Page 159 - Build Your Own Combat Robot

Build Your Own Combat Robot
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Relay-based drive systems are better implemented on slower robots, which are
more likely to be proceeding at full speed whenever they move anyway. With the
difficulty of accurately aiming a weapon on a relay-based robot, the only weapons
used should be those that do not require aiming, such as large shell-type spinners.
Any other type of robot—especially those that require accurate steering—are going
to need a variable-speed motor control. Hence, using simple relay control for
drive motors is not recommended.

Controlling Speed = Controlling Voltage
To control your robot’s drive motors, you need to change not only the direction
but the speed of the drive motors. In a DC motor, speed is proportional to voltage,
so the output speed of the motor can be controlled by controlling the voltage.
Some small and low-powered R/C cars use a simple resistance method for con-
trolling the drive motor’s speed. A hobby servo driven by the throttle signal from
the radio drives a mechanism to vary the resistance in series with the motors. Ei-
ther a sliding wiper arm on a variable resistance strip or a set of contacts to switch
the motor power through fixed resistors is used to give a varying speed. This
method works for small motors with large amounts of airflow available for cool-
ing, but it should never be considered for combat robot systems. A motor used in a
combat robot could draw continuous currents in the tens to hundreds of amps—a
variable resistor or bank of fixed resistors large enough to handle the required
power levels would be impractically large and fragile.
One method for changing the voltage to a motor is to use a bank of batteries
tapped at multiple locations within the battery bank to obtain multiple voltage
levels, and to use relays to switch by which voltage point the motor is driven. For
example, if your robot is powered by a 24-volt motor that is broken down into
two 12-volt packs, you could use a single Type C relay to switch your motor be-
tween running off a single 12-volt battery or both in series. This would give you
high- and low-speed settings. If you break your battery pack into more segments
and add additional relays for each voltage tap, you can approximate the effect of
continuous control over your robot’s speed. This method has been used by several
teams, with usually only two or three different speeds.
It does have the advantage of reliability if done correctly. The downside is that
each relay must be rated for the full stall current of the robot’s drive motors, and
the large number of relays needed for good multistep control can make this an ex-
pensive approach. The wiring and control logic involved can also get pretty com-
plex when combined with an H-bridge setup for direction control. In addition,
unless the robot is operating at full speed most of the time, the extra batteries are
just dead weight that could otherwise be better put to use in weapons or armor.

Pulse-Width Modulation

Most combat robots use a method known as pulse-width modulation (PWM) for
controlling motor speed. A PWM control fools the motor into thinking it’s being