Page 329 - Robot Builder's Bonanza

298 BUILD ROBOTS WITH WHEELS AND TRACKS

Caster Caster

Caster
Figure 26- 1 Center- versus front- drive motor mounting. With the center- drive arrangement, you typically
need a caster on each end, though you can use just one if the robot is properly balanced.

and right steering. If you stop the left motor, the robot turns to the left. By reversing the
motors relative to one another, the robot turns by spinning on its wheel axis (“turns in place”).
You use this forward- reverse movement to make “hard” or sharp right and left turns.

Centerline Drive Motor Mount
You can place the wheels— and, hence, the motors— just about anywhere along the length of
the platform. If they are placed in the middle, as shown in Figure 26- 1, you should add casters
to either end of the platform to provide stability. Since the motors are in the center of the
platform, the weight is more evenly distributed across it.
A benefit of centerline mounting is that the robot has no “front” or “back,” at least as far
as the drive system is concerned. Therefore, you can create a kind of multidirectional robot
that can move forward and backward with the same ease. Of course, this approach also com-
plicates the sensor arrangement of your robot. Instead of having bump switches only in the
front of your robot, you’ll need to add some in the back in case the robot is reversing direction
when it strikes an object.

G Depending on the size of the robot and its weight distribution, you may be able to get by with
just one caster. By placing slightly more weight over the caster, the bot will favor tipping to that
side.

Front- Drive Motor Mount
You can also position the wheels on one end of the platform. In this case, you add one caster
on the other end to provide stability and a pivot for turning, also shown in Figure 26- 1. Obvi-
ously, the weight is now concentrated more on the motor side of the platform. Even out the
weight distribution by putting the batteries in the center of the platform.
One advantage of front- drive mounting is that it simplifies the con-
struction of the robot. Its “steering circle,” the diameter of the circle in
which the robot can be steered, is still the same diameter as the centerline
drive robot. However, it extends beyond the front/back dimension of the
Spinning in place robot. This may or may not be a problem, depending on the overall size
of your robot and how you plan to use it.

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