Initial Quadrotor Design

 

I have been researching a variety of multi-rotor helicopter setups for some time.  I’ve been trying to identify the strengths and weaknesses of each design type.  I have come up with an initial design for my quad-rotor helicopter.  In attempts to create a very aggressive aircraft, I have designed the propellers to be a close as possible, as large as possible, and the motors to have a high power to weight ratio.  All of my assumptions about quadcopter design are very spectulative since I haven’t ever built one before. 

My first matter of design is a powerful CPU.  My fly-by-wire T-Rex 600 project used 4 Arduinos and they left a sour taste in my mouth.  Arduinos are great for quick prototyping but anything with substance needs a better processor.  Besides, I’m a Computer Engineer so I can’t justify using someone else’s poorly designed microcontroller libraries.  My microcontroller of choice for this project is the LPC1768 ARM Cortex-M3 by NXP Semiconductors.  It is a powerhouse!  I’ve written most of the low-level hardware drivers and a few of the higher level routines, such as PID controllers and Collective Cyclic Throttle Mixing (CCTM).  The combination of the ARM’s Cortex-M3 core with NXP’s hardware peripherials makes this as amazingly powerful design.  ARM+NXP=Happiness!

For aircraft attitude measurement, I’m planning on using the 9DOF Sensor Stick from Sparkfun.  Version 2 is still under design so I’ll have to wait on that.  I’m going to implement 3 different types of sensor fusion algorithms and see which type works the best.  The 3 algorithms are:

  • Complimentary Filter
  • Direction Cosine Matrix
  • Extended Kalman Filter

In the animation above, the two circles representing the propellers show the two sizes I will test.  The inner circles are a 12×3.8″ APCprop and the outer circles are a 14×4.7″ APCprop.  I haven’t seen another helicopter use the 14″ props before so I’ll get the 12″ props working first.

This is an explanation of the animation per level:

Level One (bottom):

  • LiPo battery
  • Receiver

Level Two (between the two metal plates):

  • 4x Electronic Speed Controllers

Level Three (top):

  • CPU
  • 3-Axis of Gyroscopes, Accelerometers, and Magnemeters

Outer Arms:

  • 4x Motors
  • 4x Props

I know that having the props close together makes the attitude harder to stabilize.  On the other hand, having the props close together will (I hope) induce much more torque on the frame from the motors.  This will help me overcome the lame yaw response of most quadrotor helicopters.  I’m banking on the fact that my CPU will be running at 100 MHz and I’ll hopefully have the sensor fusion filters and PID controller running at 400+Hz.  This should allow me to precisely adjust each axis of stabilization.  For better discrete calculus computations (integration and differentation) I’ll try running the sensor fusion algorithms above 1kHz and only commanding the ESCs at their maximum speed (50Hz-400Hz).  This will make the computations more accurate because each time step will produce less error.

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Starting a quad-rotor helicopter project

Past:
I recently graduated from the University of Utah in Computer Engineering. My senior project was a fly-by-wire system for an unmanned helicopter. We used an Align T-Rex 600 ESP helicopter for the project. Designing a stabilization system for this size of helicopter presents many problems. Given that the tips of the blades travel at 350 mph, physical danger was obviously the biggest concern. We successfully implemented our design.

Even though we felt moderate success, we didn’t feel that our system performed as well as it could have. All the supporting hardware and software was implemented but the time needed to calibrate the many features of stabilization caused us to end the project before all features could be utilized. The excessive calibration time is a result of calibrating a 53 inch helicopter during flight. Any changes in the system had to be modified very slowly.

This project gave me a lot of experience with inertial measurement and feedback control. It also gave me a HUGE desire to make something better!

Future:
Now I begin a multi-rotor helicopter project. I aim to fix all the faults in the previous design while overcoming many of the short comings of current multi-rotor designs. I will start by designing a 4 rotor system because it is the cheapest. Once I master the quadcopter, I’ll try a hexacopter or an octocopter. I have been impressed by many multicopter designs. Some are:

  • HexaKopter by MikroKopter
    This design shows a lot of intelligent software engineering. The GPS capabilities of this system are phenomenal. The supporting stabilization system is also very intelligent (see the oscillating coke bottle).
  • GRASP Labs Quadrotor
    Having the entire room and obstacles marked with position sensors is kind of cheating for autonomous vehicles, but I must admit that these helicopters are amazing! Recovering from severe initial conditions is very impressive.

My goals are to:

  • overcome the inheritly slow yaw response of current quad-rotor designs.
  • design an extremely aggressive stabilization system.
  • find a good balance between size and payload capability.
  • make the chassis rigid enough to survive moderately severe crashes.
  • create a quadcopter that is ridiculously fun to drive!

My initial design will be very minimal. I will start off with only a basic quad-rotor design controlled by a transmitter/receiver pair. No other communication will be used. I’m designing this to be minimal so that I can focus on the inertial measurements and feedback control. Once these are mastered, I’ll add fluffy features like:

  • software ground station
  • GPS hold and navigation
  • video downlink