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ArtBot II Design Concepts
Like its forerunner, the ArdBot II uses two standard size
servo motors for propulsion. The motors are conventional
R/C servos that have been modified to operate in
continuous rotation. That is, instead of a regular servo that
moves to a specific angular position, then stops, the motors
on the ArdBot II continuously spin clockwise or counterclockwise, depending on a control signal your Arduino
You can get standard size R/C motors that have been
designed for or modified at the factory for continuous
rotation (CR), or hack the servos yourself. CR servos are
now fairly common, thanks to the growing popularity of
educational robots. Among my favorites is the Parallax
continuous rotation servo. Other brands of CR servos
include GWS and Spring RC.
The motors and their wheels are mounted on the sides
of the bot. This makes the ArdBot II differentially steered,
meaning that movement is dependent on the difference in
Other Battery Options
An alternative to using separate sets of batteries is to use one
larger rechargeable pack. Whether or not you can go this route
depends on the servo motors you use, or whether you want the
extra hassle of adding a high current voltage regulator for the
Off the shelf, most servo motors (being designed for use in
radio control models with 4. 8 volt power packs) are rated for up to
six volts. Some brands and models can be successfully used at up
to 7. 2 volts, though the manufacturer may not specifically indicate
it due to liability concerns. At higher voltages, the motor can get
hotter, drawing more current and possibly damaging its
electronics. Servo makers don't want to go about replacing their
products, so they tend to be conservative with their ratings.
With this in mind and if you're sure the servo motors you use
operate satisfactorily at 7. 2 volts, you can power the Arduino from
a single robust set of rechargeable cells. For 7. 2 volts, you'll need
six cells — 1.2 x 6 = 7. 2 volts. You can either buy a six-cell pack
ready-made or construct your own.
Note! Some digital servos are designed for 8. 4 volts and
higher. By digital, these servos incorporate digital circuitry in order
to add special functionality such as speed control. Servos that are
digital say so, and often cost considerably more. But take heed.
Many types of digital servos are not adaptable for continuous
rotation use. You're better off using the old fashioned — and less
expensive — analog servo, such as the Parallax model mentioned
towards the beginning of the article.
Here's the reason to avoid digital servos as the main drive
motors in your robot: Servos are controlled by sending them
pulses; the length of the pulses communicates its desired position
to the motor. Longer pulses move the servo to a position one way,
and shorter pulses move the servo the other way.
For robotics, the same technique is used but rather than
specify a precise angular position, the duration of the pulses
determines the direction the motors turn and their speed. In
analog servos, stopping the pulses altogether — something you
may have reason to do — causes the motor to coast to a stop.
Without pulses, the motors are effectively depowered. In a typical
digital servo, stopping the pulses may have no effect. Digital
circuitry inside the servo "remembers" the last pulses it receives,
and continues to apply those to the motor.
FIGURE 2. By controlling the servo motors, the robot
can be made to travel in straight lines or make turns.
By reversing the motors relative to one another,
the robot will spin in place.
direction and speed of the motors. The robot can go
forward and backwards, of course, as well as turn and spin
in place. Figure 2 shows various turning modes, depending
on how the motors on each side are controlled. The robot
can spin in place, for example, by reversing one motor
relative to the other.
For balance, the ArdBot II uses simple skids as front
and back casters. I chose static skids because they’re small,
easy to find, and best of all, cheap. The robot is
lightweight, so there’s no problem of causing scratches or
other marks on floors.
It does mean, however, that the robot requires a fairly
flat surface for traveling — carpets and kitchen floors are
ideal, just as long as you stay away from trying to go over
old socks, magazines, electrical cords, gravel, and other
raised objects. There isn’t enough clearance between the
bottom of the robot and the ground to trounce over rough
The ArtBot II uses two decks for expandability, and an
additional deck can be easily added if you need more
mounting space. The bottom and top decks share the same
dimensions: 5” by 7” inches. Because they are the heaviest
components, motors and batteries are located on the
bottom deck. This keeps the center of gravity of the ArdBot
II low, where it should be. The Arduino and other
electronics go on the top deck.
Arduino and Power
For the prototype ArdBot II, I’ve used a standard
Arduino Uno board, mounted toward one end of the
robot’s top deck. A separate mini breadboard is stuck into
place nearby. The breadboard serves as a convenient “patch
panel” of sorts to connect the Arduino to the motors,
sensors, and other parts.
Most active robotics components typically require at
least three wire connections: power, ground, and signal.
The Arduino has only one 5V power pin, and the
arrangement of its other pins makes attaching to things like
servo motors awkward, at best. The breadboard provides a
quick and easy way to consolidate the wiring, allowing you
to merely plug in the motor or other component.
I chose a separate Arduino breadboard shield mainly
because of cost — the mini breadboard goes for about $4
and attaches to the robot in seconds. The typical solderless
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