Page 482 - Mechatronics with Experiments

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Note that x spring = x preload +Δx and x preload is adjusted by the lever motion. As
v
the operator moves the lever, x preload changes, hence p preload changes. This means that the
output pressure,
p = p = p (x ) (7.184)
pilot,c out preload l
≈ K ⋅ x (7.185)
l
changes as a function of lever displacement, where x is the lever displacement. This valve
l
functions as a proportional pressure control valve using a pressure reducing valve. The
output pressure limits are defined by the pilot supply pressure and tank pressure, and the
actual value of the output pressure is proportional to the lever displacement,
p tank ≤ [ p pilot,c = p out = p preload (x )] ≤ p pilot,s (7.186)
l
taking on a value between pilot supply pressure (p pilot,s ) and tank pressure (p tank ).
A pressure compensator valve that maintains a constant pressure drop between its
input and output ports is shown in Figure 7.49. Force balance between feedback pressures
and spring determines the steady-state position of the spool
p = p − p = p − k ⋅ (x +Δx ) (7.187)
out in spring in spring preload v
and the spool position effect on the output pressure due to the change in orifice opening
(notice the difference between this equation and the similar equation for pressure reducing
valve),
p out = p − K vp ⋅ Δx v (7.188)
in
If we eliminate the Δx from the above two equations to find a steady-state relationship
v
between input and output pressure, it can be shown that
( ) ( )
k spring k spring
1 − p out = 1 − p − k spring ⋅ x preload (7.189)
in
K vp K vp
p out ≈ p − k spring ⋅ x preload ; K vp ≫ k spring (7.190)
in
Hence, the valve spool will move to regulate the output pressure to maintain the following
relationship,
p − p = k ⋅ x (7.191)
in out spring preload
If p increases, p out has to increase in order to maintain the force balance. This is accom-
in
plished by the small movements of the valve spool.
An unloading valve is a pressure relief valve where the pilot pressure which activates
the valve to open comes from a line past the valve. For instance, a fixed displacement pump
may need to charge an accumulator which maintains a line pressure. When the accumulator
pressure exceeds the pump output pressure, the pilot pressure opens the relief valve, the
check valve closes, and the pump flow is sent to the tank without having to work against
the load, hence the name unloading valve (Figure 7.45a).
A sequence valve is a pressure relief valve used in a circuit to make sure that a
hydraulic line does not open until a certain pressure requirement at another location is met
(Figure 7.45b). For instance, a pressure relief valve can be used to open or close a hydraulic
line between two points based on the pressure feedback from another third location in the
circuit. Until the pressure in the third location reaches a preset value, the line between
location one and two is blocked. Such a valve may be used between a single directional
control valve and two cylinders: clamp and drill cylinders. The objective is that the drill
cylinder should not move until the clamp cylinder pressure reaches a certain value. This
is accomplished by a pressure relief type sequence valve between the drill cylinder and