<s>
In	O
control	O
engineering	O
,	O
a	O
servomechanism	O
,	O
usually	O
shortened	O
to	O
servo	B-Algorithm
,	O
is	O
an	O
automatic	O
device	O
that	O
uses	O
error-sensing	O
negative	O
feedback	O
to	O
correct	O
the	O
action	O
of	O
a	O
mechanism	O
.	O
</s>
<s>
A	O
common	O
type	O
of	O
servo	B-Algorithm
provides	O
position	O
control	O
.	O
</s>
<s>
They	O
operate	O
on	O
the	O
principle	O
of	O
negative	O
feedback	O
,	O
where	O
the	O
control	O
input	O
is	O
compared	O
to	O
the	O
actual	O
position	O
of	O
the	O
mechanical	O
system	O
as	O
measured	O
by	O
some	O
type	O
of	O
transducer	B-Algorithm
at	O
the	O
output	O
.	O
</s>
<s>
Any	O
difference	O
between	O
the	O
actual	O
and	O
wanted	O
values	O
(	O
an	O
"	O
error	B-Algorithm
signal	I-Algorithm
"	O
)	O
is	O
amplified	O
(	O
and	O
converted	O
)	O
and	O
used	O
to	O
drive	O
the	O
system	O
in	O
the	O
direction	O
necessary	O
to	O
reduce	O
or	O
eliminate	O
the	O
error	O
.	O
</s>
<s>
Positioning	O
servomechanisms	O
were	O
first	O
used	O
in	O
military	O
fire-control	B-Application
and	O
marine	O
navigation	O
equipment	O
.	O
</s>
<s>
A	O
hard	B-Device
disk	I-Device
drive	I-Device
has	O
a	O
magnetic	O
servo	B-Algorithm
system	O
with	O
sub-micrometer	O
positioning	O
accuracy	O
.	O
</s>
<s>
thumb|right|200px|Small	O
R/C	O
servo	B-Algorithm
mechanism	O
.	O
</s>
<s>
A	O
servomotor	B-Algorithm
is	O
a	O
specific	O
type	O
of	O
motor	O
that	O
is	O
combined	O
with	O
a	O
rotary	B-Algorithm
encoder	I-Algorithm
or	O
a	O
potentiometer	B-Device
to	O
form	O
a	O
servomechanism	O
.	O
</s>
<s>
A	O
potentiometer	B-Device
provides	O
a	O
simple	O
analog	O
signal	O
to	O
indicate	O
position	O
,	O
while	O
an	O
encoder	O
provides	O
position	O
and	O
usually	O
speed	O
feedback	O
,	O
which	O
by	O
the	O
use	O
of	O
a	O
PID	O
controller	O
allow	O
more	O
precise	O
control	O
of	O
position	O
and	O
thus	O
faster	O
achievement	O
of	O
a	O
stable	O
position	O
(	O
for	O
a	O
given	O
motor	O
power	O
)	O
.	O
</s>
<s>
Potentiometers	B-Device
are	O
subject	O
to	O
drift	O
when	O
the	O
temperature	O
changes	O
whereas	O
encoders	O
are	O
more	O
stable	O
and	O
accurate	O
.	O
</s>
<s>
Servomotors	B-Algorithm
are	O
used	O
for	O
both	O
high-end	O
and	O
low-end	O
applications	O
.	O
</s>
<s>
On	O
the	O
high	O
end	O
are	O
precision	O
industrial	O
components	O
that	O
use	O
a	O
rotary	B-Algorithm
encoder	I-Algorithm
.	O
</s>
<s>
On	O
the	O
low	O
end	O
are	O
inexpensive	O
radio	B-Algorithm
control	I-Algorithm
servos	I-Algorithm
(	O
RC	O
servos	O
)	O
used	O
in	O
radio-controlled	O
models	O
which	O
use	O
a	O
free-running	O
motor	O
and	O
a	O
simple	O
potentiometer	B-Device
position	O
sensor	O
with	O
an	O
embedded	O
controller	O
.	O
</s>
<s>
The	O
term	O
servomotor	B-Algorithm
generally	O
refers	O
to	O
a	O
high-end	O
industrial	O
component	O
while	O
the	O
term	O
servo	B-Algorithm
is	O
most	O
often	O
used	O
to	O
describe	O
the	O
inexpensive	O
devices	O
that	O
employ	O
a	O
potentiometer	B-Device
.	O
</s>
<s>
Stepper	B-Algorithm
motors	I-Algorithm
are	O
not	O
considered	O
to	O
be	O
servomotors	B-Algorithm
,	O
although	O
they	O
too	O
are	O
used	O
to	O
construct	O
larger	O
servomechanisms	O
.	O
</s>
<s>
Stepper	B-Algorithm
motors	I-Algorithm
have	O
inherent	O
angular	O
positioning	O
,	O
owing	O
to	O
their	O
construction	O
,	O
and	O
this	O
is	O
generally	O
used	O
in	O
an	O
open-loop	O
manner	O
without	O
feedback	O
.	O
</s>
<s>
A	O
standard	O
RC	O
receiver	O
(	O
or	O
a	O
microcontroller	O
)	O
sends	O
pulse-width	B-Algorithm
modulation	I-Algorithm
(	O
PWM	O
)	O
signals	O
to	O
the	O
servo	B-Algorithm
.	O
</s>
<s>
The	O
electronics	O
inside	O
the	O
servo	B-Algorithm
translate	O
the	O
width	O
of	O
the	O
pulse	O
into	O
a	O
position	O
.	O
</s>
<s>
When	O
the	O
servo	B-Algorithm
is	O
commanded	O
to	O
rotate	O
,	O
the	O
motor	O
is	O
powered	O
until	O
the	O
potentiometer	B-Device
reaches	O
the	O
value	O
corresponding	O
to	O
the	O
commanded	O
position	O
.	O
</s>
<s>
Steam	O
steering	O
engines	O
had	O
the	O
characteristics	O
of	O
a	O
modern	O
servomechanism	O
:	O
an	O
input	O
,	O
an	O
output	O
,	O
an	O
error	B-Algorithm
signal	I-Algorithm
,	O
and	O
a	O
means	O
for	O
amplifying	O
the	O
error	B-Algorithm
signal	I-Algorithm
used	O
for	O
negative	O
feedback	O
to	O
drive	O
the	O
error	O
towards	O
zero	O
.	O
</s>
<s>
The	O
Ragonnet	O
power	O
reverse	O
mechanism	O
was	O
a	O
general	O
purpose	O
air	O
or	O
steam-powered	O
servo	B-Algorithm
amplifier	O
for	O
linear	O
motion	O
patented	O
in	O
1909	O
.	O
</s>
<s>
World	O
War	O
II	O
saw	O
the	O
development	O
of	O
electrical	O
fire-control	B-Application
servomechanisms	O
,	O
using	O
an	O
amplidyne	O
as	O
the	O
power	O
amplifier	O
.	O
</s>
<s>
Vacuum	O
tube	O
amplifiers	O
were	O
used	O
in	O
the	O
UNISERVO	B-Device
tape	O
drive	O
for	O
the	B-Operating_System
UNIVAC	I-Operating_System
I	O
computer	O
.	O
</s>
<s>
Modern	O
servomechanisms	O
use	O
solid	O
state	O
power	O
amplifiers	O
,	O
usually	O
built	O
from	O
MOSFET	B-Architecture
or	O
thyristor	O
devices	O
.	O
</s>
<s>
Small	O
servos	O
may	O
use	O
power	O
transistors	B-Application
.	O
</s>
<s>
type	O
0	O
servos	O
:	O
under	O
steady-state	O
conditions	O
they	O
produce	O
a	O
constant	O
value	O
of	O
the	O
output	O
with	O
a	O
constant	O
error	B-Algorithm
signal	I-Algorithm
;	O
</s>
<s>
type	O
1	O
servos	O
:	O
under	O
steady-state	O
conditions	O
they	O
produce	O
a	O
constant	O
value	O
of	O
the	O
output	O
with	O
null	O
error	B-Algorithm
signal	I-Algorithm
,	O
but	O
a	O
constant	O
rate	O
of	O
change	O
of	O
the	O
reference	O
implies	O
a	O
constant	O
error	O
in	O
tracking	O
the	O
reference	O
;	O
</s>
<s>
type	O
2	O
servos	O
:	O
under	O
steady-state	O
conditions	O
they	O
produce	O
a	O
constant	O
value	O
of	O
the	O
output	O
with	O
null	O
error	B-Algorithm
signal	I-Algorithm
.	O
</s>
<s>
The	O
servo	B-Algorithm
bandwidth	O
indicates	O
the	O
capability	O
of	O
the	O
servo	B-Algorithm
to	O
follow	O
rapid	O
changes	O
in	O
the	O
commanded	O
input	O
.	O
</s>
