Vibration Considerations

When designing a system that integrates SPAN, achieving excellent navigation performance requires considering the vibration environment in which the IMU will operate. IMU sensors measure motion, which may consist of real platform motion but also vibration. The INS mechanizes these measurements into a full position, velocity, and attitude solution. Vibration can significantly degrade INS performance by introducing noise or errors into the IMU sensor measurements. It can be especially impactful when unfavorable INS performance conditions are present, like poor GNSS reception, low dynamics and non-optimal configurations. Managing vibration is very important to achieve optimal SPAN system performance.

Sources of Error

Vibration affects an INS in several different ways that should be considered.

Noise and errors in the raw IMU measurements can arise from:

Aliasing – signals at frequencies higher than the sampling rate incorrectly appear at low frequencies or as a bias.
Clipping – the measurement range of the sensor is exceeded, inducing error in measurements and a bias in the smoothed measurements.
Vibration Rectification Error (VRE) – changes in vibration induce a bias in the measurements that cannot be filtered out.
Amplification of standard IMU errors – as measurements approach the range limits of the sensor, even when oscillating around a mean near zero.

INS solution quality experienced at the output from a SPAN system is impacted by:

IMU lever arm rigidity and accuracy – vibration can cause more flexing and make rigid body assumptions less applicable.
USER lever arm configuration – oscillation of the INS solution attitude due to vibration amplifies noise and apparent error when the solution is translated to the USER offset.
SPAN logging configuration (i.e. solution output sampling rate) – insufficient logging rates can cause aliasing in the output solution if oscillation frequency in the INS solution due to vibration exceeds half the output logging rate.

Mitigation Methods

Vibration is best mitigated in the mechanical domain prior to IMU exposure. Key methods to mitigate in the mechanical domain include:

Selecting the IMU mounting location to achieve subsystem separation, structural rigidity considerations, and optimal locations for system dynamics and lever arms.
Appropriate IMU mounting methods that prioritize rigidity and dimensional stability.
Carefully designed custom IMU mounts that incorporate vibration isolation features.
Characterization and minimization of unwanted resonance in the system in the context of system vibration sources.

When the IMU will be exposed to vibration, robust IMU specification design is important to ensure the INS performs adequately in the overall system. For example, the IMU measurement range to avoid signal clipping when exposed to the combination of vibration, maximum system dynamics and gravity. The needs of individual applications will vary and may benefit from an iterative system design approach to manage tradeoffs.

The interface design between the INS and overall system must support the logging configuration and rates needed to minimize solution signal distortion and support any monitoring, post-processing or control loop requirements. Using an anti-aliasing filter at the solution output may be necessary. Contact Customer Support for additional information.

Additional Notes

The unique characteristics of each SPAN system integration requires a tailored vibration management approach within the context of the application. A general solution is not appropriate given the range of vibration environments and system requirements.

An important aspect of the vibration management approach is that changing vibration conditions are most impactful to INS performance.

With careful consideration of vibration early in the system design cycle, optimal SPAN performance can be achieved.

For in-depth information about vibration, please review NovAtel Application Note APN-112: Inertial Navigation System and Vibration.