Tech Briefs Magazine - March 2021 - 41

Keeping Drones Flying When a Motor Fails
Onboard cameras can be used to keep damaged quadcopters in the air and flying stably,
even without GPS.
University of Zurich, Switzerland

F

or drones with four propellers - also
known as quadcopters - the failure of
one motor is a big problem. With only
three rotors working, the drone loses stability and inevitably crashes unless an
emergency control strategy sets in.
Researchers have found a solution to this
problem: information from onboard
cameras can be used to stabilize the
drone and keep it flying autonomously
after one rotor suddenly gives out.
When one rotor fails, the drone begins
to spin on itself like a ballerina. This
high-speed rotational motion causes
standard controllers to fail unless the
drone has access to very accurate position measurements. In other words, once
it starts spinning, the drone is no longer
able to estimate its position in space and
eventually crashes.
One way to solve this problem is to provide the drone with a reference position
through GPS. But there are many places
where GPS signals are unavailable. The
researchers solved this issue without relying on GPS, instead using visual information from different types of onboard cameras. They equipped the quadcopters with
two types of cameras: standard ones,
which record images several times per second at a fixed rate, and event cameras,
which are based on independent pixels
that are only activated when they detect a
change in the light that reaches them.
The team developed algorithms that
combine information from the two sensors and use it to track the quadrotor's
position relative to its surroundings. This
enables the onboard computer to control
the drone as it flies and spins with only
three rotors. The team found that both

types of cameras perform well in normal
light conditions. When illumination decreases, however, standard cameras begin
to experience motion blur that ultimately
disorients the drone and crashes it,

whereas event cameras also work well in
very low light.
For more information, contact Professor
Davide Scaramuzza at sdavide@ifi.uzh.ch;
+41 044 635 24 07.

When one rotor fails, the drone begins to spin
on itself like a ballerina. (Picture: UZH)

Tech Briefs, March 2021

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Tech Briefs Magazine - March 2021

Table of Contents for the Digital Edition of Tech Briefs Magazine - March 2021

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