10.03
A.1 Servo solenoid valves
A-242
 Siemens AG, 2003. All rights reserved
SINUMERIK 840D/SIMODRIVE 611 digital, HLA Module (FBHLA) - 10.03 Edition
A continuous valve must have zero overlap around its mid-position if it is to be
used in a position control loop.
A positive overlap will be perceived negatively in the form of a dead zone of the
final control element.
In contrast, a negative overlap results in a marked increase in oil leakage.
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).
O
T T
B
Ü = -
P
O
T T
B
Ü = +
P
O
T T
B
Ü = 0
P
Q
U
Q
U
Q
U
Fig. A-4 Zero overlap in mid-position
The quality of zero overlap in the mid-position is represented by the pressure
intensification characteristic.
This states what percentage of the control spool deflection from the hydraulic
zero point is needed to achieve a pressure differential of 80% system pressure
at the closed load ports. The values of this characteristic are typically in the
1...3% range.
The following graphical representation of the measurement, which covers all 4
control edges, shows this clearly.
80
60
40
20
20
40
60
80
∆p/p
pu
[%]
-U
E
[%] U/U
nom
[%]
123321
∆p
p
A
p
B
p
PU
B
O
p
Fig. A-5 Pressure amplification
Zero overlap in
mid-position
Pressure
intensification
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