Build Your Own Combat Robot

Chapter 7: Controlling Your Motors 137
Servo switching was quite common in the early days of robotic combat, but using
it has many drawbacks and is not recommended.
■ The response time of a servo is fast, but the time it takes the servo to
rotate and trigger the lever switch will add a perceptible lag to the motor’s
activation. A half-second lag in your robot’s response can make a big
difference in the arena.
■ Servo switching introduces extra moving parts into your control system
that can break or jam and cause the motor to stop working or, even
worse, turn on and refuse to turn off.
■ A servo switching system will have trouble meeting fail-safe requirements
present in most competition rules. Depending on your radio type, loss of
signal may result in all servos connected to the radio simply locking in
place. If the motor was on when contact was lost, it’ll stay on until you
can switch the bot off manually. Even if your radio has the feature of
returning all the servos to the neutral position if radio signal is lost, loss
of power in your radio receiver or a severed connection between the
receiver and the servo can still result in a motor stuck running.
caution For safety reasons, servo switching should not be used for controlling drive
motors or weapons that can injure someone if the servos or relays should fail.
Remember that you must have absolute control of your robot at all times and
you must be able to shut it off remotely even if internal control parts break inside.
Servo switching can be used for applications in which failures are not safety issues,
such as for an arm that turns your robot right side up or an electrically driven lifting
arm.
Solid-State Logic A better method to control the relays is to use solid-state logic
to interpret the control signal from the radio and trigger the relays when the appropriate
signal is received. You can use a programmable microcontroller, such as
the Basic Stamp from Parallax, Inc., and program it to receive the command signal
from the R/C receiver and convert that signal into an output signal. The output
signal is then used to turn a transistor on or off, and the transistor is used to supply
power to the relay coils.
Figure 7-9 shows a simple schematic that illustrates transistor-relay control. In
the figure, a low-voltage signal is used to turn a transistor on and off. The schematic
drawing shown on the left is an NPN transistor. A positive voltage to the
transistor base (shown asaBonthetransistor) will turn it on and the relay will be
energized. The schematic to the right uses a PNP transistor. In this schematic, the
relay coil is energized when there is no voltage signal to the base. An NPN transistor
is analogous to a NO-SPST switch, and a PNP transistor is analogous to a NC-SPST
switch. A “flyback” diode is required to protect the transistor when the relay is