Page 480 - Mechatronics with Experiments

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P P
out
P in
P P
in out P
sp
1 P
1 out
P
sp
P P Time
sp in
(a) (b) (c)
FIGURE 7.49: Pressure compensator valve: (a) symbol with pressure feedback and spring
pressure, (b) input and output pressure relationship in steady state, (c) input, output, and
spring pressure relationship over time.

As the spool moves, it restricts the flow orifice between the input and output port. Eventually,
when the output pressure is high enough to further move the spool to close off the orifice
between the input and output ports, the output pressure is limited to the maximum value set
by the spring and orifice design of the valve. The valve then regulates within the vicinity
of this point and maintains a constant output pressure which is set by the spring as long as
the input pressure is larger than the set (desired output) pressure.
A modified version of the same valve concept (pressure reducing valve) is used as an
adjustable pressure reducing valve in lever or solenoid operated pilot valves. Figure 7.50
shows a modification of the pressure reducing valve. The base of the spring is moved by
a lever (or an electric actuator in the case of an EH type pressure reducing valve), hence
changing the set spring pressure. Therefore, the output pressure will be proportional to the
lever displacement as long as the input pressure is larger than the set pressure. A pair of
such pressure reducing valves is used as the pilot stage in two-stage proportional valves.
In mechanically actuated versions, a lever moves a pair of pressure reducing valves. In
forward motion of the lever, one of the pilot valves is actuated, in reverse motion the other
is actuated. Similarly, in an EH conrolled version, each pressure reducing valve is actuated
by a solenoid based on lever command. The net result of such a valve is that the output

Lever
P pilot,C
x P pilot,S
S
P pilot,C
x
L
P pilot,C
Lever

P pilot,S
(a) (b) (c)
FIGURE 7.50: Lever controlled pilot valve: a pressure reducing valve modified to be used as a
pilot valve. The output pressure is always smaller or equal to the input pressure. The output
∝ x , while P ≤ P .(a) Valve
pressure is proportional to the lever displacement, P pilot,c L pilot,c pilot,s
design concept, (b) symbol, (c) inport and outport pressures and control lever relationship. The
lever movement sets the desired output pressure. The feedback from the output pressure to the
spool moves to balance that and regulate to the set pressure.