As the term itself suggests accelerometers are devices that have been built to measure acceleration amongst other principles of motion for bodies. By definition acceleration is the rate of change of velocity according to Newton's laws of motion. Apart from measuring acceleration only the accelerometers have been used to measure shock, vibration, inertia for application in vast industries. In the early usage of accelerometers they were mechanical & analogue but due to the advent of micro electronics they have been embedded in integrated circuits (ic's) and can be used in a variety of digital devices e.g. video game controllers for virtual reality input control i.e. in the Nintendo Wii.
As a consequence an accelerometer at rest relative to the Earth's surface will indicate approximately 1 g upwards, because any point on the earth's surface is accelerating upwards relative to a local inertial frame. To obtain the acceleration due to motion with respect to the earth, this "gravity offset" should be subtracted.
The reason for the appearance of a gravitational offset is Einstein's equivalence principle, which states that the effects of gravity on an object are indistinguishable from acceleration of the reference frame. When held fixed in a gravitational field by, for example, applying a ground reaction force or an equivalent upward thrust, the reference frame for an accelerometer (its own casing) accelerates upwards with respect to a free-falling reference frame. The effect of this reference frame acceleration is indistinguishable from any other acceleration experienced by the instrument.
An accelerometer will read zero during free fall. This includes use in a spaceship orbiting earth, but not a (non-free) fall with air resistance where drag forces reduce the acceleration until terminal velocity is reached, at which point the device would once again indicate 1 g acceleration upwards.
The principle behind their navigation application is called inertial navigation; from the physics term inertia is the tendency of a body to resist changes in motion. An inertial reference platform is used & consists of accelerometers & gyroscopes to calculate the position, orientation & velocity of a body using dead reckoning which is a principle used to calculate a the position of an object based on its previous position e.g. in animal navigation, used to track animals based on their previous habitat.
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.
At first, measuring tilt and acceleration doesn't seem all that exciting. However, engineers have come up with many ways to make really useful products using them.
An accelerometer can help your project understand its surroundings better. Is it driving uphill? Is it going to fall over when it takes another step? Is it flying horizontally or is it dive bombing your professor? A good programmer can write code to answer all of these questions using the data provided by an accelerometer. An accelerometer can help analyze problems in a car engine using vibration testing, or you could even use one to make a musical instrument.
In the computing world, IBM and Apple have recently started using accelerometers in their laptops to protect hard drives from damage. If you accidentally drop the laptop, the accelerometer detects the sudden freefall, and switches the hard drive off so the heads don't crash on the platters. In a similar fashion, high g accelerometers are the industry standard way of detecting car crashes and deploying airbags at just the right time.
What things should I consider when buying an accelerometer?
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 that you are interfacing the accelerometer with. 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 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.
If you are using a BASIC Stamp, or any other microcontroller with purely digital inputs, you will most likely need to go for a digital output accelerometer. The disadvantage here is that it requires you to use the timing resources of the microcontroller to measure the duty cycle, as well as performing a computationally intensive division operation.
Applications of Accelerometers
An accelerometer has been used in collaboration with a Richter scale to detect the severity of earth quakes.
Types of accelerometers
There is much different type of accelerometers and each has unique characteristics, advantages and disadvantages. The different types include:
A piezoelectric sensor (piezo sensor for short) is a device that uses the piezoelectric effect to measure pressure, acceleration, strain, or force by converting these factors to an electrical signal. Piezo sensors use a phenomenon called piezoelectricity, the ability of some materials (notably crystals and certain ceramics) to generate an electric potential in response to physical stress. What this means is that the material of a piezo sensor, usually a crystalline coating over metal, returns more current when bent or otherwise disturbed. This lets you detect very slight changes in the sensor, for example, a breeze or the touch of a finger.
Micro electronics use of accelerometers
Memsic 2125 Accelerometer
The Memsic 2125 (datasheet) is a two-axis accelerometer capable of measuring acceleration up to plus or minus 2g. It has a simple digital interface: two pins (one for each axis) emit pulses whose duration corresponds to the acceleration of that axis.
It can be used as an add-on module to the arduino board, to build devices that measure tilt, acceleration etc.
Conclusion
The accelerometer has proved to a widespread used device and has managed to uniquely set out its existence; it will likely be improved but will be more and more in different applications in engineering & science.
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