Page 468 - Mechatronics with Experiments

JWST499-Cetinkunt
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difference P − P gets smaller (i.e., P is larger), the flow rate from the load sensing valve
L
s
L
gets smaller, hence the control piston moves to the right to increase the swash plate angle.
This in turn increase the pump output (pressure and flow) to increase the P .
s
Maintaining a constant pressure drop across the valve for all load conditions up to the
maximum pump outlet pressure limit makes the flow rate through the valve proportional
to its spool displacement regardless of the load pressure as long as maximum pump outlet
pressure is not reached. This gives good controllability since the flow rate through the
valve, and hence the cylinder (actuator) speed, will be proportional to the displacement
of the directional spool regardless of the load. Load sensing is usually mounted together
with pressure limiting devices. The swash plate is controlled so that the pressure difference
between the pump output pressure and load is maintained at a constant value. Hence, this
type of control increases or decreases the pump output pressure depending on the load, up
to a maximum limit set by the pressure limiting valve.
The combination of the load sensing valve and pressure limiting valve is referred as
the load sensing valve (also called the load sensing compensator valve, Figures 7.38, 7.39).
The load sensing compensator has four ports: pump output pressure input, load pressure
input, tank port, and output port to swash plate control piston. The pressure limiting valve
overrides the load sensing valve control only when the pump outlet pressure reaches a pre-
set limit set by the spring in the pressure limiting valve. When p > p (i.e., the directional
s limit
valve is in metering position and the load is very large), the flow from the pressure limiting
valve to control piston will de-stroke the swash plate, and reduce the output pressure.
Furthermore, if the cylinder has reached its travel limit and the directional valve is not
in neutral position, pump output pressure will reach its limit but it will also de-stroke to
almost zero flow. This is called the high pressure cut-off condition. Otherwise, the load
sensing valve controls the swash plate angle and the pressure limit valve does not make a
change in the swash plate angle control as long as the pump output pressure is less than
the maximum limit. The pump output pressure is constantly fed to the bias piston and to
the valves for the load sensing and pressure limiting. The bias piston puts the pump at
maximum displacement until no flow is restricted by the actuator or the directional valve

Load pressure Pump pressure: P
signal: P l s Large
actuator
Yoke pad
Swash plate
P c

P t

Drive
shaft
Load sensing
valve Pressure
limiting valve Small actuator
Tank & bias spring

FIGURE 7.39: Cross-sectional component drawing of a variable displacement axial piston
type pump and its load sensing control system components (load sensing valve and pressure
limiting valve, Source: Sauer-Danfos.com).