Appendix
A.3 Hydraulic
Hydraulic Drive
System Manual, 04/2015, 6SL3097-4BA00-0BP1
247
Precontrolled servo solenoid valves
The principle of precontrol is applied in order to be able to handle higher flow rates.
A directional control valve with appropriate control edges at the valve spool is used as main
stage. Like the piston of a cylinder, this stage is hydraulically clamped and positioned by a
precontrol valve.
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.
Hydraulic fluid intake and discharge
The control fluid can be supplied and removed either internally via ports P and T or, as often
is the case in practice, externally via additional ports X and Y. The unit is converted over
using suitable plugs.
Precontrolled servo solenoid valves have only three stepless-transition valve positions.
The 4th fail-safe position is omitted. If the supply voltage is disconnected, the spring force of
the main spool causes the valve to assume an indifferent middle position.
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