Page 181 - Anatomy of a Robot

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166 CHAPTER SEVEN
Volts
Battery Discharge Curve

Time

FIGURE 7-1 Battery voltage varies during a discharge cycle.

This is especially true if no power supply circuitry exists and the robot is running
off the battery directly.
Internal resistance Batteries will all have different internal resistances. This
behaves much like a resistor in series with the battery. As the battery ages, this
resistance may change. When a motor or other heavy load, places a sudden
demand on the battery for current, the voltage of the battery will change quickly.
Make sure the rest of the robot’s control circuitry and sensitive instrumentation
can take the sudden voltage transient on the power supply.
Lifetime Don’t forget that the ability of batteries to store energy will change
over time. Many types of batteries (with different internal chemistry) will lose
their capability to store power as the battery ages. Within the battery, chemicals,
gases, and metals migrate or slowly corrode so they are no longer able to fully
contribute to energy storage. Make sure the robot’s circuitry will be able to func-
tion just as well when the robot and its batteries reach old age (see Figure 7-2).

Power Requirements

If we are trying to power a robot using just the battery as our power supply, we need to
limit the number of different voltages that will be needed within the robot. This may
mean that all the electrical components must be selected so they can work off the same
voltage. This becomes quite a challenge when we try to pick motors, sensors, and com-
puters that all have similar requirements for voltage. So what voltage should we try for?
High voltage, for example, is not a good choice for running computers or most sensors.

Motors To complicate things further, low voltage does not work well to move motors.
We can use very low voltage drop Field Effect Transistors (FETs) to control the motor