Page 599 - Mechanical Engineers' Handbook (Volume 2)

590 Servoactuators for Closed-Loop Control

Another important characteristic of an electrohydraulic servovalve is its hysteresis due
to the characteristics of the permanent magnets in the torque motor. The hysteresis charac-
teristic is determined from a measurement of the output flow rate as a function of the input
current for a constant (usually zero) pressure drop across the valve (load pressure drop). A
typical hysteresis characteristic is shown in Fig. 35. The slope of the flow–current curve is
the ‘‘flow sensitivity’’ of the valve [i.e., K in Eq. (63)].
1
It is often convenient in the dynamic analysis of servoactuators to have an approximate
dynamic model for the servovalve. Experience has shown that linearized transfer functions
based on empirical approximations from measured servovalve responses are adequate for
most system designs. Reference 39 outlines considerations underlying the determination of
approximate transfer function models for electrohydraulic servovalves. Figure 36 shows typ-
ical frequency response plots for an electrohydraulic flow control servovalve, along with
approximate transfer functions. For a frequency range of 0–50 Hz, the following first-order
expression has been found to be adequate for two-stage electrohydraulic servovalves:
Q (s) K 1 (65)
m
I(s) s 1
where Q (s) Laplace transform of control flow rate to the servomotor
m
I(s) Laplace transform of the current input to the servovalve
K servovalve flow sensitivity at P 0, I 0
m
1
apparent servovalve time constant (s)
Typical time constants for electrohydraulic flow control servovalves are given in Table 9.
If a good approximation is desired over a wider frequency range, the following second-
order model may be preferred:
Q (s) K 1 (66)
m
2
I(s) (s/ ) (2 / )s 1
n
n
where 2 ƒ apparent natural frequency (rad/s)
n
n
apparent damping ratio (dimensionless)
Typical values of ƒ and for two-stage electrohydraulic flow control servovalves are given
n
in Table 9.
9 ELECTROMECHANICAL AND ELECTROHYDRAULIC SERVOSYSTEMS
9.1 Typical Configurations of Electromechanical Servosystems
An electrical servomotor may be combined with an electrical or electronic modulator to form
an electromechanical servoactuator. The addition of a feedback transducer forms a servo-
system. Figure 2 shows an electromechanical linear-motion servosystem which incorporates
a rotary brushless dc servomotor and tachometer feedback; the electronic modulator is not
shown.

9.2 Typical Configurations of Electrohydraulic Servosystems
A servoactuator comprising an electrohydraulic servovalve and a servomotor may be com-
bined with an electronic servoamplifier (or modulator) and an appropriate feedback trans-
ducer to form a high-performance servosystem. Schematic diagrams of three typical
electrohydraulic servosystems are shown in Fig. 37.