Page 516 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
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Let us assume that the supply pressure is constant. When the load pressure is constant, the
flow rate is linearly proportional to the current in the solenoid. Similarly, for a constant
solenoid current (equivalently a constant spool orifice opening), the flow rate is related
to the load pressure with a square root relationship. As the load pressure increases, the
available pressure drop across the valve decreases and hence the flow rate decreases. When
the load pressure is the same as the tank pressure, all of the supply pressure is lost across
the valve and maximum flow rate is achieved, which is called the no-load flow. However,
the flow at the output of the valve has no pressure to exert force or torque to the actuator
(cylinder or rotary hydraulic motor). Therefore, in a good design, part of the supply pressure
is used to support the necessary flow rate across the valve, and part of it is used to provide
a load pressure to generate actuator force/torque,
P − P =ΔP valve +ΔP l (7.252)
s
t
In practice, a valve flow rating (Q ) is given at a standard pressure drop across the
r
valve (ΔP = 1000 psi for servo valves and 150 psi for proportional valves) when the spool
r
is fully shifted. The flow rate of the valve (Q) for a different pressure drop (ΔP valve ) across
the valve when the spool is fully shifted can be obtained approximately by
√
Q = Q ⋅ ΔP valve ∕ΔP r (7.253)
r
For valve measurements, P and P are set to constant values, and pressure drop is set
t
s
to a number of discrete values (Figure 7.75). For each value of the load pressure (ΔP ),
L
current (i) is varied from zero to maximum value (i max ), and flow rate (Q) is measured.
The results are plotted as valve flow rate, load pressure, and current relationship as shown
in Figure 7.76 for a fixed P and P values. In Figure 7.76, the valve deadband is clearly
s
t
observed.
The flow–current–pressure relationship given above neglects the leakage flow in the
valve. Servo and proportional valves used in precise positioning applications generally

25
P - A B - T Direction of flow P - B A - T
10 bar
P-A P-B
Flow rate (in /s) 15 B-T A-T
20
3 5 bar

10 P-A P-B
B-T A-T
5

0
–2.5 –2.0 –1.5 –1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5
Input signal: solenoid current (Amp)
FIGURE 7.76: Valve flow rate versus solenoid current for different pressure drops across
ports. A four-way proportional directional valve is considered for different pressure drop
values. Servo valves have much smaller deadband and the flow rate-current relationship for a
constant pressure drop is more linear compared to those of proportional valves.