Page 259 - Robot Builder's Bonanza

228 CHOOSING THE RIGHT MOTOR

Dealing with Voltage Drops

On most robots it’s the motors that draw the most current. As the motors stop and start, the
voltage provided to them can change. This happens because under heavy current draw, the
voltage provided by the batteries can momentarily sag. The sag may be for only a fraction of
a second, but it can be long enough to cause problems.
If your robot’s control computer is connected to the same battery source as the motors, the
voltage drop can cause what’s known as a brownout; if the brownout causes the voltage to
drop below a certain threshold (Figure 21- 9), the control electronics may not operate cor-
rectly. Quite literally, your robot can go berserk!
Brownouts are particularly troublesome when using microcontrollers (see Part 7), which
are small computers that run a program you devised. During a brownout, the microcontroller
may spontaneously reset, causing it to rerun its programming from the beginning. This can
actually occur several dozens or even hundreds of times a second.
During these brownouts your robot may become inactive (that’s good), or it may lurch or
spin or do something else unpredictable (that’s bad).
There are several ways of avoiding voltage drops caused by motors:

• Add more volts to your robot’s batteries. If the batteries normally supply 6 volts, but may
“sag” to 4.5 volts during heavy motor draw, add another cell to bring the voltage to 7.2 or
7.5. The extra margin might prevent a voltage sag that causes a brownout.
• Add bigger batteries with a higher capacity (amp hours). The volts may be the same, but
the added current capability can provide a reserve against voltage drops.
• Add a second battery pack to operate the electronics. This is my preferred method,
because it effectively isolates the power supplies for the motor and electronics. As the
motors draw current, they pull it from their own batteries and not from the pack powering
the electronics.

For small robots you can often use a AAA- to D- size battery pack for the motors, and a
9- volt battery for the robot control electronics. If you have lots of electronics— microcontroller,
multiple sensors, maybe a video camera— you might need to beef up the second battery. Use
a separate multicell battery pack selected to provide the current needed for the electronics.
G In order for the dual power supply technique to work, the ground (–, negative) lead from both
battery packs must be connected together. This topic is discussed in more detail in Chapter 22.

5 Operation OK
Brownout
4 threshold
Possible robot
3 malfunction
Volts
2 Complete Figure 21- 9 A brownout may occur
system failure
1 when the system voltage of your robot falls
below a certain minimum level. As the
0 voltage falls under the brownout threshold,
Low voltage condition operation of the robot becomes
causing system brownout unpredictable.

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