Multi-rotor

A multi-rotor helicopter is a flying vehicle with more than one rotor.

The nice people at http://www.quadheli.com/ have asked for our help designing and building a multi-rotor helicopter.

Several people are using small unmanned helicopters with 4 rotors ("quadheli" or "quadcopter").

BLDC

It appears that most modern small electric aircraft use so-called "brushless DC motors", each one driven by its own "BLDC ESC". (These are easily recognized -- BLDC motors have exactly 3 equally-fat wires that go into them, which come from the BLDC ESC -- as opposed to most electric aircraft a few years ago, which used brushed DC motors with exactly 2 equally-fat wires).

While it is probably not cost-effective to build your own BLDC motor or BLDC ESC, many of us are insatiably curious about what goes on inside these things, and so build one anyway:

• Wikipedia: brushless DC electric motor
• Atmel AVR444: Sensorless control of 3-phase brushless DC motors using ATmega48 (also works without change for ATmega88 and ATmega168). Assumes you've already read Atmel AVR443: Sensor-based control of three phase Brushless DC motor
• AVR194: Brushless DC Motor Control using ATmega32M1: BLDC motor control application using Hall effect position sensors to control commutation sequence.
• MikroKopter: brushless motor controller was designed to give lower latency than off-the-shelf PWM ESCs.
• OpenServo: Brushless DC Servo "The thing that will make our board different from other ESC's is that we are closing the feedback loop with a ... outside position reference."

unnecessarily complicated equations

In hover, each rotor gives (equations from Paul Pounds et. al 2004?)

 T = 2 p A v_i^2
 P_i = sqrt( T^3 / 2 p A )

where

 T is the thrust produced
 p is the density of air, approximately 1.2 kg/m^3 at sea level and 20 'C.
 A is the area of the rotor disk
 v_i is the induced air velocity at the rotor
 P_i is the power induced in the air.

For a quad-rotor helicopter weighing 4 kg, with a 30 per cent control margin, and a rotor radius of 0.165 m, such as the Australian X-4 Flyer, the above equation results in about 101 W of power induced in the air per rotor. With a shaft-to-air rotor efficiency of 90% that requires 112 W of shaft power. With a battery-to-shaft motor efficiency of 50%, each rotor pulls about 224 W of power from the battery at full thrust.

To double thrust requires either pulling almost 3 times as much power from the batteries, or using rotors with almost 3 times the diameter.

unsorted

way too many links here. Please delete the ones not relevant to multi-rotor helicopters.

• Wikipedia: quadrotor is a nice introduction. But what are these "three rotor craft" it mentions? (TriCopter? Tri-copter?)
• the OpenPilot Wiki [1]: open source community
• The Gluonpilot wiki (autopilot) mentions "Quadrocopter" [2]
• MikroKopter wiki [3] semi-open-source "for noncommercial use"
• QC-Copter Wiki [4] : updates all motor speeds at 500 Hz.
• the NG Multikopter Project wiki [5]: a open source community project to build a modern autonomously flying Multicopter.
• the Wolferl Open Source QuadCopter (Universal Aerial Video Platform) wiki [6]. Apparently NS Rana at DIY Drones uses it in a very low-cost-frame quadcopter[7].
• QuadroCopter Wikia [8]
• ARM-o-Kopter wiki [9]
• comparing some currently known projects of airborne non-commercial or open community projects of multicopters[10]
• DIYdrones: "There are a zillion quad- and tri-copters out there" [11]
• DIYdrones: Quadcopters discussion forum [12]
• DIYdrones: Return to Home Quadrocopter (UAVX) [13]
• DIYdrones: "There are loads of open source quadcopters out there, but they're all ..." [14] Is it possible to design a helicopter that avoids this problem?
• the "ChRoMicro - Cheap Robotic Microhelicopter HOWTO" [15], [16] describes "how to build a 300 g helicopter with embedded Linux and Bluetooth datalink from off-the shelf components for less than 500 EUR." Can these ideas be adapted to helicopters with more rotors?
• Quadrotto: Project Quadcopter [17], [18]. Is there any way to avoid making the same mistakes all over again, and instead make fresh new mistakes? :-).
• RCgroups: Multi Rotor Helis discussion forum [19]
• Make magazine How-To: Quadrocopter based on Arduino[20] "The Quaduino NG & AeroQuad RC projects both make use of Arduino boards"
  • Quaduino NG[21]
• AeroQuad discussion forum [22]: dedicated to the design and construction of the AeroQuad, a remote controlled four rotor helicopter ... that uses the Arduino (Mega or Duemilanove with 328P) microcontroller as the flight control board, with a "AeroQuad Shield" that connects to all the other electronics -- radio receiver, gyros, accelerometers, and off-the-shelf ESCs. An excellent tutorial showing how it all goes together with whatever frame you have; it claims "A good motor-to-motor distance to start with is around 60cm." (2 foot)
• microdrones [23]
• whatnick blog: "quadcopter taking shape"; and other quadcopter posts ... he apparently has a Gumstix Verdex and a BeagleBoard -- are either one of these going on the quadcopter?
• WSN wiki: wireless sensor node platforms -- perhaps we could use one of these boards for our wireless communication, or perhaps make incremental improvements, rather than designing yet another one from scratch?
• Dr. Igor Bensen designed the eight rotor helicopter[24] on the front page of Popular Mechanics 1982 September.
• Google: "Real-time stabilization of an eight-rotor UAV using optical flow"[25]
• kapteinkuk built a low-cost quadrotor flight stabilizer based on a Atmel AVR ATMega48 [26]; connected to a standard RC receiver, 3 gyros with ordinary analog output, and 4 ESCs. That's all the electronics.
• the Quadrotto project[27] uses an ARM-based gumstix + an Atmel AVR ATMega128-based robostix
• Project Quadcopter [28] "altimeter is ... not our only altitude sensing device. We ... plan ... an ultrasound sensor for landing and low altitude flights. ... they work pretty well out to about 4 or 5 feet." ... apparently using an ARM cortex-m3 microcontroller
• ArduPilot [29] is a full-featured autopilot based on the Arduino open-source hardware platform. It uses infrared (thermopile) sensors or an IMU for stabilization and GPS for navigation. Optionally uses XBee modules for wireless telemetry. Jose Julio at DIY Drones [30] uses it in his two quadcopters. He uses 4 standard props (No counter-rotating !).
• "Towards Dynamically-Favourable Quad-Rotor Aerial Robots"[31] by Paul Pounds, Robert Mahony, Joel Gresham (2004?): "the Australian National University’s ‘X-4 Flyer’ platform." "The use of inverted rotors [pusher props] is shown to produce favorable stability properties"

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