Also known as inertial sensors, accelerometers are critical elements in key applications such as automotive air-bag deployment, smartphone motion tracking, and industrial predictive maintenance. These varying needs have resulted in an even more varied array of accelerator products from which designers can choose. Fortunately, by focusing on a handful of key decisions, developers can quickly zero in on the right kinds of devices for their application.
Despite the multitude of options available, accelerometers are all based on the same basic principle: inertia. A proof mass within the accelerometer’s structure can readily move in at least one dimension. Because of inertia, that proof mass will tend to stay in place when the surrounding structure undergoes acceleration (i.e., changes its motion) along that same direction. Sensing systems within the accelerator detect the proof mass’s movement relative to the surrounding structure, and interface circuits deliver a corresponding signal to the outside. A spring of some kind provides a restoring force to return the proof mass to its initial position once the acceleration has ended.
Fig. 1: Accelerometers leverage the inertia of an internal proof mass to sense changes in motion.
The many variations among commercial accelerometers are the result of differing mechanical and material designs for the proof mass and surrounding structure, the choice of sensing technique, and the type of signal that the interface provides. Options such as the use of microelectromechanical system (MEMS) structures or the choice of capacitive versus piezoelectric sensing offer various pros and cons.
In practice, however, these choices are not, in themselves, particularly relevant to designers. What is important to designers is the performance result that the vendor achieves from its choices among these options. Key performance specifications for designers include measurement range, sensitivity, precision, and accuracy, along with a variety of operating characteristics.
When choosing an accelerometer, then, designers need to first think through their application’s needs. How much acceleration will their device normally experience? What extremes might it see? What is the operating environment like? Are there dimensional or mounting constraints? What kind of interface is needed, analog or digital?
Answering these kinds of questions first will make it easier to narrow down candidates. Developers should always bear in mind, too, that everything on or near the planet is continually undergoing an acceleration of 1 g (9.8 m/s 2 ) toward Earth’s center, creating measurement offset in that direction.
With application needs in mind, a place to begin sifting through the many options is with the functional parameters: number of axes, range, and mounting. The first functional decision that developers need to make is how many axes, or orthogonal directions, the accelerometer must sense. Devices are available for one- (X or Z), two- (X-Y), and three-axis (X-Y-Z) sensing, with the X-Y plane generally referring to the device’s mounting surface. There are also accelerometers such as the TDK/InvenSense ICM-20600 that are described as six- or nine-axis devices, but these are not just accelerometers. A six-axis device typically includes gyroscopic sensing of rotation in the three linear axes, and a nine-axis device also includes magnetic field sensing in the three linear axes.
In general, a price-constrained application will use only as many sensing axes as the application requires to save cost in both the sensor and the electronics that convert the sensor signal into a useful measurement. A safety monitor on an elevator, for instance, needs to sense only in the vertical direction. A tilt monitor, on the other hand, needs two dimensions — X and Y — to determine the angle between the sensing plane’s vertical (Z) and the pull of gravity.
Applications requiring full three-axis sensing, such as determining a system’s orientation in space, can be served using a single X-Y-Z accelerometer or a combination of one- and two-axis sensors as cost and placement needs dictate.
Three-axis sensors used in smartphones and automobiles are an exception to the cost generalization, though. Volume production has driven cost down for such sensors, making them possibly the least expensive option for certain applications. Typically, however, they operate in the low sensing range.
Fig. 2: Their inclusion in consumer devices such as smartphones and automobiles has dropped the size and price of three-axis accelerometer chips such as this IAM-20381 from TDK/InvenSense. (Image: TDK/InvenSense)
Sensing range is the second key functional decision that designers need to make. Broadly described as low, medium, or high, the sensing range for accelerometers is specified in multiples of g — the acceleration of gravity — and generally is symmetric around zero and the same for all axes.
Accelerometers used in smartphones fall in the low range for accelerometers, typically ±3 g or so, as they are concerned primarily with human movement. Sensors for monitoring machinery may be more demanding and need the wider medium range of about ±30 g.
For the most aggressive types of movement, devices with a high range are also available like the STMicroelectronics H3LIS331DL , capable of sensing ±400 g. Often, devices are available in families encompassing varying ranges. For instance, the Analog Devices ADXL344 family offers ±2-g, 4-g, 8-g, and 16-g options. A rough rule of thumb is to choose a range more than twice the maximum acceleration expected to allow for unexpected conditions.