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SPECIAL- PURPOSE SERVO TYPES AND SIZES 253

The actual length of the pulses used to position a servo to its full left or right positions var-
ies among servo brands, and sometimes even among different models by the same manufac-
turer. You need to do some experimenting to find the optimum pulse width ranges for the
servos you use. This is just part of what makes robot experimenting so much fun!
The 1-to- 2- ms range has built- in safety margins to prevent possible damage to the servo.
Using this range provides only about 100° of turning, which is fine for many tasks.
But if you want a full stop- to- stop rotation, you need to apply pulses shorter and longer
than 1 to 2 ms. Exactly how long depends entirely on your specific servo. Full rotation (to
the stop) for one given make and model of servo might be 0.730 ms in one direction and
2.45 ms in the other direction.
You must be very careful when using shorter or longer pulses than the recommended
1-to- 2- ms range. Should you attempt to command a servo beyond its mechanical limits, the
output shaft of the motor will hit an internal stop, which could cause gears to grind or chatter.
If left this way for more than a few seconds, the gears may be permanently damaged.
The 1.5 ms “in- between” pulse may also not precisely center all makes and models of
servos. Slight electrical differences even in servos of the same model may produce minute
differences in the centering location.

Timing signals for R/C servos are often stated in milliseconds, but a more accurate unit of
measure is the microsecond— or millionth of a second. In the programming chapters that follow
G you’ll more often see timing pulses for servos stated in microseconds.
To convert milliseconds to microseconds, just move the decimal point to the right three digits.
For example, if a pulse is 0.840 milliseconds, move the decimal point over three digits and you
have 0840, or 840 microseconds (lop off the leading zero; it’s not needed).

The Role of the Potentiometer

The potentiometer of the servo plays a key role in allowing the motor to set the position of its
output shaft, so it deserves a short explanation of its own.
The potentiometer is mechanically attached to the output shaft of the servo (in some servo
models, the potentiometer is the output shaft). In this way, the position of the potentiometer
very accurately reflects the position of the output shaft of the servo.
The control circuit in the servo compares the position of the potentiometer with the pulses
you feed into the servo. The result of this comparison is an error signal. The control circuitry
compensates by moving the motor inside the servo one way or the other. When the potenti-
ometer reaches its final proper position, the error signal disappears and the motor stops.

Special- Purpose Servo Types and Sizes

While the standard- size servo is the one most commonly used in both robotics and radio-
controlled models, other R/C servo types, styles, and sizes also exist.

• Quarter- scale (or large- scale) servos are about twice the size of standard servos and are sig-
nificantly more powerful. Quarter- scale servos make perfect power motors for a robot arm.
• Mini- and micro servos are diminutive versions of standard servos and are designed to be

used in tight spaces in a model airplane or car— or robot. They aren’t as strong as standard
servos.

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