How To Build A Quadcopter – Choosing Hardware
In this article I will be talking about quadcopter components and how
to choose them. This is part of the tutorial series on how to build a
quadcopter. In the next post I will be talking about software, how to go
about the algorithm and programming.
If you are planning on building a quadcopter but not sure how, this
is the right place for you. Doing research is pretty boring, so I am
trying to put together a comprehensive tutorial about quadcopter, hope
it helps you as much as it helped me.
Building a quadcopter from scratch takes a lot of time and effort. If
you are inpatient, afraid of programming/maths and has a good budget,
you can just buy a pre-built kit. You could get it assembled within
minutes before it’s flying (For example, like
this one).
But I have to say, you are missing the fun part of building a
quadcopter. From choosing the parts, designing the circuits,
to programming, you will be involved in every aspect of building a
quadcopter, and it’s FUN!
If you have any questions, feel free to ask on this
quadcopter message board.
This blog post is divided into a three parts
- What is a Quadcopter and How It Work
- Quadcopter Components Introduction
- Conclusion
What Is A QuadCopter and How It Works
A
QuadCopter is
a helicopter with four rotors, so it’s also known as quadrotor. Because
of its unique design comparing to traditional helicopters, it allows
a more stable platform, making quadcopters ideal for tasks such as
surveillance and aerial photography. And it is also getting very popular
in UAV research in recent years.
The Quadcopters exist in many different sizes. From as small as a CD up to something as big as one meter in width.
On a regular helicopter has one big rotor to provide all the lifting
power and a little tail rotor to offset the aerodynamic torque generated
by the big rotor (without it, the helicopter would spin almost as fast
as the propeller)
Unlike a helicopter, a quadrotor has four rotors all work together to
produce upward thrust and each rotor lifts only 1/4 of the weight, so
we can use less powerful and therefore cheaper motors. The quadcopter’s
movement is controlled by varying the relative thrusts of each rotor.
These rotors are aligned in a square, two on opposite sides of the
square rotate in clockwise direction and the other two rotate in the
opposite direction. If all rotors turn in the same direction, the craft
would spin would spin just like the regular helicopter without tail
rotor. (if you are not sure what I mean, check out
this video)
Yaw is induced by unbalanced aerodynamic torques. The
aerodynamic torque of the first rotors pair cancelled out with the
torque created by the second pair which rotates in the opposite
direction, so if all four rotors apply equal thrust the quadcopter will
stay in the same direction.
To maintain balance the quadcopter must be continuously taking
measurements from the sensors, and making adjustments to the speed of
each rotor to keep the body level. Usually these adjustments are done
autonomously by a sophisticated control system on the quadcopter in
order to stay perfectly balanced. A quadcopter has four controllable
degrees of freedom:Yaw, Roll, Pitch, and Altitude. Each degree of
freedom can be controlled by adjusting the thrusts of each rotor.
- Yaw (turning left and right) is controlled by turning up the speed
of the regular rotating motors and taking away power from the counter
rotating; by taking away the same amount that you put in on the regular
rotors produces no extra lift (it won’t go higher) but since the counter
torque is now less, the quadrotor rotates as explained earlier.3.-
control becomes a matter of which motor gets more power and which one
gets less.
- Roll (tilting left and right) is controlled by increasing speed on one motor and lowering on the opposite one.
- Pitch (moving up and down, similar to nodding) is controlled the
same way as roll, but using the second set of motors. This may be kinda
confusing, but roll and pitch are determined from where the “front” of
the thing is, and in a quadrotor they are basically interchangeable; but
do take note that you have to decide which way is front and be
consistent or your control may go out of control.
For example, to roll or pitch, one rotor’s thrust is decreased and
the opposite rotor’s thrust is increased by the same amount. This causes
the quadcopter to tilt. When the quadcopter tilts, the force vector is
split into a horizontal component and a vertical component. This causes
two things to happen: First, the quadcopter will begin to travel
opposite the direction of the newly created horizontal component.
Second, because the force vector has been split, the vertical component
will be smaller, causing the quadcopter to begin to fall. In order to
keep the quadcopter from falling, the thrust of each rotor must then be
increased to compensate.
This illustrates how the adjustments made for each degree of freedom
must work together to achieve a desired motion. Now, building and flying
a quadrotor from a remote control is simple and fun and stuff, but
people noting the inherently stable flight (in theory with equal speed
of the motors the thing keeps itself level) and ease of control (only
three functions and they are all basically take speed from one and put
in the other), people love to make them autonomous (flies itself) and
semi-autonomous (at least keeps itself level by responding to
disturbances and error).
Quadcopter Components Introduction
There are sensors connected to a microcontroller to make the decision
as to how to control the motors. Depending on how autonomous you want
it to be, one or more of these sensors are used in combination.
In this section, I will talk about these essential quadcopter components:
- Frame – The structure that holds all the components together. They need to be designed to be strong but also lightweight.
- Rotors – Brushless DC motors that can
provide the necessary thrust to propel the craft. Each rotor needs to be
controlled separately by a speed controller.
- Propeller
- Battery – Power Source
- IMU – Sensors
- Microcontroller – The Brain
- RC Transmitter
- Optional
Before we go into explaining how to choose each components, we can
take a look some quadcopters that people have built, and the parts they
used to get a rough idea. I didn’t build these planes, so I can’t
guarantee their performance.
Multicopter Examples Page
Frame
Frame is the structure that holds all the components together. The
Frame should be rigid, and be able to minimize the vibrations coming
from the motors.
A QuadCopter frame consists of two to three parts which don’t necessarily have to be of the same material:
- The center plate where the electronics are mounted
- Four arms mounted to the center plate
- Four motor brackets connecting the motors to the end of the arms
Most available materials for the frame are:
- Carbon Fiber
- Aluminium
- Wood, such as Plywood or MDF (Medium-density fibreboard)
Carbon fiber is most rigid and vibration absorbent out of the three materials but also the most expensive.
Hollow aluminium square rails is the most popular for the
QuadCopters’ arms due to its relatively light weight, rigidness
and affordability. However aluminium could suffer from motor vibrations,
as the damping effect is not as good as carbon fiber. In cases of
severe vibration problem, it could mess up sensor readings.
Wood board such as MDF plates could be cut out for the arms as they
are better at absorbing the vibrations than aluminium. Unfortunately the
wood is not a very rigid material and can break easily in quadcopter
crashes.
Although it is not as important as for the arms which of the three
material to use for the center plate, plywood is most commonly seen
because of its the light weight, easy to work with and good vibration
absorbing features.
As for arm length, the term “motor-to-motor distance” is sometimes
used, meaning the distance between the center of one motor to that of
another motor of the same arm in the QuadCopter terminology.
The motor to motor distance usually depends on the diameter of
the propellers. To make you have enough space between the propellers and
they don’t get caught by each other.
[Give example]
Brushless Motors
A little background of Brushless motor. They are a bit similar to
normal DC motors in the way that coils and magnets are used to drive the
shaft. Though the brushless motors do not have a brush on the shaft
which takes care of switching the power direction in the coils, and this
is why they are called brushless. Instead the brushless motors have
three coils on the inner (center) of the motor, which is fixed to the
mounting.
On the outer side it contains a number of magnets mounted to a
cylinder that is attached to the rotating shaft. So the coils are fixed
which means wires can go directly to them and therefor there is no need
for a brush.
Generally brushless motors spin in much higher speed and use less
power at the same speed than DC motors. Also brushless motors don’t lose
power in the brush-transition like the DC motors do, so it’s more
energy efficient.
Brushless motors come in many different varieties, where the size and
the current consumption differ. When selecting your brushless motor you
should take care of the weight, the size, which kind of propeller you
are going to use, so everything matches up with the current
consumption. When looking for the brushless motors you should notice the
specifications, especially the “
Kv-rating“.
The Kv-rating indicates how many
RPMs (Revolutions
per minute) the motor will do if provided with x-number of volts. The
RPMs can be calculated in this way: RPM=Kv*U An easy way to calculate
rating of motor you need, check out the online calculator
eCalc. It’s an amazing tool that helps you decide what components to purchase depending on the payload that you want to carry.
Make sure you buy the counter-rotating to counteract the torque effect of the props.
I have written a more
complete guide on how to choose Motor and propeller.
Propellers
On each of the brushless motors there are mounted a propeller.
You might not have noticed this on the pictures, but the 4 propellers
are actually not identical. You will see that the front and the back
propellers are tilted to the right, while the left and right propellers
are tilted to the left.
Like I mentioned before, 2 rotors rotates in the opposite directions
to the other two to avoid body spinning. By making the propeller pairs
spin in each direction, but also having opposite tilting, all of them
will provide lifting thrust without spinning in the same direction. This
makes it possible for the QuadCopter to stabilize the yaw rotation,
which is the rotation around itself.
The propellers come in different diameters and pitches (tilting). You
would have to decide which one to use according to your frame size, and
when that decision is made you should chose your motors according to
that. Some of the standard propeller sizes used for QuadCopters are:
- EPP1045 10 diameter and 4.5 pitch this is the most popular one, good for mid-sized quads
- APC 1047 10 diameter and 4.7 pitch much similar to the one above
- EPP0845 8 diameter and 4.5 pitch regularly used in smaller quads
- EPP1245 12 diameter and 4.5 pitch used for larger quads which requires lot of thrust
- EPP0938 9 diameter and 3.8 pitch used in smaller quads
Aerodynamics is just way too complex for non-academic hobbyists. It’s
even unlikely we can explain all that theory stuff in a few words. But
in general when selecting propellers you can always follow these rules:
- The larger diameter and pitch the more thrust the propeller can
generate. It also requires more power to drive it, but it will be able
to lift more weight.
- When using high RPM (Revolutions per minute) motors you should go
for the smaller or mid-sized propellers. When using low RPM motors you
should go for the larger propellers as you can run into troubles with
the small ones not being able to lift the quad at low speed.
Analysis of Propeller Pitch, Diameter, and RPM
Pitch VS Diameter: the diameter basically means area while pitch
means effective area. So with the same diameter, larger pitch propeller
would generate more thrust and lift more weight but also use more power.
A higher RPM of the propeller will give you more speed and
maneuverability, but it is limited in the amount of weight it will be
able to lift for any given power. Also, the power drawn (and rotating
power required) by the motor increases as the effective area of the
propeller increases, so a bigger diameter or higher pitch one will draw
more power at the same RPM, but will also produce much more thrust, and
it will be able to lift more weight.
In choosing a balanced motor and propeller combination, you have to
figure out what you want your quadcopter to do. If you want to fly
around stably with heavy subject like a camera, you would probably use a
motor that manages less revolutions but can provide more torque and a
longer or higher pitched propeller (which uses more torque to move more
air in order to create lift).
ESC – Electronic Speed Controller
The brushless motors are multi-phased, normally 3 phases, so direct
supply of DC power will not turn the motors on. Thats where the
Electronic Speed Controllers (ESC) comes into play. The
ESC generating three high frequency signals with different but
controllable phases continually to keep the motor turning. The ESC is
also able to source a lot of current as the motors can draw a lot of
power.
The ESC is an inexpensive motor controller board that has a battery
input and a three phase output for the motor. Each ESC is controlled
independently by a PPM signal (similar to PWM). The frequency of the
signals also vary a lot, but for a Quadcopter it is recommended the
controller should support high enough frequency signal, so the motor
speeds can be adjusted quick enough for optimal stability (i.e. at least
200 Hz or even better 300 Hz PPM signal). ESC can also be controlled
through I2C but these controllers are much more expensive.
When selecting a suitable ESC, the most important factor is
the source current. You should always choose an ESC with at least 10 A
or more in sourcing current as what your motor will require. Second most
important factor is the programming facilities, which means in some ESC
you are allowed to use different signals frequency range other than
only between 1 ms to 2 ms range, but you could change it to whatever you
need. This is especially useful for custom controller board.
Battery
As for the power source of the quadcopter, I would recommend LiPo
Battery because firstly it is light, and secondly its current ratings
meet our requirement. NiMH is also possible. They are cheaper, but it’s
also a lot heavier than LiPo Battery.
Battery Voltage
LiPo battery can be found in a single cell (3.7V) to in a pack of
over 10 cells connected in series (37V). A popular choice of battery
for a QuadCopter is the 3SP1 batteries which means three cells connected
in series as one parallel, which should give us 11.1V.
Battery Capacity
As for the battery capacity, you need to do some calculations on:
- How much power your motors will draw?
- Decide how long flight time you want?
- How much influence the battery weight should have on the total weight?
A good rule of thumb is that you with four EPP1045 propellers and
four Kv=1000 rated motor will get the number of minutes of full throttle
flight time as the same number of amp-hours in your battery capacity.
This means that if you have a 4000mAh battery, you will get around 4
minutes of full throttle flight time though with a 1KG total weight you
will get around 16 minutes of hover.
Battery Discharge Rate
Another important factor is the discharge rate which is specified by
the C-value. The C-value together with the battery capacity indicates
how much current can be drawn from the battery.
Maximum current that can be sourced can be calculated as:
MaxCurrent = DischargeRate x Capacity
For example if there is a battery that has a discharge rate of 30C
and a capacity of 2000 mAh. With this battery you will be able to source
a maximum of 30Cx2000mAh = 60A. So in this case you should make sure
that the total amount of current drawn by your motors won’t exceed 60A.
IMU – Inertial Measurement Unit
The Inertial Measurement Unit (IMU) is an electronic sensor device
that measures the velocity, orientation and gravitational forces of the
quadcopter. These measurements allow the controlling electronics to
calculate the changes in the motor speeds.
The IMU is a combination of the 3-axis accelerometer and
3-axis gyroscope, together they represent a 6DOF IMU. Sometimes there is
also an additional 3-axis magnetometer for better Yaw stability (in
total 9DOF).
How does IMU work
The accelerometer measures acceleration and also force, so the
downwards gravity will also be sensed. As the accelerometer has three
axis sensors, we can work out the orientation of the device.
A gyroscope measure angular velocity, in other words the rotational speed around the three axis.
Using Only Accelerometer?
With the accelerometer alone, we should be able to measure the
orientation with reference to the surface of earth. But the
accelerometer tends to be very sensitive and unstable sometimes, when
motor vibration is bad, it could mess up the orientation. Therefore we
use a gyroscope to address this problem. With both the accelerometer and
gyroscope readings we are now able to distinguish between movement and
vibration.
Using Only Gyroscope?
Since the gyroscope can tell us the rotational movement, why can’t we just use the gyroscope alone?
The gyroscope tends to drift a lot, which means that if you start
rotating the sensor, the gyroscope will output the angular velocity, but
when you stop it doesn’t necessarily go back to 0 deg/s. If you then
just used the gyroscope readings you will get an orientation that
continues to move slowly (drifts) even when you stopped rotating the
sensor. This is why both sensors has to be used together to calculate a
good and useful orientation.
Magnetometer
The accelerometer cannot sense yaw rotation like it can with roll and pitch, and therefore a magnetometer is sometimes used.
A magnetometer measures the directions and strength of the magnetic
field. This magnetic sensor can be used to determine which way is south
and north. The pole locations are then used as a reference together with
the Yaw angular velocity around from the gyroscope, to calculate a
stable Yaw angle.
I am trying to keep the theory and maths minimal here, and I will go into more detail in the next couple of tutorials.
Buying an IMU
These three sensors are available individually on the market. But it
is easier for development to get an IMU sensor board with the first two
sensors (6DOF) or all three sensors (9DOF).
The raw sensor boards can communicate with the microcontroller via
I2C or analogue. Digital boards that support I2C is easier and faster
for development, but Analogue ones are cheaper.
There are even complete IMU units with processor available. Usually
the processor is a small 8-bit microprocessor which runs computations
some kind of algorithms to work out the Pitch, Roll and Yaw. The
calculated data will then be put out on a serial bus or sometimes also
available by I2C or SPI.
The choice of IMU is going to narrow down what type of controller
board you can use. So before purchasing an IMU boards you should find
out information about the controller boards. Some controller boards even
comes with built-in sensors.
Some commercially available IMU sensors boards:
IMU with processor:
Flight Controller – Controlling electronics
You can either buy a controller board that is specially designed
for quadcopter or buy all the parts and assemble one yourself. Some of
the controller boards already contain the required sensors while other
requires you to buy these on a separate board.
Here is a comprehensive list of ready to go flight controller boards:
http://robot-kingdom.com/best-flight-controller-for-quadcopter-and-multicopter/
The AeroQuad MEGA Shield The AeroQuad board is a shield for the
Arduino, either the Arduino UNO or the Arduino MEGA. The AeroQuad board
requires the Sparkfun 9DOF stick which is soldered to the shield.
The ArduPilot board contains an ATMEGA328, the same as on the Arduino
UNO. Like the AeroQuad shield this board doesn’t contain any sensors
either. You would have to buy the ArduIMU and connect it to the board to
use it.
The OpenPilot is a more advanced board which contains a 72MHz ARM
Cortex-M3 processor, the STM32. The board also includes a 3-axis
accelerometer and 3-axis gyroscope. Together with the board comes a
great piece of software for the PC to calibrate, tune and especially set
waypoints for your QuadCopter if you have installed a GPS module which I
will be talking more about in the next section.
Make You Own Quadcopter Controller Board
Alternatively you can also use general purpose microcontroller, such as Arduino.
[Coming soon]
RC Transmitter
QuadCopters can be programmed and controlled in many different ways
but the most common ones are by RC transmitter in either Rate
(acrobatic) or Stable mode. The difference is the way the controller
board interprets the orientations feedback together with your RC
transmitter joysticks.
In Rate mode only the Gyroscope values are used to control the
quadcopter. The joysticks on your RC transmitter are then used to
control and set the desired rotation speed of the 3 axes, though if you
release the joysticks it does not automatically re-balance. This is
useful when doing acrobatics with your quadcopter as you can tilt it a
bit to the right, release your joysticks, and then your quadcopter will
keep that set position.
For the beginners the Rate mode might be too difficult, and you
should start with the Stable mode. All the sensors are used to determine
the quadcopters orientation in the stable mode. The speed of the 4
motors will be adjusted automatically and constantly to keep the
quadcopter balanced. You control and change the angle of the quadcopter
with any axis using the joystick. For example to go forward, you can
simply tilt one of the joysticks to change the pitch angle of the
quadcopter. When releasing the joystick, the angle will be reset and the
quadcopter will be balanced again.
Check here for a
more detailed RC transmitter article.
Optional Components
After buying all the necessary parts, and you are still not broke,
you might consider other popular optional components such as GPS
modules, ultrasonic sensors, barometers etc. They can enhance the
performance of your quadcopter, and bring more features.
A GPS module talks to the satellite and retrieve accurate location
information. We can use this information to calculate speed and path. It
is especially useful for autonomous quadcopters which needs to know its
exact position and which way to fly.
An ultrasonic sensor measures the distance to the ground, i.e.
altitude. This is useful if you want to keep your quadcopter a certain
distance from the ground without having to adjust the height it’s flying
at constantly yourself. Most of these sensors has a range between 20cm
to 7m.
When you gets higher, you might want to use a barometer. This sensor
measures humidity and pressure to work out the altitude, so when the
quadcopter is close to the ground (where these two factors doesn’t
change much), it becomes ineffective. Therefore it is also common to use
both of them at the same time.
Conclusion
Hopefully this article has given you a better understanding what each
part of the quadcopter does, and how to go about selecting the right
product for your quadcopter.
Please do not hesitate writing a comment or giving us some feedback
on this article. The next post will be about the software side of the
quadcopter.
If you are into FPV and Video taking, you might find this
collection of FPV videos interesting
