The everyday definition for an accelerometer is a device which measures the acceleration of a system or its rate of change of velocity. Something which is increasing or decreasing its velocity or changing direction over a given time interval is accelerating. The devices then which measure this are accelerometers. Another name for an accelerometer is a g-meter, measuring g-forces. A g-force is the force caused by gravity, 9.81 m/s^2. Highly accurate g-meters can detect variances in Earth's gravity at different locations; this has been used in mining applications in finding deposits of minerals.
An interesting fact about accelerometers which are not designed to be g-meters is that an accelerometer in free-fall will always read zero. In contrast, a g-meter is calibrated so that when it experiences no gravity, it reads negative one g. This is a result of two forces acting on the accelerometer in question; the very real force of gravity and the "imaginary" force of acceleration caused by gravity. The latter force is considered imaginary because it is really a phenomenon caused by Newton's laws: an object in motion wants to stay in motion at it's given speed. It's no different from when you're in a car; you speed up and you slam against the seat, you break and you fly forward towards the windshield.
Some Basic Commercial Designs:
Capacitive: A capacitive accelerometer measures acceleration as a result of the delta capacitance in a capacitor as a result of acceleration.
Piezoelectric: A piezoelectric accelerometer creates an electric charge proportional to the amount of stress applied to a certain material (such as crystals). This allows acceleration to be known based on the voltage created by the stress.
Hall Effect: The acceleration of the system causes a change in voltage as a result of the changing magnetic field around the accelerometer.
Piezoresistive: Works in the same way as a piezoelectric accelerometer except that it measures resistance rather than voltage.
Heat Transfer: Changes in temperature inside of the accelerometer are a result of acceleration. The acceleration of the system causes heat transfer inside of the accelerometer.
Magnetoresistive: Works in the same way as the Hall Effect accelerometer except that it measures resistance rather than voltage.
MEMS-Based: (Micro-Electro Mechanical System) Allows for the measurement of acceleration on the micrometer scale.
Spring-Mass: A spring-mass system is able to detect acceleration due to the restorative force the spring puts on the mass.
Pendulum: A pendulum designed to be an accelerometer is able to detect acceleration as a result of the angle a string or rod with a mass on the end makes with respects to gravity as the system the pendulum is in accelerates. As the system accelerates or decelerates, the mass will pull the pendulum backward (acceleration) or forward (deceleration).
Accelerometer Applications: Accelerometers are generally used as safety measures in certain situations or to collect and analyze data for some other purpose.
Simultaneous 3-D Accelerometers: Three accelerometers arranged perpendicular to each other in a system are able to measure acceleration in any direction in space through the use of mathematical formulas based on the acceleration in each of the three directions.
1-D and 2-D Motion: 1-D accelerometers measure the change in velocity along one axis, with respects to the orientation of the accelerometer; their 2-D counterparts simply measure acceleration on a plane rather than a single axis. The many applications of these commonly used accelerometers are listed below.
Tilting or Rolling: To know the change in direction of a system due to tilting or rolling, an accelerometer can be used. Acceleration caused by a change in direction is the basic principle behind satellites and other systems which periodically or continuously change direction.
Vibration: The vibration of a bridge or machine or some other device is important in knowing how long the device will last under such strains. Tuners for musical instruments are also used to measure vibrations at high frequencies; these contain accelerometers to do such a task. Accelerometers measuring vibration are similar to those measuring tilt, just on a smaller scale.
Feedback Loops: Any system which requires knowing the acceleration of the system as a whole; a rocket or a robot use accelerometers to know when to do something.
Automobile Performance: The measure of acceleration can be useful in preventing skidding as well as determining when an impact has occurred to know when to employ the air-bags.
Laptops: Knowing when a laptop or other electronic device is in free-fall is important so as to know when to shut down the hard-drive in order to protect it from electrical shock on impact.
The Future of Accelerometers: The future of accelerometers is in nano-technology. Producing smaller and more accurate accelerometers is the key to making them viable in certain applications, specifically in small devices. In the case of electromechanical accelerometers, advancement has slowed down due to their already highly accurate performance abilities which do not require much further accuracy in most situations. It is lower performance ones which are advancing rapidly, in attempts to produce simpler and more cost effective accelerometers for the applications they are being used. This expansion is mainly directed for the commercial market rather than scientific and military applications. Ipods, laptops, the Wii and other commercial electronics are the main focus for further development rather than the already highly accurate accelerometers used in missiles, laboratories and other government applications.
Sources:
Daisley, M. Patrick. “Dual Spring Accelerometer.” 1998. http://physicsday.net/DualSpring.pdf.
Doherty, Paul. “Accelerometer.” 1999. http://www.exo.net/~pauld/books/car_science/accelerometer.html.
eFunda. “Accelerometer.” 2009. http://www.efunda.com/formulae/vibrations/sdof_eg_accelerometer.cfm.
Johnson, D. Curtis. Process Control Instrumentation Technology. Prentice Hall. New York, New York. August 27, 2008.
SENSR. “Selecting an Accelerometer.” 2008. http://www.sensr.com/.
Society of Robots. "SENSORS - ACCELEROMETER." How to Build a Robot Tutorial - Society of Robots. 2005. http://www.societyofrobots.com/sensors_accelerometer.shtml.
“Build an Accelerometer”. http://library.thinkquest.org/2745/data/meter.htm.