Single-Acting Hydraulic Cylinder

Hydraulic actuator exerting force in one direction

Library

Hydraulic Cylinders

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Description

The Single-Acting Hydraulic Cylinder block models a device that converts hydraulic energy into mechanical energy in the form of translational motion. Hydraulic fluid pumped under pressure into the cylinder chamber forces the piston to move and exert force on the cylinder rod. Single-acting cylinders transfer force and motion in one direction only. Use an external device, such as a spring, weight, or another opposite installed cylinder, to move the rod in the opposite direction.

Ports R and C are mechanical translational conserving ports corresponding to the cylinder rod and cylinder clamping structure, respectively. Port A is a hydraulic conserving port associated with the cylinder inlet.

The energy through port A is directed to the Translational Hydro-Mechanical Converter block. The converter transforms hydraulic energy into mechanical energy and accounts for the fluid compressibility in the cylinder chamber. The rod motion is limited with the mechanical Translational Hard Stop block in such a way that the rod can travel only between cylinder caps. The piston position is conveyed to the physical signal port p.

Displacement

The piston displacement is measured as the position at port R relative to port C. The Cylinder orientation identifies the direction of piston displacement. The piston displacement is neutral, or 0, when the chamber volume is equal to the chamber dead volume. When displacement is received as an input, ensure that the derivative of the position is equal to the piston velocity. This is automatically the case when the input is received from a Translational Multibody Interface block connection to a Simscape Multibody joint.

When displacement is set by the cylinder, it is calculated as:

xpst = x0 + xp

(1)

where

xpst	Rod displacement output by the physical signal port
x0	Initial distance between piston and cap
xp	Rod displacement with respect to its initial position

Composite Structure

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

Basic Assumptions and Limitations

• No leakage, internal or external, is taken into account.

• No loading on piston rod, 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 cylinder port R.

Parameters

Piston area

Effective piston area. The default value is 0.001 m^2.

Piston stroke

Piston maximum travel between caps. The default value is 0.1 m.

Piston displacement

Method for determining the piston position. The default value is Calculate from velocity of port R relative to port C, which calculates the position internally and reports the position at port p. The block can also receive the position at port p from a Multibody block when set to Provide input signal from Multibody joint.

Initial distance between piston and cap

The distance between the piston and cap at the beginning of simulation. This value cannot exceed the piston stroke. The default value is 0, which corresponds to the fully retracted position. To enable this parameter, set Piston displacement to Calculate from velocity of port R relative to port C.

Dead volume

Fluid volume that remains in the chamber after the rod is fully retracted. The default value is 1e-4 m^3.

Chamber initial pressure

Pressure in the cylinder chamber at the beginning of simulation. The default value is 0.

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 Translational 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.

Contact damping

Specifies dissipating property of colliding bodies for the Translational 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*s/m.

Hard stop model

Modeling approach for hard stops. Options include:

• Stiffness and damping applied smoothly through transition region (default) — Scale the magnitude of the contact force from zero to its full value over a specified transition length. The scaling is polynomial in nature. The polynomial scaling function is numerically smooth and it produces no zero crossings of any kind.

• Full stiffness and damping applied at bounds, undamped rebound — Apply the full value of the calculated contact force when the hard-stop location is breached. The contact force is a mix of spring and damping forces during penetration and a spring force—without a damping component—during rebound. No smoothing is applied.

• Full stiffness and damping applied at bounds, damped rebound — Apply the full value of the calculated contact force when the hard-stop location is breached. The contact force is a mix of spring and damping forces during both penetration and rebound. No smoothing is applied. This is the hard-stop model used in previous releases.

Transition region

Distance below which scaling is applied to the hard-stop force. The contact force is zero when the distance to the hard stop is equal to the value specified here. It is at its full value when the distance to the hard stop is zero. The default value is 1 mm..

Cylinder orientation

Specifies cylinder orientation with respect to the globally assigned positive direction. The cylinder can be installed in two different ways, depending upon whether it exerts force in the positive or in the negative direction when pressure is applied at its inlet. If pressure applied at port A exerts force in negative direction, set the parameter to Pressure at A causes negative displacement of R relative to C. The default value is Pressure at A causes positive displacement of R relative to C.

 Restricted Parameters

When your model is in Restricted editing mode, you cannot modify the following parameter:

• Cylinder orientation

All other block parameters are available for modification.

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.

Ports

The block has the following ports:

A

Hydraulic conserving port associated with the cylinder inlet.

R

Mechanical translational conserving port associated with the cylinder rod.

C

Mechanical translational conserving port associated with the cylinder clamping structure.

p

Physical signal input port that receives rod extension from a Multibody block. To expose this port, set Piston displacement to Provide input signal from Multibody joint.

p

Physical signal output port that returns the rod position with respect to port C. To expose this port, set Piston displacement to Calculate from velocity of port R relative to port C.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.