Page 112 - Introduction to Autonomous Mobile Robots

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Perception
The above sections have presented a terminology with which we can characterize the
advantages and disadvantages of various mobile robot sensors. In the following sections,
we do the same for a sampling of the most commonly used mobile robot sensors today.

4.1.3 Wheel/motor sensors
Wheel/motor sensors are devices used to measure the internal state and dynamics of a
mobile robot. These sensors have vast applications outside of mobile robotics and, as a
result, mobile robotics has enjoyed the benefits of high-quality, low-cost wheel and motor
sensors that offer excellent resolution. In the next section, we sample just one such sensor,
the optical incremental encoder.

4.1.3.1 Optical encoders
Optical incremental encoders have become the most popular device for measuring angular
speed and position within a motor drive or at the shaft of a wheel or steering mechanism.
In mobile robotics, encoders are used to control the position or speed of wheels and other
motor-driven joints. Because these sensors are proprioceptive, their estimate of position is
best in the reference frame of the robot and, when applied to the problem of robot localiza-
tion, significant corrections are required as, discussed in chapter 5.
An optical encoder is basically a mechanical light chopper that produces a certain
number of sine or square wave pulses for each shaft revolution. It consists of an illumina-
tion source, a fixed grating that masks the light, a rotor disc with a fine optical grid that
rotates with the shaft, and fixed optical detectors. As the rotor moves, the amount of light
striking the optical detectors varies based on the alignment of the fixed and moving grat-
ings. In robotics, the resulting sine wave is transformed into a discrete square wave using a
threshold to choose between light and dark states. Resolution is measured in cycles per rev-
olution (CPR). The minimum angular resolution can be readily computed from an
encoder’s CPR rating. A typical encoder in mobile robotics may have 2000 CPR, while the
optical encoder industry can readily manufacture encoders with 10000 CPR. In terms of
required bandwidth, it is of course critical that the encoder be sufficiently fast to count at
the shaft spin speeds that are expected. Industrial optical encoders present no bandwidth
limitation to mobile robot applications.
Usually in mobile robotics the quadrature encoder is used. In this case, a second illumi-
nation and detector pair is placed 90 degrees shifted with respect to the original in terms of
the rotor disc. The resulting twin square waves, shown in figure 4.2, provide significantly
more information. The ordering of which square wave produces a rising edge first identifies
the direction of rotation. Furthermore, the four detectably different states improve the res-
olution by a factor of four with no change to the rotor disc. Thus, a 2000 CPR encoder in
quadrature yields 8000 counts. Further improvement is possible by retaining the sinusoidal