Make: Drones by David McGriffy

Author:David McGriffy
Language: eng
Format: epub, mobi
Publisher: O'Reilly Media, Inc.
Published: 2016-10-20T16:00:00+00:00

Motor Controller

We will use the simplest possible motor controller circuit for the Visible Drone. Most small motor controllers use metal oxide semiconductor field effect transistors, called MOSFETs or just FETs, but some require more complex circuits. Ours will use a single FET and a single resistor, as we will see—but first, some explanation of what is not included.

To go forward and in reverse requires at least four FETs per motor instead of the single FET we use, but forward is enough for this application. Some designs would use a pull-down resistor on the input to make sure the motor doesn’t come on when the processor boots up in a strange state. This hasn’t proven to be a problem with the fast-booting and reliable Arduinos, though, and perhaps more to the point, the consequences of accidentally running a motor in a quad this size are not too serious.

Finally, this motor controller design does not include flyback or protection diodes. When driving an inductive load—that is, one that uses coils like motors and solenoids—the same mechanism that we drive with current will often produce current when pushed back the other way. Solenoids can easily produce three times their original drive voltage, in reverse, when released. This current needs somewhere to go! A diode placed across the load the right way will take this flow instead of the FET, which may not be able to take this higher, negative voltage.

In our application, the reverse voltage should never get higher than what we drive the motors with, since they are unlikely to spin faster by themselves than when we are driving them. Also, the FET we have chosen has a body protection diode. Putting this current through that diode doesn’t keep it out of the FET overall, but it seems to be enough for this application. Few brushed motor drivers for small drones include protection diodes.

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