Page 180 - Anatomy of a Robot

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ENERGY CONTROL AND SOFTWARE 165
in heatsinks creates two problems. First, it’s a direct waste of energy. Second, the heat
must be dissipated. In battery-powered robots, these considerations are especially
important.
Regulation
A couple of different types of power supply exist, but the bottom line is that they all
waste some energy in an attempt to regulate the voltage going into the robot’s control
circuitry. The robot may be powered from power lines or from batteries. In either case,
the motors and control systems generate electrical noise as they turn on and off. Motors,
in particular, cause large power surges when they turn on and off. We all have seen these
sorts of surges when a large appliance starts up and the lights flicker temporarily. The
point is, even battery voltages may vary unacceptably during the operation of a robot.
We may not be able to feed the battery voltage directly into sensitive control circuits.
We also know that power from the power lines is not well behaved either; it needs clean-
ing up. For all of these reasons, robots generally have some sort of voltage regulation
circuitry or power supply. That said, before we go into a discussion of what type of
power supply to put in, let’s explore not putting a power supply in. What are the tech-
niques we can use to avoid a power supply?

Battery Power

Batteries are fairly stable voltage sources all by themselves. They do, however, vary
under certain conditions:
Charge level The voltage on batteries will decrease as the level of energy in the
batteries decreases. Different battery technologies will decrease at different rates
over time. They have characteristic discharge curves with a voltage that decreases
in a predictable manner as the charge is drained out of them. Most rechargeable
batteries decrease rapidly during the first 5 percent of the discharge curve, level
out for 85 to 90 percent of the curve, and decrease rapidly in the last 10 to 15 per-
cent of the curve. Figure 7-1 displays a representative battery discharge curve.
Voltage levels A few points need to be made about voltage levels. First, if the bat-
tery is being charged while it’s inside the robot, great care must be taken. An inex-
pensive battery charger, charging an open battery, doesn’t limit voltage. Instead, it
would attempt to deliver a much higher voltage, which spreads throughout the
robot. Batteries act like voltage limiters when they are being charged up and will
limit the voltage of a cheap charger. But if the battery opens up (or is simply
removed), the charger may fry the rest of the robot with abnormally high voltage!