FIGURE 2. Grey Walter’s Elsie.
FIGURE 3. Elsie’s parts.
like tank treads — but I’ll cover that later.) Grey
Walter’s ‘Elsie’ robot in Figure 2 from the late
1940s is an excellent example of an early
experimental robot. You can see the worm drive
motor used to steer the front wheel. Figure 3
shows how the front wheel is driven by a motor
mounted above the wheel. The steered single-wheel design proved to be quite functional since
Elsie was used to demonstrate Pavlov’s ‘stimulus
and response’ theories.
This design was used long before the
differential robot drives that many robot
experimenters use today. Back in the mid ‘80s,
many robot builders used a 6 VDC motorized
wheel from kid’s riding toys as a single driving
The Ackermann wheel and steering
arrangement uses four wheels with either the
front or back two wheels doing the steering.
Some really great robot bases have been made
with this setup and are especially popular in Robo-Magellan contests where the robots must travel
over uneven outdoor terrain. This arrangement
works very well if the robot is radio controlled and
the operator is using one of the ‘gun type’
transmitters with a trigger to control speed and a
steering wheel feature to control the car’s
Figure 4 shows Mark Curry’s Nomad from the
Robo-Magellan contest held last year at the Seattle
Robotics Society (SRS) Robothon at Seattle Center.
It was built on an R/C car chassis with Ackermann
steering and was last year’s winner.
Autonomous steering of this type of robot
requires that several physical characteristics of the
robot’s base layout be identified. For example,
wheel diameter must be known to determine the
distance traveled with one revolution of the wheel.
Shaft encoders are usually placed on both drive
wheels and an average number of encoder pulses
determines the distance. True Ackermann steering
— as in automobiles — actually has the two steered
wheels turned at different angles. Figure 5 shows
how the turned inner wheel is angled more
sharply due to the shorter turning radius of that
FIGURE 4. Top
view of Nomad
SERVO 04.2010 77