Page 173 - Build Your Own Combat Robot

Build Your Own Combat Robot
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At the moment, using the OSMC controller successfully means committing to
learning the ins and outs of the system in some detail and being prepared to do
your own programming and modification.
The OSMC shows great potential as a high-powered motor controller; but at the
time of writing this book, the OSMC lacks significant combat testing. If the current
momentum on the project is maintained, the OSMC could become the choice for
high-power motor control. Keep your eye on this one in the coming years.

caution When using any ESC, you must carefully inspect and test all the wiring before
powering up your robot for the first time. It takes only a momentary short circuit, reversed polarity,
or over voltage to destroy the controller, batteries, and in some cases even the motor—which
can cost hundreds of dollars and weeks of precious time to replace.

Most combat robots will use a traditional radio control system that was origi-
nally designed for R/C airplanes, cars, and boats for controlling the robot’s motion
and actuators. Because they are so widely available, combat robot components
are being designed to accept the standard R/C servo command signal, such as the
Vantec and Victor speed controllers. Some robot builders prefer to build their own
remote control units but use regular R/C servos and speed controllers that accept the
standard R/C servo control signal.
Some robot builders even build servo-mixing circuits to help improve the driv-
ing control of the robot. Servo mixing is common with robots that use tank-type
steering. Instead of having one stick on the radio transmitter controlling the speed
and direction of one motor and the other stick on the transmitter controlling the
other motor, by combining both of the signals together, one stick on the transmitter
can be used to control the velocity of the robot and the other stick can be used to
control the direction of the robot. In fact, one joystick on a transmitter can be used
to control both direction and speed. This frees up the robot driver’s other hand to
control weapons on the robot. Servo mixers are commonly called elevon mixers,
veetail mixers, or v-tail mixers.
To develop custom controls for driving R/C servos or speed controllers, you
must understand how the R/C command signal works. Many people call the R/C
command signal a pulse-width modulated signal. Though technically correct, it is
nothing like the true variable-duty-cycle–controlled PWM signal that is used to
vary the speed to a motor. A true PWM signal is a square wave signal that has a
duty cycle that can range from 0 to 100 percent. The R/C control signal is a vari-
able 1 to 2 millisecond pulse that must be repeated every 15 to 20 milliseconds.
The internal circuitry of a R/C servo is designed to interpret the 1- to 2-millisec-
ond pulse and convert it into a position command. A pulse width of 1.5 milliseconds
represents the neutral position of the servo, or zero degrees. R/C servos rotate ap-
proximately +/– 60 degrees from the neutral position. A 1.0-millisecond pulse
width represents an approximate –60 degree position, and a 2.0-millisecond