Helicopter SAS: Overview

This post will be an overview of my helicopter Stability Augmentation System (SAS) project.

The Story

This last spring, I decided to build a stability augmentation system (SAS) for an R/C helicopter, with the goal of giving my dad an easy-to-fly R/C helicopter that still had the coolness factor of a single-rotor design.

I spent the next two months designing, building, and debugging the SAS. I don't think I could have flown the helicopter without the SAS active, but then again I haven't tried.

I did crash it a few times, though, and the unbalanced blades and bent shafts definitely degraded performance. I also broke the landing gear and cyanoacrylated it back together. During my demo for my dad (my family came to town for graduation), I touched down too hard and one side of the landing gear broke, causing the helicopter to flop over onto that side.

My dad took it home; I think he's repaired the crash damage but I don't think he's flown it. So, I haven't been quite successful in that regard. Maybe next time...

The Parts

The SAS is implemented as this foil-shielded box that sits where the original heading-hold gyro used to be. Inside the box are sensors to measure the helicopter's motion and orientation. The lid of the box is the control board that uses those measurements to send stabilizing control signals to the helicopter servos.

Barely visible in this photo is the wireless module taped to the rear strut of the right (starboard?) landing skid. I used the telemetry data extensively during testing and debugging.

Suggestions for Next Time

• Velocity feedback (optical flow)
• Heading measurement (magnetometer w/ offset for body-fixed fields)
• Transformation of cyclic commands from pilot axes to body-fixed axes
• Different mounting location/method to minimize vibration at accelerometers
• Don't crash it as much