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260 USING SERVO MOTORS

latitude in input voltage, and 6 volts from a four- pack of AAs provides more than enough
juice. As the batteries drain, however, the voltage will drop, and you will notice your servos
won’t be as fast or strong as they used to be.
But what about going beyond the voltage of typical rechargeable batteries used for R/C
models? Indeed, some servos can be operated at 7.2 volts, but always check the datasheet
that comes with the servos you are using. Unless you need the extra torque or speed, it’s best
to keep the supply voltage to your servos at no more than 7.2 volts, and preferably between
the rated 4.8- to 6- volt range specified in the manufacturer’s literature.

WORKING WITH AND AVOIDING THE “DEAD BAND”

References to the Grateful Dead notwithstanding, all servos exhibit what’s known as a dead
band. The dead band of a servo is the maximum time difference between the incoming con-
trol signal and the internal reference signal produced by the position of the potentiometer. If
the difference equates to less than the dead band— say, 5 or 6 microseconds— the servo will
not bother trying to nudge the motor to correct for the error.
Without the dead band, the servo would constantly “hunt” back and forth to find the exact
match between the incoming signal and its own internal reference signal. The dead band
allows the servo to minimize this hunting so it will settle down to a position close to, though
maybe not exactly, where it’s supposed to be.
The dead band amounts vary and are often listed as part of the servo’s specifications. A
typical dead band is 5 s. If the servo has a full travel of 180° over a 1000- s range, then the
5- s dead band equates to 1 part in 200. If your control circuitry has a resolution higher than
the dead band, then small changes in the pulse width values may not produce any effect. For
instance, if the controller has a resolution of 2 s, and if the servo has a dead band of 5 s,
you must make changes in the pulse width in no less than 5- s increments.

Using Continuously Rotating Servos

So far I’ve only talked about servos that are meant to turn a portion of a circle. These are used
when precise angular positioning is required, such as scanning a sensor from side to side.
But R/C servos can also rotate continuously, either by design or via a modification that you
perform yourself. R/C servos make terrific drive motors for your robot. They tend to be less
expensive than comparable DC gear motors of the same specification, and they come with
their own driver electronics. They’re definitely worth considering for your next robot.
Servos that rotate continuously act like an ordinary geared DC motor, except they’re still
controlled by sending the motor pulses.

• To make the motor go in one direction, send it 1 ms pulses.
• To make the motor go in the other direction, send it 2 ms pulses.
• To make the motor slow to a stop, send it 1.5 ms pulses.
• To make the motor stop altogether, stop sending it pulses.
Stopping the motor by ceasing the pulses works for all except digital servos. With most digital
G servos, when pulses are stopped the servo will merely continue with the last good position
information it received.
You’re not likely to use digital servos for continuous rotation, so this problem seldom comes
up in real life. But keep it in mind just in case.

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