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Quad(copter) of the Damned

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This post introduces the concepts and vocabulary related to quad-copters, and overview of what is involved from an embedded software perspective to build one of these flying contraptions. Progress will be documented as incremental steps in posts that follow.

Introduction

As part of my coursework in the pursuit of a Masters degree in Cybersecurity I took a course called Software Engineering for Real-Time Embedded Systems. The course focused on the concepts and challenges that are encountered when developing software for systems that have hard real-time deadlines for the system to function properly. The homework was a series of projects that led up to the development of a quad-copter with streaming video.

Many of the developers that I follow on Twitter have asked for resources on how to get started with embedded development. I thought this would be a great project to build and document the process for those of you that are interested in learning what is required to create one of these machines. I plan to build a general-purpose drone that is suited for aerial-photography with longer-than-average flight-times. However, the software that I develop can be adapted to all types of multi-rotor air-vehicles. This means that by changing the components such as frame and motors, you could easily build your own drone suitable for FPV racing.

There are plenty of pre-built and DIY (do-it-yourself) kits available to amateurs and enthusiasts. If I simply wanted to fly model aircraft I would purchase one of these kits and I wouldn't be writing this series. My chosen career is Software Engineering and it just so happens that is also my hobby. While I will be using the same components found in the DIY kits, I will be foregoing the flight-controller. Instead, I will be designing and documenting my journey as I develop this controller and integrate it with the drone. If you aren't interested in writing your own flight controller from scratch, you can still build a drone based on an embedded controller and use the open-source software Ardupilot. This development for this software is definitely active and contains support for many different types of air-vehicles. Alternatively, you could purchase a flight controller circuit-card that has all of the required functionality built into the hardware.

I will definitely cover all of the software aspects required to build a drone for yourself. I also plan to write some entries that will be of interest to you if you are interested in embedded development. For example, reading component data sheets, setting up interrupts and other aspects important to real-time embedded development.

What is a Quad-copter?

A quad-copter is considered a multi-rotor air-vehicle. Other common multi-rotor configurations include 3, 6 and 8 rotors. Four rotors provides a relatively stable configuration that is symmetric and won't be as expensive as the 6 or 8 rotor configurations.

There are two other types of radio-controlled air-vehicles common with hobbyists.

  1. Fixed-Wing (Airplanes):

    These are relatively efficient craft that allow for much longer flight times. However, they typically require a large amount of setup/teardown time when moving to the flight zone, and these craft require large open areas where the craft can maintain constant forward momentum to create lift over the wings.

  2. Rotary Wing: The Traditional Helicopter

    This is a single-rotor aircraft. The flight direction is controlled by changing the attitude of the main rotor and a secondary tail rotor is required to counteract the gyroscopic rotation caused by the primary blade.

Gyroscopic Rotation

I mentioned that a traditional helicopter requires a tail-rotor to counteract the gyroscopic rotation introduced by the primary blade. A quad-copter is subject to the same forces as its rotors spin. To counteract this effect, we will spin two of the motors clockwise and the other two motors counter-clockwise. At this point I do not know if it matters which motors that we command to rotate in each direction. Unless I find a definitive reference or someone comments with the definitive answer, this seems like something worth experimenting with when I reach that point.

This is the rotation configuration that we will start with as we work towards the final quad-copter:

Rotation Orientation
Rotation Orientation

We will alternate the direction of rotation for each motor

How does a quad-copter change direction?

The four rotors on a quad-copter allow for a relatively stable and very maneuverable aircraft. A quad-copter has 6-degrees of freedom in movement, 1) up, 2) down, 3) left (port), 4) right (starboard), 5) forward, and 6) backwards (aft). Additionally, the craft is also capable of hovering in-place and rotating in both the clockwise and counter-clockwise directions.

Attitude

A different set of terminology is used in navigation of ships and aircraft, and I will be using these terms moving forward.

Thrust

This describes the overall rotation speed of the rotors. Thrust adjusts the relative speed of each rotor equally. Therefore, if the thrust is increased, the speed of the rotors will increase and the air-vehicle will ascend. Alternatively, if the thrust is decreased the air-vehicle will descend. Assuming the air-vehicle is in the air, if the thrust is left at a steady neutral position, the vehicle will hover in place. This will require the calibration and selection of the hover level for thrust. The hover level is the neutral thrust level.

Roll

This causes the craft to rotate either to the left or the right. This rotation will cause the drone the move to the side as well. To introduce this motion the two rotors on the side in the direction of the roll should be reduced, while the rotors on the opposite side are increased.

Pitch

This causes the craft to move forward or backward by rotating the orientation of the craft. Again, to introduce this motion the rotors on the opposite side of the desired direction of movement should be increased.

Yaw

This is the rotation of the craft around the vertical access. The gyroscopic forces of the spinning rotors are used to control this aspect of flight. Two of the rotors spin clockwise and two spin counter-clockwise. Increasing the speed of the clockwise motors relative to the counter-clockwise motors will induce a force that causes the craft to yaw to the right (clockwise). The craft moves in the opposite direction when the rate of the counter-clockwise motors is increased relative to the clockwise motors.

Components

The list below contains a description of the components that are required to build a basic quad-copter / quad-rotor. I have also included the parts that I am using for my build.

Frame

There are many types of frames, built with a variety of materials such as wood, plastic and carbon fiber. It is important to get a frame that is light and well-balanced. I chose to work with the Tarot - IronMan 650, which is built from carbon fiber. This frame collapses easily to make it more portable. It has a solid core with two plates to protect the internal components. This frame was about 100 dollars. It is a larger frame, so it is not suited for FPV-racing. I am more interested in developing a platform that I can additional sensors to perform interesting tasks remotely.

Tarot IronMan 650
Tarot IronMan 650

Motors

Multi-rotor copters typically use outrunner type motors because of their high efficiency. Outrunners are a type of motor where the internal coil is mounted fixed and the entire brushless outer bell-housing rotates with the shaft, attached to the propeller. Motors are rated by a unit represented as KV. This means Kilo-rotations / Volt. Typical batteries run at 12 volts. Therefore, the 1000KV motor would rotate at a top speed of 12000 RPM.

I selected the Turnigy - Multistar Elite 4006 740KV motor for my drone. This is a slower motor at 740KV that has a larger number of magnetic poles to produce more torque. I will pair this with a steeper pitch of propeller to create a drone that can handle a heavier payload with the additional sensors that I will eventually add. The specs for these motors indicate they are capable of handling from 10" to 17" rotors. To start with, I have selected 11" carbon-fiber rotors. If you are purchasing equipment to build your own drone, I suggest you order extra rotors. Because you will break a few before you attain stable flight.

Multistar Elite 4006 740KV
Turnigy - Multistar Elite 4006 740KV

Electronic Speed Controller (ESC)

The motors are ultimately controlled by an Electronic Speed Controller (ESC). They are colloquially called "Escapes". The motor is controlled by 3 input wires, which the ESC uses to adjust the voltage across the different wires to cause the motor to spin. The controller side of the ESC has two wires that the flight controller uses Pulse-Width Modulation (PWM) signals to signal the desired speed. I will elaborate on the ESC in later posts when I describe its integration with the flight controller.

Most ESCs are designed to support a single motor. These ESCs are attached to each arm of the multi-rotor so they can connect to the flight controller in the center of the frame, and the motor on the edge of the arm. A separate ESC is required for each motor. This electronic component is one of the most important elements of your drone. So don't go cheap on this component. ESCs are rated by the current that they control. I have seen ratings in increments of 5A and 10A. I would go one level greater than the rating of the battery that you plan to use. ESCs can support additional features to protect the other electronics in your system, such as a low-voltage protector for your lithium-polymer battery. These features are often programmable.

When searching for the ESC that I wanted to use, I discovered a 4-in-1 module that is installed at the center of the drone and supports up to 4 motors. This module also has a Battery Elimination Circuit (BEC) that can be used to power a 5 volt flight-controller. I chose the Q Brain 4x25A Brushless Quadcopter ESC. I wanted to simplify the circuitry that I would have to create for the drone. I am more interested in the software that plan to write than the physical circuits that are created.

QBrain ESC
Q Brain 4x25A Brushless Quadcopter ESC

Lithium Polymer Battery

The current drones are powered by Lithium-Polymer (Lipo) type batteries. They are very dense and can store quite a bit of energy. There are a few metrics used to rate these batteries:

Current Output Rate (C)

Lipo batteries are rated by the rate of current that can be drawn from the battery. This quality will be indicated with a "C" in the battery description. For example, a battery that is capable of sustaining a current of 10 amps will be marked 10C.

Total Stored Power (mAh)

The amount of energy contained in a fully charged battery is indicated in milli-amp hours (mAh). Smaller batteries may only contain 100 mAh, while the types of batteries used to control a multi-rotor drone store between 2500 mAh to 12000 mAh. This rating typically determines the amount of flight time you will get per charge. I don't know if there is a calculation to approximate your flight times. I will be sure to post it if I run across one.

Number of Cells (S)

Most entry-level drones uses batteries with 3 cells. When you read a batteries rating, it will indicate the number of cells with an "S". Therefore a 3 cell battery will be described as 3S. More cells generally means more stored power to provide longer flight times or support a stronger current draw.

I will be using a Turnigy MultiStar battery rated at 5200 mAh 4S 14.8V @ 10C. note: I couldn't find an image to match the rating of the battery that I purchased. (I also didn't look that hard).

Turnigy Battery
Turnigy MultiStar Battery

Inertial Measurement Unit (IMU)

This is the most important sensor on a quad-copter. This sensor is also found in smart phones. An IMU contains a collection of accelerometers and gyroscopes to deduce the orientation and current motion of your drone. The absolute orientation of the drone can also be determined by locating magnetic North when the IMU contains a magnetometer. All of these sensors are typically packaged into a relatively inexpensive MEMS unit and integrated on a circuit board. The processor board that I selected is designed for robotics applications and it contains an IMU. If you want to use a different processor board, you can pick up an inexpensive IMU from AdaFruit.

Radio Transmitter / Receiver and Controller

To control the drone, you will need a radio transmitter and receiver pair. The most straight-forward solution is to simply buy a RC controller from a hobby shop or online. The transmitter is typically a controller with a channel selector and joysticks. The radio receiver is a small component that integrates with the Analog-Digital Converters (ADC) of your processor board to read the signal levels for each control.

While I initially develop my drone, I am going to use a standard 802.11 wifi network that connects my tablet to the wifi radio in AP mode on the flight controller processor board. I capture the control inputs with an Xbox 360 controller. Using this method allows me to capture useful information and display it on a display on my computer on the ground while I am testing. This wifi link will also facilitate streaming of live video once basic flight is achieved. We can go back at any point in time and integrate support for a traditional RC controller if desired.

Flight Controller

This control board is the component that coordinates all of the information required to make a multi-rotor drone capable of flight. As I have already mentioned, many flight controllers exist and can be purchased just like all of the other components that I have mentioned. Except, that is not why we are here. We are going to explore the embedded world, and take on the challenge of developing our own flight controller.

I originally planned to use the BeagleBone Black. This is a very cool open-source prototype board that has lots of capabilities related to embedded systems development. It is very similar to the Raspberry Pi. The BeagleBoard runs an image of Debian Linux. I really enjoy working on these boards, because I can eliminate the need for setting up a cross-compiling toolchain. I compile the BeagleBoard's software directly on the BeagleBoard. I will add another post soon to help get you started with this platform.

Finally, this board requires a 5v power-supply. When I first assembled the drone, this configuration was great, in that I was able to use the 5v BEC from the ESC controller to power the Beagle board. But then...

I discovered the recently released (March 2017) BeagleBoard Blue. This is a version of the board is specifically designed for robotics applications. It contains an embedded IMU, and a wifi radio. This meant that I would no longer have to integrate those two components into the drone as they would be contained within this single board. However, this board requires a 12v power supply, or a 2 cell battery. I will be powering the flight controller with a separate 2 cell battery.

Beagle Bone Blue
Beagle Bone Blue

Summary

That covers the basic concepts of a quad-copter and hardware that I plan to start with. The next few posts will focus on getting started with the BeagleBone processor board and developing basic electronics projects. This will set the stage to integrate all of the components I described in this post. The final goal is to have a working quad-copter with supporting software that is ready to be expanded for custom purposes. The software will be available from GitHub. So if you are interested, check back soon. Follow me on Twitter to receive tweets when I post new updates.

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