Electronic Products - January 2019

A

lso known as inertial sensors, ac-

celerometers are critical elements

in key applications such as auto-

motive air-bag deployment, smartphone

motion tracking, and industrial predic-

tive maintenance. These varying needs

have resulted in an even more varied

array of accelerator products from which

designers can choose. Fortunately, by fo-

cusing on a handful of key decisions, de-

velopers can quickly zero in on the right

kinds of devices for their application.

Despite the multitude of options avail-

able, 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 surround-

ing structure undergoes acceleration (i.e.,

changes its motion) along that same direc-

tion. Sensing systems within the accel-

erator 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.

The many variations among commer-

cial accelerometers are the result of differ-

ing 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 microelectro-

mechanical system (MEMS) structures or

the choice of capacitive versus piezoelec-

tric sensing offer various pros and cons.

In practice, however, these choic-

es 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 mea-

surement range, sensitivity, precision,

and accuracy, along with a variety of

operating characteristics.

When choosing an accelerome-

ter, 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/Inven-

Sense 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

Designer’s guide to accelerometers:

choices abound

Understanding key functional needs

will jumpstart the selection process

BY RICHARD QUINNELL

Editor, Special Projects, Technical

m

accel

0

(Rest)

X

nine-axis device also includes magnetic

field sensing in the three linear axes.

In general, a price-constrained appli-

cation 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 com-

bination of one- and two-axis sensors as

cost and placement needs dictate.

Three-axis sensors used in smart-

phones and automobiles are an excep-

tion 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.

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 con-

cerned primarily with human movement.

Sensors for monitoring machinery may

be more demanding and need the wider

Fig. 1: Accelerometers leverage the inertia

of an internal proof mass to sense changes

in motion.

Designer’s Guide: Accelerometers FEATURE 13

ELECTRONIC PRODUCTS • electronicproducts.com • JANUARY 2019