torque (Figure 10). It’s a 180 degree standard servo, also
available in 360 degree and continuous rotation versions.
WHERE TO BUY
Servos are widely available at local hobby shops and
online. Standard size analog servos are generally priced
from $5-$65. ServoCity mainly sells big brands like Futaba
and Hitec, along with their wide variety of servo
accessories. Discounter Hobby King sells all brands,
including smaller Turnigy, BMS, and Tower Pro/Hextronic
brands. Hobby King has an exceptional value in its $2.69,
nine gram HXT900 version.
It is very popular and often on backorder. Hobby
robotics sellers Adafruit, Parallax, Pololu, RobotShop,
Solarbotics, and SparkFun all sell servos.
Not surprisingly, a servo’s power output gets stronger
and faster as the supply voltage increases, but going much
above 6V risks damage as mentioned earlier. Some servos
can operate down to three or four volts with
correspondingly lower power and speed, and often with
less accuracy and repeatability. Some of my small robots
use a single 3.7V Li-Ion battery to power everything directly,
including their CR servos.
Some applications have very specific power
requirements. A six-axis arm I built worked very well, but it
required a very specific 4.0 volts for proper operation. The
servos would oscillate at higher voltages and lost precision
at lower voltages.
As mentioned initially, servos consume most of the
power on a robot. I bench tested a dozen servos of various
sizes and under ideal stationary, quiet, no-load conditions,
they all draw 5-10 mA. Sounds very efficient. As soon as
they “growl” (still stationary with no load applied), however,
current jumps up to 80-100 mA just sitting there. Try slow
motion with no load and it goes up to 100-150 mA. Any
loads or just accelerating an unloaded servo can pull an
amp or more per servo.
I have generally found that a single battery can power
a robot’s servos and microcontroller if filter/decoupling caps
are used. A small 0.1 µF disk cap and a 10+ µF electrolytic
cap right at the microcontroller power pins works great. Of
course, the battery needs to be able to maintain voltage
while delivering high current.
Choosing the right battery and servo combination for a
robot takes experience and testing. Rechargeable lithium-ion
(Li-Ion) and lithium polymer (Li-Po) batteries offer better
power to weight ratios than older NiMH or NiCad batteries,
and make disposable alkaline cells look ridiculous in
comparison. Perhaps the most common “noob” problems
discussed in Parallax’s very helpful robotics forum stem from
weak alkaline batteries. Things appear to function properly
until the servos move — drawing an amp or more — causing
the old batteries’ voltage to drop and reset the
microcontroller. New alkaline batteries almost always solve
An outstanding example of servo power management
is the walking "Tai Chi" robot (Figure 11) shown at
discussed at www.picaxeforum.co.uk/showthread
.php?27524-9g-Tai-Chi-Stepper. The builder is running 12
small 9 g servos from just four AAA alkaline batteries.
SERVOS FROM A
Hobbyists use several different microprocessors in their
74 SERVO 07.2015