Page 116 - Robot Builder's Bonanza

VARIATIONS ON A THEME 85

4-40 hardware

Plywood base

Ball caster
Figure 8-8 Ball caster (a.k.a. ball transfer) as
Edge view
specified in Table 8-1. You can substitute any other
Figure 8-7 Mounting the ball caster using 4-40 caster mechanism that is the same approximate
hardware fasteners. size.

The PlyBot is “invertible,” meaning you can turn it over, and the side with the motors becomes
the top of the bot. If you do this, there’s isn’t enough clearance for a ball caster. Instead,
construct a skid using an 8-32 3/4″ machine screw and matching 8-32 hex nut and acorn
G (cap) nut. The acorn nut provides a smooth surface for the skid to glide against. You can adjust
the height of the skid using the hex nut.
Drill a hole just smaller than the diameter of the screw, so that the screw
makes it own threads when you insert it. Be careful not to make the hole
too small, or else the wood may chip as the screw is tightened.

Using the PlyBot

The Tamiya motors used in the PlyBot are rated for 3 to 6 volts. You can rig them up to
switches to manually control the motor (on/off and direction) or use electronic control. These
topics are covered in Chapter 22, “Using DC Motors,” as well as the My First Robot lessons
found on the RBB Support Site.

We’re getting a bit ahead of ourselves here, but this needs to be mentioned now: When run at
4.5 volts, the motors draw less than 100 milliamps of current. But if the motors stall— meaning
they are physically stopped while voltage is still applied to them— current consumption goes up
to about 1.5 amps.
Bear this in mind when using electronic control of the motors. Be sure the drive electronics
can handle the current. See Chapters 21 and 22 for more details on current consumption of
motors and what it means.

The PlyBot has plenty of room on top (and underneath, too) for mounting electronics, bat-
teries, sensors, and other paraphernalia. Because the base is made of wood, it’s easy to drill
additional holes for mounting components.
Variations on a Theme

The PlyBot is the basic “T- bone” robot, where there’s a pair of two motors on one end of the
bot and a supporting caster or skid on the other. Figure 8-9 shows the concept, whereby you
can readily visualize the “T” shape of the bot.

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