<s>
Servo	B-Algorithm
control	I-Algorithm
is	O
a	O
method	O
of	O
controlling	O
many	O
types	O
of	O
RC/hobbyist	B-Algorithm
servos	I-Algorithm
by	O
sending	O
the	O
servo	B-Algorithm
a	O
PWM	O
(	O
pulse-width	B-Algorithm
modulation	I-Algorithm
)	O
signal	O
,	O
a	O
series	O
of	O
repeating	O
pulses	O
of	O
variable	O
width	O
where	O
either	O
the	O
width	O
of	O
the	O
pulse	O
(	O
most	O
common	O
modern	O
hobby	O
servos	O
)	O
or	O
the	O
duty	O
cycle	O
of	O
a	O
pulse	O
train	O
(	O
less	O
common	O
today	O
)	O
determines	O
the	O
position	O
to	O
be	O
achieved	O
by	O
the	O
servo	B-Algorithm
.	O
</s>
<s>
The	O
PWM	O
signal	O
might	O
come	O
from	O
a	O
radio	O
control	O
receiver	O
to	O
the	O
servo	B-Algorithm
or	O
from	O
common	O
microcontrollers	B-Architecture
such	O
as	O
the	O
Arduino	O
.	O
</s>
<s>
Given	O
the	O
rotation	O
constraints	O
of	O
the	O
servo	B-Algorithm
,	O
neutral	O
is	O
defined	O
to	O
be	O
the	O
center	O
of	O
rotation	O
.	O
</s>
<s>
This	O
is	O
a	O
form	O
of	O
pulse-width	B-Algorithm
modulation	I-Algorithm
.	O
</s>
<s>
The	O
typical	O
RC	B-Algorithm
servo	I-Algorithm
expects	O
to	O
see	O
a	O
pulse	O
every	O
20ms	O
,	O
however	O
this	O
can	O
vary	O
within	O
a	O
wide	O
range	O
that	O
differs	O
from	O
servo	B-Algorithm
to	O
servo	B-Algorithm
.	O
</s>
<s>
The	O
low	O
time	O
(	O
and	O
the	O
total	O
period	O
)	O
can	O
vary	O
over	O
a	O
wide	O
range	O
,	O
and	O
vary	O
from	O
one	O
pulse	O
to	O
the	O
next	O
,	O
without	O
any	O
effect	O
on	O
the	O
position	O
of	O
the	O
servo	B-Algorithm
motor	O
.	O
</s>
<s>
Modern	O
RC	B-Algorithm
servo	I-Algorithm
position	O
is	O
not	O
defined	O
by	O
the	O
PWM	O
duty	O
cycle	O
(	O
i.e.	O
,	O
ON	O
vs	O
OFF	O
time	O
)	O
but	O
only	O
by	O
the	O
width	O
of	O
the	O
pulse	O
.	O
</s>
<s>
The	O
period	O
of	O
20ms	O
(	O
50Hz	O
)	O
comes	O
from	O
the	O
days	O
where	O
the	O
signal	O
was	O
encoded	O
in	O
PPM	O
(	O
pulse-position	B-Device
modulation	I-Device
)	O
format	O
to	O
be	O
sent	O
over	O
the	O
air	O
.	O
</s>
<s>
The	O
PPM	O
period	O
was	O
around	O
22.5ms	O
,	O
and	O
the	O
conversion	O
to	O
PWM	O
was	O
trivial	O
:	O
the	O
time	O
of	O
the	O
PWM	O
high	O
state	O
was	O
the	O
time	O
position	O
of	O
the	O
PPM	O
pulse	O
for	O
that	O
servo	B-Algorithm
.	O
</s>
<s>
Most	O
RC	O
receivers	O
send	O
pulses	O
to	O
the	O
RC	B-Algorithm
servo	I-Algorithm
at	O
some	O
constant	O
frame	O
rate	O
,	O
changing	O
only	O
the	O
high	O
time	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
possible	O
to	O
command	O
an	O
RC	B-Algorithm
servo	I-Algorithm
to	O
move	O
over	O
its	O
entire	O
range	O
with	O
a	O
function	O
generator	O
set	O
to	O
a	O
constant	O
10%	O
duty	O
cycle	O
by	O
changing	O
only	O
the	O
frequency	O
(	O
frame	O
rate	O
)	O
.	O
</s>
<s>
If	O
an	O
external	O
force	O
pushes	O
against	O
the	O
servo	B-Algorithm
while	O
the	O
servo	B-Algorithm
is	O
holding	O
a	O
position	O
,	O
the	O
servo	B-Algorithm
will	O
resist	O
from	O
moving	O
out	O
of	O
that	O
position	O
.	O
</s>
<s>
The	O
maximal	O
amount	O
of	O
force	O
the	O
servo	B-Algorithm
can	O
exert	O
is	O
the	O
torque	O
rating	O
of	O
the	O
servo	B-Algorithm
.	O
</s>
<s>
Servos	O
will	O
only	O
hold	O
their	O
position	O
for	O
their	O
timeout	O
duration	O
,	O
though	O
;	O
the	O
position	O
pulse	O
must	O
be	O
repeated	O
,	O
usually	O
within	O
20ms	O
,	O
to	O
instruct	O
the	O
servo	B-Algorithm
to	O
stay	O
in	O
position	O
.	O
</s>
<s>
When	O
a	O
pulse	O
is	O
sent	O
to	O
a	O
servo	B-Algorithm
that	O
is	O
less	O
than	O
1.5ms	O
,	O
the	O
servo	B-Algorithm
rotates	O
to	O
a	O
position	O
and	O
holds	O
its	O
output	O
shaft	O
some	O
number	O
of	O
degrees	O
counterclockwise	O
from	O
the	O
neutral	O
point	O
.	O
</s>
<s>
The	O
minimal	O
and	O
maximal	O
widths	O
of	O
pulse	O
that	O
will	O
command	O
the	O
servo	B-Algorithm
to	O
turn	O
to	O
a	O
valid	O
position	O
are	O
functions	O
of	O
each	O
servo	B-Algorithm
.	O
</s>
