Understanding key functional needs will jumpstart the selection process

By Richard Quinnell, Editor, Special Projects, Technical

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/s2) 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.

0119_Feature_Designers-Guide-Accelerometers_Fig-2

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.

Sensing range
should not be confused with the accelerometer’s shock range. Operating outside of
the sensing range results only in distortion or clipping of the output signal.
The shock range is the maximum acceleration that the device can experience
without being damaged.

The accelerometer’s
packaging and mounting style are a third key functional choice for designers to
make. Many accelerometers are available in chip form for mounting on a circuit
board. Single-axis accelerometers can have the sensing direction either in the
chip’s mounting plane or normal to it. Two-axis devices typically sense in the
mounting plane only.

Another mounting
style available is the encased accelerometer that mechanically attaches to a non-electronic
supporting surface such as machinery housing. Such devices can be attached
using bolts, screws, adhesives, magnets, or clamps or simply be hand-held.

0119_Feature_Designers-Guide-Accelerometers_Fig-3

Fig. 3: Encased accelerometers are available with a variety of
mounting options, but the choice can affect the achieved sensing bandwidth. (Image: IDS Innomic)

The mounting
style chosen has an important effect on an accelerometer’s frequency response.
As with any kind of sensor, accelerometers have limits on how rapidly they can
respond, and as with any mechanical system, they can resonate with vibration at
the right frequency. The key performance parameters to consider, then, are the
sensor’s bandwidth and resonant frequency.

Bandwidth is the
frequency range over which the sensor’s measurements will be consistent (i.e.,
a “flat” response) and is usually specified as a tolerance band in terms of
percentage deviation from the accelerometer’s performance at a reference
frequency — often 100 Hz. To get a full picture of a device’s behavior,
however, designers should look for a frequency response curve in the data
sheets.

For many
applications, it is essential that the accelerometer’s frequency response extends
all the way to DC. This is essential for applications that are measuring
orientation using gravity to determine which way is “down.” It is also
important for those applications that are integrating acceleration to determine
velocity or movement.

There are
numerous other performance parameters for designers to explore as well. These
include:

  • Sensitivity.
    This is a measure of how strong a signal the accelerometer generates for a
    given acceleration, often measured in mV/g for analog devices or as the number
    of bits representing 1 g in a digital device. Sensitivity and range often go
    hand in hand, with a greater range typically implying reduced sensitivity. This
    parameter is not to be confused with transverse, or cross-axis, sensitivity. Cross-axis
    sensitivity refers to the amount of signal that appears in the sensor for one
    dimension (say, X) when the acceleration is strictly orthogonal to that
    dimension — Y or Z. Such signals are error sources and should be as small as
    possible.
  • Noise.
    This parameter can be expressed in several different ways, including RMS or as
    a spectral value. Designers should consider noise specifications in conjunction
    with sensitivity.
  • Linearity.
    This parameter indicates measurement consistency over the accelerometer’s
    sensing range. Ideally, a unit change in acceleration should produce a unit
    change in the output signal regardless of how strong the absolute acceleration
    may be. In practice, calibration may be required to correct some non-linear
    operation.
  • Temperature
    sensitivity.
    Because they are mechanical systems,
    accelerometers are susceptible to temperature, which causes dimensional changes
    as it varies. Temperature sensitivity measures how significant an impact a
    temperature change will have on the output signal.

There is also a
host of practical considerations that affect an accelerometer choice in the
final design. These include the device’s supply voltage, current draw, package
size, degree of robustness — commercial-, industrial-, or ruggedized-grade —
and the like. Sensors may generate an analog output signal or include an
integrated ADC to provide a digital signal. The digital signal interface may
conform to an industry standard such as I2C or SPI or, in rare cases, produce a
pulse-width modulated (PWM) output. The accelerometer may be a raw sensor or an
integrated device with signal conditioning and controls built in.

With all of these
parameters and variations to consider, it should come as no surprise that there
are many vendors offering accelerometers, often focusing on a few types and
application spaces. The table below provides a partial list of representative
vendors that developers can use to jumpstart their search for the best match to
their needs.

0119_Feature_Designers-Guide-Accelerometers_Table

Many
of the above vendors offer directly comparable products for common
applications, while others specialize in niche devices for which there are few
competitors. Regardless, the right choice of accelerometer is so highly
application-dependent that developers should consider enlisting help in making
a final selection once the essential needs are well-defined and the choices narrowed
down. The vendor’s support can prove an essential final factor in choosing the
right accelerometer for a design.