The accelerometer is a device that is used to measure forces experienced by a body.
Accelerometers generally contain four basic elements
· a case, which houses the other elements
· a mass, sometimes referred to as the proof mass
· a suspension or restraint, which maintains some relationship between the mass and the instrument case a pickoff, which produces a signal output related to the acceleration experienced.
There exist many differing types of accelerometer which have been designed to combat the causes of error growth and to take advantage of governing law principles which allow the accelerometer to function to its full capacity. They all operate according to Newton's Second Law of motion which relates the force incident upon its mass to its acceleration. Accelerometers are incapable of differentiating the forces that are applied to it so the inertial force that is related to its position is combined with the other forces such as gravity. Some accelerometers are also susceptible to measuring forces as a result of rotational accelerations.
No accelerometer is perfect and each has its own strengths and weaknesses in certain situations, all of which can have an effect upon the operation of an integrated INS/GPS System.
The gyroscope is the name given to a wide range of sensors that are used to provide information about attitude, attitude rates and specific force. The methods employed, construction and governing principles used by gyroscopes to obtain information varies widely. This can be attributed to the widespread use of gyroscopes in a number of applications in many differing fields. The variations in construction and operation determine the characteristics of the gyroscope. The characteristics that are important to implementation within INS are the error growth, sensitivity and range of input the gyroscope can interpret per unit of time, known as the dynamic range.
The most basic and original forms of gyroscope make use of the inertial properties of a rotating mass, these are known as conventional or mechanical gyroscopes. Optical rotation rate sensors are a more recent technology that operates using a different principle to that of the conventional gyroscope. In optical gyroscopes the properties of electromagnetic radiation are used, rather than a rotating mass, in order to sense rotation.
As with accelerometers the gyroscope technology has been able to create gyroscopes that exhibit traits that are beneficial to certain applications. Optical sensors can handle large fast rotation rates and a high dynamic range but is less than ideal in situations which have low rotation rates and require high sensitivity to small rotations, in which case conventional gyroscopes are superior.