Support: Knowledge Base
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What size fasteners or bolts
are used with 420/440 Series products?
Question: The NAV420 and 440 Series Inertial
Products all have a 0.19" bolt hole which allows for the use of a
#10 or #8 bolt. A metric equivalent would be an M5 series. If your
inertial system uses an internal magnetometer, it is important to
make sure that you use non-ferrous material that will not have an
affect on the magnetometer readings as noted in the user
manual.
Solution Detail: The NAV420 and 440 Series
Inertial Products all have a 0.19" bolt hole which allows for the
use of a #10 or #8 bolt. A metric equivalent would be an M5
series. If your inertial system uses an internal magnetometer, it
is important to make sure that you use non-ferrous material that
will not have an affect on the magnetometer readings as noted in
the user manual.
How long does GPS lock take
on the 440 Series
Question: If you are using the built in GPS
receiver on the NAV440 series or the previous generation NAV420,
you can expect full GPS lock within 60 seconds assuming that you
have a high quality antenna and are located in an open area with
multiple satellites available.
Solution Detail: If you are using the built in
GPS receiver on the NAV440 series or the previous generation
NAV420, you can expect full GPS lock within 60 seconds assuming
that you have a high quality antenna and are located in an open
area with multiple satellites available.
NAV420/440 Hard Iron
Alignment Tips
Question: What are some useful tips for
performing the Hard/Soft Iron alignment with the NAV420/440 series
inertial systems? When you complete a hard iron alignment with the
latest NAV-VIEW firmware you will be provided with values for the
X and Y Offset and the Soft Iron Ratio. As a general rule, these
values should be as follows: X-Offset = <0.10 Y-Offset =
<0.10 Soft Iron Ratio = >0.97 If your values are outside of
these ranges, you will need to evaluate your current mounting
location and either move ferrous material near the NAV420/440 or
relocate the NAV420/440 to a better location. Magnetometer
performance is critical for heading and roll/pitch
calculations.
Solution Detail: What are some useful tips
for performing the Hard/Soft Iron alignment with the NAV420/440
series inertial systems?
When you complete a hard iron
alignment with the latest NAV-VIEW firmware you will be provided
with values for the X and Y Offset and the Soft Iron Ratio. As a
general rule, these values should be as follows:
X-Offset =
<0.10
Y-Offset = <0.10
Soft Iron Ratio =
>0.97
If your values are outside of these ranges, you
will need to evaluate your current mounting location and either
move ferrous material near the NAV420/440 or relocate the
NAV420/440 to a better location. Magnetometer performance is
critical for heading and roll/pitch calculations.
Synchronizing ITOW (2
bytes)
Question: Interval Time of the Week (ITOW) is
provided in the NAV packet in the 440 series products. Due to
packet size limitations however, only the last two bytes of this
time are provided. In order to synchronize this time with a known
UTC time, you will need to convert the UTC time given on the
B-port by the GPS receiver into the current ITOW time for that
week (this varies based upon the number of days in the month and
leap seconds). Once you have converted the values from the B-port
you can synchronize them with the last two bytes that you are
given in the NAV packet. This will give you allow you to know when
the data was taken by the 440 series inertial system.
Solution Detail: Interval Time of the Week
(ITOW) is provided in the NAV packet in the 440 series products.
Due to packet size limitations however, only the last two bytes of
this time are provided.
In order to synchronize this time
with a known UTC time, you will need to convert the UTC time given
on the B-port by the GPS receiver into the current ITOW time for
that week (this varies based upon the number of days in the month
and leap seconds). Once you have converted the values from the
B-port you can synchronize them with the last two bytes that you
are given in the NAV packet. This will give you allow you to know
when the data was taken by the 440 series inertial
system.
3rd Party GPS Connection to
440 Series
Question: How can I connect a third party GPS
system using the supplied cable that comes with the kit? You can
use the "GPS" end of the cable to plug into the output from your
GPS devices serial port. In some cases, a null modem cable will
need to be used for proper operation. Please also make sure that
your GPS receiver meets the criteria noted in the user manual for
acceptable baud rate and communication protocols.
Solution Detail: You can use the "GPS" end of
the cable to plug into the output from your GPS devices serial
port. In some cases, a null modem cable will need to be used for
proper operation. Please also make sure that your GPS receiver
meets the criteria noted in the user manual for acceptable baud
rate and communication protocols.
What is the difference
between VG400 and VG700 series products?
Question: What is the difference between VG400
and VG700 series products?
Solution Detail: The VG700 uses higher quality
FOG sensors which have inherently better bias stability and lower
noise, whereas VG400 uses MEMS based gyros.
In addition,
the VG400 uses an advanced Kalman filter algorithm to correct for
bias drift in the sensors. The VG400 also automatically adjusts
for dynamic situations, which means the user does not have to
manually adjust the erection rate (T setting.) The VG400 can be
used without any user-settable parameters.
The VG700 lets
the user control the erection rate (T setting) and thereby lets
take advantage of the bias stability. It works best when the user
can send zeroing commands and erection rate
commands.
What is the difference
between the VG400CC and the VG700CA?
Question: What is the difference between the
VG400CC and the VG700CA?
Solution Detail: The VG700CA uses a different
technology than the VG400CC for the angular rate sensors. The
VG700CA uses fiber-optic gyros. FOG technology is an order of
magnitude more stable than the silicon based technology used in
the VG400CC. The VG400CC Uses Kalman Filter algorithm while
VG700CA uses Adaptive-T algorithm to calculate the stabilized
pitch and roll output.
The fiber-optic gyro technology
allows for a more accurate angle calculation in dynamic
environments.
What is the accuracy of the
stabilized pitch and roll angles?
Question: What is the accuracy of the
stabilized pitch and roll angles?
Solution Detail: We quote a typical accuracy
of 2 deg RMS in a "dynamic" situation. The actual accuracy is very
application dependent. Achieving the accuracy you want will
require some experimentation to find the correct erection rate for
your system. Plan on some experimentation when you are first
integrating the VG700CA into your application. And please contact
Moog Crossbow with questions before you buy the DMU and during your
integration process. We will help guide you in finding the best
way to use our products for your application.
How do the VG700CA's analog
outputs work?
Question: How do the VG700CA's analog outputs
work?
Solution Detail: The VG700 measures the
voltages from the rate sensors and accelerometers and uses this
data in its calculations. The calculations include calibration and
linearization routines, as well as angle calculations. The
calibrated data is then converted back to an analog signal and
presented as a fully conditioned signal on the analog outputs.
Please refer to the Analog outputs verification procedures
documentation.
What is the bandwidth of
the sensors in the VG700CA?
Question: What is the bandwidth of the sensors
in the VG700CA?
Solution Detail: The bandwidth of the
accelerometers is 75 Hz and that of the rate gyros is 100 Hz.
However, we usually add a digital filter that limits the
accelerometers to 10 Hz as well. This is because the VG700CA uses
the accelerometers as a gravity reference. This means the VG700CA
gets the best performance from the angle calculation when the
accelerometers have vibrations filtered
out.
What is the VG700CA's
definition and notation of Euler angle?
Question: What is the VG700CA's definition and
notation of Euler angle, i.e., order of yaw versus pitch versus
roll in going from stationary (level surface) to body frame, and
vice versa?
Solution Detail: For all DMUs and orientation
sensors, Moog CrossbowÕs Euler angle is defined as follows: To go from
a level surface to the DMU's body frame, with given roll, pitch,
and yaw, we follow a standard Euler Angle 3-2-1 scheme. Namely, we
yaw first, then pitch, and then roll.
What is the data format for
the VG700CA?
Question: What is the data format for the
VG700CA?
Solution Detail: In voltage mode, the DMU will
output a 12-bit unsigned number that represents the sensor
voltage. In scaled sensor mode, the DMU will output a 2's
complement signed 16-bit number representing the data scaled to
actual engineering units. In VG mode, the DMU will output in the
same format as the scaled sensor mode. The structure of the data
packet is specific to each mode. Look at the DMU Data Sheet,
User's Manual, or the Moog Crossbow catalog to see the data packet
structure in each mode.
The lowest T-setting for
VG700
Question: The lowest T-setting I can set
(without accumulating roll/pitch errors) for VG700 series
products
Solution Detail: The lowest T you can set for
VG700CA is 1 and that for VG700AA is 0.
What are the differences
between the VG700CA and the VG700AA DMUs?
Question: What are the differences between the
VG700CA and the VG700AA DMUs?
Solution Detail: The main difference is in
firmware. The VG700CA only outputs sensor data in the DMU body
coordinate system. This means that the DMU outputs exactly what
the sensors actually measure. For example, if the DMU is level,
the accelerometers will measure (0, 0, 1) g on the X, Y, Z axes.
If you roll the unit 45 deg, the accelerometers will measure, and
the VG700CA will output (0, 0.707, 0.707) g. This is the body
coordinate system. SAE refers to this as the "vehicle axis
system." You can think of this as "forward, right,
floor".
The earth coordinate system is what the DMU would
measure in the given situation, if it were kept fixed level
relative to earth. You can imagine the DMU mounted in the vehicle
in a device that always keeps it level, even when the vehicle is
pitching or rolling. The VG700AA does this in a virtual sense by
converting the sensor measurements from the body frame to the
earth coordinate frame using the measured pitch and roll angles.
Taking the previous example, with the VG700AA level, the
accelerometers will output (0, 0, 1) g. If you roll the VG700AA 45
degrees, the output is still (0, 0, 1) g. This is because the
VG700AA is converting the actual sensor measurements to what they
would be if the DMU were kept level. SAE refers to this as the
"earth-fixed axis system." You can think of this as "north, east,
down."
Automobile testing uses this feature because they
are often interested in the dynamical acceleration, and not the
gravitational acceleration. For example, in a skid pad test, the
car is driven in a circle of constant diameter as fast as the car
will go, and the test wants to see how much lateral acceleration
the car can take before it starts to skid out. As the car is
turning, the lateral forces cause the car to roll out on its
suspension. So a simple accelerometer will aligned along the
left/right axis of the car will measure cosine(roll)*A +
sin(roll)*g. The test technicians only want A, but now they have a
complicated combination of A, g, and the roll. The VG700AA can
calculate all of this and report just A as if the DMU were kept
level during the test.
Is there any automatic
"zeroing" procedure for VG700?
Question: Since the DMU is able to output roll
and pitch measurements at power up, even without prior sending of
any "Zero" command, we would like to know what actually happens at
power up : is there any automatic "zeroing" procedure?
Solution Detail: Yes, the rate sensor zero is
not stored in the EEPROM after you switch the power off and hence
you need to issue the zero command every time you power up. The
way to work around this problem is to program your unit to perform
auto-zero every time you power up. This needs the switch settings
change in EEPROM and you will need to send the unit back to the
factory. But, be advised that you need to have your unit still and
leveled when every time you power up.
Default T value used by the
VG700 to calculate roll and pitch data
Question: Is there any default T value used by
the VG700 to calculate roll and pitch data even when the user does
not send any erection rate command?
Solution Detail: The default value for the
Errection rate (T-setting) used by VG700 is 20.
How to calculate stabilized
roll and pitch in VG700AA-202?
Question: How to calculate stabilized roll and
pitch in VG700AA-202?
Solution Detail: VG700AA provides you
stabilized Roll and Pitch angles. You can refer to the product application
note. Anything more is considered proprietary and can not be
disclosed.
How to calculate stabilized
yaw in VG700AA-202?
Question: How to calculate stabilized yaw in
VG700AA-202?
Solution Detail: In order to calculate the
stabilized yaw angle from VG700AA, you need a yaw angle reference
such as magnetometer, GPS etc combined with some algorithm like
Kalman Filter. That is exactly what we do in our AHRS series of
products with magnetometers using Magnetic North as a
reference.
Calculation of yaw angle
from VG700
Question: My VG700CA returns an absolute pitch
and roll angle, but not a yaw. I do have a compass for period
drift cancelation, but it's not accurate during high velocity
motions. Can I integrate the angular yaw rate to get the yaw
angle? I'm concerned this won't work during large pitch and roll
angles since the measured angular rate around the z-axis of the
gyro (what you're calling yaw) won't match with the world
coordinate z-axis.
Solution Detail: You will need to first
transform the yaw rate into world co-ordinate frame. This can be
done by, Rt=-S(pitch)*Rx+C(pitch)*S(roll)*Ry+C(pitch)*C(roll)*Rz
Then, you will need to integrate it to obtain the yaw
angle and correct for any drift using
magnetometers.
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