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Technology Primer for Pilots and the Makers of Flying Machines
Photograph by Chris Thompson
Background
Solid State Inertial Systems and The Crossbow Solid State Gyro
Displays
Installation and Performance
FAA Certified
Contacting Crossbow
Background
Most aircraft have several instruments that are traditionally driven by mechanical
Gyros. These instruments assist in flying and navigation of an aircraft. These instruments
are the Attitude Indicator (also known as the Vertical Gyro), the Directional Gyro,
and the Turn and Bank Indicator. Aircraft also typically have a compass, and in
some cases a Flux Valve (also known as a Magnetometer) to which the Directional
Gyro is connected or slaved to cancel long term drift. If the aircraft does not
have an electronic Flux Valve, then the Directional Gyro or DG has to be manually
reset to the compass reading during straight and level flight (when the compass
is accurate) on a periodic basis.
The three Gyro instruments, Attitude Indicator, Directional Gyro and Turn and Bank
Indicator are ¡®gyro¡¯ driven. What does gyro-driven mean? A gyro is a spinning wheel
(mass) that obeys the Laws of Physics. The spinning wheel is spun up either electrically
(electric gyros) or via air flow (vacuum gyros) to high rotational speeds and a
high angular momentum. The spinning wheel is mechanically isolated from the casing
of the instrument thru a series of gimbals. Due to the conservation of angular momentum,
the spinning wheel will try to maintain its orientation, via the gimbals, as the
outer casing moves. The outer casing is of course connected to the airframe. The
gimbals, move by the amount the aircraft has rolled, pitched, or changed heading,
and in some cases directly connect to the display. The display provides an indication
of the aircraft attitude. In the case of a remote gyro and also with many electric
gyros, the gimbals provide an analog electrical output proportional to aircraft
orientation change. See Figure 1 for a picture of a mechanical gyro and its guts.
Figure 1: Vacuum Attitude Indicator / Vertical Gyro
While mechanical gyros have been used in aircraft for many years, there are a number
of problems that make mechanical gyros less than ideal, and have driven the need
for more accurate and reliable instruments. The first and foremost problem is long-term
reliability. Because mechanical gyros are constructed with many moving parts with
close tolerances, they break easily. As the ball bearings that support the high-speed
wheel and the gimbals begin to wear, they contribute to precession errors. Compounding
the issue with vacuum gyros, is that dirt and dust in the vacuum line that destroys
the bearings. Another common problem is that long periods of inactivity can also
cause the mechanical gyro to stop functioning altogether or reduce accuracy and
increase drift rates. The recommended operating life of most mechanical gyros is
only several hundred hours.
A second class of problems is the limited accuracy and resolution of most mechanical
gyros. The design of the majority of mechanical gyros used in general aviation today
was done in the 1950s or before, and the manufacturing techniques have not kept
pace with technology. The result is limited accuracy and resolution, especially
in dynamic maneuvers. Of course, all pilots know that if you do an aerobatic or
other very aggressive maneuver the majority of mechanical gyros generally lose their
mind and in some cases break!
Solid State Inertial Systems and The-Solid-State-Gyro by Crossbow
Ring Laser Gyros
Some time ago, the designers of navigation systems recognized the need for improved
instrumentation for navigating and controlling aircraft in a reliable and more accurate
way. Research and development of Inertial Navigation Systems began, and soon a better
way to navigate was invented. A major breakthrough was the development of a technology
called the ring laser gyro. The ring laser gyro is a highly accurate way to measure
changes in angular position ( or angular rate) without the use
of any spinning things. An angular rate sensor, however, does not directly measure
attitude like a gimbaled mechanical gyro does; instead it measures the rate at which
an object rotates in degrees per second. Strapping three ring laser gyros together
on the X,Y, and Z axes of an airplane, and doing some math, allows for the continuous
calculation of a level reference and the change in roll, pitch and heading. The
ring laser gyro systems, with their simple ¡°strapdown¡± construction that uses no
spinning wheels or gimbals, replaced the mechanical gyro systems in most military
and commercial aircraft. The gotcha is that ring laser gyro inertial navigation
systems are very expensive. Perfect quality glass machined cavities, precision mirrors,
high voltages (> 1kV), lasers, and inert gases are all required to build a laser
gyro. The resulting system is well over $100K.
The MEMS Breakthrough
Another breakthrough occurred when techniques in silicon fabrication technology
allowed for the creation of accurate inertial sensors in silicon. This technology
is known as Micro Electro-Mechanical Sensors (MEMS), and is in high volume production
today. In fact, Crossbow has shipped over 250,000 sensors that utilize MEMS technology.
The Crossbow Solid-State Gyro
Crossbow has been developing and selling low cost solid-state gyros that measure
Roll, Pitch, and Heading using MEMS technology in commercial, industrial and aerospace
markets since 1998. The Crossbow Solid State Gyro, known in our product lingo as
an Attitude-Heading Reference System, or AHRS, uses a 3-axis accelerometer and a
3-axis rate sensor to make a complete measurement of the dynamics of your system.
The addition of a 3-axis magnetometer inside the Crossbow AHRS allows it to make
a true measurement of magnetic heading without an external flux valve. The Crossbow
AHRS is a solid-state equivalent of a vertical gyro/artificial horizon display combined
with a directional gyro and flux valve. The Crossbow AHRS units are low power (<
0.3A), reliable (> 20,000 hr MTBF) and accurate (better than 2 degrees in roll
and pitch). The
AHRS400CC, shown in figure 2, is ideal for driving the AI and DG
displays in uncertified applications. It is a standard in the guidance and control
of unmanned aircraft, and has flown in numerous aircraft under varied conditions.
Figure 2: AHRS400CC
Piezo Gyro Myths
Besides the silicon MEMS rate sensors in Crossbow¡¯s products, there are also piezo
gyros and other technologies which also measure angular rate, but are not really
MEMS. Inexpensive piezo gyros are frequently used for stabilizing remote controlled
aircraft and in some cases limited automotive applications. These hobby-grade rate
sensors are inexpensive, not accurate, and have serious drift problems, especially
with Temperature and Vibration. The rate sensors used in the Crossbow Solid-State
Gyro products have 10 times the accuracy and repeatability of these devices. In
the past, Crossbow built instruments using these piezo sensors, which have been
useful in some applications. However, even with the best compensation and software
in the world, these devices are just not for suitable for attitude measurement in
aircraft.
The Importance of Stand Alone Operation
An additional benefit of the Crossbow AHRS400CC is that it is designed to operate
in a stand-alone mode. Unlike some other gyro systems, it does not need input from
external air data, magnetometers, or GPS. This makes installation easy, and improves
the reliability of your attitude display. A gyro system that relies on external
input has an inherent reliability multiplier problem: lose the external system,
and you lose your gyro.
Displays
The Crossbow Gyro has a digital computer compatible output (RS-232). Packets of
digital information containing roll angle, pitch angle, and heading angle are sent
out in standard serial format up to 70 times per second. This makes it easy to connect
to digital displays ¨C like those in the new glass cockpit systems. Several companies
are now releasing General Aviation glass cockpits systems that will work with the
Crossbow Gyro.
However, if you are just looking for a simple back up instrument, we have developed
software to allow display on standard computing devices. The software is called
GyroView and it runs on Windows-based laptops and Windows CE based handhelds, like
the Compaq IPAQ. Figures 3 and 4 are pictures of GyroView running on a Windows-based
laptop and GyroView running on an IPAQ Windows CE handheld.
Figure 3: GyroView on Windows Laptop
Figure 4: GyroView CE on IPAQ Handheld
Installation and Performance
As with any instrument, the Crossbow AHRS has to be installed properly to get the
desired performance.
The AHRS must be securely fastened to the airframe. The AHRS can be installed just
about anywhere in the airframe, but it should be installed on a level surface (with
respect to Earth), and with the connector facing towards the aft. Because the Magnetometer
is built into the instrument, installing near large amounts of magnetic material,
or moving magnetic objects should be avoided. A good example of a moving magnetic
object is retractable landing gear.
The unit is powered by 9-30 VDC and uses about 0.3 A. The AHRS can be easily tied
into an airplane¡¯s power bus. It is a good idea to maintain a battery back up on
the power line.
A final installation step is to perform a heading calibration. Heading calibration
compensates for any magnetic field created by the aircraft. It consists of putting
the unit in calibration mode, and rotating the vehicle in a circle. Most airports
have a compass rose, which is the best place to perform the calibration. Unlike
some other heading systems, you do not need to position or point the plane in any
specific direction. Just turn at least one complete circle while in the calibration
mode.
Once properly installed and calibrated, the performance of the system is pretty
stunning. The following graphs show the roll, pitch, and heading performance of
a Crossbow AHRS compared to a Ring Laser Gyro navigation system that costs $125K.
Crossbow purchased the Ring Laser Gyro instrument for the sole purpose of proving
that its instruments rival the performance of the very very best. During flight
tests, we have also had the opportunity to compare the accuracy of our Solid State
Gyro to a mechanical gyro. The graphs show the Crossbow in red, the mechanical gyro
in green, and the ring laser reference system in blue.
Figure 6: Roll Performance on a Test Flight
Crossbow (Red), Mechanical (Green), Ring Laser Reference (Blue)
Figure 7: Pitch Performance on a Test Flight
Crossbow (Red), Mechanical (Green), Ring Laser Reference (Blue)
FAA Certification
The AHRS400CC is not approved for use in aircraft by the FAA. Crossbow has recognized
the need for a certified system and has developed the AHRS500GA system for FAA certified
applications. The AHRS500GA is shipping now. Figure 8 is a Picture of AHRS500GA.
The unit is designed from the ground up for FAA certification, including the TSO
requirements (TSO 4 and 6), DO-178B software certification to level C, and DO-160D
environmental tests for class 1, 2, and 3 aircraft.
Figure 8: AHRS500GA
WARNING:
Contact Crossbow prior to installing one of its products for IFR operations.
Contacting Crossbow:
To request additional info or have sales person call:
mailto:sales@xbow.com
Call: 408-965-3300
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