IMU Shock Rejection¶

Overview¶

The IMU Shock Detection feature enables the system to identify and mitigate mechanical shock events that adversely affect Inertial Measurement Unit (IMU) measurements. Because impulse shocks can saturate sensors and exceed their effective bandwidth---producing invalid data that would otherwise corrupt Attitude and Heading Reference System (AHRS) and Inertial Navigation System (INS) estimates---the system employs specialized algorithms to detect, isolate, and reject shock-induced disturbances. When a shock is detected, the Extended Kalman Filter (EKF) is rewound to remove corrupted measurements and maintain navigation solution integrity.

Without Shock Rejection¶

In the absence of a shock rejection mechanism on platforms such as the IMX-5, the attitude solution (comprising roll, pitch, and heading) can exhibit significant degradation, manifesting as attitude offset deviations of up to +10 degrees from the true value. This degradation underscores the critical need for a robust mitigation strategy.

Performance With Shock Rejection¶

Shock Rejection Enabled¶

When the shock rejection feature is active, a substantial impulse can be observed on the IMU accelerometer's X-axis. The true peak magnitude of this shock is often unidentifiable due to the large impulse force exceeding the sensor's bandwidth limitations.

During the event, the IMU status flags provide crucial diagnostic information, recording both sensor saturation and successful shock detection. Critically, because the shock event is accurately identified and mitigated, the attitude estimate (roll, pitch, and yaw) remains largely impervious to the shock input, preserving the accuracy and reliability of the navigation solution.

Because the shock is properly identified, the attitude estimate (roll, pitch, yaw) is relatively unaffected by the shock input.

How to Enable Shock Rejection¶

The shock rejection feature is configured through specific bit settings within the Software Development Kit (SDK) and must be applied to the DID_FLASH_CONFIG.imuShockOptions register:

IMU_SHOCK_OPTIONS_ENABLE (0x01): Activates the IMU shock rejection functionality.

IMU_SHOCK_OPTIONS_FAST_RECOVERY (0x02): This option is particularly beneficial when impulse shocks occur concurrent with platform rotation. It minimizes the post-shock integration error and substantially reduces the time required for attitude convergence. However, it is important to note that this setting can marginally increase the attitude error induced directly by the shock event itself. Therefore, its use is generally discouraged for applications where the platform is stationary and not undergoing rotation during shock occurrences.

Configuration¶

The shock rejection can be tuned if needed using the following parameters.

Parameter	Type	Units	Purpose	Typical Range	Notes / Impact
imuShockDetectLatencyMs	uint8_t	ms	EKF rewind window after shock detection	10–100 ms	Larger values protect more history but add navigation latency
imuShockRejectLatchMs	uint8_t	ms	Time shock rejection remains active after shock ends	100–500 ms	Prevents rapid on/off oscillation; ensures stabilization
imuShockOptions	uint8_t	—	Bitfield controlling shock detection behavior	—	Enables detection, algorithm selection, EKF rewind behavior
imuShockDeltaAccHighThreshold	uint8_t	m/s²	Acceleration difference to trigger shock start	2–10 m/s²	Lower = more sensitive; higher = fewer false positives
imuShockDeltaAccLowThreshold	uint8_t	m/s²	Acceleration difference to confirm shock end	0.5–5 m/s²	Must be lower than high threshold for hysteresis
imuShockDeltaGyroHighThreshold	uint8_t	deg/s	Angular rate difference to trigger shock start	5–20 deg/s	Detects rotational shocks
imuShockDeltaGyroLowThreshold	uint8_t	deg/s	Angular rate difference to confirm shock end	1–10 deg/s	Lower than high threshold to avoid oscillation

Usage Example¶

Configuration for Vehicle Dynamics¶

config.imuShockDetectLatencyMs = 50;           // 50ms EKF rewind window
config.imuShockRejectLatchMs = 200;            // 200ms latch time after shock end
config.imuShockOptions = 0x01;                 // Enable shock detection
config.imuShockDeltaAccHighThreshold = 5;      // 5 m/s² to trigger
config.imuShockDeltaAccLowThreshold = 2;       // 2 m/s² to confirm end
config.imuShockDeltaGyroHighThreshold = 10;    // 10 deg/s to trigger
config.imuShockDeltaGyroLowThreshold = 4;      // 4 deg/s to confirm end

Tuning Guidelines¶

Increase imuShockDetectLatencyMs to provide better protection
Lower acceleration/gyroscope thresholds to catch subtle shocks
Longer latch time to ensure full stabilization

For Rugged Environments (e.g., off-road vehicles)¶

Consider reducing thresholds further to detect frequent impacts
Balance latch time against desired responsiveness
Monitor EKF rewind performance in field testing

For Minimal Interference¶

Increase thresholds to only catch significant shocks
Shorter latch times for faster recovery
Reduce imuShockDetectLatencyMs if rewind latency is unacceptable

Shock Detection Algorithm¶

The system detects shocks by monitoring inter-IMU differences:

Shock Start: When differences exceed either the acceleration OR gyroscope high thresholds
Active Shock: Shock rejection remains active for the duration of the shock event
Shock End: When ALL differences drop below their respective low thresholds AND the latch timer expires
EKF Rewind: The navigation state is rewound by imuShockDetectLatencyMs to remove shock-corrupted estimates

Performance Considerations¶

Latency Impact: EKF rewind introduces latency proportional to imuShockDetectLatencyMs
Computational Load: Shock detection adds minimal overhead (difference calculations and comparisons)
IMU Health: Requires all 3 IMUs to be operational for reliable shock detection