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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 499
the valve operates mostly in the vicinity of the null-position. However, the valve also
operates in its full range of spool displacement when it is commanded to make a fast, large
displacement or force changing motion. As a result, the valve spool position versus orifice
opening area function of a valve for the full range of spool displacement is important. In ideal
proportional and servo valves, this relationship is linear as shown in Figure 7.73a. Many
open loop applications, especially the ones with an operator in the loop, use valves with
deadband, as shown in Figure 7.73b. Other applications require valve spool geometry with
dual gain, as in Figure 7.73c and d or nonlinear gain, as in Figure 7.73e. It is the application
requirement that dictates the type of valve that should be used. Furthermore, the spool
position versus orifice area opening curve does not have to be symmetric between positive
and negative direction of the spool displacement, as in Figure 7.73f. For instance, injection
molding applications commonly require valves with non-symmetric spool geometry. It is
important to note that the nonlinear relationship in the valve spool geometry (i.e., nonlinear
function of orifice opening as a function of spool displacement, Figure 7.73e) can be inverted
in real-time valve control software so that the command signal to flow rate relationship
can be made linear by effectively using a control gain that is inversely proportional to
the valve spool position-flow rate gain. In other words, we can achieve the steady-state
valve characteristics shown in Figure 7.73a by using a valve shown in Figure 7.73e plus
a real-time control algorithm that adjusts the current signal to the valve in a way that is
inversely proportional to the actual valve gain. The transient response would be a little
different due to the control algorithm and spool motion transient delays. The adjustable
gain in the control algorithm is calculated as the ratio of the desired gain (Figure 7.73a)
divided by the actual valve gain (Figure 7.73e) at the current signal level. This is referred
to as the inverse valve gain compensation in control.
The null-position is defined as the position of the spool where the pressure versus
spool position curve (ΔP versus x or ΔP versus i) goes through zero value at the pressure
L
s
L
axis. A null position test is conducted on a valve by fully blocking the ports between P-T
and A-B. The null position of a valve is usually adjusted mechanically under a zero current
condition. The mechanical adjustment of the null position may be provided by an adjustable
screw on the valve which is used to move the spool around the neutral position by a small
amount. This is part of the valve calibration procedure.
Null position performance is highly dependent upon the machining tolerances of the
spool lands, sleeve and valve orifices, pressure, and temperature. Even high accuracy servo
valves exhibit variation in their flow gain as a function of input current (under constant
pressure drop) in the null-position vicinity. Recall that the flow is function of spool position
and pressure,

Q = Q(x spool , ΔP ) (7.239)
v
The valve characteristics around the null-position are represented by flow gain, pressure
gain, flow-pressure gain (also called the leakage coefficient). The flow gain

Q
K = (7.240)
q
x spool
can vary between 50 to 200% of the nominal gain within the ±2.5% of maximum current
value. The pressure gain of a valve is defined as the rate of change of output pressure as a
function of solenoid current when output ports (A, B) are blocked (Figure 7.74),

P L
K = (7.241)
p
x spool