SimHydraulics^®

Hydraulic Double-Acting Valve Actuator - Simulate double-acting hydraulic valve actuator

Library

Valve Actuators

Description

Use the Hydraulic Double-Acting Valve Actuator block as a pilot actuator for directional, pressure, or flow control valves in applications where all the forces, except spring force, and flow consumption can be neglected. The actuator consists of two single-acting actuators acting against each other. Each single-acting actuator consists of a piston, centering spring, and centering washer. When control pressure is applied to either hydraulic port, only one centering spring is compressed by its washer while the other butts against the valve body and exerts no force on the spool. When both control pressures are released, the springs force the washers against the valve body, and the spool centers between them. This design allows each actuator to have a different spring, preload force, and piston area.

As pressure applied to the piston develops enough force to overcome the spring preload, the piston moves to the opposite position until it reaches its maximum stroke. Pressure applied at port X shifts the valve in the x-direction, overcoming the spring located in the Y chamber. Pressure applied at port Y shifts the valve in the y-direction, overcoming the spring located in the X chamber.

The actuator is simulated according to the following equations:

If F >= 0,

If F < 0,

where

F	Force acting on the valve
s	Piston displacement
p_x	Pressure in the actuator X chamber
p_y	Pressure in the actuator Y chamber
A_x	Valve face area in the X chamber
A_y	Valve face area in the Y chamber
str_x	Valve stroke in `x`-direction
str_y	Valve stroke in `y`-direction
F_prx	Chamber X spring preload force
F_maxx	Chamber X spring maximum force
F_pry	Chamber Y spring preload force
F_maxy	Chamber Y spring maximum force
or	Actuator orientation with respect to the globally assigned positive direction. If pressure applied at port X moves the piston in positive direction, or equals 1. If pressure applied at port X moves the piston in negative direction, or equals –1.

Connections X and Y are hydraulic conserving ports associated with the valve chambers. Connection P is a physical signal port whose output corresponds to piston displacement. Pressure applied at port X moves the piston in the positive or negative direction depending on the value of the Actuator orientation parameter.

Basic Assumptions and Limitations

The model is based on the following assumptions:

The flow consumption associated with the valve motion is assumed to be negligible.
The inertia, friction, and hydraulic axial forces are assumed to be small and are not taken into account.
The clearances between the valve and the washers are not taken into account.

Dialog Box and Parameters

Piston area at port X: Effective piston area at port X. The parameter value must be greater than zero. The default value is 2e-4 m^2.
Piston area at port Y: Effective piston area at port Y. The parameter value must be greater than zero. The default value is 2e-4 m^2.
Preload force at port X: Spring preload force at port X. The default value is 0.
Preload force at port Y: Spring preload force at port Y. The default value is 0.
Spring maximum force at port X: Chamber X spring maximum force. The parameter value must be greater than the spring preload force. The default value is 50 N.
Spring maximum force at port Y: Chamber Y spring maximum force. The parameter value must be greater than the spring preload force. The default value is 50 N.
Piston stroke at port X: Piston stroke in chamber X. The parameter value must be greater than or equal to zero. The default value is 5e-3 m.
Piston stroke at port Y: Piston stroke in chamber Y. The parameter value must be greater than or equal to zero. The default value is 5e-3 m.
Actuator orientation: Specifies actuator orientation with respect to the globally assigned positive direction. The actuator can be installed in two different ways, depending upon whether it moves the piston in the positive or in the negative direction when pressure is applied at its inlet. If pressure applied at port X moves the piston in the negative direction, set the parameter to Acts in negative direction. The default value is Acts in positive direction.

Restricted Parameters

Ports

The block has the following ports:

X: Hydraulic conserving port associated with the valve X chamber.
Y: Hydraulic conserving port associated with the valve Y chamber.
P: Physical signal port that outputs piston displacement.

Examples

The following illustration shows a typical control unit of a variable-displacement pump that provides load sensing and pressure limiting (see [1]). In the unit, the load-sensing compensator varies the pump displacement to maintain a preset pressure differential across the variable orifice, while the pressure-limiting compensator does not allow the pump pressure to exceed the pressure limit.

The Hydraulic Actuator with Load-Sensing Variable-Displacement Pump demo (sh_hydraulic_actuator_load_sensing_pump) implements this type of control. The next illustration shows the schematic of the Load-Sensing and Pressure-Limiting Control block in the demo.

There are three hydraulic valve actuators in the model:

SA1 — A single-acting actuator that controls the Pressure-Limiting Valve.
SA — A single-acting valve actuator that acts on the pump displacement control device (yoke control).
DAA — A double-acting valve actuator that controls the Load-Sensing Valve. Its output is proportional to the difference between the pump pressure (port P) and the load pressure (port A).

Open the demo model to see the parameter settings for the blocks.

References

[1] F. Yeapple, Fluid Power Design Handbook, Marcel Dekker, Inc., 1995