Optical Flow Sensor Testing and Setup

Testing the sensor

With the sensor connected to the autopilot, connect to the autopilot with the Mission Planner and open the Flight Data screen’s Status tab. If the sensor is operating you should see non-zero opt_m_x, opt_m_y and an opt_qua values.


Calibrating the sensor

  1. Connect to your autopilot and ensure that logging while disarmed is enabled (In Copter-3.3 set LOG_BITMASK to 131071.

  2. Find a location with a textured surface and good lighting (natural light or strong incandescent)

  3. Remove Propellers (safety first)

  4. Power on the vehicle and hold level away from your body and at eye level

  5. Rotate about the vehicle in roll through a range from -15 to +15 degrees in about a second and then back again. Repeat this 5 to 10 times. By closing one eye you will be able to keep the centre of the sensor stationary against the background while you do the rotation.

  6. Repeat about the vehicle pitch axis.

  7. Download the data flash logs and plot the OF.flowX, OF.bodyX and IMU.GyrX data. It should look something like this:

  8. If OF.flowX is larger or smaller than OF.bodyX, then it can be adjusted by changing the FLOW_FXSCALER parameter

  9. IF OF.bodyX is uncorrelated or opposite sign to IMU.GyrX, the FLOW_ORIENT_YAW parameter is probably set incorrectly or you do not have the flow sensor pointing downwards

  10. Plot the OF.flowY, OF.bodyYband, IMU.GyrY data. It should look something like this:

  11. If OF.flowY is larger or smaller than OF.bodyY, then it can be adjusted by changing the FLOW_FYSCALER parameter

  12. IF OF.bodyY is uncorrelated or opposite sign to IMU.GyrY, the FLOW_ORIENT_YAW parameter is probably set incorrectly or you do not have the flow sensor pointing downwards

Range Sensor Check

Check the EKF5.meaRng message in the flashlog from your flow sensor calibration test. Check the following:

  1. There is continuous range measurement.
  2. It outputs a range on the ground that is 10cm of the expected value (remember that measured range will increase when you roll or pitch the vehicle because the laser is then measuring on a slant)

Pre-Arm checks

To allow arming and taking off in Loiter without a GPS the GPS arming check should be turned off as shown below. Unchecking “All” and “GPS” and leave all other options checked.


Because optical flow requires good sonar/range finder data when the optical flow is enabled, an additional pre-arm check is enforced.

While the vehicle is disarmed you should lift the vehicle straight up to at least 50cm but no higher than 2m (if the rangefinder sees a distance of over 2m you will need to restart the autopilot).

The error message when arming fails this check is “PreArm: check range finder”

This check can be disabled by unchecking the “Parameter/Sonar” arming check.

First Flight

  1. Set EK2_GPS_TYPE = 0 (we don’t want the optical flow being used by the EKF at this stage)
  2. Perform a short test flight hovering in STABILIZE or AltHold for copter, or QSTABILIZE or QHOVER for QuadPlane, at small lean angles at heights ranging from 50cm to 3m with
  3. Download the flash log and plot the following in mission planner
  4. EKF5.meaRng should correlate with the change in vehicle height
  5. OF.flowX and OF.flowY should be varying
  6. OF.bodyX and OF.bodyY should be consistent with IMU.GyrX and IMU.GyrY

Second Flight


You will need at least 15m of clear space around the vehicle to do this flight safely. If the optical flow velocity estimates are bad, you will have little warning and the copter may lean to its maximum lean angle very quickly.

  1. Set EK2_GPS_TYPE = 3 to make the EKF ignore GPS and use the flow sensor
  2. Ensure you have a loiter and hover mode available on you transmitter.
  3. Set “EKF Origin” on Ground Control Station map. In Mission Planner, right click, select “Set Home here”, and choose to set “set EKF origin here”.
  4. Take-off in loiter and bring the Copter/Quadplane to about 1m height
  5. If the vehicle starts to accelerate away or there is erratic pitch or roll movement, then switch to hover and land. You will need to download the log file and share it on the forums to understand why.
  6. If it holds location then congratulations, you have succeeded and can now start experimenting with height changes and moving it around in the loiter mode

Example Video (Copter-3.4)