Siemens SINAMICS S120 System Manual page 236

Appendix
A.3 Hydraulic
Solenoid actuation with valve spool position control
For standard servo solenoid valves, the valve spool is actuated directly by a stepless
solenoid. This converts a current I into a force F, which is compared to the force of the reset
spring. This comparison of forces finally produces a travel s, and thus an opening cross-
section at the control edges of the valve spool.
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. Any deviations from the spool position setpoint are thus continuously corrected. This
method is particularly successful in reducing the valves' sensitivity to dirt.
Very small control deviations, such as those caused when the valve spool sticks, can be
corrected by mobilizing the entire available magnetic force.
A wear-resistant, proximity-type differential transformer (LVDT) is used as the spool travel
sensor.
Figure A-2
236
Solenoid actuation with valve spool position control
System Manual, 04/2015, 6SL3097-4BA00-0BP1
Hydraulic Drive