An accelerometer is an instrument that measures constant acceleration. Constant acceleration is that of a vehicle at its absolute rest point; this is clearly different from GPS or inertial acceleration, which is the increase in velocity in a fixed coordinates system. The accelerometer does not require a data source in order to function; it computes acceleration directly by measuring changes in acceleration from the position of the vehicle to the time the vehicle is stationary. There are basically three types of accelerometers: passive systems using mechanical or spring balances, active systems using battery power and hybrid systems using electric motors.
The mechanical accelerometer consists of two parts: a mass sensor and a tachometer. Mass sensors determine the weight of the vehicle by measuring the force of rotation that the meter applies to the body. Tachometers determine the velocity by measuring the force of attraction that the meter applies to the vehicle. In addition, the mass sensor can also measure transmission of motor torque to the wheel. Depending on your needs you will choose between a hybrid accelerometer or an electronic one.
A passive accelerometer has only a motor as the source of acceleration; thus the name passive. This type is used more often in automobiles. It uses a pinion or rack and pinion gear; this gear changes the amount of force applied to the accelerator. Most of them are made of high precision engineering plastics. These devices are very accurate and they are extremely durable, making them suitable for use in rugged conditions.
Another type of accelerometer is the piezoelectric accelerometer. Piezoelectric means ‘with Piezo’ that is when it contains a charge. Capacitive accelerometers are commonly used in hand held devices such as phones, PDAs, tablet computers and gaming consoles. The advantage of capacitive is that it does not require mechanical motion of the finger to be effective; instead the electrical signal provided is an effective indication of acceleration. As such, this type of accelerometer can offer a faster response time and greater sensitivity than its piezoelectric counterpart.
Vibration sensitive accelerometers measure the change in capacitance across a capacitance series. The change in capacitance is proportional to the vibration frequency of the device. Therefore, they measure acceleration or deceleration at various speeds relative to a constant potential value. They are most commonly used for measuring vibration.
There are two types of vibratory measurement apparatus – electrical and electromagnetic. Electrical device vibrates when there is an electric field across it. Electromechanical device vibrates with a change in voltage across it. A common application of these two devices is vibration sensing. There are many types of vibration measuring device available in the market today; however all of them are used to measure acceleration forces.