Hello. Today I’ll describe an additional
devices, which could be useful during quadrocopter’s flight.
1)
Accelerometer/Gyroscope
Nowadays, there’s increasing amount of both
devices “hidden” in one package, so I’ll describe it together.
Accelerometer is a kind of devices, which
measure the acceleration in different directions. The main force, which act on
us every time is gravity, but by moving something we can produce acceleration.
This force is visible for example during moving by cars or other means of
communication, and physics have described it well for hundred year.
Accelerometer can measure force acts on different directions, but most common
type is three-axis device. It measures acceleration like in Cartesian
coordinate system (z axis corresponds to gravity force). Also, if accelerometer
rest relative to the Earth's surface, it will indicate approximately 1 g force.
There’s a few kind of electrical accelerometers:
piezoelectric, piezoresistive, capacitance, and MEMS (micro electro-mechanical systems), and all of devices acts in different
way. In piezoelectric accelerometers every change of acceleration change a
charge of piezoelectric crystal. Changes can be observed by an implemented
circuit and then transmitted to the user. Piezoresistive devices change its
resistivity due to outer forces, and the Wheatstone bridge implemented in core
helps to detect it. Capacitance devices act by changing a capacitance of
built-in sensitive capacitor. It’s changing its charge depending on accelerate
force. Then, the very precise analog-to-digital converter reads this value
(after an amplification) and sends it to user. MEMS sensors works in the other
idea. It contains a small heater at the bottom of a very small dome, which
heats the air inside the dome to cause it to rise. A thermocouple on the dome
determines the levels of signal - temperature (like ADC before) and in this way
the acceleration is detected.
Accelerometers can be used to measure vibration
on cars, machines, buildings, process control systems and safety installations.
They can also be used to measure gravity, seismic activity, inclination,
machine vibration, dynamic distance and speed with or without the influence of
gravity. Nowadays, accelerometers are more and more popular in smartphones,
tablets and other kind of electronic amusement, where movement has a
significant influence of contest. There is a lot of software applications which
react on move of user (and there are usually connected with some gyroscope).
Nevertheless, now almost every new phone, tablet has accelerometer (it has also
gyroscope, which is described below).
Gyroscopes measure angle of rotation or
movement. Mathematically, it is easier
to works with accelerometer in Cartesian coordinates. Using gyroscope, it is
easier to use cylindrical coordinate system. Gyroscope is more complicated than
accelerometer, so if anyone is interested in: how it exactly works, please
refer into some more specialist literature. Electronic devices are usually
based on similar constructions like those mentioned earlier. Applications also
are similar.
Gyroscope also can act in three dimensions (due
to different coordinate system, there are not similar to accelerometer’s one,
but the main rule is exactly the same). And if our device has three axis
accelerometer and three axis gyroscope, we can call it 6-axis motion device.
Why am I describing accelerometer and
gyroscope? Because it is most important external device, which we will use in
our quadrocopter project. Now I can tell that it is important to build reliable
PID controller, and it is necessary during flight. We are using MPU-9150 from
Invensense. It is MEMS sensor, including accelerometer, gyroscope and
accelerometer and also can measure temperature . It is 9-axis motion device (3 axis for accelerometer, 3 axis for
gyroscope and 3 axis for magnetometer); it is very precise low-power sensor with
changeable ranges. It could communicate by I2C serial bus (100kHz and 400kHz). The
most important thing is, that it has implemented DMP (Digital Motion Processor)
with FIFO buffer and possibility of data fusion in few ways (for example: raw
data, quaternions, Euler angles etc.) and sending it. It is very interesting
device.
2)
GPS
Tracking
GPS (Global Positioning System) is well known
now, cause its popularity. It can be found in mobile phones, tablets,
navigation devices, buses and trams – it is everywhere! The main rule of
working such a system is comparing times of propagating and phases of
electromagnetic waves transmitted from space satellites. Then, some complicated
algorithms are able to determine the exact position. If anyone is interested
how it exactly works – please refer to some special literature.
Why could it be useful in quadrocopter? To
determine the position of vehicle in any time of flight (but please still
remember that GPS device cannot refresh with higher frequency than 10 Hz), and
then for example save it in memory (SD or MMC card) and display coordinates as
a points on map on personal computer.
3)
Altitude
meter
Altitude meter, or altimeter, is useful device
to determine approximately altitude of flight. In some cases, it depends of Above
Mean Sea Level, but not always. The
most reliable way to determine altitude in quad is to measure air pressure
differences (higher altitude=lower pressure) during the flight. The simplest
way to obtain this is using some barometer or other sensor which can measure
air pressure. Producers sometimes gives an equations to determine exact
altitude while pressure is changing. Approximately, air pressure
decreases with an increase of altitude—100 hectopascals per 800 meters. If we
had respectively sense device, we can achieve even less than 1 meter accuracy
of changing the altitude! I will not describe how barometer works, I can add
that in some cases GPS can determine altitude (if 4 or more satellites are
reachable in our reception, but it is not very precise way).
It is everything for today, in nearest future
we will show our progresses with more detail. We will publish some schematics,
PCB projects, how PS2 controller works and more details about PID controller
implemented in microcontroller. Stay tuned with us!
References:
More sophisticated links:
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