Page 244 - Robot Builder's Bonanza

LOCOMOTION USING WHEELS 213

Steering Steering
wheels wheel

Drive Drive
wheels
wheels

Figure 20- 4 Car- type steering offers a workable Figure 20- 5 In tricycle steering, one drive motor
solution for a robot used outdoors, but it’s less useful powers the robot; a single wheel in front steers the
for a robot used indoors or in places where there are robot. Beware of short wheelbases, as they can
many obstacles to steer around. introduce tipping when the robot turns.

THREE- WHEELED TRICYCLE STEERING
Car- type steering makes for fairly cumbersome indoor mobile robots; a better approach is to
use a single drive motor powering two rear wheels, and a single steering wheel in the front;
the arrangement is just like a child’s tricycle (Figure 20- 5).
The robot can be steered in a circle just slightly larger than the width of the machine. Be
careful of the wheel base of the robot (distance from the back wheels to the front steering
wheel). A short base results in 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 subdegree 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; servo motors used a “closed- loop feedback” system that provides a high degree
of positional accuracy.
There are two basic variations of tricycle drives:
• Unpowered steered wheel. The steering wheel pivots but is not powered. Drive for the
robot is provided by one or two other wheels.
• Powered steered wheel. The steering wheel is also powered. The two other wheels freely
rotate.

A subvariant of the tricycle base design reverses the functionality of the wheels: two wheels
in the front of the robot steer, and a third wheel in the back provides support. The third wheel
can even be a simple caster or omnidirectional ball (see the section on caster types, below).

OMNIDIRECTIONAL (HOLONOMIC) STEERING
All of the steering methods described so far are known as nonholonomic. This basically (and
simplistically) means that in order for the robot to turn, it has to change the orientation of its
body. A good example of nonholonomic steering is a car. It can turn, but only by following a
circle described by the axis of its four wheels. The car cannot instantaneously move in any
direction of the compass.

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