Moog Crossbow GNAV540 - Manual (Page 25)

7430‐0808‐01 Rev. B
25 GNAV540 User Manual Page
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 GNAV540 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 GNAV540 product 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
Crossbow website:
http://www.moog‐crossbow.com .
Advanced Settings
The GNAV540 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, and custom axes configuration.
The unit
ds: s can be configured via two metho
• ation: NAV‐VIEW 2.2, a GUI applic
• C Language Programming:
Chapter 7. Configuring GNAV540 with NAV­VIEW 2.2
Chapter 10. Programming Guidelines.
IMU Function
Inertial Measurement Unit (IMU) Function provides inertial rate and acceleration data in the forms of changes in
velocity and rotation angle (∆ѵ and ∆ф). The IMU Function signal processing chain consists of the 6‐DOF sensor
cluster, 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 IMU data. The 100Hz IMU data is continuously being maintained inside the unit. Digital
IMU data is output over the RS‐422 or Ethernet link at a selectable fixed rate (100, 50, 25, 20, 10, 5 or 2 Hz) or an on
request basis using the GP (Get Packet) command.
The digital IMU data is available in one of several measurement packet formats including Scaled Sensor Data (S1
Packet) and Delta‐Theta, Delta‐V (S2 Packet). In the Scaled Sensor Data (S1 Packet) data is output in scaled
engineering units. In the Delta‐Theta, Delta‐V format (S2 Packet) scaled sensor data is integrated with respect to
the time of the last output packet and the data is reported in units of accumulated (i.e., delta) degrees and
meters/second. For information about full packets, refer to Chapter 9. Communicating with the GNA540 Unit and
Chapter 10. Programming Guidelines.