TMHP51 Servomechanisms (HT2012)
Lecture 04
Sensors for feedback
Servo-Valve internals
Multi-stage Valves
Magnus Sethson
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1
1
The Servo Valve
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2
2
760 SERIES
TWO STAGE SERVOVALVES
760 SERIES SERVOVALVES
The 760 Series flow control
servovalves are throttle valves
for 3-, and preferably 4-way
applications.They are a high
performance, two-stage design
that covers the range of rated
flows from 1 to 15 gpm at
1000 psi valve drop.The
output stage is a closed center,
four-way, sliding spool.The pilot
stage is a symmetrical doublenozzle and flapper, driven by a
double air gap, dry torque
motor. Mechanical feedback of
spool position is provided by a
cantilever spring.The valve
design is simple and rugged for
dependable, long life operation.
These valves are suitable for
electrohydraulic position,
speed, pressure or force control systems with high dynamic
response requirements.
Principle of operation
An electrical command signal
(flow rate set point) is applied
to the torque motor coils and
creates a magnetic force which
acts on the ends of the pilot
stage armature.This causes a
deflection of armature/flapper
assembly within the flexure
tube. Deflection of the flapper
restricts fluid flow through one
nozzle which is carried through
to one spool end, displacing
the spool.
Movement of the spool opens
the supply pressure port (P) to
one control port while simultaneously opening the tank port
(T) to the other control port.
The spool motion also applies
a force to the cantilever spring,
creating a restoring torque on
the armature/flapper assembly.
Once the restoring torque
becomes equal to the torque
from the magnetic forces, the
armature/flapper assembly
moves back to the neutral
position, and the spool is held
open in a state of equilibrium
until the command signal
changes to a new level.
In summary, the spool position
is proportional to the input
current and, with constant
pressure drop across the valve,
flow to the load is proportional
to the spool position.
VALVE FEATURES
➣ 2-stage design with dry torque motor
➣ Rugged, long-life design
➣ Low friction double nozzle pilot stage
➣ High resolution, low hysteresis
➣ High spool control forces
➣ Completely set-up at the factory
➣ High dynamics
➣ Optional fifth port for separate pilot supply
➣ Intrinsically safe or flameproof valve versions are available
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The actual flow is dependent
upon electrical command signal
and valve pressure drop.The
3
Q = QN
Δp
ΔpN
3
D791 and D792 Series
D791 and D792 Series
Three stage servovalves
The flow control servovalves D791
and D792 Series are throttle valves
for 3-way and preferably 4-way
applications. These three stage
servovalves have been especially
developed for such demanding
applications where high flow rates
and at the same time extreme
dynamic performance requirements must be met. The design of
these valves is based on the well
known D079 Series. The integrated electronics has been
replaced by a new design applying
SMD technology. The valves are
offered with pilot valves of D761
or D765 Series, optional standard
response or high response versions
are available. Series D791 can deliver rated flow up to 250 l/min,
Series D792 is available with rated
flow up to 1000 l/min.
These valves are suitable for pressure or force control, position and
velocity control systems with high
dynamic response requirements.
Operational features
❒
❒
❒
Electrical position feedback with pressure isolated position
transducer (LVDT), no wear
Integrated SMD electronics with false polarity protection
Optional external pilot supply and return connections via fifth
and sixth port in valve body
Low threshold and hysteresis, excellent null stability
Preadjusted at factory
❒
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❒
Principle of operation
An electrical command signal (set
point, input signal) is applied to
the integrated control amplifier
which drives a current through
the pilot valve coils. The pilot valve
produces differential pressure in
its control ports. This pressure
difference results in a pilot flow
which causes main spool displacement.
The position transducer which is
excited via an oscillator measures
the position of the main spool
(actual value, position voltage).
This signal then is demodulated
and fed back to the control
amplifier where it is compared
with the command signal. The
control amplifier drives the pilot
valve until the error between
command signal and feedback
signal is zero. Thus, the position of
the main spool is proportional to
the electrical command signal.
The valves D791 and D792 Series
described in this catalogue have
successfully passed EMC tests
required by EC Directive. Please
take notice of the respective
references in the electronics
section.
4
4
MOOG Inc
F1 servo valve, 93gram, 3.5kW power control
Source:
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5
5
Two-Stage Servo Valve
Source:
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6
P
A
T
B
Classical Nozzle-Flapper Controlled Servo Valve
Source:
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7
7
3 stage Servovalve D792
with Pilot valve D765 Series
T
B
P
A
Three Stage Servo Valve
Source:
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8
8
The Torque Motor
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9
Torque
Motor
TORQUE
MOTOR
Upper
Polepiece
➣ Charged permanent magnets polarize the polepieces.
➣ DC current in coils causes increased force in diagonally
opposite air gaps.
➣ Magnetic charge level sets magnitude of decentering force
gradient on armature.
NOZZLE FLAPPER SERVOVALVE OPERATION
N
TORQUE MOTOR
Armature
Upper
Polepiece
S
➣ Charged permanent magnets polarize the polepieces.
➣ DC current in coils causes increased force in diagonally
opposite air gaps.
N
Armature
Lower
SPolepiece
Permanent
➣ Magnetic charge level setsMagnet
magnitude of decentering force
gradient on armature.
Coil
Lower
Polepiece
Permanent
Magnet
S
Permanent
Magnet
Flux
HYDRAULIC AMPLIFIER
N
S
Permanent
Magnet
Permanent
Attractive
Magnet
Flux
Force
S
➤
S
➤
N
S
N
S
N
S
S
Coil Flux
S
S
HYDRAULIC AMPLIFIER
VALVE SPOOL
➣ Armature and flapper rigidly joined and supported by thin-wall
flexure sleeve.
➣ Spool slides in bushing (sleeve) or directly in body bore.
➣ Bushing contains rectangular holes (slots) or annular grooves
Coil
➣ Fluid continuously flows from pressure PS, through both
inletFlux that connect to supply pressure PS and tank T.
orifices, past nozzles into flapper chamber, through drain orifice
to tank T.
➣ At “null” spool is centered in bushing; spool lobes (lands) just
cover PS and T openings.
➣ Rotary motion of armature/flapper throttles flow through one
nozzle or the other.
➣ Spool motion to either side of null allows fluid to flow from PS
to one control port and from other control port to T.
➣ This diverts flow to one end of the spool.
VALVE SPOOL
Spool at Null
T
Ps
FLAPPER
PS
➣ Rotary motion of armature/flapper throttles flow through one
T
Ps
➣ Spool slides in bushing (sleeve) or directly in body bore.
INLET
ORIFICE
➣ Fluid continuously flows from pressure PS, through both inlet
orifices, past nozzles into flapper chamber, through drain orifice
to tank T.
Spool
Feedback Spring
Bushing
ARMATURE
➣ Armature and flapper rigidly joined and supported by thin-wall
flexure sleeve.
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nozzle or the other.
N
➤
Permanent
Magnet
Attractive
Force
➤
➤
N
Torque to
Rotate
Armature
N
➤
N
N
➤
N
Torque to
Rotate
Armature
➤
Coil
FLEXURE
SLEEVE
➣ Bushing contains rectangular holes (slots) or annular grooves
that connect to supply pressure PS and tank T.
A
B
PS
T
Spool Dispaced to Left
T
Ps
T
Ps
Æ
➣ At “null” spool is centered in bushing; spool lobes (lands) just
cover PS and T openings.
16
A
B
10➣ Spool motion to either side of null allows fluid to flow from PS
to one control port and from other control port to T.
10
Operation
Armature
S
Torque Motor & Flapper-Nozzle Operation
Force
S
Permanent
Magnet
Flux
S
Coil Flux
Valve Responding
to Change in
Electrical Input
Valve Condition
Following Change
HYDRAULIC AMPLIFIER
N
VALVE SPOOLN
N
S
S
T
A
T
➣ At “null” spool is centered in bushing; spoolP lobes (lands) just
cover PS and T openings.
➣ Rotary motion of armature/flapper throttles flow through one
nozzle or the other.
➣ This diverts flow to one end of the spool.
S
PS
S
DPL
S
➣ Bushing contains rectangular holes (slots) or annular grooves
P
P
that connect to supply pressure PS and tank T.
S
S
S
➣ Spool slides in bushing (sleeve) or directly in body bore.
P
P
➣ Fluid
continuously flows from pressure PS, through both inlet
orifices, past nozzles into flapper
chamber,
through drain orifice
T
T
P
P
to tank T.
S
N
N
➣ Armature and flapper rigidly joined and supported by thin-wall
S
S
flexure sleeve.
S
➣ Spool motion to either side of null allows fluid to flow from PS
Flow to Actuator
to one control port
and
from other control
port to T.
B
A
B
Spool at Null
T
Ps
➣ Electrical current in torque motorFLAPPER
coils creates magnetic
forces on ends of armature.
INLET
FLEXURE
P
Spool
Feedback Spring
Bushing
ARMATURE
OPERATION
ORIFICE
S
T
Ps
➣ As feedback torque becomes equal to torque from magnetic
forces, armature/flapper moves back to centered position.
A
SLEEVE
P
➣ SArmature and flapper assembly rotates about flexure sleeve S
support.
T
➣ Spool stops at a position where feedback spring torque
Spool Dispaced to Left
equals torque
due to input current.
Ps
➣ Flapper closes off one nozzle and diverts flow to that end of
spool.
➣ Spool moves and opens PS to one control port; opens other
control port to T.
B
T
T
Ps
➣ Therefore, spool position is proportional to input current.
Æ
➣ With constant pressures, flow to load is proportional to
A
B
spool position.
16
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➣ Spool pushes ball end of feedback spring creating a restoring
11
torque on the armature/flapper.
11
The Servo System
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12
12
Servo System
DesignAreospace LLC
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13
13
Direct Drive Pilot Stage
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14
Single Stage Servo Valve
Source:
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15
15
MOOG D636 Single-Stage Servo Valve
Fieldbus connector X4
Valve connector X1
Service connector X10
Fieldbus connector X3
Spool
Digital electronics
Bushing
Linear force motor
Status LEDs
Position transducer (LVDT)
Ports
Single-Stage Servo Valve
Source:
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16
16
Permanent magnets
Centering springs
e motor is proportional to the coil
Bearing
Coil
Armature
Screw plug
Permanent Magnet Linear Force Motor
Source:
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17
Permanent Magnet Linear Force Motor
Operation
➣ When
current is applied to the coil with one polarity, the flux
Coil
Armature
in one of the air gaps surrounding the magnets is increased,
cancelling out the flux in the other.
➣ Without a current being applied to the coil, the magnets and
➣ This dis-equilibrium allows the armature to move in the
springs hold the armature at equilibrium.
direction of the stronger magnetic flux.
P A T B X
N S
S N
N S
OPERATION
S N
Direction
of Armature
N S
➣ An electrical signal corresponding to the desired spool pos
is applied to the integrated electronics and produces a puls
N S
S N current in the linear force motor
width modulated
(PWM)
S N
➣ The current causes the armature to move which then dire
activates the spool.
➣ When current is applied to the coil with one polarity, the flux
➣ The armature
is moved
inand
the opens
opposite
direction
byone
changing
➣
The
spool
moves
pressure
P
to
control po
in one of the air gaps surrounding the magnets is increased, the polarity of the current in the coil.
while the other
control port is opened to tank T.
Neutral
Actuated
cancelling out the flux in
the other.
19
➣ The position transducer (LVDT), which is mechanically atta
to the spool, measures the position of the spool by creatin
electrical signal that is proportional to the spool position.
➣ This dis-equilibrium allows the armature to move in the
direction of the stronger magnetic flux.
N S
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Direction
of Armature
➣ The demodulated spool position signal is compared with
the command signal, and the resulting electrical error drive
current to the force motor coil.
S N
18
➣ The spool moves to its commanded position and the spoo
position error is reduced to zero.
18
Valve Performance
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19
19
Flow Charateristics
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20
100
75
3
- 150
0
- 120
+/– 05%
- 90
-3
50
+/– 25%
+/– 10%
-6
- 60
25
+/– 90%
-9
- 30
0
0
5
10
15
-12
20
Time [ms]
PrESSurE SigNAl CurVE
(valve with zero lap)
1
10
0
1000
Frequency [Hz]
100
Phase lag [degrees]
Amplitude ratio [dB]
Stroke [%]
STEP rESPoNSE
Δ pAB
[%]
pP [%]
VAlVE Flow SigNAl CurVE
(valve with zero lap)
100
80
60
40
20
0
-20
-40
-60
-80
-4 -3 -2 -1
0 1 2 3
-100
4
-60
-20
20
60
100
Command signal [%]
Command signal [%]
MOOG D636 Characteristics
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21
21
3000 psi DTE-24
at 100˚F (38˚C)
Rated Current:
±40%
±100%
-10
100
80
60
40
20
10
20
30
50 70 100
200 300
500
-6
-8
-10
20
10
1000
20
3000 psi DTE-24
at 100˚F (38˚C)
Rated Current:
±40%
±100%
120
100
80
60
40
20
30
50 70 100
200 300
500
Frequency (Hz)
Frequency Response
of 1, 2.5, and 5 gpm Servovalves
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1000
Amplitude Ratio (dB)
-4
Phase lag (degrees)
Amplitude Ratio (dB)
-2
20
50 70 100
200 300
500
-4
120
-6
3000 psi DTE-24
at 100˚F (38˚C)
Rated Current:
±40%
±100%
-8
-10
10
20
-4
120
3000 psi DTE-24
at 100˚F (38˚C)
Rated Current:
±40%
±100%
100
80
60
40
20
10
20
30
50 70 100
200 300
Frequency (Hz)
Frequency Response
of 10 gpm Servovalves
30
50 70 100
200 300
500
1000
Frequency (Hz)
-2
-10
60
Frequency Response
of 15 gpm Servovalves
0
-8
80
20
1000
+2
-6
100
40
Frequency Response
of 10 gpm Servovalves
0
10
30
-2
Frequency (Hz)
+2
-10
80
40
Frequency Response
of 1, 2.5, and 5 gpm Servovalves
-8
100
60
Frequency (Hz)
-6
120
3000 psi DTE-24
at 100˚F (38˚C)
Rated Current:
±40%
±100%
500
1000
Amplitude Ratio (dB)
-8
120
Phase lag (degrees)
-6
-4
0
Phase lag (degrees)
-4
-2
+2
+2
0
-2
-4
-6
120
-8
100
3000 psi DTE-24
at 100˚F (38˚C)
Rated Current:
±40%
±100%
-10
80
60
40
20
10
20
30
50 70 100
200 300
500
Phase lag (degrees)
-2
0
Amplitude Ratio (dB)
0
+2
Phase lag (degrees)
+2
Amplitude Ratio (dB)
Servo Valve Performance
Phase lag (degrees)
Amplitude Ratio (dB)
00%
000Nozzle-Flapper
1000
Frequency (Hz)
Frequency Response
of 15 gpm Servovalves
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22
LVDT
Linear Variable Differential Transformer
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23
23
LVDT, Linear Variable Differential Transformer
Source: Design News Inc.
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24
24
OSCILLATOR
13
Front Panel
VAC
2
2
14
LVDT
9
+1
SECONDARY
AMPLIFIER
10
SECONDARY
SIGNAL
TEST POINT
DEMODULATOR
11
(P.C.B.)
TP2
ADJUST FOR MINIMUM
PHASE DIFFERENCE
12
PHASE ADJUSTMENT CIRCUIT
10K
6.8nF 15nF 47nF
S2-1
1K5
S2-3 S2-5
100K
10K
SECONDARY
DEMOD
TEST POINT
R6
6.8nF 15nF 47nF
S2-2 S2-4 S2-6
10K
+1
S2-7
680
pF
S2-8
TP3
(P.C.B.)
2200
pF
LAG
LEAD
LVDT Phase & Oscillation Circuit
Source:
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25
25
Jet-Pipe Pilot Stage
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26
the annular area around the nozzle to tank T.
Jet-Nozzle Pilot Stage
from its
h the special
om both
he control
hich in turn
n is through
➣ The current through the coil displaces the jet pipe from its
neutral position.
➣ The displacement of the jet directs the flow to one end of
the spool.
➣ Spool moves and opens P to one control port, while the
other control port is open to tank T.
Annular
Area
Nozzle
Jet
Pipe
Receiver
Jet
Pipe
VALVE SPOOL
Receiver
➣ Spool slides in bushing (sleeve) or directly in body bore.
X T
A
P
B
T2
Y
➣
➣ Bushing contains rectangular holes (slots) or annular grooves
that connect to supply pressure PS and tank T.
➣
➣ At “null,” spool is centered in bushing; spool lobes (lands)
just cover PS and T openings.
➣ Spool motion to either side of null allows fluid to flow from
PS to one control port, and from other control port to T.
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y bore.
➣ The position transducer (LVDT), which is excited via
27an
oscillator, measures the position of the main spool (actual
➣
27
The Jet-Pipe Function
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28
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Next Lecture;
10:15, 2012-11-12, P34
Magnus Sethson
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