An accelerometer measures acceleration
(change in speed) of anything that it's mounted on. How does it work? Inside an
accelerator MEMS device are tiny micro-structures that bend due to momentum and
gravity. When it experiences any form of acceleration, these tiny structures
bend by an equivelent amount which can be electrically detected. Today,
accelerometers are easily and cheaply available, making it a very viable sensor
for cheap robotics hobbyists like you and me.
Watch
a demo I made of using a microcontroller and an accelerometer to control a
servo:
Applications
for Accelerometers
Accelerometers are very important in
the sensor world because they can sense such a wide range of motion. They're
used in the latest Apple Powerbooks (and other laptops) to detect when the
computer's suddenly moved or tipped, so the hard drive can be locked up during
movement. They're used in cameras, to control image stabilization functions.
They're used in pedometers, gait meters, and other exercise and physical
therapy devices. They're used in gaming controls to generate tilt data. They're
used in automobiles, to control airbag release when there's a sudden stop.
There are countless other applications for them.
Possible
uses for accelerometers in robotics:
Self balancing robots
Tilt-mode game controllers
Model airplane auto pilot
Alarm systems
Collision detection
Human motion monitoring
Leveling sensor, inclinometer
Vibration Detectors for Vibration Isolators
G-Force Detectors
Axis
of Acceleration
The
tiny micro-structures can only measure force in a single direction, or axis of
acceleration. This means with a single axis measured, you can only know the
force in either the X, Y, or Z directions, but not all. So if say your X-axis
accelerometer endowed robot was running around and ran into a wall (in the X
direction). Your robot could detect this collision. But if say another robot
rammed into it from the side (the Y direction), your robot would be oblivious
to it. There are many other situations where a single axis would not be enough.
It is always a good idea to have at least 2 axes (more than one axis).
Gravity
Gravity is an acceleration. A such, your
accelerometer will always be subject to a -9.81 m/s^2 acceleration (negative
means towards the ground). Because of this, your robot can detect what angle it
is in respect to gravity. If your robot is a biped, and you want it to always
remain balanced and standing up, just simply use a 2-axis accelerometer. As
long as the X and Y axes detect zero acceleration, this means your robot device
is perfectly level and balanced.
Accelerometers,
Rated G
When you buy your accelerometer, you will
notice it saying something like 'rated at 2g' or '3g accelerometer.' This is
the maximum g force your sensor can report. Gravity accelerates objects at 1g,
or 9.81 m/s^2. For example, if your robot is moving at 1g upwards, then that
means you sensor will detect 2g. For most robotics applications a 2g rating
will be fine. So why not just get the highest rating possible? The lower the
rating, the more sensitive it will be to changes in motion. You will always
have a more fine tuned sensor the lower the rating. But then again, more
sensitive sensors are more affected by vibration interference.
Calculate
Acceleration and Angle wrt Gravity
To calculate the magnitude of
acceleration for a
single-axis
accelerometer
acceleration_max = sqrt(x^2) = x
2-axis
accelerometer
acceleration_max = sqrt(x^2+y^2)
3-axis
accelerometer
acceleration_max = sqrt(x^2+y^2+z^2)
To
calculate the detected force on an accelerometer due to gravity:
Force_gravity = -g*cos(angle) (depends on
starting axis of sensor)
http://www.societyofrobots.com/images/sensors_accelerometer_angle.jpg
Chances are you would have no need to
measure the force, but if you reverse the equation you can calculate the angle
by knowing the detected force:
cos(sensor_value*conversion_constant /
-g)^-1 = angle
Availability
and Cost
The MEMS IC's are easily available and
very affordable. However they all require support circuitry and come as surface
mounts. I highly discourage buying an IC and doing your own wiring. However
there are many already setup accelerometer packages you can buy. For example,
Dimension Engineering has a great plug and play dual axis accelerometer which
requires no additional support circuitry. There are several other great sensors
out there, some as a 3-axis, and now some even with built in rotation sensor
gyros!
Wiring
Requirements
Any accelerometer package will have a
power and ground line, and a single output analog pin for each axis of
acceleration. Some of the sensors come with additional features/pins, read
their datasheets.
Additional
Tips and Uses
Placing an accelerometer on a mobile
robot that experiences bumps can trigger the accelerometer unintentionally. Use
a capacitor to smooth out output over several hundred milliseconds (testing
required) to prevent this. Also, read the interpret sensor data tutorial to
enhance your accelerator sensor accuracy.
