Page 243 - Robot Builder's Bonanza

212 MOVING YOUR ROBOT

Base

Figure 20- 2 “Dually” wheels should be
placed close to one another for maximum
steering. If they are placed far apart, the
Wheels robot cannot steer as easily.

wheels on each side are located close together. This aids in steering, where the robot will
pivot at a virtual point midway between the two wheels on each side.
• Another technique in 4WD systems, easier to implement but more expensive, is to power
each wheel with a separate motor. The motors on each side are powered as if they were
one unit: both motors on the left turn on or off at the same time.
• And yet another technique for 4WD robots is to power only one motor per side and let the
other one simply rotate freely.
Please don’t call this differential drive! Only robots that use a differential gearbox, like the one
G in an automobile, use “differential drive.” The correct term is differential steering— it’s how the
robot steers that defines how it putt- putts down the hall. In fact, you might have noticed that all
of the drive geometries discussed in this section actually describe the way the robot steers.

One of the key benefits of differential steering is that the robot can spin in place by revers-
ing one wheel relative to the other, as shown in Figure 20- 3.

CAR- TYPE STEERING

Pivoting the wheels in the front is yet another method of steering a robot. Robots with
car- type steering (see Figure 20- 4) are not as maneuverable as differentially steered bots, but
they are better suited for outdoor use, especially over rough terrain.
Why even bother with car- type steering? It seems so twentieth century. Well, there are a
number of valid reasons to use it. One of the greatest drawbacks of the differentially steered
robot is that it will veer off course if one motor is even a wee bit slow. You can compensate
for this by monitoring the speed of both motors and ensuring that they operate at the same
RPM. This, of course, adds to the complexity of the robot.
Somewhat better traction and steering accuracy are obtained if the wheel on the inside
of the turn pivots to a greater extent than the wheel on the outside. This technique is called
Ackermann steering, and it is found on most cars but not as many robots.

Figure 20- 3 Differentially steered
robots can turn in a circle within
itself. This is called the steering circle,
and the size of the circle depends on
the dimensions of the robot and the
Spinning in place
placement of the wheels.

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