A servo valve is a directional control valve that can be precisely positioned using an electrical signal to control the amount and direction of fluid flow. Coupled with sensors, servo valves provide very accurate control of position, velocity, or acceleration. Compared to proportional valves, servo valves use closed-loop control for higher accuracy and frequency response but require cleaner fluid and more maintenance. Servo valves are the most sensitive to contamination and require fluid cleanliness of 5 microns or less.

1. 1
SERVO VALVES

2. 2
SERVO VALVE
A SERVO VALVE IS A DIRECTIONAL VALVE
IT MAY BE INFINITELY POSITIONED TO PROVIDE
CONTROL OF BOTH THE
AMOUNT AND THE DIRECTION OF FLUID FLOW.
A SERVO VALVE COUPLED WITH PROPER
FEEDBACK SENSING DEVICES PROVIDES VERY
ACCURATE CONTROL OF THE POSITION,
VELOCITY OR ACCELERATION OF AN ACTUATOR
.

3. 3
CONVENTIONAL DIRECTIONAL VALVE
P A
B
T

4. 4
SERVO VALVE
- Compare with a Proportional Valve
PROPORTIONAL
VALVE
SERVO VALVE
Pressure , flow,
Direction control
by electrical signal
Pressure, flow,
Direction control
by electrical signal
Controller &
Amplifier is used
Controller &
Amplifier is used
Based on Control Method

5. 5
SERVO VALVE
PROPORTIONAL
VALVE
SERVO VALVE
For general
electrical control
system.
High Accuracy, High
Frequency
applications.
Open loop or
Closed loop circuit
Closed loop circuit
- Compare with a Proportional Valve Based on Application

6. 6
PROPORTIONAL VALVES & SERVO VALVES
OPEN LOOP & CLOSED LOOP
Open Loop circuit :
Accuracy, Response : POOR
Controllability : GOOD
Cost : LESS
Controller controls the hydraulic valve according to the input signal
which is given from the controller to the valve.
SLOW
FAST

7. 7
PROPORTIONAL VALVES & SERVO VALVES
Closed Loop Circuit :
According to the requirement of speed or position or pressure ,
input signal is given and compared with output value through a
sensor and necessary adjustments are made.
Input signal and output values are compared and differences are
corrected. This gives high accuracy & high Response
( Feed back Control System )
CONSTANT
SPEED

8. 8
SERVO VALVE
Control method in a Proportional Valve

9. 9
SERVO VALVE
Control method in a Proportional Valve
By Providing Spool between Magnetic Solenoid
and Spring it is possible to control Hydraulic
energy.

10. 10
ELECTRO-HYDRAULIC TWO STAGE SERVO VALVE
NULL ADJUST MAGNET
UPPER POLEPLATE
LOWER POLEPLATE
FLAPPER
NOZZLE
FEEDBACK SPRING
BUSHING
SPOOL
INLET ORIFICE
INLET FILTER
P B
A R
ARMATURE
COIL

11. 11
ELECTRO-HYDRAULIC TWO STAGE SERVO VALVE
MAGNET
Side Sectional View
P
R
Graphic Symbol
P R
A B

12. 12
SERVO VALVE - OPERATION
P
R
Service Service
No Signal to Valve

13. 13
SERVO VALVE - OPERATION
Signal applied to
torque motor &
spool about to
move.
P
R
Service Service

14. 14
SERVO VALVE - OPERATION
Equilibrium
condition with
signal applied.
P
R
Service
Service

15. 15

16. 16

17. 17
ELECTRO-HYDRAULIC THREE STAGE SERVO VALVE
PILOT VALVE
L.V.D.T
P B
A R
Drain
Port
SPOOL
SLEEVE

18. 18
SERVO PILOT PROPORTIONAL VALVE
Input
Voltage
SERVO
AMPLIFIER
L.V.D.T
SPOOL
SPRING
PILOT VALVE
P B
A
R
Pilot Pr. Port

19. 19
MECHANICAL SERVO VALVE
P B
A
T
Drain
Port
Pilot Pr.
Port
STYLUS
SPOOL
SLEEVE

20. 20
PROP. VALVE SERVO VALVE
According to
required function
many types of
valves are available
- Prop.Pr. Ctrl.Vv
- Prop.Flow Ctrl.Vv
- Prop. Pr. & Flow
Control Valve
- Prop. Dir. & flow
Control Valve.
Construction same
as Dir. Valve.
Type of feedback
sensor, function of
the servo valve is
determined.
Use pr. Sensor and
give feedback, then
become pr. Control
Servo Valve.
SERVO VALVE
Based on Function
- Compare with a Proportional Valve

21. 21
SERVO VALVE
Based on Function
- Compare with a Proportional Valve
PROP. VALVE SERVO VALVE
Rated flow same as
Conventional valve for
Pr. Control & flow control
Valve.
For Dir. Control depends
on accuracy and
frequency response.
Rated flow gives flow at
10 ~ 30 Kg/Cm2 pr. Diff.
To get high frequency
response, high pressure
response should be
given.
Generally 70 Kg/Cm2 of
pres. Diff. Is given.
Rated flow indicates flow
at pr. drop of 70 Kgf /Cm2
Take care about power
loss & heat generation.

22. 22
FREQUENCY RESPONSE
Amplitude
Ratio
(
Gain
)
(
dB
) +2
0
-2
-4
-6
-8
Phase
Lag
(
Degrees
)
-10 
-30 
-50 
-70 
-90 
-110 
5 10 20 30 50 100 200
Frequency ( Hz )
38 L/min
63 L/min

23. 23
Type of valve. Proportional valve.
Item.
Standard.
High
performance
Servo valve.
Pressure
difference and
rated flow.
10
Kgf / Cm2
15 ~ 30
Kgf / Cm2
.
70
Kgf / Cm2
.
Hysterisis. Less 5%. 1 ~ 3%. Less 1%.
Repeatability. Less 1%. Less 1 %. Less 0.5 %.
Gain
at
– 3db.
6 ~ 10 Hz. 20 ~ 40 Hz. 70 ~ 150 Hz.
Frequ
-ency
response Phase
at
90o
.
6 ~ 9 Hz. 15 ~ 45 Hz. 150 ~170 Hz.
SERVO VALVE
- Compare with a Proportional Valve
Performance comparison

24. 24
SERVO VALVE
- Most Sensitive to Contamination
Application Cleanli-
ness
Stds.
Filter
General Hydraulic System NAS 12 20 ~ 30  ab
Proportional Valve &
Flow Control Valve to
control small flow
NAS 11 10  ab.
SERVO VALVE NAS 7 5  ab.