hydraulic servo solinoid valves
TRANSCRIPT
Servo Valves
HYDRAULIC SERVO SOLENOID VALVES
CONTENTS
HYDRAULIC SERVO SOLENOID VALVES............................................................................................2General.......................................................................................................................................................2
Spool principle.......................................................................................................................................2Solenoid actuation with position control of valve spool........................................................................3Graphic symbol......................................................................................................................................4
Zero overlap in mid-position......................................................................................................................5Flow characteristic, linear......................................................................................................................6Flow characteristic, knee-shaped...........................................................................................................7Linearization of knee-shaped characteristic...........................................................................................8Rated flowrate........................................................................................................................................8
Dynamic response......................................................................................................................................9Filtration grade.....................................................................................................................................10Hysteresis, response sensitivity, range of inversion.............................................................................10
Servo solenoid valves of sizes 6 and 10...................................................................................................11Mechanical construction......................................................................................................................11
Valve amplifier.........................................................................................................................................12Pilot-actuated sizes 10 and 16 servo solenoid valves...................................................................................13
Mechanical construction......................................................................................................................13Inlet and outlet of hydraulic fluid.........................................................................................................13Positions...............................................................................................................................................13Valve amplifier.....................................................................................................................................13
HRV valves..............................................................................................................................................16General.................................................................................................................................................16Features................................................................................................................................................16
Cylinder........................................................................................................................................................18General.................................................................................................................................................18
Quality of seals.........................................................................................................................................19Dead volume............................................................................................................................................19
Page 1 of 21
Servo Valves
HYDRAULIC SERVO SOLENOID VALVES
General
The servo solenoid valve is the final control element in the electro-hydraulic control loop. It converts electrical quantity U=–10...+10 V into hydraulic quantities pressure p and flow Q, and thus into a cylinder movement.
Spool principle
These valves are of the sliding-spool type. A valve spool with 4 control edges moves inside a steel sleeve on which the control bore is connected to the 4 ports in the valve housing.
The ports in the valve housing are:
P: Pressure port (inlet)
T: Tank port (return)
A and B: Working ports (cylinder)
The valve spool slides stepless through 3 switching positions (continuous valve).
Sliding spool principle
Page 2 of 21
A B
T
P
Valve-spindle
Housing
Steel-sleeve
Servo Valves
Solenoid actuation with position control of valve spool
On the standard servo solenoid valves of sizes 6 and 10, the valve spool is actuated directly by a stepless actuating solenoid. This converts a current I into a force F, which is compared with the force of the reset spring. This comparison of forces finally produces a travel s, and thus an opening cross-section on the control edges of the valve spool.
In order to compensate for disturbance forces acting on the valve spool (flow forces) and to reduce the hysteresis and response sensitivity or range of inversion, the position of the armature, and therefore the spool travel, is scanned and applied to a position control loop as an actual value. Deviations from the spool position setpoint are corrected continuously in this way. This method is particularly successful in reducing the valve sensitivity to dirt.
Very small control deviations, such as those caused when the valve spool “sticks”, can be corrected through mobilization of the total available magnetic force.
A wear-resistant, proximity-type differential transformer (LVDT) is employed as the spool travel sensor.
Solenoid actuation with position control of valve spool
Page 3 of 21
Reset spring
Valve amplifierUE
Valve spool FMFF
Actuating Solenoid
Actuating Solenoid
F
FM
S
US
FF
Servo Valves
Graphic symbol
The operating principle of the servo solenoid valve is represented by a symbol in the hydraulic circuit diagram. The symbol comprises a series of different squares denoting the valve positions.
The three stepless-transition valve positions are represented by additional lines. The symbol also indicates the relevant mode of actuation, in the example below, direct solenoid actuation with spring return at one end.
In the de-energized state, the valve assumes a 4th fail-safe position. Two, alternative fail-safe positions are available.
The symbol also reveals the principle of position control applied to the valve spool.
Page 4 of 21
Graphic symbol
Actuating SolenoidPosition Sensor
Reset Spring
Fail-safe Disabled Fail-safe Enabled
A B
TP
Valve amplifier with position control
SetpointUE=0…10V
A B
TP
A B
TP
Servo Valves
Zero overlap in mid-position
Zero overlap by a continuous valve around its mid-position is an important requisite for its application in a position control loop.
A positive overlap has a disturbing effect because of the final control element “dead zone”.
In contrast, a negative overlap results in a marked increase in oil leakage. In order to achieve zero overlap, valve spools, spool housings, and spool sleeves must be manufactured with extreme precision and made of wear-resistant materials. The production costs incurred are correspondingly high.
To maintain the zero overlap over prolonged operating periods, it is essential to ensure that a clean pressure medium is used (to prevent erosion of control edges).
Zero overlap in mid-position
Pressure intensification
The quality of zero overlap in the mid-position is represented by the so-called pressure intensification characteristic.
This states what percentage of the setpoint signal is needed to achieve a pressure difference corresponding to 80 % system pressure at the closed load ports. The values of this characteristic are typically in the 1...3 % range.
The following graphic representation of the measurement, which acquires all 4 control edges, provides precise information.
Pressure amplification
Page 5 of 21
+Q
-Q
-UE+UE
+Q
-Q
-UE+UE
+Q
-Q
-UE+UE
A B
T P T
A B
T P T
A B
T P TÜ = - Ü = 0 Ü = +
Dp
-UE[%] +UE[%]
PA PB
A B
PPPU
806040
3
80
20
204060
2 3211
P [% of pPU]
Servo Valves
Flow characteristic, linear
The stepless slide movement, and thus the change in the throttle cross-section at the control edges, results in a corresponding flow, which is represented as a function of spool travels or of electrical input signal U (manipulated variable). The linearity is determined by the geometry of the control edges of the spool sleeve.
The flow is not only dependent on the opening cross-section, but also on the pressure drop in accordance with the law of flow:
Flow characteristic
Control edge on spool sleeve
Page 6 of 21
Flow Q PNOM per control edge = 35 bar
PA toBT
A B
P TA B
P TPB toAT
Spool Travel sInput signal I
Window in spool sleeve
Spool Stroke
Q
UE
Servo Valves
Flow characteristic, knee-shaped
In addition to valves with a linear characteristic, values with a knee-shaped characteristic are used in many cases to increase the finer resolution in the positioning range to better control the effects of static/sliding friction. These are achieved by means of corresponding windows (control edges) in the steel valve sleeve. A constant machining velocity of the drive directly at the knee-point of the valve is not recommended.
Definition for the position of the knee-point, e.g. 40% knee, up to 40 % of the spool value or setpoint signal, only 10 % of the maximum opening cross-section (flow) is released.
Definition of flow characteristic
Valve with knee-shaped characteristic
Page 7 of 21
+Q
-Q
-UE+UE
Machining velocity
Rapid traverse
10%
40%
Spool stroke
Q
UE40% UE
10%
Qm
ax
Servo Valves
Linearization of knee-shaped characteristic
The knee-shaped characteristic of the valve is linearized in the HLA module to match it to the closed-loop control of the overall drive (cylinder). No steady-state operating point should be defined in the knee-point area.
The corresponding valve data are stored in the HLA module and automatically parameterized when the order number is entered.
Rated flowrate
This describes the maximum flowrate with fully opened valve in relation to a pressure drop of ∆p=35 bar per control edge.
Typical values are:
Size 6: Q=4...40 l/min
Size 10: Q=50...100 l/min
With pilot-controlled valves, the rated flowrate is related to ∆p=5 bar per control edge. (Reduction in losses with higher flow rates.)
The values for sizes 10 and 16 are then as follows:
Size 10: Q=55...85 l/min
Size 16: Q=120...200 l/min
The flow under other pressure conditions is calculated according to the law of flow by the following
formula:
Asymmetrical characteristicsThe speed of the cylinder is determined by a combined throttling effect at the inlet and outlet. The cylinder is clamped hydraulically at both ends.In order to ensure adequate effectiveness of the outlet throttle on cylinders with single-ended piston rod and draw load, larger servo solenoid valves with asymmetrical characteristics are available as alternatives.
Page 8 of 21
Linearization of knee-shaped characteristic
+Q
+Q
+UE-UE
Valve characteristic
Linearized characteristic
Compensation
10%
40%
Asymmetrical characteristics
G
QA
QB
-UE+UE
V
A B
P T
PA
PB
-Q
+Q
Servo Valves
Dynamic response
The dynamic response characteristics indicate how fast the valve can react to setpoint changes.
One characteristic that is indicative of dynamic response is the actuating time. This defines how much time the valve spool needs to open the valve in response to a step change in the input signal from 0 V to 10 V.
More exact information about the dynamic response is provided by the Bode diagram or frequency response. In this case, a sinusoidal setpoint is applied to the valve. The frequency up to which the actual value (valve spool) can follow the setpoint is then noted. The amplitude and phase displacement must be plotted logarithmically as a function of frequency.
Dynamic response of valve
Page 9 of 21
AE
[dB] j [5]
5%
100
Signal 100 Umax
Signal 5% Umax
at -905
f[Hz]
at -3 dB-180
-160
-40
-20
-0
-80
-60
-140
-120
-100
3002001008060402010
Servo Valves
Filtration grade
In order to achieve the longest possible control edge life, thus assuring the quality of the zero overlap, a certain degree of purity of the hydraulic medium (fluid) must be maintained.
The objective is contamination class 7...9 to NAS 1638, this can normally be achieved with a pressure filter
10 = 75.
Hysteresis, response sensitivity, range of inversion
These three terms express similar characteristics.
The term “Hysteresis” refers to the greatest difference in the input signal for identical outputs signals on traversal of a full signal range.
On a servo solenoid valve, the hysteresis is caused by mechanical friction, magnetic hysteresis of electromagnetic signal transducer and the play between transmission elements.
The position control corrects the hysteresis.
The hysteresis on Bosch closed-loop proportional values is less than 0.2 % and is compensated for in the closed control loop.
The terms “Response sensitivity” and “Range of inversion” refer to the signal level required to set a valve in motion again after it has stopped. The values of these characteristics correspond to about half the hysteresis.
To overcome residual hysteresis or initial valve friction, a friction compensation function can be activated in the HLA module.
Hysteresis, response sensitivity, and range of inversion of servo solenoid valve
Page 10 of 21
Hysteresis
Response sensitivity
Range of inversion
Q
U
Q
U
Q
U
Servo Valves
Servo solenoid valves of sizes 6 and 10
Mechanical construction
The standard series of size 6 and 10 servo solenoid valves shown in the following diagram are designed according to the same principle.
The valve spool in its steel sleeve is pushed in contact with the reset spring by the actuating solenoid. The armature axis of the solenoid is mechanically coupled to the ferrite core of the position sensor integrated in the solenoid. This sensor is a proximity-type, wear-resistant differential transformer (LVDT). The housing of the integrated valve amplifier (On Board Electronic OBE) is bolted directly onto the solenoid/position sensor module.
Electrical power is supplied and the setpoint injected via a 7-pin connector.
If the valve is operating around the mid-position, the solenoid is energized by about 50%. When the supply voltage is disconnected, the valve assumes a 4th position, i.e. the “fail-safe position”. On connection and disconnection of the supply, it slides through the crossed position.
The valves are available with a variety of rated flows and two different fail-safe positions.
Sizes 6 and 10 servo solenoid valves, directly actuated
Page 11 of 21
Size 6 Size 10
7-pin connector Valve amplifier Fail-safe enabled
Fail-safe disabled
P T
A B
P T
A B
Reset spring
Steel sleeve
Valve spoolActuating solenoid coil
Actuating solenoid armature
LVDT ferrite coreLVDT coils
Servo Valves
Valve amplifier
The functions of the integrated valve amplifier are implemented with analog electronics and illustrated in a block diagram (see the following figure).
The main amplifier functions are:
Supply and evaluation of the position sensor (AC/DC converter)
Comparison of setpoint input signal with spool actual value
Formation of manipulated variable via a PID controller for the output stage
Timing output stage with pulse length modulation
The amplifier is calibrated to match the valve at the factory. A zero-point adjustment is made via the NC during start-up.
Block diagram of sizes 6 and 10 directly actuated servo solenoid valves
Page 12 of 21
2.5 AF+24V=
0V
0V
Supply
Reference point Actual value spool valueSetpoint0...10V
Sign Actual value spool value
Protective earth
Screening
ABCDEF
100k100k10k
Logic
PID
+UB
+UB
+15V-15V
S
U
Servo Valves
Pilot-actuated sizes 10 and 16 servo solenoid valvesIn order to control higher flow rates, the principle of pilot control has been applied.
Mechanical construction
A directional servo solenoid valve of size 10 or 16 with corresponding control edges on the valve spool functions as the main stage. Like the piston rod of a cylinder, this stage is hydraulically clamped and positioned by a size 6 pilot valve (see Section A.1.2).
The position of the main spool is scanned by another position sensor and the corresponding actual value applied to a second, subordinate position control loop.
Inlet and outlet of hydraulic fluid
The control fluid can be inlet and outlet either internally via ports P and T or, as often the case in practice, externally via additional ports x and y. Plugs are used to convert the assembly to the desired method.
Positions
Pilot-actuated servo solenoid valves have only the three stepless-transition positions.
They do not have the 4th fail-safe position. If the supply voltage is disconnected, the spring force of the main spool causes the valve to assume an indifferent mid-position.
Valve amplifier
The integrated valve amplifier is mounted on the pilot valve assembly and contains both position control loops. A cable is used to connect the position sensor on the main stage to the amplifier.
Page 13 of 21
Servo Valves
Pilot-actuated sizes 10 and 16 servo solenoid valves
Page 14 of 21
Graphic symbol in detailed form
Graphic symbol in simplified form
Servo Valves
Page 15 of 21
2.5A
F
Ref
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ce p
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t 1
) A
ctu
al v
alu
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ool
valu
e
Su
pp
ly+
24V
=
0V0V Sig
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etp
oin
t 0…
10
VA
ctu
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alu
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ool
valu
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tect
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Scr
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1) D
o n
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onn
ect
to
0V s
up
ply
A B C D E FD
iff.
am
p
100k
100k
10k
Log
ic
+U
B
+U
B
Pil
ot
stag
e
PID
Mai
n
stag
e
PD
+15
V-1
5V
-15V
ref.
0+
15V
Sig
nal
1 2 3 4
SU
SU
DC D
C
Servo Valves
Block diagram of pilot-actuated sizes 10 and 16 servo solenoid valves
Page 16 of 21
Servo Valves
HRV valves
General
HRV (High Response Valve) is a series of servo solenoid valves with excellent dynamic and static characteristics and therefore extends the product range to cover particularly demanding applications.
The core of the series is the size 6 valve (HRV 1), which is also used as a pilot valve for the size 10...size 25 (HRV 2) series.
Both valve stages on pilot-actuated valves operate under position control.
Features
In comparison to other valves, the HRV series has the following features:
Significantly better dynamic response More compact dimensions A higher hydraulic switching capacity.
The HRV valve comprises the following components:
Flow-force-compensated hydraulic section with wear-resistant steel sleeve and spring centered control spool,
Double-stroke solenoid with inductive position encoder (LVDT) and integrated electronics (OBE).High-response valves do not have the fail-safe position provided on other servo solenoid valves. Many applications therefore require external non-return valves, such as those available as sandwich-plate valves.
High-response valves
Page 17 of 21
12 pin connector
Valve amplifierA B
P T
Steel sleeve Valve spoolDouble-stroke solenoid
Position encoder
Servo Valves
Page 18 of 21
1 2 3 40…
10
V
+ -P
D
Log
ic
VH
RV
S
DC D
C
+15
V-1
5V
+U
B
+U
B
+U
B
100k
100k
10k
PID
V
1 2 3 4 5 6 7 8 9 10 11
2.5A
F+
24V
=
0V
24V
Ou
tpu
t st
age
sup
ply
En
able
Sig
nal
Set
poi
nt
0…
10V
Act
ual
val
ue
spoo
l va
lue
24V
En
able
ac
kn
owle
dge
men
t
Ele
ctro
nic
s su
pp
ly0V 24
V
+24
V=
/<=
0.5A
Err
or m
essa
ge
Pro
tect
ive
eart
h
Scr
een
ing
Servo Valves
Block diagram of HRV valves
Page 19 of 21
Servo Valves
CylinderGeneral
The cylinder acts as the drive element in the electro hydraulic control loop.
It converts the flow to a linear movement. In this case, high velocities are required for rapid traverse movements as well as slow velocities for machining operations
Through-rod or differential cylinder
On a through-rod cylinder, a piston rod of the same diameter is mounted at both ends as a power transmission element. Consequently, the piston areas at the A and B ends are identical. Likewise, at a constant piston speed, the incoming flow is equal to the displaced flow in the settled state. The piston rods on the through-rod cylinder thus travel in and out symmetrically.
In contrast to the through-rod cylinder, a differential cylinder has either a power- transmission piston rod at one end only or the piston rods at its two ends have different diameters. In the latter case, the piston areas at the A and B ends are different. Furthermore, at a constant piston speed, the displaced flow is not the same as the incoming flow. The maximum piston travel-in and travel-out speeds are not the same on a differential cylinder.
This asymmetry can, however, be compensated by means of the piston area adaptation function (MD 5112: VALVE_FLOW_FACTOR_A_B).
Apart from the piston diameter, it is necessary to specify the rod diameters at the A and B ends. On a differential cylinder, both rod diameters are different, one of the rods might even have a zero diameter. The maximum piston stroke and cylinder dead volume are also required.
Cylinder principle
Page 20 of 21
Through-rod cylinder
Differential cylinder
D0
D2
D2
D0
D0
D1
D1
D1A B
A B
A B
D1=D2
D1□D2
Servo Valves
Quality of seals
In order to ensure the lowest possible frictional forces, the quality of the seals and guides on the piston, and the piston rod itself, must be particularly high.
Transitions from static to sliding friction have a particularly adverse affect on the quality of control accuracy. A friction compensation setting has been provided in the HLA module (MD 5460: FRICTION_COMP_RADIENT) for the purpose of counteracting initial friction.
Cylinders
Dead volume
The dead volume is the volume between the cylinder and servo solenoid valve that is not displaced by one piston stroke. It reduces the natural frequency of the drive and should be avoided wherever possible.
Cylinder pipe work should be kept as short as possible, i.e. the servo solenoid valve should be mounted directly on the cylinder.
The dead volume is parameterized in the HLA module (MD 5135, MD 5136 and
MD 5141...5143).
Page 21 of 21
Piston
Piston Rod
2 x guide ring (PTFE)
Seal (FPM+PTFE)
2 x guide ring (PTFE)
2 x sealing ring (FPM+PTFE)
Stabilizing ring (FPM)