1. AHRS & ITS APPLICATIONS
Erfan Dashti
2015-2016
MUT TEHRAN
AEROSPACE ENGINEERING DEPARTMENT
2. What is an AHRS tech?
Attitude heading reference system or better known as
AHRS is 3-axis inertial measurement unit (IMU) which is
combined with 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 gyroscope aircraft flight instruments
and provide better reliability and accuracy. Typically an
AHRS will consist of eighter of fiber optic or MEMS 3-axis
angular rate gyro triad, a 3-axis MEMS accelerometer, and
a 3-axis magnetic sensor known as a magnetometer.an
onboard kalman filter is used to compute the orientation
solution using three various measurements.some AHRS
sensors will also use GPS to help stabilize the gyro drift and
provide more accurate estimate of the inertial acceleration
vector.
3. Parts that make up an AHRS
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 sensor package to measure magnetic field vector. A 32-
bit processor or DSP is added to provide a platform to run
a kalman filter attitude estimation algorithm.
Examples of high-end AHRS systems
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 7*4*4.5 inches
and a weight of 4.5 lb. (Libra balance-as pounds).
4. This AHRS is used on several aerial platforms like 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
(<10$k) AHRS sensors on the market. All of these sensors
utilize MEMS gyros, accelerometers, and magnetometers.
The main difference between 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. 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
5. measurements to estimate the time-varying gyro bias in
real-time. These systems offer the high 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.
Gyroscope
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
advantages of the latest automotive grade accelerometers
made by companies such as analog devices. Newer
companies such as sensors and silicon sensing offer state
of the art MEMS gyros that claim bias stabilities that offer
near tactical level performance.
6. Accelerometer
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
accelerometers bias error translates into a 1/10th
degree
pitch/roll orientation error in static conditions. This accuracy
ensures adequate vertical alignment during the initializing
process. High-end systems typically require
accelerometers that exhibit very linear outputs, or
alternatively require higher order sensor model 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 the
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 within 1.5mg
over one year, and
+-10g acceleration full scale range.
7. Magnetometer
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 platform prior to use to
compensate for the effects of nearby objects.
Uses for an AHRS
8. 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 tried into 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.
The VectorNav VN-100 AHRS
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 -40 Celsius degree to 80c, and a highly
competitive price, the VN-100 is sure to further expand the
current lists of applications which utilize AHRS tech.
9. All attitude inertial sensor system
o Provides attitude, heading and flight dynamics
info to aircraft systems and displays
o Interfaced to EFIS through high speed data bus
Attitude Heading Reference Unit (AHRU)
o Computer located in right avionics bay
o Provides EFIS w/ pitch, roll and yaw rate data
from accelerometers and gyro
o Provides EFIS w/ heading data from magnetic
sensor unit
o Power routed through both FCP circuit breaker
panels (redundant)
AHRS Controller (located in FCP)
o Mode switch
Slaved (SLVD): Gyro stabilized magnetic
heading info
Primary mode
Degraded when flying near disturbances
in earth’s magnetic field or when flying
in vicinity of the magnetic poles
Direct Gyro (DG):
Back up mode
Decoupled from magnetic sensor unit
Free directional gyro
Only as good as the heading slewed to
by the pilot
Requires updates
Drift rate less than 9 degrees per hour
Meter displays slaving error
10. Pilot alerted via “FHDG” message, and
heading warning flag (red HDG in red
box) on the EHSI display when AHRS
computer notes failure in heading
sensing system. AHRS computer
automatically selects DG mode. If pilot
manually selects DG mode, heading
warning flag disappears, but “FHDG”
message remains displayed.
Magnetic Sensor Unit
o Located in right wing
o Provides raw data to AHRU
Total AHRS Failure
o Confirm indications in both cockpits
Crosscheck standby instruments
EFIS related fault annunciations
Red “ATTITUDE FAIL” enclosed in red
box – EADI unusable
Red “HDG” enclosed in red box and
“FHDG” message on EHSI display –
heading info defaulted to DG mode
o Check circuit breakers (FR L/R CSL)
Assess severity before reset
o If not already popped, pull and reset both circuit
breakers
Must allow at least .5 second power
interruption (both breakers out)
Fly straight and level un-accelerated for
minimum of 30 seconds
