In the default configuration, a zero displacement corresponds to a fully closed valve between positions I and II. A positive displacement signal shifts the spool toward valve position I. A negative displacement shifts the spool toward valve position II. The spool displacement acts indirectly by setting the spool position relative to each flow path—a length known here as the orifice opening.
The orifice opening in turn determines the opening area of the respective flow path. The orifice opening of a flow path depends partly on its opening offset —the orifice opening of a flow path at zero spool displacement. The block models only the dynamic effects of the opening offsets. An offset can be due to a change in distance between ports or spool lands—the thick disks built into the spool to obstruct flow.
It can also be due to a change in the thicknesses of the spool lands. The orifice openings are computed separately for each flow path in terms of the respective opening offset:. The orifice openings are computed during simulation. The opening offsets are specified in the Valve opening offsets tab. The spool displacement is specified through physical signal port S.
The figure shows the effects of the opening offsets on the orifice openings. Plot I corresponds to the default configuration with both opening offsets equal to zero.
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Plot II corresponds to a valve with both opening offsets greater than zero and plot III to a valve with both opening offsets smaller than zero. The valve schematics to the right show what the offsets might look like for the A-T flow path. An underlapped valve is always partially open and allows some flow at all spool displacements. An overlapped valve is fully closed over an extended range of spool displacements and requires longer spool travel to open. The table summarizes the opening offsets for zero-lapped, underlapped, and overlapped valves. Other configurations are possible—e.
The Model parameterization setting determines the calculations used for the opening areas of the flow paths—or, in the Pressure-flow characteristic case, the volumetric flow rates. The calculations are based on orifice parameters or tabulated data sets specified in the Model Parameterization tab.
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The block uses the same data for both flow paths if the Area characteristics parameter in the Basic Parameters tab is set to Identical for all flow paths and different data otherwise. Model parameterizations that you can select include:. Maximum area and opening — Specify the maximum opening area and the corresponding orifice opening. The opening area is a linear function of the orifice opening,.
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The subscript Max refers to a fully open orifice and the subscript Leak to a fully closed orifice—one with internal leakage flow area only. The figure shows a plot of the linear function A h.
The opening area is computed for a given orifice opening by interpolation or extrapolation of the tabulated data. The figure shows a conceptual plot of the tabulated function A h. Pressure-flow characteristic — Specify the volumetric flow rate at discrete orifice openings and pressure differentials as a 2-D lookup table. The opening area is computed for a given orifice opening and pressure differential by interpolation or extrapolation of the tabulated data. The figure shows a conceptual plot of the tabulated function q h , p.
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Volumetric flow rates are computed analytically in the Maximum area and opening and Area vs. The calculations are based on additional block parameters such as the flow discharge coefficient and account for the effects of flow regime—laminar or turbulent. Regime transition occurs at the specified critical laminar flow ratio or critical Reynolds number. The Maximum area and opening and Area vs.
The leakage area ensures that portions of the hydraulic network never become isolated when a flow path is closed. The effects of flow regime and internal leakage are assumed to be reflected in the tabulated flow rate data specified directly in the Pressure-flow characteristic parameterization.
The block is a composite component with two Variable Orifice blocks driven by a single physical signal. The physical signal is specified through Connection Port block S. All valve orifices are assumed identical in size unless otherwise specified. Hydraulic isothermal liquid port associated with the supply line orifice. Hydraulic isothermal liquid port associated with the return line orifice. Hydraulic isothermal liquid port associated with the actuation orifice. Choice of different or identical flow path opening characteristics.
Select Different for each flow path to specify flow path parameters or tabulated data separately for each flow path. By maximum area and opening — Specify the maximum orifice opening and opening area. The opening area varies linearly with the spool displacement specified at physical signal port S.
The opening area is computed by interpolation or extrapolation of the tabulated data. By pressure-flow characteristic — Specify the flow path volumetric flow rates at discrete orifice openings and pressure differentials. The flow rate is computed by interpolation or extrapolation of the tabulated data. Method of computing values inside the tabulated data range. The Linear method joins adjacent data points with straight line or surface segments with generally discontinuous slope at the segment boundaries.
Surface segments are used in the 2-D lookup table specified in the Pressure-flow characteristic model parameterization. The Smooth method replaces the straight segments with curved versions that have continuous slope everywhere inside the tabulated data range. The segments form a smooth line or surface passing through all of the tabulated data points without the discontinuities in first-order derivatives characteristic of the Linear interpolation method.
This parameter is active when the Model Parameterization parameter is set to By area vs.
Method of computing values outside of the tabulated data range. The Linear method extends the line segment drawn between the last two data points at each end of the data range outward with a constant slope. The Nearest method extends the last data point at each end of the data range outward as a horizontal line with constant value.
Ratio of the actual and theoretical flow rates through the valve. This parameter depends on the geometrical properties of the valve. Values are usually provided in textbooks and manufacturer data sheets.
This parameter is active when the Model Parameterization parameter is set to By maximum area and opening or By area vs. Total area of internal leaks in the completely closed state. The purpose of this parameter is to maintain the numerical integrity of the fluid network by preventing a portion of that network from becoming isolated when the valve is completely closed. Select the parameter to base the laminar-turbulent transition on.
Pressure ratio — Flow transitions between laminar and turbulent at the pressure ratio specified in the Laminar flow pressure ratio parameter. Use this option for the smoothest and most numerically robust flow transitions. Reynolds number — The transition occurs at the Reynolds number specified in the Critical Reynolds number parameter.
Flow transitions are more abrupt and can cause simulation issues at near-zero flow rates. Pressure ratio at which the flow transitions between the laminar and turbulent regimes. The pressure ratio is the fraction of the outlet pressure over the inlet pressure. Maximum Reynolds number for laminar flow. This parameter depends on the orifice geometrical profile.