<s>
Servos	O
(	O
also	O
RC	O
servos	O
)	O
are	O
small	O
,	O
cheap	O
,	O
mass-produced	O
servomotors	B-Algorithm
or	O
other	O
actuators	B-Algorithm
used	O
for	O
radio	O
control	O
and	O
small-scale	O
robotics	O
.	O
</s>
<s>
Most	O
servos	O
are	O
rotary	B-Algorithm
actuators	I-Algorithm
although	O
other	O
types	O
are	O
available	O
.	O
</s>
<s>
Linear	B-Algorithm
actuators	I-Algorithm
are	O
sometimes	O
used	O
,	O
although	O
it	O
is	O
more	O
common	O
to	O
use	O
a	O
rotary	B-Algorithm
actuator	I-Algorithm
with	O
a	O
bellcrank	O
and	O
pushrod	O
.	O
</s>
<s>
Some	O
types	O
,	O
originally	O
used	O
as	O
sail	B-Language
winches	O
for	O
model	O
yachting	O
,	O
can	O
rotate	O
continuously	O
.	O
</s>
<s>
A	O
typical	O
servo	B-Algorithm
consists	O
of	O
a	O
small	O
electric	O
motor	O
driving	O
a	O
train	O
of	O
reduction	O
gears	O
.	O
</s>
<s>
A	O
potentiometer	B-Device
is	O
connected	O
to	O
the	O
output	O
shaft	O
.	O
</s>
<s>
The	O
position	O
of	O
the	O
output	O
,	O
measured	O
by	O
the	O
potentiometer	B-Device
,	O
is	O
continually	O
compared	O
to	O
the	O
commanded	O
position	O
from	O
the	O
control	O
(	O
i.e.	O
,	O
the	O
radio	O
control	O
)	O
.	O
</s>
<s>
Any	O
difference	O
gives	O
rise	O
to	O
an	O
error	B-Algorithm
signal	I-Algorithm
in	O
the	O
appropriate	O
direction	O
,	O
which	O
drives	O
the	O
electric	O
motor	O
either	O
forwards	O
or	O
backwards	O
,	O
and	O
moving	O
the	O
output	O
shaft	O
to	O
the	O
commanded	O
position	O
.	O
</s>
<s>
When	O
the	O
servo	B-Algorithm
reaches	O
this	O
position	O
,	O
the	O
error	B-Algorithm
signal	I-Algorithm
reduces	O
and	O
then	O
becomes	O
zero	O
,	O
at	O
which	O
point	O
the	O
servo	B-Algorithm
stops	O
moving	O
.	O
</s>
<s>
If	O
the	O
servo	B-Algorithm
position	O
changes	O
from	O
that	O
commanded	O
,	O
whether	O
this	O
is	O
because	O
the	O
command	O
changes	O
,	O
or	O
because	O
the	O
servo	B-Algorithm
is	O
mechanically	O
pushed	O
from	O
its	O
set	O
position	O
,	O
the	O
error	B-Algorithm
signal	I-Algorithm
will	O
re-appear	O
and	O
cause	O
the	O
motor	O
to	O
restore	O
the	O
servo	B-Algorithm
output	O
shaft	O
to	O
the	O
position	O
needed	O
.	O
</s>
<s>
Radio	O
control	O
servos	O
are	O
connected	O
through	O
a	O
standard	O
three-wire	O
connection	O
:	O
two	O
wires	O
for	O
a	O
DC	O
power	O
supply	O
and	O
one	O
for	O
control	O
,	O
carrying	O
a	O
pulse-width	B-Algorithm
modulation	I-Algorithm
(	O
PWM	O
)	O
signal	O
.	O
</s>
<s>
Each	O
servo	B-Algorithm
has	O
a	O
separate	O
connection	O
and	O
PWM	O
signal	O
from	O
the	O
radio	O
control	O
receiver	O
.	O
</s>
<s>
This	O
signal	O
is	O
easily	O
generated	O
by	O
simple	O
electronics	O
,	O
or	O
by	O
microcontrollers	B-Architecture
such	O
as	O
the	O
Arduino	O
.	O
</s>
<s>
This	O
,	O
together	O
with	O
their	O
low-cost	O
,	O
has	O
led	O
to	O
their	O
wide	O
adoption	O
for	O
robotics	O
and	O
physical	B-General_Concept
computing	I-General_Concept
.	O
</s>
<s>
Pulse	O
widths	O
outside	O
this	O
range	O
can	O
be	O
used	O
for	O
"	O
overtravel	O
"	O
-	O
moving	O
the	O
servo	B-Algorithm
beyond	O
its	O
normal	O
range	O
.	O
</s>
<s>
Each	O
PWM	O
defines	O
a	O
value	O
that	O
is	O
used	O
by	O
the	O
servo	B-Algorithm
to	O
determine	O
its	O
expected	O
position	O
.	O
</s>
<s>
The	O
"	O
absolute	O
"	O
type	O
allows	O
up	O
to	O
eight	O
servos	O
to	O
share	O
one	O
communication	O
channel	O
by	O
multiplexing	B-Architecture
control	O
signals	O
using	O
relatively	O
simple	O
electronics	O
and	O
is	O
the	O
basis	O
of	O
modern	O
RC	O
servos	O
.	O
</s>
<s>
The	O
two	O
types	O
are	O
both	O
PWM	O
because	O
the	O
servo	B-Algorithm
responds	O
to	O
the	O
width	O
of	O
the	O
pulse	O
.	O
</s>
<s>
However	O
,	O
in	O
the	O
first	O
case	O
a	O
servo	B-Algorithm
may	O
also	O
be	O
sensitive	O
to	O
pulse	O
order	O
.	O
</s>
<s>
The	O
servo	B-Algorithm
is	O
controlled	O
by	O
three	O
wires	O
:	O
ground	O
,	O
power	O
,	O
and	O
control	O
.	O
</s>
<s>
The	O
servo	B-Algorithm
will	O
move	O
based	O
on	O
the	O
pulses	O
sent	O
over	O
the	O
control	O
wire	O
,	O
which	O
set	O
the	O
angle	O
of	O
the	O
actuator	B-Algorithm
arm	O
.	O
</s>
<s>
The	O
servo	B-Algorithm
expects	O
a	O
pulse	O
every	O
20	O
ms	O
in	O
order	O
to	O
gain	O
correct	O
information	O
about	O
the	O
angle	O
.	O
</s>
<s>
The	O
width	O
of	O
the	O
servo	B-Algorithm
pulse	O
dictates	O
the	O
range	O
of	O
the	O
servo	B-Algorithm
's	O
angular	O
motion	O
.	O
</s>
<s>
A	O
servo	B-Algorithm
pulse	O
of	O
1.5	O
ms	O
width	O
will	O
typically	O
set	O
the	O
servo	B-Algorithm
to	O
its	O
"	O
neutral	O
"	O
position	O
(	O
typically	O
half	O
of	O
the	O
specified	O
full	O
range	O
)	O
,	O
a	O
pulse	O
of	O
1.0	O
ms	O
will	O
set	O
it	O
to	O
0°	O
,	O
and	O
a	O
pulse	O
of	O
2.0	O
ms	O
to	O
90°	O
(	O
for	O
a	O
90°	O
servo	B-Algorithm
)	O
.	O
</s>
<s>
The	O
physical	O
limits	O
and	O
timings	O
of	O
the	O
servo	B-Algorithm
hardware	O
varies	O
between	O
brands	O
and	O
models	O
,	O
but	O
a	O
general	O
servo	B-Algorithm
's	O
full	O
angular	O
motion	O
will	O
travel	O
somewhere	O
in	O
the	O
range	O
of	O
90°	O
–	O
180°	O
and	O
the	O
neutral	O
position	O
(	O
45°	O
or	O
90°	O
)	O
is	O
almost	O
always	O
at	O
1.5	O
ms.	O
</s>
<s>
This	O
is	O
the	O
"	O
standard	O
pulse	O
servo	B-Algorithm
mode	O
"	O
used	O
by	O
all	O
hobby	O
analog	O
servos	O
.	O
</s>
<s>
A	O
hobby	O
digital	O
servo	B-Algorithm
is	O
controlled	O
by	O
the	O
same	O
"	O
standard	O
pulse	O
servo	B-Algorithm
mode	O
"	O
pulses	O
as	O
an	O
analog	O
servo	B-Algorithm
.	O
</s>
<s>
Some	O
hobby	O
digital	O
servos	O
can	O
be	O
set	O
to	O
another	O
mode	O
that	O
allows	O
a	O
robot	O
controller	O
to	O
read	O
back	O
the	O
actual	O
position	O
of	O
the	O
servo	B-Algorithm
shaft	O
.	O
</s>
<s>
Common	O
battery	O
packs	O
are	O
either	O
NiCd	O
,	O
NiMH	B-Device
or	O
lithium-ion	O
polymer	O
battery	O
(	O
LiPo	O
)	O
type	O
.	O
</s>
<s>
Two	O
figures	O
are	O
typically	O
stated	O
:	O
angular	O
speed	O
of	O
servo	B-Algorithm
shaft	O
rotation	O
and	O
mechanical	O
torque	B-General_Concept
produced	O
on	O
the	O
shaft	O
.	O
</s>
<s>
Speed	O
is	O
expressed	O
as	O
a	O
time	O
interval	O
that	O
a	O
servo	B-Algorithm
requires	O
in	O
order	O
to	O
rotate	O
the	O
shaft	O
for	O
60°	O
angle	O
.	O
</s>
<s>
Torque	B-General_Concept
is	O
expressed	O
as	O
weight	O
that	O
can	O
be	O
pulled	O
up	O
by	O
the	O
servo	B-Algorithm
if	O
it	O
hangs	O
from	O
a	O
pulley	O
with	O
a	O
certain	O
radius	O
mounted	O
on	O
the	O
shaft	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
servo	B-Algorithm
model	O
is	O
described	O
as	O
"	O
0.2s	O
/	O
2kg	O
"	O
,	O
that	O
should	O
be	O
interpreted	O
as	O
"	O
This	O
servo	B-Algorithm
rotates	O
the	O
shaft	O
for	O
60°	O
in	O
0.2seconds	O
,	O
and	O
it	O
is	O
able	O
to	O
pull	O
up	O
2kg	O
weight	O
using	O
a	O
1cm	O
radius	O
pulley	O
"	O
.	O
</s>
<s>
That	O
is	O
,	O
that	O
particular	O
servo	B-Algorithm
model	O
rotates	O
the	O
shaft	O
with	O
the	O
angular	O
speed	O
of	O
(	O
2π	O
/	O
6	O
)	O
/	O
0.2s	O
=	O
5.2rad/s	O
while	O
producing	O
2kg	O
×	O
9.81m/s2	O
=	O
19.6N	O
force	O
at	O
1cm	O
distance	O
,	O
i.e.	O
</s>
<s>
it	O
produces	O
19.6N	O
×	O
0.01m	O
=	O
0.196Nm	O
torque	B-General_Concept
.	O
</s>
<s>
They	O
can	O
be	O
thought	O
of	O
as	O
a	O
motor	O
and	O
gearbox	O
with	O
servo	B-Algorithm
input	O
controls	O
.	O
</s>
<s>
A	O
smaller	O
value	O
should	O
turn	O
the	O
servo	B-Algorithm
clockwise	O
and	O
a	O
higher	O
one	O
counterclockwise	O
.	O
</s>
<s>
The	O
earliest	O
form	O
of	O
sequential	O
(	O
although	O
not	O
proportional	O
)	O
actuator	B-Algorithm
for	O
radio	O
control	O
was	O
the	O
escapement	O
.	O
</s>
<s>
The	O
fly-ball	O
actuator	B-Algorithm
was	O
introduced	O
to	O
R/C	O
modelling	O
in	O
1951	O
by	O
Brayton	O
Paul	O
,	O
and	O
consisted	O
of	O
an	O
electric	O
motor	O
and	O
a	O
centrifugal	O
governor	O
connected	O
to	O
a	O
free-running	O
axis	O
that	O
could	O
,	O
with	O
the	O
motor	O
running	O
,	O
pull	O
a	O
rudder	O
control	O
rod	O
by	O
varying	O
degrees	O
.	O
</s>
