Accelerometers used to be expensive devices used in aircraft and missiles but now they are very cheap devices found everywhere – for example in your mobile phone. Their use in a mobile phone is not as critical as part of an aircraft navigation system and the accuracy and cost reflect that, with 3 axis devices costing less than a dollar even in small quantities. Due to MEMS (Micro Electro-Mechanical Systems) you can now buy low cost accelerometers that are made directly on Silicon, so using the low cost manufacturing facilities used for making normal integrated circuits (with some modifications).
Before MEMS there were several ways of making an accelerometer but they tended to use the same principle – a proof mass attached to a support and its effect on that support is measured, for example with a strain gauge. Such a structure will be resonant so they need a damping system. Also, gravity will produce a continuous 1g (9.81m/s) acceleration in one direction which is why accelerometers always have a full scale range of 1g or more. MEMS accelerometers are similar and work by having a small proof mass which displaces a micro-machined structure and that movement is measured, often by capacitive means. The devices have internal damping from the air trapped in the IC cavity or a fluid could be used.
The Analog Devices ADXL50 is shown below.
An analog accelerometer will simply have a voltage output such as the NXP FXLN83xx.
You must add capacitance to the outputs to filter them (see datasheets for details).
Digital accelerometers are more common and actually cheaper such as the STMicroelectronics LIS3DE which is only $0.62. It even has an auxiliary 10 bit 3 channel ADC built in, but the accelerometer data is only 8 bit. A 16 bit version would cost around $1.10 and doesn’t have the auxiliary ADC. You will typically have an SPI or I2C interface.
You need to bear in mind the different resolution available when choosing a digital accelerometer (from 8 bit to 16 bit) but also the zero g offset which can be quite significant, for example 0.1g which is 5% of full scale on a 2g accelerometer. Even 16 bit accelerometers can have a significant zero offset, amounting to maybe 40 LSB (least significant bit) counts.
Some devices have a high pass filter which can remove any offset but at the expense of not being able to measure steady acceleration. You will not detect static gravity and hence orientation, only changes in acceleration so the use of high pass filters is limited to a few applications.
Typical bandwidths will be typically 1kHz or so and can be unequal so you might have a lower bandwidth in one axis (typically the Z-axis). However, for the typical use of accelerometers such as monitoring human movement, the bandwidth is unlikely to be a limiting factor. With an airbag sensor you will be more concerned about the response time because you cannot wait half a second for an air bag to trigger, but again, the bandwidth shouldn’t be an issue because you would be choosing a device certified for safety critical automotive applications if you were designing an airbag trigger so that would restrict your choice to devices which will also have a suitable bandwidth/speed.
These images are of the Analog Devices ADXL202 in their Analog Dialogue article “Using MEMS Accelerometers as Acoustic Pickups in Musical Instruments” which also has some other useful information about the structure of MEMS accelerometers. It shows the sensing element in the sensor and the processing and interface electronics around it.
Accelerometers typically have 10000g shock survivability and if plastic packaged they are molded with a protective cavity to allow the sensor to move slightly and avoid contact between the plastic and the sensing element.
So, what would you use accelerometers for? You probably already have a couple or more in your possession – one in the airbag of your car and another in your smartphone or tablet for changing the display orientation when you rotate the device. They can also be used on pedometers; gaming (such as Wii) or virtual reality devices; as an input device in place of piezo where the input device is tapped or struck; vibration monitoring for industrial applications; motion detection for power saving in electronics devices and automatic wake-up; seismology; navigation; free fall detection to protect disk drives; car stability control; image stabilization and orientation detection in cameras, and drone stabilization. They seem to be everywhere and are low cost when you consider the complex technology involved. It opens up the possibility of new applications where they hadn’t been considered before. Also, their use in consumer products in particular smartphones, tablets and cars, has really helped to bring the price down from something which would have been a very expensive part 20 years ago. With low power, low voltage operation and digital interfaces they are simple to use. In the past accelerometers were large, heavy devices which could not have found their way into a smartphone. There are numerous manufacturers – simply look at one of the main distributors to find NXP, Analog Devices, STMicroelectronics, Murata, Bosch, Epson etc.