3-20
auto goes into a turn, you tend to be forced away from the direction of the turn-that is, if your auto turns
left, you are forced to the right, and vice versa.
If we replace the human in an auto with a mass suspended in an elastic mounting system, as shown
in figure 3-19, any acceleration of the auto will cause movement of the mass relative to the auto. The
amount of displacement is proportional to the force causing the acceleration. The direction the mass
moves is always opposite to the direction of the auto's acceleration.
Figure 3-19.—Auto with spring-suspended now.
The mass moves according to Newton's second Law of Motion which states: when a body is acted on
by force, its resulting acceleration is directly proportional to the force and inversely proportional to the
mass of the body.
When no acceleration is present, the mass will be at rest. When acceleration is present, the mass will
lag in proportion to the acceleration force. In other words, the car moves but the mass wants to remain at
rest.
Accelerometers are used principally in inertial navigation systems. They are used in aircraft and
missile navigation systems as well as aboard ship. Some common types of accelerometers are described
briefly in the following paragraphs.
THE BASIC ACCELEROMETER
Figure 3-20 is a simplified drawing of a basic accelerometer. It consists of a mass that is free to slide
along the sensitive axis within the case. The movement of the mass is limited by the springs. When the
case is accelerated, the mass, because of its inertia, tends to remain stationary. This results in a relative
movement of the mass with respect to the case. When the stretch of the springs overcomes the inertia of
the mass, the springs cause the mass to stop moving with respect to the case. The displacement of the
mass with respect to the case is directly proportional to the acceleration of the case. When the case stops
accelerating, the springs return the mass to its zero position (the reference position). To keep the springs
from causing the mass to overshoot and oscillate about the reference position, some form of damping is
needed. This is usually provided by an oil-filled case with vanes for oil to bypass the mass.
