Page 506 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
492 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
Permanent magnet
armature N N
Double nozzle
S S
Flapper Flexure sleeve
P s P s
Feedback spring T T
P s P s

Main spool P L
A B
FIGURE 7.68: Double nozzle flapper type two-stage servo valve – the most popular two-stage
servo valve type. Courtesy Moog Inc.

The double nozzle flapper design is the most common two-stage servo valve used
in high performance applications. This is the most accurate, but expensive, type of servo
valve. Stage 1 is the torque motor plus the double nozzle flapper mechanism, stage 2 is the
main valve spool. Figure 7.68 shows the cross-sectional picture and operating principle of a
two-stage double nozzle flapper type valve. A torque motor is a limited rotation permanent
magnet electric motor with a coil winding. The direction of the current in the winding
determines the direction of the torque generated. The permanent magnet strength and air
gap between the armature and the winding determines the current to torque gain. Hence,
the current magnitude determines the torque magnitude. The armature and flexure tube are
mounted around a low friction pivot point about which they rotate as the motor generates
torque.Whencurrentisappliedtothetorquemotorwinding,thepermanentmagnetarmature
rotates in the direction based on the current direction, and by an amount proportional to
the current magnitude. The flexure sleeve allows the armature–flapper assembly to tilt.
As a result, the nozzle at the tip of the flapper is changed and hence a different pressure
differential is created between the two sides. The pressure on the side of the nozzle where
the flapper is closer gets larger. This pressure differential moves the main spool and along
with it the feedback spring until the pressure is balanced on both sides of the flapper at the
double nozzle point. At this time, the main spool is positioned in proportion to the input
current. The input current is translated proportionally into the main spool displacement.
Notice that due to the small nozzles at the double nozzle and flapper interface, the filtration
of the fluid at the pilot stage is very important in order to avoid contamination related
failures of the valve operation at the pilot stage.
The jet pipe valve design is very similar to the double nozzle flapper design, except
that the pilot stage amplification mechanism is different (Figure 7.69). Stage 1 is the torque
motor plus the jet pipe nozzle and two receiver nozzles. Stage 2 is the main spool. The pilot
pressure is fed through the jet pipe nozzle which directs a very fine stream of fluid to two
receivers. When the current in the torque motor is zero (null position), the jet pipe directs
this very fine stream of “jet” flow equally to both receivers and there is a balance between
the two control ports holding the main spool in null position. When the current is applied,
the torque motor deflects the armature and the jet pipe proportionally. As a result, the jet
pipe directs different amounts of fluid stream to two receivers, and hence creates a pressure
differential in the control ports (two sides of the main spool). This pressure differential
moves the main spool until the pressure differentials in the control ports are stabilized. The
feedback from the main spool movement to the jet pipe is provided either mechanically by