Copyright © Shimmer 2016
Realtime Technologies Ltd IMU User Manual
All rights reserved Rev 1.4
5
3. Inertial Measurement Units
This sections provides a brief introduction to IMUs (sometimes known as kinematic sensors) and the
signals that they measure.
3.1. Accelerometer
The acceleration, a, of a body can be defined as its rate of change of velocity and it is directly
proportional to the forces, F, acting on the body:
.
An accelerometer is a device which measures acceleration due to all forces acting on the device.
Forces acting on a device include both the gravitational force due to the mass of the earth as well as
any inertial forces which may be applied to the device.
The two primary components of acceleration are, thus, inertial and gravitational acceleration. Thus,
the total acceleration,
, measured by the device is the vector sum of these components:
,
where
is the inertial component and
is the gravitational component.
Inertial acceleration
Inertial acceleration occurs due to the application of a force other than gravity to a body. Unless a
body is completely motionless or moving with a constant velocity, there are inertial forces acting on
it. These forces give rise to inertial acceleration. This acceleration is defined as the rate of change of
velocity of the body in motion. It is measured in units of m/s2.
Acceleration due to gravity
Gravity is a natural phenomenon by which physical bodies attract each other with a force
proportional to their masses. Gravity is most familiar as the agent that gives weight to objects with
mass and causes them to fall to the ground when dropped.
The units of gravity are m/s2. Thus, it is a form of acceleration and is measured by an accelerometer.
When an accelerometer is completely stationary (i.e. there is no inertial acceleration acting on the
device), it measures a constant acceleration equal in magnitude to the acceleration due to gravity
(9.81 m/s2approx). This is often referred to in units labelled “g”, where 1 g ≈ 9.81 m/s2.
It is a common misconception to assume that the direction of the gravity vector measured by an
accelerometer is vertically downwards; this is incorrect. In fact, the measured vector of acceleration
due to gravity points vertically upwards from the Earth’s surface.
A simple example to help you remember that this is the case is the observation that an
accelerometer in free-fall records an acceleration of zero. In this case, the downward inertial
acceleration due to motion equals the upward gravitational acceleration.
