An accelerometer is an instrument for measuring acceleration. There are two types, one for measuring linear (straight-line) accelerations, the other for measuring angular (twisting) accelerations.
Accelerometers may also be used to measure deceleration, such as the braking of a car. They are much used in the motor industry both to measure the forward acceleration and deceleration of a car, and to measure the sideways and up-and-down accelerations caused by cornering and bumpy roads.
Other uses include testing the strength of safety belts (by measuring the deceleration force at which they break when carrying a known load) and studying vibrations in the hulls of ships and the wingtips of aircraft. They are also important components of inertial guidance systems.
The linear accelerometer usually contains a body of known mass attached to a coil spring and free to move only along the axis of the spring, which is a straight line down the middle of the coil. This line is called the sensitive axis of the accelerometer. In order to measure the acceleration of any moving object, the axis must be placed in line with the direction of movement.
The tension of the spring and the weight of the body on its end are adjusted so that when a known force is applied to the accelerometer along the sensitive axis, the body moves a known distance along the spring. The body is either connected to a dial, or its movement is detected electronically.
The dial is calibrated by placing the accelerometer on a body of known acceleration and marking the position of the indicator. If this is repeated with various known accelerations, the marks will provide enough information to allow the rest of the scale to be filled in.
Linear acceleration can be measured in various units. Moderate forces are generally measured in units such as feet or meters per second per second, written as ft/s2 or ft-2, which is the number of feet per second a body adds to its speed every second. Higher forces are generally measured in g units, that is, how many times greater they are than the force of gravity. One g, the acceleration of an object falling freely to the ground, is 32 ft/s2 or 9.8 m/s2, at the surface of Earth.
The angular accelerometer works on a similar principle, but is shaped differently so as to measure circular acceleration, such as might be given to the flywheel of an engine. The body of known mass makes the disc revolve, twisting the spring. Angular acceleration is measured in degrees/s2 or radians/s2 (27T radians being equivalent to 360°).