its own wheel. The wheels are pulled from an inexpensive
toy. Their hard plastic construction reduces “skittering”
caused by friction in turns — a problem with four- and
six-wheel drive designs.
Cost and weight can be reduced by using just a single
motor to power one wheel on each side, letting the
other(s) rotate freely. Or, the wheels on each side can be
connected to the motor by way of gears or belts. This is
harder (and more expensive) to implement, but it can
provide better traction over uneven road than by using
freely rotating wheels.
The wheel size greatly influences the traveling speed of
the robot. Larger wheels (for a given motor RPM) make the
robot go faster. Example: If the robot travels at six inches
per second with 1.5” diameter wheels, changing to 3”
wheels increases speed to 12 inches per second. The speed
increase isn’t without cost, however. Torque is reduced as
the wheels get bigger.
A main benefit of wheels is that they make measuring
distance — called odometry — very easy. Sensors are
mounted on the wheels which detect even incremental
rotation. By comparing the motion of the wheels on each
side, it’s possible to keep the robot on a straight path.
Odometry can be used to control speed, as well as measure
distance. Accurate travel distance calculations are much
more difficult with tracked and legged robots.
While differential steering is the most common drive
geometry for robots with wheels, there are other
approaches. One uses a single drive motor powering two
rear wheels and a steering wheel in the front; the
arrangement is just like a child’s tricycle (see Figure 4).
On these designs, be careful of the wheel base of the
robot — the distance from the back wheels to the front
steering wheel. A short base will cause
instability in turns, causing the robot to tip
over in the direction of the turn.
Tricycle-steered robots require a very
accurate steering motor in the front. The
motor must be able to position the front
wheel with sub-degree accuracy.
Otherwise, there is no guarantee the
robot will be able to travel a straight line.
Most often, the steering wheel is
controlled by a servo motor.
There are two basic variations of
• Unpowered steered wheel. The
steering wheel pivots but is not
powered. Drive for the robot is provided
by one or two rear wheels.
• Powered steered wheel. The steering
wheel is also powered. The two other
wheels only rotate freely.
44 SERVO 05.2014
Figure 5. Robots with tank tracks (also called
treads) can be used over rough ground.
Figure 6. Like differentially-steered wheeled robots, tracked bots
steer by changing the direction of the treads.
Figure 4. In tricycle steering, one drive motor powers the robot;
a single wheel in front steers the robot. Beware of short
wheelbases as this can introduce tipping when the robot turns.