Aerospace and Electronic Systems Magazine October 2017 - 45

Changey et al.
cal, electrical, and environmental constraints, while guaranteeing the
accuracy of measurements under real operational conditions.
The IG-500N IMU proposed by SBG Systems SAS is an integrated miniature solution that uses three gyrometers and three microelectromechanical accelerometers, a three-axis magnetometer,
a GPS, a barometer, and a data fusion filter to provide the attitude,
speed, and location of the vehicle in real time.

CHARACTERIZATION AND CALIBRATION OF SENSORS
Figure 10.

Sensor Behavior to Vibrations
The inertial sensors are sensitive to vibrations generated by the
engines and rotors of the GLMAV. A vibrator that simulates complex vibration environments was used to highlight the influence
of vibrations on the IMU. Two standard IG-500N IMUs with gyroscopes measuring ±300°/s and accelerometers measuring ±5 g,
as required, were tested: IG-500N-G4A2 (±300°/s and ±5 g) and
IG-500N-G4A3 (±300°/s and ±18 g).
A synthesis of the most revealing results are commented hereafter. Gyroscopes were not significantly affected by vibrations,
compared with accelerometers that were very sensitive to them
and, particularly, accelerometers having a measuring range of 5
g. Accelerometers quickly derived as soon as the vibration level
increased. An offset is mainly observed on the measured value that
is very detrimental, starting from 1.5 g rms for a 5 g accelerometer
and from 3 g rms for an 18 g accelerometer. The 18-g accelerometer used in the IG-500N-G4A3 is more tolerant than the 5 g one,
and the IMU will continue to provide relevant results in vibration
environments of 3 g rms.
Note that the accelerometers are used to measure both the vertical reference and the displacement of the vehicle. Accurate measurement of accelerations experienced by the drone is required for
proper operation of the IMU. For example, an error of 10 mGal
on an accelerometer can lead to a drift of attitude of about 0.6°.
According to real tests, the IG-500N-G4A2 can only operate up
to vibration levels of 3 g rms, which remains limiting in high vibration applications, such as those encountered on the GLMAV.
Mechanical insulation is necessary to avoid problems during stabilized flight. The IG-500N-G4A3 was also tested on the GLMAV
without mechanical insulation, and the results were much more
stable and were kept within specifications (Figure 10).
In conclusion, the IMU with 18-g accelerometers (IG-500N-BG4A3) allowed both to endure vibration levels encountered on the
GLMAV and to keep the launching acceleration to 2500 g.

The IMU IG-500N B-G4A3 resistant to a 2500-g launching acceleration.

interfaces for mounting the IG-500N properly (Figure 11). A new
mechanical interface was developed to ensure a better alignment
of the IMU on the calibration tools to remove many factual inaccuracies sources. This interface ensures an alignment of the IMU
better than 0.05° with the calibration tool, as well as an orthogonality better than 0.01°. Accordingly, the calibration of gyroscopes
and accelerometers were improved by a factor of two for gain and
misalignment parameters of the sensors.

Characterization and Compensation of Accelerometer Linearity
The 18-g accelerometer version of the IMU keeps both high accelerations and vibrations, but it has a larger bias and needs a better
characterization of the nonlinearity of the sensor. A characterization and correction procedure of the nonlinearity of the accelerometers was established on the basis of the effect of the centrifugal
force. These tests showed a significant nonlinearity on the 18-g
accelerometer, which degrades the measurement of attitude and
position. In Figure 12, a 3% drift of the full-scale measurement
was found, which represents 540 mGal for an acceleration of 18 g.
The error is of the order of 0.5% of the measurement range, which
corresponds to 10 mGal, which is acceptable because this value is
commonly encountered on the GLMAV.

Sensor Calibration Enhancement
SBG Systems SAS calibrates accelerometers and gyroscopes individually to improve their performance, especially when the operating temperature varies. Currently, the following effects are corrected at the factory: offset evolution of the temperature measurement,
evolution of the temperature scale factor, sensor misalignments,
nonlinearity of sensors, and effects of accelerations on gyroscopes.
This precise calibration requires both particular tools, such as
a rotating table associated with a climatic chamber and mechanical
OCTOBER 2017

Figure 11.

Vibrator, rotating table, and climatic chamber from SBG Systems SAS.

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine October 2017