Stabilization systems and how they work

Written by Lucas Weakley As seen in the Fall 2016 issue of Park Pilot.

Stabilization is a growing technology in the RC hobby. If you haven’t yet noticed, many of today’s expanding RC facets, such as multirotors, can be impossible to fly without their complex stabilization systems. You also might be surprised to learn that a lot of RTF (Ready to Fly) aircraft use some kind of stabilizer. Now, let’s take a look at how this happened and how stabilization works. Stabilization systems consist of some kind of sensor, something to interpret the signals from that sensor (usually a small microprocessor), and a way to mix those signals into the physical controls of whatever is being stabilized. These parts work together to measure changes in motion and position in, for example, an RC aircraft. The information is used to dampen or correct for those changes. The sensor is the hard part, and what has become miniaturized, which I’ll discuss later. One of the first ways to measure changes in motion was by using a physical spinning gyroscope. If a mass is spun at a fast enough rate, it can exhibit characteristics of a gyroscope, similar to many of the toys we have likely played with. The gyroscope resists moving in a single axis and can be used as a reference point to measure a fixed position relative to a moving body. Some of the first RC aircraft to need some kind of stabilization were RC helicopters, specifically in the yaw axis, where slight changes in throttle, blade pitch, translational movement, or wind could send the heli spinning out of control too fast for manual correction. Gyros, similar to what I previously described, were used to measure the changes in yaw axis and stabilize the aircraft. A set of heavy, spinning disks created the gyroscope. The position was measured using either magnetic or connecting switches. That measured change in angular acceleration—when the aircraft started to yaw—would mechanically increase or decrease the pitch of the blades on the tail rotor before the helicopter spun out of control. A flybar on an RC helicopter is also another form of a gyroscopic stabilizer. Don’t go looking for big spinning disks on your multirotor controller board, though. A simpler way of measuring changes in motion today is called MEMS (micro-electromechanical systems). As the name implies, these are tiny mechanical measuring devices etched in silicon using piezoelectric-induced oscillations instead of a spinning gyro to measure angular accelerations. Mind blowing!

This is an image of a MEMS gyroscope taken by an electron microscope. This specific sensor was taken out of an iPhone, but is similar to those found in flight controller boards. Photo credit: IFixit.com. Image used under a Creative Commons license: www.ifixit.com/Guide/Image/meta/MeunHawo54qPRY5S.

To put it more simply, just as a spinning disk likes to stay fixed in a certain axis, a vibrating bar wants to vibrate in its current plane and will resist being rotated or moved out of its position. By vibrating a tiny comblike structure using electricity, changes in how fast that comb vibrates and how much it vibrates can be detected and used to measure changes in external accelerations. There are many kinds of MEMS gyros, and I encourage you to look up the technology if you wish to learn more about them. They are truly fascinating. These new sensors started becoming available to consumers in the late 1990s and early 2000s. They were smaller and lighter than any of the mechanical predecessors. RC helicopters started using them, allowing for smaller helicopters. Later on, more gyros were added to sense changes in all three axes, creating flybarless helicopters.

When MEMS gyros came out, old physical gyroscopes were replaced with small, lightweight boxes that didn’t use much power and were more accurate.

At roughly the same time, people were taking single-axis RC helicopter yaw gyros and orienting, wiring, mixing, soldering, and hacking them together to make the first stabilization systems for RC multirotors. What was interesting about multirotors was that you could mix together a few ESCs and get the throttle, pitch, roll, and yaw controls working without gyros, but they were impossible to fly without some kind of stabilization because of the inherent instability in the design. These makeshift, cobbled-together stabilizers were crude and limited in their customizability. It didn’t take long, though, for people to start using the new MEMS gyros on their own and making dedicated flight control and stabilization boards for multirotors, but there was a problem. Gyros can only measure changes in motion; they can’t tell up from down. MEMS gyros also drift, losing their accuracy throughout the duration of a flight. If you had a multirotor with only gyros, you would still have to manually hover it in place and constantly trim it out to get it to fly level. The solution was to use another MEMS sensor called an accelerometer, which can be similar to the construction of the aforementioned MEMS gyro, but is tuned to measure linear acceleration instead of angular acceleration. An accelerometer can measure the force of gravity in all three axes, and with some trigonometry, can determine where down and up are (that’s also how your smartphone can tell what’s up and down). That information is used in conjunction with the readings from the gyros to correct for any deviations, and to add a few more stabilization modes such as horizon lock and auto-leveling. A pair of three-axis accelerometers and gyroscopes is used on almost all flight controller boards to give accurate measurements for the aircraft’s orientation. Later, fixed-wing aircraft began using gyros and controller boards, too. These helped more complicated and smaller models become easier to control and more resistant to windy conditions. Now you can find many RTF or BNF (Bind-N-Fly) airplanes from companies such as Horizon Hobby (horizonhobby.com) and HobbyKing (hobbyking.com) that have built-in stabilization systems. You can even buy RC receivers with three-axis gyros and accelerometers built in! Stabilization systems are also used in 3-D aerobatic RC aircraft. They help the airplane be more stable and easier to control while performing maneuvers such as hovering, knife-edge, flat spins, general flight in high wind, etc. Of course, this has brought up the discussion of when it is fair to use stabilization.

This is the first flight controller board that I purchased and flew with. There are three silver boxes on the front. Those are the MEMS gyroscopes on their axes of orientation.

During any kind of aerobatics or racing competition with fixed-wing aircraft, using some kind of stabilization to aid in an airplane’s performance could be inappropriate. On the other hand, multirotors for racing have to use stabilizers to fly, and the settings that each racer chooses to use are indicative to his or her individual skills and flying style. Although this could be the topic of an entirely different article, these same systems that might be considered cheating by many, allow beginner pilots to learn more quickly and get past some of the frustration of learning to fly RC. Some newer trainer airplanes, such as the Horizon Hobby Apprentice, even have ground-avoidance sensors and a built-in autopilot system that can fly the airplane for you if you lose control. Now, instead of hitting the trainer switch like what is done when using a buddy box, the airplane can self-correct and give you back the controls! This technology ultimately means that the hobby is more accessible to people who use these new beginner airplanes. These same people can then begin other projects in the hobby that might not need stabilization. Regardless, stabilization will be an integral part of shaping what RC will look like in the future, and I’m excited about the possibilities!