Page 107 - Introduction to Autonomous Mobile Robots

Chapter 4
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The sensor classes in table 4.1 are arranged in ascending order of complexity and
descending order of technological maturity. Tactile sensors and proprioceptive sensors are
critical to virtually all mobile robots, and are well understood and easily implemented.
Commercial quadrature encoders, for example, may be purchased as part of a gear-motor
assembly used in a mobile robot. At the other extreme, visual interpretation by means of
one or more CCD/CMOS cameras provides a broad array of potential functionalities, from
obstacle avoidance and localization to human face recognition. However, commercially
available sensor units that provide visual functionalities are only now beginning to emerge
[90, 160].

4.1.2 Characterizing sensor performance
The sensors we describe in this chapter vary greatly in their performance characteristics.
Some sensors provide extreme accuracy in well-controlled laboratory settings, but are
overcome with error when subjected to real-world environmental variations. Other sensors
provide narrow, high-precision data in a wide variety of settings. In order to quantify such
performance characteristics, first we formally define the sensor performance terminology
that will be valuable throughout the rest of this chapter.

4.1.2.1 Basic sensor response ratings
A number of sensor characteristics can be rated quantitatively in a laboratory setting. Such
performance ratings will necessarily be best-case scenarios when the sensor is placed on a
real-world robot, but are nevertheless useful.
Dynamic range is used to measure the spread between the lower and upper limits of
input values to the sensor while maintaining normal sensor operation. Formally, the
dynamic range is the ratio of the maximum input value to the minimum measurable input
value. Because this raw ratio can be unwieldy, it is usually measured in decibels, which are
computed as ten times the common logarithm of the dynamic range. However, there is
potential confusion in the calculation of decibels, which are meant to measure the ratio
between powers, such as watts or horsepower. Suppose your sensor measures motor current
and can register values from a minimum of 1 mA to 20 Amps. The dynamic range of this
current sensor is defined as

20
10 log⋅ ------------- = 43 dB (4.1)
0.001

Now suppose you have a voltage sensor that measures the voltage of your robot’s bat-
tery, measuring any value from 1 mV to 20 V. Voltage is not a unit of power, but the square
of voltage is proportional to power. Therefore, we use 20 instead of 10: