# Collision Prevention
Collision Prevention may be used to automatically slow and stop a vehicle before it can crash into an obstacle.
It can be enabled for multicopter vehicles in Position mode, and can use sensor data from an offboard companion computer, offboard rangefinders over MAVLink, a rangefinder attached to the flight controller, or any combination of the above.
Collision prevention may restrict vehicle maximum speed if the sensor range isn't large enough! It also prevents motion in directions where no sensor data is available (i.e. if you have no rear-sensor data, you will not be able to fly backwards).
Tip If high flight speeds are critical, consider disabling collision prevention when not needed.
Tip Ensure that you have sensors/sensor data in all directions that you want to fly (when collision prevention is enabled).
# Overview
Collision Prevention is enabled on PX4 by setting the parameter for minimum allowed approach distance (CP_DIST).
The feature requires obstacle information from an external system (sent using the MAVLink OBSTACLE_DISTANCE message) and/or a distance sensor connected to the flight controller.
Note Multiple sensors can be used to get information about, and prevent collisions with, objects around the vehicle. If multiple sources supply data for the same orientation, the system uses the data that reports the smallest distance to an object.
The vehicle restricts the maximum velocity in order to slow down as it gets closer to obstacles, and will stop movement when it reaches the minimum allowed separation. In order to move away from (or parallel to) an obstacle, the user must command the vehicle to move toward a setpoint that does not bring the vehicle closer to the obstacle. The algorithm will make minor adjustments to the setpoint direction if it is determined that a "better" setpoint exists within a fixed margin on either side of the requested setpoint.
Users are notified through QGroundControl while Collision Prevention is actively controlling velocity setpoints.
PX4 software setup is covered in the next section. If you are using a distance sensor attached to your flight controller for collision prevention, it will need to be attached and configured as described in PX4 Distance Sensor. If you are using a companion computer to provide obstacle information see companion setup.
# PX4 (Software) Setup
Configure collision prevention by setting the following parameters in QGroundControl:
| Parameter | Description |
|---|---|
| CP_DIST | Set the minimum allowed distance (the closest distance that the vehicle can approach the obstacle). Set negative to disable collision prevention. > Warning This value is the distance to the sensors, not the outside of your vehicle or propellers. Be sure to leave a safe margin! |
| CP_DELAY | Set the sensor and velocity setpoint tracking delay. See Delay Tuning below. |
| CP_GUIDE_ANG | Set the angle (to both sides of the commanded direction) within which the vehicle may deviate if it finds fewer obstacles in that direction. See Guidance Tuning below. |
| CP_GO_NO_DATA | Set to 1 to allow the vehicle to move in directions where there is no sensor coverage (default is 0/False). |
# Algorithm Description {#algorithm}
The data from all sensors are fused into an internal representation of 36 sectors around the vehicle, each containing either the sensor data and information about when it was last observed, or an indication that no data for the sector was available. When the vehicle is commanded to move in a particular direction, all sectors in the hemisphere of that direction are checked to see if the movement will bring the vehicle closer to any obstacles. If so, the vehicle velocity is restricted.
This velocity restriction takes into account both the inner velocity loop tuned by MPC_XY_P, as well as the jerk-optimal velocity controller via MPC_JERK_MAX and MPC_ACC_HOR. The velocity is restricted such that the vehicle will stop in time to maintain the distance specified in CP_DIST. The range of the sensors for each sector is also taken into account, limiting the velocity via the same mechanism.
Note If there is no sensor data in a particular direction, velocity in that direction is restricted to 0 (preventing the vehicle from crashing into unseen objects). If you wish to move freely into directions without sensor coverage, this can be enabled by setting CP_GO_NO_DATA to 1.
Delay, both in the vehicle tracking velocity setpoints and in receiving sensor data from external sources, is conservatively estimated via the CP_DELAY parameter. This should be tuned to the specific vehicle.
If the sectors adjacent to the commanded sectors are 'better' by a significant margin, the direction of the requested input can be modified by up to the angle specified in CP_GUIDE_ANG. This helps to fine-tune user input to 'guide' the vehicle around obstacles rather than getting stuck against them.
# Range Data Loss {#data_loss}
If the autopilot does not receive range data from any sensor for longer than 0.5s, it will output a warning No range data received, no movement allowed. This will force the velocity setpoints in xy to zero. After 5 seconds of not receiving any data, the vehicle will switch into HOLD mode. If you want the vehicle to be able to move again, you will need to disable Collision Prevention by either setting the parameter CP_DIST to a negative value, or switching to a mode other than Position mode (e.g. to Altitude mode or Stabilized mode).
If you have multiple sensors connected and you lose connection to one of them, you will still be able to fly inside the field of view (FOV) of the reporting sensors. The data of the faulty sensor will expire and the region covered by this sensor will be treated as uncovered, meaning you will not be able to move there.
Warning Be careful when enabling CP_GO_NO_DATA=1, which allows the vehicle to fly outside the area with sensor coverage. If you lose connection to one of multiple sensors, the area covered by the faulty sensor is also treated as uncovered and you will be able to move there without constraint.
# CP_DELAY Delay Tuning {#delay_tuning}
There are two main sources of delay which should be accounted for: sensor delay, and vehicle velocity setpoint tracking delay. Both sources of delay are tuned using the CP_DELAY parameter.
The sensor delay for distance sensors connected directly to the flight controller can be assumed to be 0. For external vision-based systems the sensor delay may be as high as 0.2s.
Vehicle velocity setpoint tracking delay can be measured by flying at full speed in Position mode, then commanding a stop. The delay between the actual velocity and the velocity setpoint can then be measured from the logs. The tracking delay is typically between 0.1 and 0.5 seconds, depending on vehicle size and tuning.
Tip If vehicle speed oscillates as it approaches the obstacle (i.e. it slows down, speeds up, slows down) the delay is set too high.
# CP_GUIDE_ANG Guidance Tuning {#angle_change_tuning}
Depending on the vehicle, type of environment and pilot skill different amounts of guidance may be desired. Setting the CP_GUIDE_ANG parameter to 0 will disable the guidance, resulting in the vehicle only moving exactly in the directions commanded. Increasing this parameter will let the vehicle choose optimal directions to avoid obstacles, making it easier to fly through tight gaps and to keep the minimum distance exactly while going around objects.
If this parameter is too small the vehicle may feel 'stuck' when close to obstacles, because only movement away from obstacles at minimum distance are allowed. If the parameter is too large the vehicle may feel like it 'slides' away from obstacles in directions not commanded by the operator. From testing, 30 degrees is a good balance, although different vehicles may have different requirements.
Note The guidance feature will never direct the vehicle in a direction without sensor data. If the vehicle feels 'stuck' with only a single distance sensor pointing forwards, this is probably because the guidance cannot safely adapt the direction due to lack of information.
# PX4 Distance Sensor {#rangefinder}
At time of writing PX4 allows you to use the Lanbao PSK-CM8JL65-CC5 IR distance sensor for collision prevention "out of the box", with minimal additional configuration:
- First attach and configure the sensor, and enable collision prevention (as described above, using CP_DIST).
- Set the sensor orientation using SENS_CM8JL65_R_0.
Other sensors may be enabled, but this requires modification of driver code to set the sensor orientation and field of view.
- Attach and configure the distance sensor on a particular port (see sensor-specific docs) and enable collision prevention using CP_DIST.
- Modify the driver to set the orientation.
This should be done by mimicking the
SENS_CM8JL65_R_0parameter (though you might also hard-code the orientation in the sensor module.yaml file to something likesf0x start -d ${SERIAL_DEV} -R 25- where 25 is equivalent toROTATION_DOWNWARD_FACING). - Modify the driver to set the field of view in the distance sensor UORB topic (
distance_sensor_s.h_fov).
Tip You can see the required modifications from the feature PR. Please contribute back your changes!
# Companion Setup {#companion}
If using a companion computer or external sensor, it needs to supply a stream of OBSTACLE_DISTANCE messages, which should reflect when and where obstacle were detected.
The minimum rate at which messages must be sent depends on vehicle speed - at higher rates the vehicle will have a longer time to respond to detected obstacles.
Info Initial testing of the system used a vehicle moving at 4 m/s with
OBSTACLE_DISTANCEmessages being emitted at 10Hz (the maximum rate supported by the vision system). The system may work well at significantly higher speeds and lower frequency distance updates.
The tested companion software is the local_planner from the PX4/avoidance repo. For more information on hardware and software setup see: PX4/avoidance > Run on Hardware.
The hardware and software should be set up as described in the PX4/avoidance repo.
In order to emit OBSTACLE_DISTANCE messages you must use the rqt_reconfigure tool and set the parameter send_obstacles_fcu to true.
# Gazebo Setup
Collision Prevention can also be tested using Gazebo. See PX4/avoidance for setup instructions.