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ENERGY CONTROL AND SOFTWARE 187
traction surface. It does no good to spin the robot’s wheels when it is accelerating. That’s
just a waste of power, time, and rubber. (The tire makers in Detroit will be glad I can-
not conceive of moving on anything other than tires.) The following discussion assumes
the robot has more than one speed or can choose between more than one torque setting
on the wheels. To counteract spinning wheels, the robot must first be able to sense the
event. The robot’s control system can sense when the tires are spinning in several ways.
The simplest method is to determine the speed of the robot over the terrain and com-
pare it to a model of the wheels. If one wheel is spinning significantly faster than the
others, it is probably not gripping the same surface. The same sensors used in ABS
brakes would work in this case.
A slightly more difficult method is to sense the torque on each wheel directly. This
can be done with spring mechanisms or by monitoring the voltages on the motor wind-
ings. A motor meeting no resistance will not consume as much power to spin the wheels
at a known rate. If the wheel is spinning, the motor control circuitry should be able to
signal that.

RECLAIMING ENERGY
One of the features that comes almost for free with an electric car is the ability to gen-
erate electricity when going downhill or braking. (A fun web site that should come in
handy and that details much of the thinking that has gone into electric cars is at
www.howstuffworks.com/electric-car.htm.) If a robot takes 100 watt-hours of energy to
climb a hill, we might think we could reclaim most of those 100-watt hours by going
down the other side of the hill. But alas the laws of thermodynamics get in the way.
Surely, we would not want the thermodynamic police to be on our tail.
The second law states that the entropy of an isolated system can never decrease. This
limits the efficiency of energy conversion between different types of energy. It’s rarely
possible to approach 25 percent efficiency converting electrical energy to kinetic energy
and back to electrical energy again. Reclaiming energy is very difficult and should only
be attempted if the equipment is virtually free and does not interfere with other
processes. It rarely pays off in a device as complex as a robot. More info on thermody-
namics and energy conversion can be found at http://members.aol.com/engware/
systems.htm.
Revisiting technology is one of the pleasures of writing a book like this. During my
search for good supplementary web sites, I often run across some odd twists on things.
For some truly interesting reading, I offer the satirical web page of the Thermodynamic
Law Party (http://zapatopi.net/tlp.html). The thermocrats among you will already rec-
ognize the principals therein. For the rest of us, read this site with care. On the site, it
states that Kelvinian meditation causes epileptic seizures “only in lab mice at extreme