How many mobile beacons do I need per drone?
Can we only have one mobile beacon per autonomous drone?
One mobile beacon is enough for basic 3D (XYZ) tracking
If you want to have have just a basic 3D (XYZ) tracking a single mobile beacon, for example, Mini-TX (for NIA), or Mini-RX with External Microphone (for IA), or Super-Beacons (with or without additional microphones and for IA, NIA, and MF NIA) is enough.
See more details in the Placement Manual.
There are many examples of basic tracking of indoor drones. To start tracing a drone indoors, you need to:
- Install four stationary beacons high on the wall or on the ceiling and face them down towards the drone
- Attach one mobile beacon on the drone and face it up towards stationary beacons
- Connect one modem to your computer with the Dashboard via a USB cable
- Build the map in 5-7 seconds automatically without any manual measurements
- Save the map and start tracking the mobile beacon on the drone, i.e. track the drone, in 5-7 seconds.
Can we fly autonomously indoors with only one mobile beacon or not?...
Yes, it is possible to fly autonomously indoors with only one beacon.
But it is difficult to fly autonomously indoors with only one mobile beacon, and we don’t recommend doing it unless absolutely necessary, for example, when your drone is tiny and there is no space or not enough base for the second mobile beacon.
If you absolutely necessarily want to fly the indoor drone with a single beacon, then, the algorithm can be like this:
- Keep the take off area 1m or without any obstacle horizontally
- Take off
- Start flying any direction for 50-100 cm but keep movement exactly linear using the gyro readings – that is easy
- Then, by knowing your take off XYZ coordinates (point 1) – before the linear and horizontal flight – and XYZ coordinates after the linear flight (point 2), you can calculate the direction of the drone against the indoor positioning system and your waypoints that you also know in terms of your indoor positioning system. If your indoor positioning system is geo-referenced – to your other coordinates, for example, indoor coordinates within the building, or outdoor coordinates – GPS, etc., then, you will know XYZ and direction against them as well. It is still easy, if you haven’t hit anything during the initial linear flight. It is easy because the IMU can be assumed ideal during this time – no drift – because it is pretty short
- Then, you start flying according to your waypoints
- This flight is not short anymore. Thus, you need to compensate the yaw drift of the gyro. When you have two mobile beacons, that what the system does automatically and you just get the IMU data without a drift. But with a single mobile beacon, it is your task. It is doable. Not extremely complex but not too simple either. It is solvable when the drone is moving and locations are updated along the way. But in static, particularly, if you haven’t locked nor created a special mechanism to remember your direction when you landed, it is impossible to know your direction after sometime because you don’t have a mechanism to compensate the yaw gyro drift. At this point, you return to the initial phase – take off and fly horizontally somewhere and pray that you don’t hit anything
Is it workable solution in some cases, particularly, when you have tiny drones? – sure, it is!
Do we recommend it to a novice users of our system? – no, it is too complex and less robust at the same time
But what is the problem and why do we need the second mobile beacon on the drone?
The task of knowing where you are facing is not simple as one may think. Note, that drone’s gyro doesn’t give you the direction. The gyro gives you the change of direction, which is not the same thing.
But why not to use compass/magnetometer?
You can try using compass/magnetometer but you risk failing your autonomous flight at the most critical moment, because magnetometers/compasses don’t work indoors reliably:
- There are a lot of ferromagnetic materials around that distort the Earth’s magnetic field
- There are lot of wires with currents that produce their own magnetic fields thus distorting the Earth’s magnetic field even more
What is the recommended solution for autonomous indoor drones?
The recommended solutions for indoor drones to have Location + Direction is to use two mobile beacons per drone with as large base between the mobile beacons as you can afford based on the drone’s size.
Configurations may slightly change depending on your particular cases. Examples for autonomous indoor drones:
- Typical PixHawk-based drone: 2 x Super-Beacon in NIA configuration if there is only 1-2 drones
- 2-8 x PixHawk-based drones: 2 x Super-Beacon in MF NIA configuration
For smaller 1-2 drones:
- 1-2 x Mini-TX per drone flying in NIA depending on whether you want Location only or Location+Direction
- Typically, if the drone is large enough to carry 2 x Mini-TX, it is larger enough Super-Beacons
For tiny drones – under 100 grams, it is possible to remove the housing and replace the default battery with something very light – 30-50 mAh or even power directly from the drone. Remember about the voltage drops, electric noise from the motors, etc. It may seriously affect. If not, you may easily safe several very valuable grams.
In IA, each mobile beacon calculates its own position independently. Thus, there could be thousands of mobile beacons in the same swarm. Location of each of them will be calculated at the same time and the update rate per drone doesn’t directly depend on the number of the mobile beacons unlike in NIA.
- 2 x Super-Beacon, if propellers are facing down, for example, like in DJI Matrice, and the mobile beacons are installed on top of the motors and facing up. In this case, the beacons’ body will be an ultrasound shield against the propellers’ noise
- 2 x (Mini-RX + External Microphone + Ultrasound deflector) – similar to this case but with two mobile beacons and a base between them of 30+ cm
Summary
- Can we track an indoor drone in XYZ with only one mobile beacon? – yes, it is easy
- Can we fly autonomously indoors with only one mobile beacon? – yes, it is possible, but difficult
- What is the easy and recommended configuration for indoor autonomous drones? – two mobile beacons/drone