Page 585 - Mechatronics with Experiments

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JWST499-Cetinkunt
JWST499-c07
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 571

w D P–C HE P–C RE
in P
P
P–T
C –T C –T
HE RE
T
Open center valve

FIGURE 7.107: Open-center hydraulic system: fixed displacement pump, main flow control
valve with pump-to-tank orifice at neutral position.

7.10 EXAMPLE: OPEN CENTER HYDRAULIC
SYSTEM – FORCE AND SPEED MODULATION
CURVES IN STEADY STATE

2
Consider the single degree of freedom hydraulic motion system shown in Figure 7.107 .
An open center hydraulic system has a fixed displacement pump and a main flow metering
valve which has an orifice opening between pump and tank ports (P–T) when its spool is in
neutral position. Since the pump is of fixed displacement type, as long as the input shaft to
the pump is rotating (i.e., typically the case in mobile equipment applications because the
pump is driven by the engine), the pump generates flow according to

Q = ⋅ D ⋅ w (7.684)
s v p in
where Q is the pump flow rate, D is the pump volumetric displacement, w input shaft
s
p
in
speed of the pump, and is the volumetric efficiency of the pump. The pump output
v
pressure is determined by the load. When the main valve is in neutral position, this flow
must go through the P–T port to the tank. Since this flow through the P–T port performs
no useful work, it is a lost power. The lost power (P loss ) is the flow rate times the pressure
drop across the valve,
Q = Q s (7.685)
v
P loss = Q ⋅ Δp PT (7.686)
v
where Q is the flow rate across the valve P–T orifice, Δp PT is the pressure drop across
v
the P–T orifice of the valve. In order to minimize the power loss, the pressure drop across
the valve through the P–T port should be minimized. The design balance is obtained by
a compromise between the desire to minimize the power loss, which requires as small
a pressure drop as possible which in turn requires as large an orifice size as possible,
while controlling accuracy and physical size (compactness requirement) limitations require
as small a valve as possible. A typical compromise design choice in mobile equipment
applications is that the valve is sized to support the rated flow with P–T pressure drop value
in the range of 100–200 psi.

2 This section is based on lectures by Dr. Richard Ingram.