Page 108 - Introduction to Autonomous Mobile Robots

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Perception
20
20 log⋅ ------------- = 86 dB (4.2)
0.001

Range is also an important rating in mobile robot applications because often robot sen-
sors operate in environments where they are frequently exposed to input values beyond
their working range. In such cases, it is critical to understand how the sensor will respond.
For example, an optical rangefinder will have a minimum operating range and can thus pro-
vide spurious data when measurements are taken with the object closer than that minimum.
Resolution is the minimum difference between two values that can be detected by a sen-
sor. Usually, the lower limit of the dynamic range of a sensor is equal to its resolution.
However, in the case of digital sensors, this is not necessarily so. For example, suppose that
you have a sensor that measures voltage, performs an analog-to-digital (A/D) conversion,
and outputs the converted value as an 8-bit number linearly corresponding to between 0 and
8
5 V. If this sensor is truly linear, then it has 2 – 1 total output values, or a resolution of
(
5 V 255) = 20 mV .
Linearity is an important measure governing the behavior of the sensor’s output signal
as the input signal varies. A linear response indicates that if two inputs x and y result in the
two outputs fx() and f y() , then for any values and , fax +( by) = af x() + bf y() . This
b
a
means that a plot of the sensor’s input/output response is simply a straight line.
Bandwidth or frequency is used to measure the speed with which a sensor can provide a
stream of readings. Formally, the number of measurements per second is defined as the sen-
sor’s frequency in hertz. Because of the dynamics of moving through their environment,
mobile robots often are limited in maximum speed by the bandwidth of their obstacle detec-
tion sensors. Thus, increasing the bandwidth of ranging and vision-based sensors has been
a high-priority goal in the robotics community.

4.1.2.2 In situ sensor performance
The above sensor characteristics can be reasonably measured in a laboratory environment
with confident extrapolation to performance in real-world deployment. However, a number
of important measures cannot be reliably acquired without deep understanding of the com-
plex interaction between all environmental characteristics and the sensors in question. This
is most relevant to the most sophisticated sensors, including active ranging sensors and
visual interpretation sensors.
Sensitivity itself is a desirable trait. This is a measure of the degree to which an incre-
mental change in the target input signal changes the output signal. Formally, sensitivity is
the ratio of output change to input change. Unfortunately, however, the sensitivity of
exteroceptive sensors is often confounded by undesirable sensitivity and performance cou-
pling to other environmental parameters.