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can determine the potential energy required to climb the ladder using the formula PE
m g h: ENERGY AND POWER SYSTEMS 155
2
PE 50 kg 9.8 m/s 6 m 2,940 joules
Since 1 joule equals .000278 watt-hours, 2940 joules equals 0.817 watt-hours.
Table 6-1 outlines the watt-hour ratings for rechargeable batteries.
Certainly, many other battery technologies are available, but the preliminary calcu-
lations show that just one AA NMH battery should carry enough energy to take a robot
up the ladder once. We require 0.817 watt-hours of energy and the battery can contain
1.8 watt hours. We have a margin of about 2 to 1. That’s not too bad, hauling a robot the
size of a 12-year-old boy up a long ladder with one battery. Clearly, to do it 10 times,
we’ll need 10 0.817 watt-hours, or 8 watt-hours. So we’ll need a couple of D-size
NMH batteries to provide 15 watt-hours. We’ll see in a bit that a margin of 2 to 1 may
not be enough, however.
The astute observer would note that adding more batteries to the robot alters the
weight! That’s quite true. Simply add the battery weight to the robot’s weight, and per-
form the calculations again. Eventually, everything will pencil out.

TABLE 6-1 Rechargeable batteries’ watt-hours

BATTERY SIZE WATT-HOURS BATTERY MATERIALS
D 4.0 Nicad
D 7.8 NMH
AA 0.6 Nicad
AA 1.8 NMH

EMPIRICAL MEASUREMENTS

One other way to estimate the power the robot will need is to literally build a model of
the robot and try it out. Practically speaking, we do not have to build the entire robot;
rather we can simulate it with a hastily built mockup. It would suffice to just build the
drive mechanisms and load down the simulated chassis with the proper amount of
weight (perhaps with bricks). Then the simulated robot can be put through its paces and
the power drain can be measured directly. This will prove to be quite an accurate way
of gauging the amount of energy that will be required. It takes into account almost all
the inefficiencies that can throw off an energy prediction that might be only calculated.