1
Chapter 11
SERVO VALVES (contd.)
Fluid Power Circuits and
Controls, John S.Cundiff,
2001
Servo Analysis
The concept of gain has
preciously been defined
G = Output / Input
Feedback principle
: We
sense the output, the error
between input and feedback
loop drives the system to the
desired zero error condition,
thus feedback helps control
the system.

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2
Servo Analysis
Feedback signal will be opposite in sign to the input
signal.
When correcting for drift in the output, we must
move it back toward the set point. This correction is
called negative feedback.
Closed loop hydraulic systems are also called servo
systems.
Servo Analysis
Feedback signal is typically a scaled DC voltage,
which is proportional to the output signal.
If the feedback signal is an AC sine wave it must be
shifted in phase by 180
o
from the input.
When the amplitudes are equal, the two signals
cancel each other, and the resulting error is zero.

3
Servo Analysis
Block diagram of closed-
loop system for servo
cylinder.
The servo valve transfer
function is the flow transfer
function, not the pressure
transfer function.
Input to the cylinder is a
flow, in
3
/s, and the output is
a linear velocity, in/s.
Servo Analysis
Transfer function is given by ,
G
cyl
= output
= in/s
=
in
=
1
=
1
input
in
3
/s
in
3
in
2
A
where A = cylinder area (in
2
).
A typical feedback transducer is the potentiometer.
It’s transfer function is V/in. A linear velocity (in/s)
drives the potentiometer to produce the feedback
signal (V), not V/s.

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