Acceleration is the amount of force needed to move a unit of mass. .
What is an accelerometer?
An accelerometer is an electromechanical device that measures acceleration forces. It senses how
much a mass presses on something when a force acts on it. These forces may be static, like the
constant force of gravity pulling at your feet, or they could be dynamic - caused by moving or
Types of accelerometers
There are many different types of accelerometers. The mechanical ones are a bit like scaled-down
versions of passengers sitting in cars shifting back and forth as forces act on them.
In piezoresistive accelerometers, the mass is attached to a potentiometer (variable resistor), a bit
like a volume control, which turns an electric current up or down according to the size of the force
acting on it
Capacitive accelerometers measure a force in a similar way. When a moving mass alters the
distance between two metal plates, measuring the change in their capacitance gives a measurement
of the force that's acting.
In piezoelectric accelerometers, piezoelectric crystals such as quartz do the clever work. You have
a crystal attached to a mass, so when the accelerometer moves, the mass squeezes the crystal and
generates a tiny electric voltage.
In Hall-effect accelerometers, a mass is unattached and free to move, like a ball in a cup, when the
accelerometer moves force is measured by sensing tiny changes in a magnetic field.
Semiconductor accelerometers have an electrical terminal with enough mass to move up and down
very slightly when you move or tilt the accelerometer - a bit like a bell ringer. The electrical terminal
sits between two electrodes separated by an air gap. The air gap between the two electrodes work
together as a capacitors. Moving the electrical terminal alters the distances between the electrodes,
measuring the changes in their capacitance gives a measurement of the force that's acting on it.
Analog vs digital - First and foremost, you must choose between an accelerometer with analog
outputs or digital outputs. This will be determined by the hardware interfacing with the
Analog style accelerometers output a continuous voltage that is proportional to acceleration. E.g.
2.5V for 0g, 2.6V for 0.5g, 2.7V for 1g.
Digital accelerometers usually use pulse width Pulse Width Modulation (PWM) for their output.
This means there will be a square wave of a certain frequency, and the amount of time the voltage is
high will be proportional to the amount of acceleration.
Sensitivity - The more sensitivity the better. This means that for a given change in acceleration, there
will be a larger change in signal. Since larger signal changes are easier to measure, you will get more
Bandwidth - This means the amount of times per second you can take a reliable acceleration
reading. For slow moving tilt sensing applications, a bandwidth of 50Hz will probably suffice. If you
intend to do vibration measurement, or control a fast moving machine, you will want a bandwidth of
several hundred Hz.
Impedance/buffering issues - This is by far the single most common source of problems in projects
involving analog accelerometers. Data sheets specify that for A-D conversion to work properly, the
connected device must have an output impedance under 10kO. Unfortunately, analog
accelerometers have an output impedance of 32kO. The solution to this is to use a low input offset
rail to rail op amp as a buffer to lower the output impedance.
By measuring the amount of static acceleration due to gravity, you can find out the angle the device
is tilted at with respect to the earth. By sensing the amount of dynamic acceleration, you can analyze
the way the device is moving. This is useful in cell phone, drone and robotic applications.
*** WARNING ACCELEROMETERS CAN BE HACKED ***
Resonant sound waves can induce fake motion into capacitive accelerometers. This make them
vulnerable to hacking. This poses a serious security threat because most devices inherently trust all
the data coming from accelerometers and don't feature protection systems to detect spoofed signals
coming from local sensors.
The principle behind how the attack works is simple. Accelerometers have a sensing mass that sits
on a series of springs. By sending finely-tuned sound waves to the accelerometer, the sound waves
induce controlled movement into
Sound waves used in these attacks could be embedded in songs played over a user's speakers. No
high-end speakers are needed, and regular $5 speakers could be used to replay the malicious