Moog Crossbow NAV440 Series - Manual (Page 23)

7430‐0131‐01 Rev. F
NAV440 User Manual Page 23
The axes form an orthogonal SAE right‐handed coordinate system. Acceleration is positive when it is oriented
towards the positive side of the coordinate axis. For example, with a 440Series unit sitting on a level table, it will
measure zero g along the x and y‐axes and ‐1 g along the z‐axis. Normal Force acceleration is directed upward,
which would be defined as negative for the 440 Series z‐axis.
The angular rate sensors are aligned with the same axes. The rate sensors measure angular rotation rate around a
given axis. The rate measurements are labeled by the appropriate axis. The direction of a positive rotation is
defined by the right‐hand rule.
With the thumb of your right hand pointing along the axis in a positive direction, your fingers curl around in the
positive rotation direction. For example, if the 440Series unit is sitting on a level surface and you rotate it clockwise
on that surface, this will be a positive rotation around the z‐axis. The x and y‐axis rate sensors would measure zero
angular rates, and the z‐axis sensor would measure a positive angular rate.
The magnetic sensors are aligned with the same axes definitions and sign as the linear accelerometers. For example,
when oriented towards magnetic North, you will read approximately +0.25 Gauss along X, 0.0 Gauss along Y, and
+0.35 Gauss along Z direction (North America). Magnetic values at other geographic locations are available at
http://www.ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml .
Pitch is defined positive for a positive rotation around the y‐axis (pitch up). Roll is defined as positive for a positive
rotation around the x‐axis (roll right). Yaw is defined as positive for a positive rotation around the z‐axis (turn
right).The angles are defined as standard Euler angles using a 3‐2‐1 system. To rotate from the body frame to an
earth‐level frame, roll first, then pitch, and then yaw.
The position output from GPS is represented in Latitude, Longitude, and Altitude (LLA) convention on the WGS84
Ellipsoid. This is the most commonly used spherical coordinate system. The GPS velocity is defined in North, East
and Down reference frame, which can be converted to the Cartesian coordinate system: Earth‐Centered, Earth‐Fixed
(ECEF). ECEF uses three‐dimensional XYZ coordinates (in meters) to describe the location of a GPS user or satellite.
Several online resources are available to help users with this transformation. Application notes are available on the
Moog Crossbow website:
http://www.moog‐crossbow.com .
Advanced Settings
The 440Series Inertial Systems have a number of advanced settings that can be changed. The specific settings
available vary from unit to unit, and a detailed description of each unit is found in the subsequent sections of this
manual. All units support baud rate, power‐up output packet type, output rate, sensor low pass filtering, and
custom a
be configured via two methods: xes configuration. The units can
• application, ( NAV‐VIEW 2.2, a GUI
• Program commands,
Chapter 7. Configuring the 440 Series with NAV‐VIEW 2.2)
Chapter 10. Programming Guidelines)
IMU440 Function
IMU440 (Inertial Measurement Unit) provides the inertial measurement unit functionality that the unit provides by
outputting inertial rate and acceleration data in 6‐DOF (six degrees of freedom). The IMU440 signal processing chain
consists of the 6‐DOF sensor cluster, programmable low‐pass filters , analog to digital conversion, and the DSP signal
processor for sensor error compensation. The rate and acceleration analog sensor signals are sampled and
converted to digital data at 1 kHz.
The sensor data is filtered and down‐sampled to 100Hz by the DSP using FIR (finite impulse response) filters. The
factory calibration data, stored in EEPROM, is used by the DSP to remove temperature bias, misalignment, scale
factor errors, and non‐linearities from the sensor data. Additionally any advanced user settings such as axes
rotation are applied to the IMU440 data. The 100Hz IMU440 data is continuously being maintained inside the unit.