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Single-Acting Rotary Actuator

Single-acting hydraulic rotary actuator


Hydraulic Cylinders


The Single-Acting Rotary Actuator block models a single-acting hydraulic rotary actuator, which directly converts hydraulic energy into mechanical rotational energy without employing intermediary transmissions such as rack-and-pinion, sliding spline, chain, and so on. Single-acting actuators generate torque and motion in a single direction only. Use an external device, such as a spring or another opposite installed actuator, to move the shaft in the opposite direction.

The model of the actuator is built of Simscape™ Foundation library blocks. The schematic diagram of the model is shown below.

The blocks in the diagram perform the following functions:

Rotational Hydro-Mechanical ConverterConverts hydraulic energy into mechanical rotational energy and vice versa, while accounting for fluid compressibility.
Rotational Hard StopImposes limits on shaft rotation.
Linear Hydraulic ResistanceAccounts for leakages.

Connection A is a hydraulic conserving port corresponding to the actuator chamber. Connection S is a mechanical rotational conserving port associated with the actuator shaft.

The block directionality is adjustable and can be controlled with the Actuator orientation parameter.

Basic Assumptions and Limitations

No loading, such as inertia, friction, spring, and so on, is taken into account. If necessary, you can easily add them by connecting an appropriate building block to port S.


Actuator displacement

Effective displacement of the actuator. The default value is 4.5e-5 m^3/rad.

Shaft stroke

Shaft maximum travel between stops. The default value is 5.1 rad.

Shaft initial angle

The position of the shaft at the beginning of simulation. You can set the shaft position to any angle within its stroke. The default value is 0, which corresponds to the shaft position at the very beginning of the stroke.

Dead volume

Fluid volume that remains in the chamber when the shaft is positioned at the very beginning of the stroke. The default value is 1e-4 m^3.

Leak coefficient

Leak coefficient for the Linear Hydraulic Resistance block. The default value is 1e-14 (m^3/s)/Pa.

Specific heat ratio

Gas-specific heat ratio for the Hydraulic Piston Chamber block. The default value is 1.4.

Contact stiffness

Specifies the elastic property of colliding bodies for the Rotational Hard Stop block. The greater the value of the parameter, the less the bodies penetrate into each other, the more rigid the impact becomes. Lesser value of the parameter makes contact softer, but generally improves convergence and computational efficiency. The default value is 1e6 N*m/rad.

Contact damping

Specifies dissipating property of colliding bodies for the Rotational Hard Stop block. At zero damping, the impact is close to an absolutely elastic one. The greater the value of the parameter, the more energy dissipates during an interaction. Keep in mind that damping affects slider motion as long as the slider is in contact with the stop, including the period when slider is pulled back from the contact. For computational efficiency and convergence reasons, MathWorks recommends that you assign a nonzero value to this parameter. The default value is 150 N*m/(rad/s).

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 generates torque in the positive or in the negative direction when pressure is applied at its inlet. If pressure applied at port A generates torque in the negative direction, set the parameter to Acts in negative direction. The default value is Acts in positive direction.

 Restricted Parameters

Global Parameters

Parameter determined by the type of working fluid:

  • Fluid bulk modulus

Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.


The block has the following ports:


Hydraulic conserving port associated with the actuator inlet.


Mechanical rotational conserving port associated with the actuator shaft.

Introduced in R2007a

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