There are a host of sensors available as explained earlier. In this post I shall list the ones I will include in my drone.
Optical flow sensor with sonar:
An optical flow sensor measures the movement of an imagine it sees. It is the same sensor that is used in optical computer mouses (the ones without mouse balls) to measure the movement of the mouse accurately. It does this by using a low resolution camera and detecting the imagine moving under the camera.
The reason I am including this in my drone is because I can detect the movement of the drone in the same way. As a mouse sees the table move under it my drone will see the ground moving under it. Why can’t I use GPS? Not everywhere has GPS, such as indoors, and GPS is not very accurate, it is about accurate to a meter. Thus for landing, it is preferable that this be used.
The sonar component measures the distance from the sensor to the ground. It does this by buzzing an ultra high frequency buzzer which is aimed outwards. This sound will take a certain amount of time to travel to the object (the ground in this case) then it will be reflected off and back to the sensor after a certain time. The sensor has a microphone that is tuned to listen for this sound. It is this time delay that is used to calculate the distance since the speed of sound is a relatively constant 340.29 m/s.
It is the same method as used by whales to locate fish.
So for example is the sound took 0.01 seconds to return we would use the equation:
340.29m/s * 0.01s = 3.4m
So the object would be 3.4 meters away.
This data will be then fed to the flight controller, and will be very useful for landing.
Here is an example of an optical flow sensor in use:
GPS and Magnometer
GPS – Global Positioning System is used to find the location of any GPS receiver anywhere in the world. It works by using a series of 27 satellites which emit signals that can be trilateralated by the receiver to calculate its position. (this particular system also uses the Russian and European GPS equivalents and combines the data of all 3 for increase accuracy and reliability)
Each satellite sends a pulse signal, which carries a series of information. It includes these two things:
- The time the signal was sent
- The location of the satellite
With the the receiver can match it’s internal clock to that of the GPS, then time how long the next signal takes to reach it, which like with the sonar can be used to calculate the distance to the satellite. This can then be used to trilaterate the position as described below.
More information on GPS can be found here:
The magnetometer is basically an advanced electronic compass. It contains a hall effect sensor, which measures the slight fluctuations in magnetic fields. This can be used to measure earth’s magnetic field, and used in conjunction with GPS can be used to calculate true north, and angular data – just like a compass always points north!
There is a few problems with a magnetometer, which is isolating it from other magnetic fields, like those produced by the electric motors. So for my drone I shall have to keep the GPS and Magnetometer on a mast away from the rest of the done (this will also help the GPS have a clear view of the sky).
All the other sensors are included on the control unit.