Detecting Optical Flow Sensor Failures in ArduPilot Logs

TL;DR: To diagnose optical flow sensor failures in ArduPilot logs, check for the absence of readings or erratic behavior in OPTICAL_FLOW messages. Look for ERR Subsys=18 ECode=2 indicating sensor issues. Ensure sensor parameters are correctly configured and connections are secure to minimize problems.

What Optical Flow Data Actually Is in the Log

In ArduPilot logs, optical flow data is recorded in the OPTICAL_FLOW message. This message captures critical information about the drone's movement and helps in stabilizing its position during flight. Understanding these parameters is essential for troubleshooting navigation issues.

OPTICAL_FLOW time   flow_x   flow_y   quality   sensor_id

• time: Timestamp of the message in microseconds since the system started.
• flow_x: The flow measurement in the x direction (pixels per second).
• flow_y: The flow measurement in the y direction (pixels per second).
• quality: A value indicating the quality of the flow measurement, with higher values representing better quality.
• sensor_id: Identifier for the optical flow sensor used.

Confirming It in Mission Planner

To verify optical flow data in Mission Planner, follow these steps:

• Open Mission Planner.
• Select the Flight Data tab.
• Navigate to DataFlash Logs → Review a Log.
• Load the relevant log file.
• In the Data View section, plot OPTICAL_FLOW.flow_x and OPTICAL_FLOW.flow_y for visual analysis.

Confirming It in MAVExplorer

In MAVExplorer, you can visualize optical flow data using the following command:

MAV> graph OPTICAL_FLOW.flow_x OPTICAL_FLOW.flow_y

This plot reveals the relationship between the x and y flow measurements, which can indicate sensor performance issues if the readings are inconsistent or absent.

Why Optical Flow Sensor Failures Happen — Ranked by What We See Most Often

1. Zero Readings: If both flow_x and flow_y are zero, the sensor may be disconnected or malfunctioning. This often leads to erratic flight behavior and could be a sign of hardware failure.
2. Poor Quality Readings: Low values in the quality field suggest that the flow data is unreliable. This could stem from environmental factors like poor lighting or dirt on the sensor lens.
3. Sensor Misalignment: If the sensor is not properly aligned with the ground, it can produce incorrect readings. This misalignment often results in drifting during loiter mode.
4. Firmware Issues: Outdated or incorrect firmware can lead to improper sensor configuration, causing failure in data logging. Always ensure your firmware is updated to the latest stable version.
5. Cable and Connection Problems: Loose or damaged connections between the sensor and flight controller can prevent accurate data transmission, leading to sensor failures.

Remediation Steps for Optical Flow Sensor Failures

1. Check and secure all connections to the optical flow sensor.
2. Ensure that the sensor is clean and free from obstructions.
3. Verify that the sensor is properly calibrated and aligned with the frame of the drone.
4. Update the firmware to the latest version and ensure that all relevant parameters are set correctly.
5. Replace the sensor if it continues to fail after troubleshooting.

When This Isn't Actually an Optical Flow Sensor Failure

• Inconsistent GPS readings could cause similar symptoms, so always check GPS.HDOP values for accuracy.
• Camera issues can sometimes mimic optical flow problems, especially if relying on visual-based navigation.
• Improperly configured PID settings can lead to erratic behavior that may be misinterpreted as optical flow failure.

When LogHat Helps — and When It Doesn't

LogHat can assist in identifying optical flow sensor failures by providing clear visualizations and error message interpretations from the logs. However, it cannot diagnose physical hardware issues or connection problems; those require hands-on troubleshooting.

What are common signs of optical flow sensor failure?

Common signs include zero readings in logs or drifting during loiter mode.

How can I troubleshoot my optical flow sensor?

Check connections, validate firmware settings, and analyze log data.

What is the role of the quality parameter in optical flow data?

The quality parameter indicates the reliability of the flow measurement; higher values mean better quality.

How can I confirm the optical flow readings in Mission Planner?

Plot OPTICAL_FLOW.flow_x and OPTICAL_FLOW.flow_y in the Data View section.

When should I replace my optical flow sensor?

If all troubleshooting efforts fail and the sensor continues to provide erroneous readings, it may need replacement.

Can firmware updates fix optical flow sensor issues?

Yes, updating to the latest firmware can resolve compatibility and configuration issues that affect sensor performance.

If you're experiencing issues with your optical flow sensor, consider using LogHat for advanced log analysis and visualization tools to aid your troubleshooting process. For more insights and tips, check out our other posts on RTL issues in ArduPilot and GPS glitch diagnosis.