactured in Europe and has a size of 7x4x4.5 inches and a weight of 4.5 pounds.
Figure 2 - LISA 200 AHRS
This AHRS is used on several aerial platforms including the UH-60 Blackhawk. Another popular AHRS is the LCR 92 and LCR 93 AHRS units also manufactured in Europe. Both systems utilize fiber optic gyros. The LCR 92 uses a bubble level for sensing gravity direction, while the LCR 93 uses MEMS based accelerometers.
Low Cost AHRS Systems
There are numerous companies that now offer low cost (<$10K) AHRS sensors on the market. All of these sensors utilize MEMs gyros, accelerometers, and magnetometers. The main difference between the low-cost AHRS sensors on the market is the type of attitude estimation algorithm utilized on the sensor. These different sensors can be divided into two groups. On one end you have a few AHRS sensors on the market that fully utilize a similar Kalman filter algorithm to that used on the high-end AHRS systems. This type of algorithm uses the magnetic and acceleration measurements to estimate the time-varying gyro bias in real-time. These systems offer the higher performance and provide a means of optimally tuning the sensor for enhanced performance in certain applications. The higher-end algorithms will typically use quaternion math internally in order to ensure complete operation in any orientation without mathematical singularities. The other group of sensors use modified non-Kalman filter based algorithms to compute an estimate of the orientation of the sensor in real-time. This type of algorithm differs from the ones used on high-end AHRS systems, and the ultimate quality of performance is highly manufacturer dependent.
Typically very precise gyros are used on an AHRS as the quality of the gyros has an enormous impact on the overall performance of the resulting sensor performance. Tactical grade AHRS use fiber optic gyros to provide very stable angular rate measurements. Recently AHRS manufactures have begun to use MEMS gyros due to their low power and low costs. Recent advances in MEMS gyros have reached performance levels that are comparable with low end fiber optic gyros. All low cost AHRS sensors on the market take advantage of the latest automotive grade accelerometers made by companies such as Analog Devices. Newer companies such as Sensonor and Silicon Sensing offer state-of-the-art MEMS gyros that claim bias stabilities that offer near-tactical level performance.
High-end AHRS systems typically use solid-state silicon accelerometers. The accelerometers used on an AHRS are chosen to have exceptionally good long-term bias stability. A stable accelerometer is important in an AHRS since it provides the onboard algorithm with a measurement of the horizontal level plane. High-end AHRS systems typically use tactical grade accelerometers where better than 2mg over all full operating conditions is required. A 2mg accelerometer bias error translates into a 1/10th degree pitch/roll orientation error in static conditions. This accuracy ensures adequate vertical alignment during the initialization process. High end systems typically require accelerometers that exhibit very linear outputs, or alternatively require higher order sensor models to compensate for the non-linearity. The lower class of AHRS are typically not used for flight display systems use consumer grade accelerometer and gyros and provide adequate performance for many applications such as civilian airplane post-flight data recorders, and some UAV's. Companies such as Colibrys are pushing the limits of MEMS accelerometer performance, and offer several accelerometer sensors that provide ideal performance for AHRS applications. The Colibrys IRIS - RS9010.A offers tactical level performance with bias stabilities to within 1.5mg over one year, and +-10g acceleration full scale range.
Figure 2 - LISA 200 AHRS
Uses for an AHRS
The magnetic sensors used on an AHRS are typically flux-gate magnetometers for high-end AHRS applications and MEMS Anisotropic Magneto Resistive (AMR) sensors for the low-end AHRS systems. Flux-gate magnetometers provide exceptionally low non-linearity, high accuracy and reliability. Typically on aircraft systems the magnetometer is mounted in a separate remote location from the AHRS. Since the magnetometer can be affected by external magnetic disturbances such as magnetic fields produced by electric motors, it is typically mounted in a selectively chosen location where the magnetic disturbances are minimal. Many low-cost AHRS systems do not provide a means of connecting a remote magnetometer. MEMS based magnetometers offer lower power requirements and a much smaller package size than the traditional flux-gate designs. Either magnetometer will need to be calibrated once installed on an aircraft or platform prior to use to compensate for the effects of nearby objects.
The VectorNav VN-100 AHRS
AHRS sensors are typically used in one of three ways. First of all they can be used as an instrument to provide flight data recording of the orientation of the platform as a function of time. Secondly an AHRS can be used for platform stabilization. The angular rate outputs can be directly tied into a control loop to maintain platform stability. An AHRS can also be used as an attitude control system. Aircraft, helicopters, quad-rotors, blimps, and even underwater robotic vehicles all can benefit from the use of an AHRS to form a closed-loop attitude control system. The angular rate outputs at a high bandwidth along with the estimated orientation angles make it possible to command a platform directly with desired attitude angles. The quaternion output that many units provide make singularity free attitude control a reality for many applications.
In June 2009 VectorNav released the world's first AHRS as a single surface mountable flat module design. Utilizing a robust Extended Kalman filter that estimates the gyro bias in real-time, the VN-100 offers a true high performance AHRS attitude estimation algorithm. With its extremely compact surface mountable footprint, a high quality individual level sensor calibration conducted over the full operating range of -40C to 80C, and a highly competitive price, the VN-100 is sure to further expand the current lists of applications which utilize AHRS technology.