Page 483 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 469
Outport
Shuttle P = max (P , P )
piston 0 i1 i2

Inport 1 Inport 2
P i1 P i2
(a) (b)

FIGURE 7.51: A shuttle valve is used as a maximum function. It selects the larger of the two
pressures at its input ports and feeds it to its output port.

directional control valve output line. The control signal for the sequence valve comes from
the pressure in the clamp cylinder. When the directional valve opens, the flow first goes to
the clamp cylinder since the sequence valve makes the flow path to the drill cylinder have
more resistance. When the clamp cylinder extends and pressure in the line increases, then
the sequence valve opens and guides the flow to the drill cylinder. It should be noted that the
sequence of motion between the two cylinders is based on pressure sensing, not based on
position. Therefore, if the pressure increases in the clamp cylinder for some reason before
the actual clamping operation is done, the drill cylinder will still actuate.
The counterbalance valve (also called load holding valve) is a pressure relief valve. It
does not open until a desired pressure differential is met in order to avoid sudden movements
(Figure 7.45c). They are used in load lowering applications such as lift trucks and hydraulic
presses. The valve can be used to control the back pressure between the cylinder and tank
port so that the speed of the cylinder does not move uncontrollably due to large load, but is
controlled by the valve on the cylinder to tank line.
The so-called shuttle or resolver valves are commonly used as selector valves in
hydraulic circuits (Figure 7.51). The valve has two input ports and one output port. The
output port has the larger of the two input port pressures,
p =max(p , p ) (7.192)
o
i2
i1
7.5.2 Example: Multi Function Hydraulic Circuit
with Poppet Valves
Figure 7.52 (also see Figure 7.13) shows the hydraulic circuit schematics of the bucket
control for the wheel loader model L120 by Volvo. The load sensing signal is used in the
same way to control the pump displacement. That is the maximum load pressure signal is
resolved by use of a series of shuttle valves, and the resulting pressure is used to control the
displacement of the pump. The maximum load signal is determined by comparing the load
signals from the bucket hydraulics and steering hydraulic system (the steering hydraulic
system is not shown in the figure). That way, the pump is able to support the circuit with
the most demanding load.
The tilt and lift valves are closed-center, proportional flow control valves (MV1,
MV2). The lift valve also has a so-called float position which is used when the bucket is
riding on the ground surface and desired to track the natural contour of the ground. At the
float position, both sides of the cylinder are connected to the tank. This way, the cylinder
follows the natural contour of the ground surface. The shuttle valve (SV1) is used to actuate
the main valve spool to the float position when the command signal is larger than a certain