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Chapter 2: SumoBot Locomotion · Page 11
Chapter 2: SumoBot Locomotion
The first task of any Mini-Sumo robot is to move – most competition rules do not allow
the robot to stop (without competitor contact) for more than a few seconds. In this
experiment you will learn how to get the SumoBot moving and learn to take control over
its motion.
HOW A SERVO WORKS
Normal (un-modified) hobby servos are very popular for controlling the steering systems
in radio-controlled cars, boats and planes. These servos are designed to control the
position of something such as a steering flap on a radio-controlled airplane. Their range
of motion is typically 90° to 270°, and they are great for applications where inexpensive,
accurate high-torque positioning motion is required. The position of these servos is
controlled by an electronic signal called a pulse train, which you'll get some first hand
experience with shortly. An un-modified hobby servo has built-in mechanical stoppers to
prevent it from turning beyond its 90° or 270° range of motion. It also has internal
mechanical linkages for position feedback so that the electronic circuit that controls the
DC motor inside the servo knows where to turn to in response to a pulse train.
SumoBot motion is controlled using two pre-modified Parallax Continuous Rotation
servo motors using a process called differential drive. The modification "tricks" the
feedback circuitry so that the servo will stop only when it receives a centering command;
it also allows the servo to continuously rotate in either direction. When both motors are
turning in the same direction, the SumoBot will move in that direction. When the
SumoBot servo motors turn in different directions, the chassis will rotate. The rate of
movement or rotation is determined by motor speeds.
TIME MEASUREMENTS AND VOLTAGE LEVELS
Throughout this text, amounts of time will be referred to in units of seconds (s),
milliseconds (ms), and microseconds (µs). Seconds are abbreviated with the lower-case
letter "s". So, one second is written as 1 s. Milliseconds are abbreviated as ms, and it
means one one-thousandth of a second. One microsecond is one one-millionth of a
second. Figure 2.1 shows how Milliseconds and Microseconds equate in terms of both
fractions and scientific notation.
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