Page 469 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 455
of the system that will create a pressure at the discharge port of the pump. The pressure
downstream the directional valve is fed back via a shuttle valve that always gives the highest
pressure of the two sides of the actuator.
Although not shown in the figure, the load sensing shuttle valve is enabled by a
check valve only when the directional control valve is not in the neutral closed center
position. When in the neutral position, the cylinder is stationary and there is no connection
between pump and cylinder ports. Therefore, the pump does not need to be concerned with
supporting the load. As a result, the signal from the shuttle valve is blocked and the holding
pressure of the load is not fed back to the pump control. In the neutral position of directional
control valve, the pump will go to its low pressure stand-by condition. The load sensing
signal line is connected to the tank line. As a result, pump output pressure is controlled to
a value which is the tank pressure plus the pressure differential.
Figure 7.39 shows a cross-section drawing of a variable displacement axial piston
type pump along with its two valves (load sensing valve and pressure limiting valve). The
pump and load sensing control valve pair are generally packaged together. For the variable
displacement pump to function, the load pressure feedback line must be connected to the
proper location associated with the load.
Figure 7.40 shows a two-axis hydraulic circuit with a variable displacement axial
piston pump which has load sensing hydro-mechanical controls. Notice that the load sensing

Double -acting cylinder

Variable displacement
axial piston pump with
load sensing control
Multi-section
load sensing
control valve
Bi-directional
gear motor

Pump pressure
Screen
Servo pressure
Heat exchanger
Filter Actuator pressure
Reservoir
Load sense pressure
Actuator return pressure
Suction/casedrain/line return
FIGURE 7.40: Two axis hydraulic motion control system: the pump is a variable displacement
axis piston pump with a load sensing control (implemented hydro-mechanically), a linear
cylinder, a bidirectional gear motor, and two manually controlled proportional valves. Pump
and tank port connections to two proportional valves are not shown in the figure for simplicity.
Reproduced with permission from Danfoss Power Solutions.