Vehicular Sensor Networks: General Aspects and Implementation Issues - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 218

Speed Sensors

A number of different technologies (fifth wheels, radar, microwave, optical, GPS)
exist to measure the vehicle speed and use that parameter to calculate the distance
traveled. Generally, speed sensors can be divided into two groups, i.e., wheel-contact
and non-contact sensors . The second ones have much better performances, are easier
to implement, and are much more recommended by today's security standards.
An interesting application of a vehicle's speed sensors is evaluation of the braking
performance, i.e., measuring the total distance traveled by a vehicle during a stop.
The method of measuring this distance needs to be accurate and repeatable in order
to detect small variations in the braking performance. Measurements like this are
suitable for testing performances of different types of speed sensors and also
contribute to the non-contact sensors superiority. Figure 10.9 represents the process
of non-contact speed measurement used by the Doppler radar speed sensor.
The Doppler radar is named after the Doppler principle, which explains the
frequency shift associated with energy waves reflected by or emanated from a mov-
ing body. A familiar example of a Doppler shift is the change in pitch in the sound
of a passing car - higher as the car approaches; lower as it leaves. The measurement
process is done as follows. A Ka band radar signal (26.5-40 GHz) is transmitted at
a specific frequency by the sensor, reflects off of a target (or targets), and returns to
the sensor (see Fig. 10.9 ). If either the sensor or the target is moving relative to one
another, the signal will be shifted in frequency when it returns to the sensor. The
fundamental Doppler frequency shift is given by:

F
FV c

0

(10.1)

= 2· · ·cos θ

d

where F d is the Doppler frequency shift (Hz), V is the velocity, q is the offset angle
of the sensor relative to the direction of the target motion, c is the speed of light and
F = 35.5 ± 0.1 GHz (Ka band).
Equation (10.1) is used to accurately obtain the velocity of the vehicle by measuring
only the Doppler frequency shift since all other variables are known in advance.

Sensor

Offset Angle Target

Distance

6°

Speed

Fig. 10.9 Non-contact speed measurement