electrical

An overview of the electrical systems powering the Tetracopter

Circuit

ADXL203 accelerometer

The Tetracopter sports one PIC 18F2455 on-board microcontroller that controls the craft (see software). We use a dual axis accelerometer (ADXL203) to sense our tilt and an ultrasound sensor (LV-MaxSonar -EZ1) mounted on the bottom to measure altitude.

LV-MaxSonar -EZ1 ultrasound rangefinder

Both the accelerometer and ultrasound sensor support analog output, so we simply run analog to digital conversion on each of those values. This results in a relatively simple circuit on the input side: three pins on the PIC connected to the outputs of the sensors.

circuit

Emergency Stop

During testing we quickly noticed that it was critical to have an emergency stop switch. Without one, the craft would tear itself apart within seconds. Our first iteration of such a switch was simple: we connected each motor to a power supply instead of the main battery and shut down the power supply if the test went awry.

Once we transitioned to an on-board battery, we needed a remote way to turn the power off. We decided to implement a wireless system using a stock R/C controller and a receiver. We connected the receiver to the PIC and wrote a few lines of code to interpret the signal. For the most part, this worked well. However, because the switch was implemented in software, it was susceptible to one critical problem. If the PIC should crash, there would no way to shutdown. Moreover, a crash on the PIC is one of the most important times to have an emergency stop switch.

Video: Why a Hardware Emergency-Stop is Important

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After learning how bad a software-implemented emergency-stop is the hard way, we installed a simpler, more reliable system. Instead of using R/C gear, we simply tied a string to the main-battery switch in such a way that when it was pulled the switch would be toggled to the off position.