Page 507 - Mechatronics with Experiments

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JWST499-Cetinkunt
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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 493
Jet pipe
Torque motor
assembly
Command signal
Position feedback Stainless steel wire
spring
mesh filter
Receiver
orifices
DSP/OP-Amp
current amplifier
Receiver
body Position sensor
X T A P B T Y
A A T B P s
Valve body Spool and sleeve
(a) (b)
Enlarged view of the jet pipe
pilot stage design

Jet pipe

Annular Area
Receivers
Nozzle

(c)
FIGURE 7.69: Jet pipe type two-stage servo valve: (a) mechanical feedback of the main stage
spool position, (b) electrical sensor feedback of main stage spool position, (c) enlarged view of
the jet pipe and receiver nozzles. In steady-state, the current of the torque motor is proportional
to the main spool displacement. Courtesy Moog Inc.

a flexure spring between the nozzle and the main spool position or electrically by a spool
position sensor (typically an LVDT) and a closed loop spool position control algorithm
used to determine the torque motor current (Figure 7.69).
In general, the jet pipe design has lower bandwidth, requires lower manufacturing
tolerances, and is able to tolerate more contamination compared with the double nozzle
flapper servo valve. The jet pipe pilot stage can support a larger pilot force than the double-
nozzle flapper valves, and hence tend to be used in large valves more often. In high flow
rate applications (i.e., 1500 l/min), a double nozzle flapper or jet pipe valve is used as a
pilot stage of a three-stage servo valve. In modern three-stage proportional or servo valves,
the position loop on the main spool (third stage) is generally closed using an electrical
position sensor. Notice that in both types of servo valves, the pressure for the hydraulic
amplification stage can be provided either from a separate pilot pressure supply source
or can be derived from the main supply line pressure using a pressure reducing valve or
orifice. Filtration of the fluid, especially in the hydraulic amplifier stage (pilot stage), is
very important since the nozzle and receivers have very small diameter dimensions and are
prone to contamination. Flow orifices in the main stage spool are rather large and less prone
to contamination. Therefore, filtration of the fluid is most important for the pilot stage.
Single-stage servo direct drive valves (DDV) currently support up to about
150 l∕min (lpm) flow rate (i.e., Moog D633 through D636 series). Two-stage jet pipe