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USING SERVO MOTORS FOR SENSOR TURRETS 265

LIMITATIONS OF MODIFIED SERVOS
Modifying a servo for continual rotation carries with it a few limitations, exceptions, and “got-
chas” that you’ll want to keep in mind:
• The average servo is not engineered for lots and lots of continual use. The mechanics of
the servo are likely to wear out after perhaps as little as 25 hours (that’s total elapsed time,
which for a robot is quite a long time), depending on the amount of load on the servos.
Models with metal gears and/or brass bushing or ball bearings will last longer.
• Standard- size servos are not particularly strong in comparison to many other DC motors
with gearboxes. Don’t expect a standard servo to move a 5- or 10- pound robot. If your
robot is heavy, consider using either larger, higher- output servos (such as 1/4- scale or sail
winch) or, better yet, DC motors with bolted- on gearboxes.
• Last and certainly not least, remember that modifying a servo voids its warranty.

Using Servo Motors for Sensor Turrets

After all this talk of using R/C servos as robot drive motors it’s easy to forget what they were
created for in the first place: for precise position and control. A common application in robot-
ics is the sensor turret, so called because it acts as a rotating turret (like a cannon gun turret)
for one or more robot sensors. Typical sensors for turrets include ultrasonic and infrared
proximity detection— these are detailed in Chapter 43, “Proximity and Distance Sensing.”
The concept of the rotating sensor turret is simple: put a sensor on top of the servo, and
then “scan” it back and forth by alternating the position of the servo left and right. A simple
way to mount a sensor is atop a servo horn. You can use double- sided foam tape, hot- melt
glue, even a rubber band (wound around a couple of times) to secure the sensor to the horn.
Or use a specialty sensor bracket, like the one in Figure 23- 10. Sensor brackets come in
all shapes and sizes; the one in the picture is designed for
the typical two- transducer ultrasonic sensor, such as the
Devantech SRF05 or the Parallax Ping. Holes in the back
of the bracket allow the connection wires to come through.
You can mount the servo to the robot using a variety of
methods. Foam tape and glue are always options, but I
prefer mechanical fasteners that can be easily removed and
reattached; the fasteners make it easier to build and change
the robot.

Figure 23- 10 A servo used to rotate a
sensor, mounted on a bracket.

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