Saturday, April 6, 2013

UAV part 4: single rotor

This is the helicopter I’ve been working on for the long period since my last post. After I got the coaxial helicopter up and running with my custom electronics, I flew it way too high outside and crashed it hard enough to break the plastic frame. Then I flew it without the frame like this:

That left the drive gears on the bottom exposed, and I broke one of the gears in another crash. That left all the mechanics pretty much useless, until I had the crazy idea of converting it into a single-rotor helicopter since I still had one working drivetrain.

The board I used for this one is a custom helicopter board I’d been designing for a while. This board put all the electronics I used on the 2.4GHz coaxial into a single PCB, but with a more capable microcontroller to allow for more functions. The board features are:
·         PIC16F1829 microcontroller
·         nRF24L01+ 2.4GHz radio transceiver
·         LY550ALH single-axis gyroscope
·         3 N-channel motor FETs
·         Breakouts for connecting ultrasonic rangefinder & optical sensor

This board is called MATH, for Mostly Autonomous Tiny Helicopter (although at this point it stands for Mildly Autonomous). The core functions are just to get me up and running on a helicopter with the radio link, but later I’ll connect in the ultrasonic rangefinder for altitude control and make it really autonomous.

The connections for the optical sensor (labeled “terrain imager” in the schematic) are for an idea I had that’s not yet tested: I have seen a few robotics projects where people used optical mouse sensors to keep track of motion by focusing them on the ground below the robot. I think it will be possible to use this technique on a helicopter provided I can find optics to focus the sensor on the ground at a height of 3 or 4 feet (and maintain altitude precisely). If this works, the helicopter could keep track of its path and position and navigate autonomously with high precision.

To make the single rotor helicopter, I stripped down the Syma S107 to minimal weight and removed one of the drive motors, gears, and rotor. The tail rotor is now needed for torque balancing so it has to aim sideways. Early tests showed that the stock motor wasn’t powerful enough for this job, so I make a custom tail boom to extend it and increase torque without increasing power draw. I later replaced this aluminum boom with a carbon fiber boom, and put a bigger propeller on the tail motor.

I was able to make the single-rotor fly, but it had to be stripped to less than 20g and run at full power just to stay aloft. Clearly I needed a bigger motor for the main rotor, but I decided to do something crazy just to make this one work, and went to a 2-cell battery to double the voltage. Now I don’t have to worry about weight so much and have power to spare. Unfortunately the motors get hot as hell and will probably burn out very soon, but for code development purposes this setup works well.

I started from scratch on the software to run this helicopter to keep everything clean. I struggled for weeks trying to stabilize the helicopter with my yaw control loop, and finally found the solution this weekend. I was using PID all along but could never seem to get my gains right, but then finally discovered that the control loop was being screwed up by noisy readings. The solution was to use a 100ms moving average (4 samples at 25ms) to get the yaw value from the gyro, and then derive the integral and derivative terms from that value. That smoothed things out and it finally stabilized.

It still jitters right and left, so there is more work to do. Once I’m happy with this I plan to work on adding a magnetic (non-contact) swashplate mechanism to this helicopter to control pitch. Here’s the code

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