Multicopters For All
We all have a pretty good idea what a helicopter is.A flying machine with a fuselage or pod, using a single or coaxial rotary "wing" to provide lift, thrust and control (a tail rotor may be used for torque compensation). A multicopter is a flying machine that uses two or more non-coaxial rotary "wings" for lift, thrust and directional control.
Full scale multicopter type hovercraft have been attempted since the 1940s, mostly for military uses, with no practical success. Noise, stabity, reliability were all problematic.
On the model front, there was little interest until recently. Developments in the fields of electric motors, batteries and micro-computers adapted for hobby robotic lead to a successful and popular new field.
Multicopter enthusiasts cross lines of interests. A large group started as robotics hobbyists. Others were attracted by the multicopters' potential as a stable aerial photography platform. Traditional model airplane modelers are probably the smallest group, probably for several reasons.- multicopters are not like "real aircraft", the technology involved is new to most modelers that have been in the hobby for a number of years.
Multicopter technology is still mainly in the non-commercial, developmental stages. There are no kits or ready to fly multicopters, from major companies, other than a few tiny micros like the Traxxas QR-1. The flight control technology is based on the hobby-robotic Arduino boards that were developed from microcomputer chips from Atmel. Microcomputer chips are everywhere - in your car, washing machine, and RC transmitter. They are perfect for hobbyists - they take inpput from the outside world and make something happen. Italians Massimo Banzi and others developed open source Arduino system with a simple programming language to make robotics and electronic control available to artists and hobbyists.
Arduino boards make robotics available on the hobby, tinkerer leve. They take input from the world and process it to cause a desired output. Sources of input can be as simple as an on-off button. Arduino can also use a variable voltage source, like a temperature sensor or barometer. Tinkerers - people that take things apart to see how they work and make other things with them - noticed the small, inexpensive sensors that were available in video gaming controllers and other comsumer hardware. Accelerometers and gyroscopes were found to lend themselves to flight control uses. An aircraft inputs usually respond to movement that create accelerations or movements away from a fixed reference. Small versions of these sensors were found in Wii controllers. Tinkerers put these on small circuit boards, linked to Arduino computers, and the army of amateur programmers took over.The resuting flight control programs are generally known as MultiWii programs, referring to the use of multiple Wii sensors.
It was not a big step to incorporate the sensors and microcomputer on a single board. All the circuit schematics are "open-source". That means shared programming for non-commercial uses, modified and improved by users, and posted for others to try. Some very talented people have contributed to various flight control programming streams. By community agreement, these programs are available free for non-commercial use, With many people providing input, these have developed quickly into reliable, sophisticated programs. Additional sensors have been added - barometers for altitude sensing, magnetometers for compass direction, GPS for geo-location, sonar for low altitude sensing.
Are multicopters autonomous, taking them into the controversial, highly publicized world of drones? At this stage of development of hobby Arduino based multicopters, the answer would be no. Hands on flying skills are required. A warning on one website cautions that "multicopters are subject to novel and unexpected" results. Even when behaving well, there is a learning curve like that of conventional model helicopters. Sensors can assist in control, flight stability, stationary hovering, and navigation. With GPS, multicopters can automatically return to launch location on command or as a fail safe. But current reliability and external variables require a hands on pilot be at the controls.
Because of the computer aided stability, there is a large variety of multicopter configurations possible. The basic flight requirements of thrust and control in three dimensions can be met in a number of ways, using combinations of electric motors with speed contollers and, in some cases, servos to tilt the motors. See the chart below for some of the current configurations.