over the last 5 years I had 2 quadcopter as a gift. the first one was basic, manual, I had to manually set the 4 motor speed for it not to crash during take off and I was alway controlling the amount of thrust myself.

1/ Why was that necessary? if you send the same thing to the 4 motor they should spin at the same rate and thus provide the same thrust no?

The second one was a smartphone controlled drone, you just hit the take off button and the thing fly and stay at the same place/altitute.

To do that I could see an ultrasonic sensor below the drone. if I made it fly above a mattress it would crash.

2/ Ok that nice but what about high altitude? what if my drone is hovering a 500m, what kind of sensor can I use to make him stay at the same place and altitude.

I already fiddled with GPS and I don't think it's fast/precise enought.



1 Answer 1


The first quadcopter probably only had a gyroscope to assist in roll/pitch/yaw control. For real motors, sending the same value will not provide the same rotation speed (it will always vary at least a little). Also, for 4 different real rotors, spinning at the same speed will not produce the same thrust. That is why you had to adjust takeoff motor speeds. In flight, unless you produce exactly, and I mean exactly, the correct thrust to offset the quadcopter's weight you will always need to adjust the thrust to maintain hover since the system itself doesn't know that it's rising/falling.

As you guessed, the second quadcopter was using an ultrasonic sensor to know it's height. It probably also had a more advanced gyroscope and algorithm to hold roll/pitch. At high altitudes where the ultrasonic is not effective, barometers are usually used. This will not be terribly accurate in an absolute sense, but it will be able to tell the computer algorithms if the system is rising or falling which is enough to hold still.

  • $\begingroup$ How about the 500 ft question ? That would require a different sensor suite (pressure or gps based). $\endgroup$ Commented Oct 19, 2017 at 19:59
  • $\begingroup$ That is the barometer I was referring to in the second paragraph $\endgroup$
    – ryan0270
    Commented Oct 19, 2017 at 23:18
  • $\begingroup$ I've somehow missed that sentence. Right, it's there. +1 for the answer. $\endgroup$ Commented Oct 20, 2017 at 5:43

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