Using Radio Waves
to Control a Robot
There are numerous methods to
control a robot. For this article, I’ll
demonstrate just one way using a radio
link. For background purposes, here are
a few other common schemes:
• Wired link. Connect a control
panel consisting of buttons and
switches directly to the robot’s motors.
The finished result is more like a
motorized toy than a real robot,
because you directly influence every
action of the bot — it has no
independent control.
• Infrared remote control. A
universal TV remote can be used to
send coded signals to a robot. A
receiver mounted on the bot receives
the signal, and a microcontroller
commands the robot based on which
buttons on the remote were pressed.
Because the robot contains a
microcontroller, it is capable of
independent control in addition to
human-influenced control. For example,
you might press the button to have the
robot go forward, but sensors on the
bot will detect collision with something.
The robot’s own programming can
reverse direction, without requiring any
corrective action from you.
• Line following. In this type of
control, a predefined line, dot, or other
shape indicates where the robot is to
go. Lines are usually drawn on paper,
but it’s also possible to interactivity
draw the lines by using such techniques
as a laser pointer, or even a large flat
screen TV resting on its back.
• Sound, light, or other sense.
These are all indirect control methods,
where the robot is influenced by some
sensory condition you manage. For
example, you might build a robot that
has a light detector that can be
“steered” using a flashlight or handheld
laser pointer. A musicbot might be able
to differentiate tones played on a piano
or guitar.
For the Telebot, I’m using the
ZigBee standard of compact and
affordable two-way radios.
Getting Started
ZigBee is a wireless data standard.
Two ZigBee radios form a link for
sending and receiving serial data over
the air. The standard ZigBee is based
on the IEEE 802.15.4 specification,
intended for low speed, low power
wireless data. ZigBee is only one of the
technologies that uses the 802.15.4
specification — MiFi and WirelessHART
are a couple others — but it’s the one
that’s been most embraced by the
robotics community.
FIGURE 2. An XBee IEEE 802.15.4
(Series 1) radio on a Parallax carrier
board. The board provides
regulated 3.3V to the XBee.
as multiple digital input/output pins. I
won’t be talking about these features
here, but you should know they’re
available if you need them.
The Telebot robot uses a pair of
low cost XBee Series 1 modules. XBee
is a popular brand of ZigBee RF radios
that support the 802.15.4 protocol.
The form factor — the physical layout —
of the XBee radios is widely supported
with various adapters that allow you to
plug them directly into solderless
breadboards. Since the transmission
range does not need to be extensive,
the low power one milliwatt (1 m W)
modules are more than adequate.
XBee modules use connector pins
that are set 2 mm apart. In order to
plug the XBee module into a solderless
breadboard, you must first connect it
to a carrier or adapter. These carriers
come in various styles with a variety of
features. Series 1 modules are simple
to use. If you don’t mind the default
9600 baud communications speed,
they provide out-of-the-box factory
settings for quick and easy setup.
While you can always modify the
factory settings for such things as
communications speed, channel, and
ID number (so more than two XBees
can link at the same time), none of this
is required if you’re only needing to
establish a basic wireless serial link
between two nodes (points).
SERVO 11.2012 51
