Page 467 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 453
Pump and load sensing control
Load sensing prsessure signal feedback
P
L
Load sensing valve Pressure limiting valve
P Shuttle valve
P t
P cl
s
P
Control piston for t
swash plate actuation
Swash plate P c
Control
pressure
Pump outlet
P pressure P s
Piston s
P Tank pressure
t
Tank
Slipper pressure
Bias piston for swash plate actuation
Main system actuator
Directional spool
Tank
(a)

P P
t s

Load sensing valve

P
margin
P P
L s

Pressure limiting valve

P
limit

P
c
(b)
FIGURE 7.38: (a) Principle of pressure limiting and load sensing control using a
hydro-mechanical feedback on an axial piston pump; control element is the swash plate angle,
(b) symbolic representation and logical relationship between the load sensing valve and
pressure limiting valve for the pump control.

valve. There are no electrical sensors nor digital controls involved. The load sensing valve
controls the flow rate to the control piston in proportion to the difference between the pump
outlet pressure (P ) and the load pressure (P ). As the pressure difference P − P gets
L
s
s
L
larger (i.e., P is lower), the flow from the load sensing valve to control piston increases.
L
The movement of the control piston to the left reduces the swash plate angle, hence the
pump output flow rate and the pump outlet pressure. This reduces the P . If the pressure
s