Despite what most folks think, the science of robotics is not based on mobility and
motion alone. Say the word “robot” to most anyone on the street and they will
assimilate the word robot to a mobile humanoid-like mechanism that has superhuman
intelligence and strength. In reality, Earth-bound robots have the ability to weld, cut,
saw, drill, wash dishes, wash your underwear, wash your car, and cook your dinner.
The typical robot doesn’t need to sleep, eat, or breathe in
the human sense. Thus, robots are often sent into space
to what we think are uninhabited planets. Planet-hopping
robots are designed to fly, land, roll around, dig in the dirt, and
sample the surrounding atmosphere. In that the interplanetary-traveling robot can’t call home with a cell phone, the robot’s
ability to transmit sensor data and receive commands is inherent.
Most of us don’t have the means or need to field a
Mars rover and the ground support it requires. However,
some of our robotic designs do have the need to sense and
communicate over short distances. Lately, we’ve been
concentrating on moving stuff in a robotic way. This month,
let’s shift our attention to communicating in a robotic way.
Many of you are much more comfortable at the
PHOTO 1. The ETRX2 is designed to stand alone or run
under the control of a personal computer or external host
microcontroller. SMT pads on the other side of the ETRX2
module provide access to its innards.
50 SERVO 09.2008
controls of a milling machine than at the entry point of a subroutine. If you choose the right communications hardware,
you won’t have to worry about the software or hardware
details behind the robotic communications process. What if
I could show you how to implement a self-healing mesh
network that is based on the ZigBee standard using a very
simple PIC microcontroller circuit, some very basic C code,
and an off-the-shelf ZigBee radio? I promise to keep it simple.
Isn’t ZigBee Complicated?
In a word, yes. However, if you don’t know a ZigBee
PAN from a ZigBee Coordinator, that is just fine. In fact,
you don’t even have to know how to spell ZigBee to get the
network we’re about to discuss up and running.
The secret to our success lies inside of an RF shield that
is physically attached to a 37. 5 mm x 20. 5 mm printed circuit
board (PCB). The goodies under the RF shield make up
an Ember EM250 ZigBee/IEEE 802.15.4-compliant radio
system. Our EM250 and supporting components can be
found under the RF shield of the Telegesis ETRX2 you see
in Photo 1. The ETRX2 comes loaded with the EmberZNet
meshing stack. If you’ve ever issued an AT command to
dial a phone with your personal computer’s modem, you
can form and control a ZigBee network with the ETRX2.
If you wish, you can take the road less traveled and
program the ETRX2 from scratch with your own driver. Rolling
your own ZigBee stack application with the ETRX2 requires
some special hardware and software, which you can obtain
from Ember. On the other hand, you can take the interstate
and employ the services of the ETRX2’s built-in AT
