Pneumo-Hydraulic Actuator

Double-acting cylinder operated by hydraulic and pneumatic power


Hydraulic Cylinders


The Pneumo-Hydraulic Actuator block simulates a pneumo-hydraulic actuator, implemented as a double-acting cylinder with one side connected to a hydraulic power supply and another side operated by pneumatic power. Such devices are widely used as pneumo-hydraulic pumps, intensifiers, and converters of various types. The following illustration shows a few examples of the pneumo-hydralic actuator: a) with rigid separator; b) with flexible separator; c) pneumo-hydraulic intensifier.

The hydraulic part of the model accounts only for fluid consumption associated with the piston velocity. The pneumatic part of the model is built with the ideal gas relationships. To simulate the limit on the piston motion, the hard stop is included in the model. The piston effective area is assumed to be constant. As a result, the model is described with the following equations:




FHS={K(xstroke)+Dvfor x>strokeKx+Dvfor x<00for 0xstroke







qHHydraulic volumetric flow rate
pPressure in the actuator chambers
AHHydraulic side effective area
APPneumatic side effective area
vPiston velocity
FHForce developed by piston on hydraulic side
FPForce developed by piston on pneumatic side
FHSHard stop force
FLForce developed by external load connected to port L
KHard stop stiffness
DHard stop damping
strokePiston stroke
xPiston displacement
VPChamber volume on pneumatic side
V0Chamber dead volume on pneumatic side
GGas mass flow rate
RGas constant
TGas absolute temperature
QPHeat flow through the pneumatic chamber
cvGas specific heat at constant volume
cpGas specific heat at constant pressure
QHEHeat flow through the thermal port E

The model is suitable for building pneumo-hydraulic or hydro-pneumatic pumps, intensifiers, and similar devices. You can simulate piston loading (such as inertia, springs, friction) by modeling the load externally and connecting it to port L. Similarly, simulate the heat exchange with the environment through the external thermal port E, which corresponds to the gas in the chamber. Use blocks from the Simscape™ Foundation library, such as the Convective Heat Transfer, Conductive Heat Transfer, Thermal Mass, and so on, depending on the actual system configuration.

Port P is the pneumatic conserving port associated with the pneumatic side of the actuator. Port H is the hydraulic conserving port associated with the hydraulic inlet.

The block directionality assumes that pressure in the hydraulic chamber causes the piston to move in the positive direction, while pressure in the pneumatic chamber tends to move the piston in the negative direction. Flow rates are considered positive if they flow into the actuator.

Basic Assumptions and Limitations

  • The effective piston area in each chamber is assumed to be constant.

  • The leakage flow between chambers is assumed to be negligible because pressures in the chambers are equal.

  • On the hydraulic side, fluid compressibility is not taken into account.

  • On the pneumatic side, the mass flow rate and heat flow computations assume that the gas is ideal.

Dialog Box and Parameters

Hydraulic side piston area

Effective piston area on the hydraulic side. The default value is 20e-4 m^2.

Pneumatic side piston area

Effective piston area on the pneumatic side. The default value is 10e-4 m^2.


Piston maximum travel between caps. The default value is 0.2 m.

Initial position

The distance between the piston and the cap on the hydraulic side at the beginning of simulation. This value cannot exceed the piston stroke. The default value is 0.

Pneumatic side dead volume

Chamber dead volume on the pneumatic side. The default value is 0.1e-3 m^3.

Hard stop stiffness

The hard stop model implemented in the block assumes that the stop resists penetration of the piston with force proportional to the penetration. This parameter sets the stiffness of the contact between the stop and the piston. The default value is 1e6 N/m.

Hard stop damping

The damping in the hard stop model accounts for dissipation in the piston-stop contact. The default value is 150 N*s/m.

Initial pressure (absolute)

The initial absolute pressure in the pneumatic chamber. The default value is 101325 Pa.

Initial temperature

The initial gas temperature in the pneumatic chamber. The default value is 293.15 K.


The block has the following ports:


Hydraulic conserving port associated with the actuator hydraulic chamber.


Pneumatic conserving port associated with the actuator pneumatic chamber.


Mechanical translational conserving port associated with the actuator piston. You can model the load on the piston, such as external force, inertia, friction, or spring, and connect it through this port.


Thermal conserving port associated with the gas in the pneumatic chamber. You can simulate the heat exchange with the environment through this port.

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