Hydraulic device model definition

@HYDRAULIC_DEVICE_MODEL_DEFINITION {
@HYDRAULIC_DEVICE_MODEL_NAME {HydModlName} {
@HYDRAULIC_DEVICE_TYPE {HydType}
@OIL_PROPERTY_NAME {OilPropName}
@HYDRAULIC_CHAMBER_NAME {Chamber0Name}
@HYDRAULIC_CHAMBER_NAME {Chamber1Name}
@HYDRAULIC_ORIFICE_NAME {Orifice0Name}
@HYDRAULIC_ORIFICE_NAME {Orifice1Name}
@PRESSURE_RELIEVE_VALVE_NAME {Prvl0Name}
@PRESSURE_RELIEVE_VALVE_NAME {Prvl1Name}
@FUNCTION_1D_NAME {Fun1DName}
@NUMBER_OF_SUB_TIME_STEPS {n}
@COMMENTS {CommentText}
}
}

Introduction

The hydraulic device model definition describes the parameters associated with various types of hydraulic devices. Such devices consist of a combination of hydraulic chambers, hydraulic orifices, and pressure relief valves. Hydraulic devices are connected to flexible multibody systems via force element interface elements. The physical properties of the oil used in the hydraulic devices are defined by the oil properties, OilPropName.

Three types of hydraulic devices can be defined.

  1. If HydType = HYDRAULIC_ACTUATOR, the hydraulic device is a linear hydraulic actuator.
  2. If HydType = HYDRAULIC_DAMPER, the hydraulic device is a hydraulic damper.
  3. If HydType = HYDRAULIC_DAMPER_WITH_RELIEF_VALVES, the hydraulic device is a hydraulic damper with pressure relief valves.

NOTES

  1. To guarantee the accuracy of the simulations, n sub-time steps are used to integrate governing equations of the device. For more details, see the formulation of force element interfaces.
  2. It is possible to attach comments to the definition of the object; these comments have no effect on its definition.

Sensors

Sensors can be defined to extract information about hydraulic devices. The following SensorType specifications are allowed for hydraulic devices: DEVICE_DATA. (Default value: DEVICE_DATA).

No u value and v value are accepted for the hydraulic device.

Linear hydraulic actuator

Figure 1. Configuration of the linear hydraulic actuator.

Linear hydraulic actuators combine two hydraulic chambers, Chamber0Name and Chamber1Name, and two hydraulic orifices, Orifice0Name and Orifice1Name, to form the configuration depicted in fig. 1. Hydraulic orifice Orifice0Name flows from NULL to chamber Chamber0Name; hydraulic orifice Orifice1Name flows from NULL to chamber Chamber1Name. A 1D function, Fun1DName, of type TIME_FUNCTION, controls the entrance pressures of the hydraulic orifices.

Hydraulic chambers Chamber0Name and Chamber1Name are under pressures p0 and p1, respectively. The hydraulic Orifice0Name and Orifice1Name generate flows Q0 and Q1 into Chamber0Name and Chamber1Name, respectively. The two orifices are opening to entrance pressures pEnt 0 and pEnt 1, respectively. To increase the length of the actuator, valves controlled by 1D function, Fun1DName, will set the entrance pressure of Orifice0Name to a high value, ph, such that pEnt 0 = ph, while the entrance pressure of Orifice1Name remains at a low value pCir (the hydraulic circuit background pressure), such that pEnt 1 = pCir. To decrease the length of the actuator, the control valves reverse the pressure level at the entrance to the two orifices.

The description and formulation of linear hydraulic actuators provides the relationships among these variables in more details.

Hydraulic damper

Figure 2. Configuration of the simple hydraulic damper.

Hydraulic dampers combine two hydraulic chambers, Chamber0Name and Chamber1Name, and one hydraulic orifice, Orifice0Name, connecting the two chambers to form the configuration depicted in fig. 2. Hydraulic orifice Orifice0Name flows from chamber Chamber0Name to chamber Chamber1Name.

Hydraulic chambers Chamber0Name and Chamber1Name are under pressures p0 and p1, respectively. The hydraulic orifice generates a flow Q from Chamber0Name into Chamber1Name. If the length of the damper increases, pressure p1 increases whereas pressure p0 decreases. This generates a pressure differential across the orifice and hence, a flow Q into Chamber0Name that tends to equilibrate the pressures in the chambers. The force generated by the damper always opposes the motion and is therefore a damping force.

The description and formulation of simple hydraulic dampers provides the relationships among these variables in more details.

Hydraulic damper with pressure relief valves

Figure 3. Configuration of the hydraulic damper with pressure relief valves.

Rotorcraft hydraulic lead-lag dampers present a configuration similar to that of the simple damper described in the previous section. However, this simple design suffers an important drawback: under a high stroking rate, the pressure differential in the chambers can be rather high, and hence, high damping forces are generated. These high forces must be reacted at the hub and at the root of the blade, creating high stresses and decreasing fatigue life. To limit the forces in the hydraulic damper, two pressure relief valves are added to the configuration, as shown in fig. 3. Hydraulic orifice Orifice0Name flows from chamber Chamber0Name to chamber Chamber1Name. To prevent low pressures to occur in the chambers, two additional hydraulic orifices, Orifice1Name and OrificeName2 connect chambers Chamber0Name and Chamber1Name, respectively, to oil supply reservoirs. For simplicity, these two orifices are not shown in fig. 3. Hydraulic orifice Orifice1Name flows from NULL to chamber Chamber1Name; hydraulic orifice OrificeName2 flows from NULL to chamber Chamber1Name.

The complete damper combines two hydraulic chambers, Chamber0Name and Chamber1Name, one orifice connecting the two chambers, two pressure relief valves, Prvl0Name and Prvl1Name, and two orifices to an oil reservoir, Orifice1Name and OrificeName2. Hydraulic Chamber0Name and Chamber1Name are under pressures p0 and p1, respectively. Hydraulic orifice Orifice0Name generates a flow Q from Chamber0Name into Chamber1Name. When open, the pressure relief valves regulate the pressures in Chamber0Name and Chamber1Name and generate flows Q0 and Q1, respectively. Finally, Orifice1Name and OrificeName2 are associated with flows Q2 and Q3, respectively.

If the length of the damper increases, pressure p1 increases whereas pressure p0 decreases. This generates a pressure differential across the orifice and hence, a flow Q into Chamber0Name that tends to equilibrate the pressures in the chambers. If the stroking rate is high, the pressure differential in the chambers will become high enough to open pressure relief Prvl1Name, resulting in an additional flow Q1 from Chamber1Name into Chamber0Name. Given the sign of the pressure differential, Prvl0Name will remain closed. The opening of the valve and the ensuing flow controls the magnitude of the pressure differential. The force generated by the damper always opposes the motion and is therefore a damping force.