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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 471
set value. Also shown in the figure is the so called ride control or boom suspension circuit
which reduces the oscillations transmitted to the machine frame between road surface and
lift cylinder.
The flow between each service port (A and B) of the main flow control valve and
the cylinder ports (head-end and rod-end) passes through a poppet valve. There are two
poppet valves per valve–cylinder pair, one for each of the valve–cylinder port connection.
Poppet valves provide a leakage free sealing on a line. Unlike a spool type valve, where
some leakage is inevitable, poppet valves are excellent for leakage free sealing. In this
configuration, the poppet valves are used as load holding valves, that is when the main
flow valve is in neutral position, the poppet valve blocks the flow and seals the line. Thus,
the load does not drift in position due to leakage problems. Hence, for lift and tilt circuits
combined, there are four poppet valves (PV1, PV2, PV3, PV4). The spool displacement of
the poppet valve is controlled by a pilot valve (PPV1, PPV2, PPV3, PPV4). When a pilot
control pressure from the lever operated pilot (PMV1) valve acts on the main spool (MV1),
it also acts on the pilot valve (PPV1) which then actuates the poppet valve.

7.5.3 Flow Control Valves

Flow rate through an orifice or restriction is a function of both the area of opening and the
pressure differential across the orifice,

√
Q = K ⋅ A(x ) ⋅ Δp (7.193)
s
where Q is the flow rate, Δp is the pressure drop across the valve, A(x )isthevalve
s
orifice opening area as function of spool displacement x , and K is a proportionality
s
constant (discharge coefficient). Non-compensated flow control valves set the orifice area
only by moving a spool based on a command signal. Figure 7.53a shows a needle valve
used as a flow control valve where the needle position is manually adjusted. The orifice
area is approximately proportional to the needle position. If the input or output pressure
change, the flow rate changes for a set needle position in accordance with the above orifice
equation.
If it is desired that the flow rate should not change with pressure variations, the stan-
dard flow control valve can be modified with a pressure compensator spool and orifice. Such
a valve is called pressure compensated flow control valve. There are two types of pressure
compensated flow control valves: the restrictor type and by-pass type (Figure 7.53b,c).
There are two spool and two orifice areas in a pressure compensated flow control
valve: one pair is the needle–orifice pair which sets the nominal orifice opening. Another
pair modulates the second orifice opening based on input–output pressure feedback signals
in order to maintain a constant pressure drop across the needle–orifice area. As a result,
a constant flow rate is maintained at a constant setting of the needle even though input
and output pressure may vary (since the second spool would compensate for it) as long
as valve operating conditions do not reach saturation. This type is called a restrictor type
pressure compensated flow control valve since flow is regulated against pressure variations
by adding restriction in the flow line (Figure 7.53b). The valve regulates the pressure drop
(tries to maintain it at a constant value) across the needle orifice.
Another type is the by-pass type where an orifice opening by-passes excess flow to
the tank port as a function of pressure feedback signal. The output pressure is maintained at
the load pressure plus the spring due to pressure, that is p out = p + p spring (Figure 7.53c).
l
Notice that the desired flow rate is set by the main orifice opening which is shown in
the figures as being controlled by a manually moved needle-screw. This mechanism can also