Optical Flow (OFS)
- Anthony Luo
How it works
An Optical Flow Sensor (OFS) takes multiple scans of the ground, and then compares pixels or features to determine relative motion! Because on optical flow sensor needs to scan the ground for features, there’s a minimum and maximum height that it can be used at! The height depends on the sensor itself, but the range is usually between 0.08-30m!
What do we use it for?
Optical flow sensor on drones allow centimeter level precision when hovering in place, since the system is capable of determining very small movement vectors. This allows the drone to hover in place, or track the ground in a very straight line in autonomous or guided flight modes.
Warg commonly uses the following 2 sensors:
Name/Link | Use case |
|---|---|
Competition Drone OFS | |
Firmware testing & Test drones |
Constraints & Mounting Requirements
Please refer to manufacturer documentation for sensor-specific orientation. Links to product pages are in the table above.
It is also recommended to always use an optical flow in conjunction with a rangefinder, this is so that differences caused by changes in altitude can be better accounted for
General mounting requirements are listed below:
Unobstructed FOV | Since the OFS generates movement vectors based on differences between scans, it’s important that stationary objects like landing gear or wires remain outside of the FOV of the sensor. |
|---|---|
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 optical flow will work, and ensure that the optical flow is always above that height when resting on the ground. This ensures that the OFS will continue to give good data all the way until landing. |
Parallel to the ground, facing downwards | This should be pretty self explanatory. When the drone is level, the OFS should also be level and facing towards the ground. |
In general, it can also be desirable to have tho OFS centered at the center of rotation of the aircraft, so that any rotation (yaw) can be rendered as pure rotation. If this is not possible, offsets for the optical flow sensor can also be added but they must be known to centimeter level. If you do not add offsets - the drone will attempt to rotate around the optical flow sensor, which may not be the natural center of rotation of the drone. This increases other sensor errors (especially GPS), and can cause unpredictable behaviour.
When pairing the optical flow with a rangefinder, it may be desirable to mount them close to each other (so data is more similar), but this is not a hard requirement.