Use of Optical Flow

Running the technology "Optical Flow" offers the possibility of POSCTL flight mode, and autonomous flight operating on a camera pointed downwards that detects changes of ground texture.

Enabling

Hint It is recommended to use special PX4 firmware for Clover.

The use of a rangefinder is essential. Connect and setup laser-ranging sensor VL53L1X, according to the manual.

Enable Optical Flow in the file ~/catkin_ws/src/clover/clover/launch/clover.launch:

<arg name="optical_flow" default="true"/>

Optical Flow publishes data in mavros/px4flow/raw/send topic. In the topic optical_flow/debug is also published a visualization, that can be viewed with web_video_server.

Info Correct connection and setup of the camera module is needed for proper functioning.

Setup of the flight controller

Hint Suggested parameters are applied automatically in our custom PX4 firmware.

When using EKF2 (parameter SYS_MC_EST_GROUP = ekf2):

EKF2_AID_MASK – flag 'use optical flow' is on.
EKF2_OF_DELAY – 0.
EKF2_OF_QMIN – 10.
EKF2_OF_N_MIN – 0.05.
EKF2_OF_N_MAX - 0.2.
SENS_FLOW_ROT – No rotation.
SENS_FLOW_MAXHGT – 4.0 (for the rangefinder VL53L1X)
SENS_FLOW_MINHGT – 0.01 (for the rangefinder VL53L1X)
Optional: EKF2_HGT_MODE – range sensor (cf. rangefinder setup).

When using LPE (parameter SYS_MC_EST_GROUP = local_position_estimator, attitude_estimator_q):

LPE_FUSION – flags 'fuse optical flow' and 'flow gyro compensation' are on.
LPE_FLW_QMIN – 10.
LPE_FLW_SCALE – 1.0.
LPE_FLW_R – 0.2.
LPE_FLW_RR – 0.0.
SENS_FLOW_ROT – No rotation.
SENS_FLOW_MAXHGT – 4.0 (for the rangefinder VL53L1X)
SENS_FLOW_MINHGT – 0.01 (for the rangefinder VL53L1X)
Optional: LPE_FUSION – flag 'pub agl as lpos down' is on (see rangefinder setup.

The selfcheck.py utility will help you verify that all settings are correctly set.

POSCTL flight

Setup POSCTL to be one of PX4 flight modes and then select POSCTL.

Autonomous flight

The module simple_offboard enables autonomous flight.

Example of take off and leveling at 1.5m above the ground:

navigate(z=1.5, frame_id='body', auto_arm=True)

Flying forward for 1m:

navigate(x=1.5, frame_id='body')

Navigation using ArUco-markers and [using VPE] are available when using Optical Flow.

Additional settings

If the copter has an unstable position, try to increase the P coefficient of speed PID controller - parameters are MPC_XY_VEL_P and MPC_Z_VEL_P.

If the copter has an unstable height, try increasing MPC_Z_VEL_P coefficient or getting better hover throttle - MPC_THR_HOVER.

If the copter is consistently yawing, try:

recalibrate gyroscopes;
recalibrate magnetometer;
different values for EKF2_MAG_TYPE parameter, that indicates how data from the magnetometer is used in EKF2;
changing values of EKF2_MAG_NOISE, EKF2_GYR_NOISE, EKF2_GYR_B_NOISE parameters.

Note

For better results perform gyro calibration directly before taking off, using appropriate snippet.

If the copter's height is deviating, try:

increasing the value of MPC_Z_VEL_P coefficient;
change the value of MPC_THR_HOVER parameter;
add MPC_ALT_MODE = 2 (Terrain following).

When using Optical Flow, the maximal horizontal speed is further limited. This is an indirect influence of the parameter SENS_FLOW_MAXR (maximal reliable "angular speed" of the optical flow). In normal flight mode, control loops will be adjusted so that Optical Flow values do not exceed 50% of this parameter.

Errors

If errors like EKF INTERNAL CHECKS occur, try to restart EKF2. To do so, enter in the MAVLink-console:

ekf2 stop
ekf2 start

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