Rangefinders (Sonar/Lidar/Radar)
- Anthony Luo
How it works
Rangefinders use by measuring the time it takes for a wave (sound, radio, or light are just different frequency waves) to reflect off of a surface. Knowing the speed of each wave in free space (air), we can determine how far away the item is.
Different waves propagate in different patterns, so different types of rangefinders will have different beam angles and effective distances! Different sensors will also cease to work on different materials (e.g. clear or thin plastics).
All rangefinders have an effective distance - outside that range they may begin to return inconsistent values, or return the absolute max or min that they are allowed to report. Check manufacturer specifications carefully!
What we use it for
One of the most common uses for rangefinders on warg is to measure the distance to the ground! Rangefinders are significantly more accurate than barometers or gps measurements, and update much faster. Knowing the distance from the ground allows for significantly improved altitude hold as well as significantly improved RTL and auto-land ability.
WARG commonly uses 2 rangefinders - one which is low quality and packaged as part of the mateksys optical flow sensor, and one which is higher quality and exists as a separate module.
Name | Link |
|---|---|
Mateksys optical flow +lidar sensor | |
Benewake TF-Mini S Lidar |
Constraints & Mounting Requirements
Please refer to manufacturer documentation for sensor-specific items. Things like FOV, offsets, mounting patterns, etc. Should be provided by the manufacturer.
General mounting requirements are listed below:
Unobstructed FOV | Since the rangefinders measure distance by bouncing waves off of a target, anything in the “fov” of the sensor will return false data. |
|---|---|
Cable/Connector Access | It’s important to be able to probe signal wires and test the device, so cable/connector access is important |
Minimum height | Please refer to the manufacturer specification for minimum height that the sensor will work. This is incredibly important for reliable data during takeoff and landing. |
Parallel to the ground, facing downwards | This should be pretty self explanatory. When the drone is level, the rangefinder should be returning the height, not the hypotenuse. |
Note that if you are using an optical flow sensor, you should mount the rangefinder close to the optical flow sensor (or with the same offsets, but applied in the opposite direction). Offsets (in 10ths of a cm) need to be known so that ardupilot can correct for pitch/roll of the drone.
Note that some rangefinders may not be suitable for use outdoors where there can be large amounts of environmental noise (foilage, similar frequencies, overexposure, etc).