Siemens SINUMERIK 840D Description Of Functions page 234

A Hydraulics
A.1 Servo solenoid valves
Solenoid actuation
with position
control of valve
spool
Graphic symbol
A-234
SINUMERIK 840D/SIMODRIVE 611 digital Descr. of Functions HLA Module (FBHLA) – 04.00 Edition
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 cur-
rent 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-sec-
tion 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 inver-
sion, 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 correctly continuously in this way. This method is particu-
larly 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.
F Valve spool
F
É É
É É
É É
Reset spring
F
Fig. A-2 Solenoid actuation with position control of valve spool
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 alter-
native fail-safe positions are available.
Valve amplifier
F
M
U
S
Actuating
Position
solenoid
sensor
F
F
F
M
S
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