Page 595 - Mechanical Engineers' Handbook (Volume 2)

586 Servoactuators for Closed-Loop Control

Table 8 Performance Specifications of Servovalves
Maximum
Maximum Flow at 1000 Frequenty at
Working psi Pressure 90 Hysteresis Resolution
Valve Type Pressure (psi) Drop (gpm) Lag (Hz) (%) (%)
Spool
One-stage 5000 3500 200 0.1 0.01
Two-stage 7000 1000 200 1 0.01
Three-stage 4500 300 200 1 0.01
Flapper-nozzle/spool
One- and two-stage 5000 1000 500 0.2 0.01
Three-stage 5000 1000 500 1 0.01
Jet pipe/spool
One- and two-stage 4500 300 500 2 0.1
Three-stage 4500 300 200 2 0.1
Sliding plate
One- and two-stage 300 40 150 3 0.1
Source: From Ref. 25.

Equation (53) gives the pressure–flow–displacement characteristics of the valve. These char-
acteristics are needed in the dynamic analysis of a servoactuator which employs the valve.
Equation (54) is used to compute the required flow rate from the source and will not be
considered further here. Equation (55) is used to calculate the force required to move the
spool (e.g., force output requirement of the torque motor in the case of an electrohydraulic
servovalve).
The steady-state pressure–flow–displacement characteristics of the spool valve are char-
acterized by the nonlinear orifice equation. 30,37
P P m
s
Q Cw(x U) (56)
d
m
where C effective coefficient of discharge
d
P supply pressure
s
P pressure drop across the servomotor (see Fig. 33)
m

Figure 33 Spool valve nomenclature.