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IMU statistics


Objectives

  • Ensure the linear acceleration and angular velocity values are within the supported ranges for the selected operation mode.

  • Identify vibration sources impacting performance by analyzing the frequency response of the IMU.

  • Evaluate if the location of the Vision-RTK2 on the platform is ideal.


Explanation

The IMU statistics file allows the user to evaluate the performance of the inertial measurements on the selected platform. The left column presents the temporal response of the IMU measurements collected throughout the recording, which include the accelerometer (i.e., linear acceleration in three dimensions) and gyroscope (i.e., angular velocity in three dimensions).

acc.png

Accelerometer

gyr.png

Gyroscope

1. For the linear acceleration:

  • The sensor expects all values to be inside the supported range for the selected operation mode (e.g., ±40 m/s2 for 'generic'). The vibrations observed on the Z-axis are particularly interesting, as we expect to measure only the gravity vector (i.e., -9.81 m/s2) under ideal conditions.

  • Any other values correspond to either vibration on the sensor's platform or to the vehicle's dynamics. For the former case, isolating the sensor from external vibrations (e.g., combustion engine) is essential.

2. For the angular velocity:

  • The sensor expects all values to be inside the supported range for the selected operation mode (e.g., ±1.5 rad/s for 'generic'). If the observed angular velocities are higher, the chosen tuning mode might not be ideal, and the sensor might present degraded performance.

The right column presents the frequency response of all measurements captured during the recording. These plots allow the detection of vibrations caused by diverse elements on the platform, such as combustion engines, pumps, and hydraulics, among others. If there is a clear response between multiple recordings, it might be necessary to isolate the Vision-RTK2 further or change its location on the platform.

If isolating the Vision-RTK2 from vibrations or changing its location is not feasible, please get in touch with support@fixposition.com for further guidance on the matter.


Expected results

1. For the temporal response:

  • All values for the linear accelerator and angular velocity in all axes should be inside the supported range for the selected operation mode.

  • Assuming the sensor is parallel to the ground in stationary conditions, only the Z-axis should be excited and present a value close to the gravity vector. If the sensor is tilted, the norm of the forces should be equal to the magnitude of the gravity vector.

2. For the frequency response:

The cause for observing peaks in the frequency spectrum depends on the conditions in which the recording was performed:

  • If the engine is off and the vehicle is stationary: An active component is likely causing vibrations on the sensor, or the platform is not rigid enough.

  • If the engine is on and the vehicle is stationary: The sensor is likely located above an active component (e.g., the engine) and not adequately isolated from it. Use some dampening mechanism or change the location of the sensor.

  • If the engine is on and the vehicle is moving: When the vehicle is moving, the IMU will be excited by the vehicle's dynamics. However, if clear peaks in the frequency spectrum do not correspond to the velocities in which the vehicle was displacing, then proceed to perform stationary tests to discard effects from the motor or the structure.


Examples

  1. IMU performance on a slow-moving vehicle: All IMU values are inside the expected range for the 'slow' operation mode, and there is no clear indication of external vibrations affecting the sensor.

image-20240109-125231.png

Good IMU performance on a slow-moving vehicle.

Example HTML:

imu_stats.html

  1. IMU performance on a tractor: All IMU values are inside the expected range for the 'lawnmower' operation mode. However, there is a clear indication of external vibrations on the frequency response of all axes on the linear acceleration plot, with the most notable frequency peaks at 30, 60, and 90 Hz.

image-20240109-132117.png

Degraded IMU performance on a tractor.

Example HTML:

imu_stats_vibrations.html


Further analysis

  • Make a recording with the vehicle stationary for about 5 minutes with the engine off.

  • Make a recording with the vehicle stationary for about 5 minutes with the engine on.

  • Make a recording with the platform moving at a low throttle.

  • Make a recording with the platform moving at a high throttle.

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