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Double-Acting Actuator (IL-G)

Linear actuator with piston motion controlled by opposing hydraulic and gas chambers

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Description

The Double-Acting Actuator (IL-G) block represents a linear actuator with piston motion controlled by two opposing fluid chambers, one filled with gas, the other with hydraulic (isothermal liquid) fluid. The actuator generates force in the extension and retraction strokes. The force generated depends on the pressure difference between the two chambers. This block constitutes an interface between four Simscape™ domains—Gas, Hydraulic, Thermal, and Mechanical Translational.

The figure shows the key components of the actuator model. Ports A and B represent the inlets to the hydraulic fluid and gas chambers, respectively. Port R represents the translating actuator piston and port C the actuator case. Port H represents the thermal interface between the gas chamber and the environment. The temperature of the hydraulic fluid is assumed constant and any heat exchange with the environment is ignored. The piston is assumed perfectly insulating.

The direction of motion of the piston depends on the mechanical orientation specified in the block dialog box. If the mechanical orientation is positive, then a higher pressure at port A yields a positive piston translation relative to the actuator casing. If the mechanical orientation is negative, then a higher pressure at port A yields a negative piston translation instead.

A set of hard stops limit the piston range of motion. The hard stops are treated as spring-damper systems. The spring stiffness coefficient controls the restorative component of the hard-stop contact force. The damping coefficient controls the dissipative component of the force. The hard stops are located at the distal ends of the piston stroke, with the exact locations partly dependent on the piston stroke (ds) and piston initial distance from the hydraulic inlet (x0):

  • If the Mechanical orientation parameter is set to Positive, then the lower hard stop is at -x0 and the upper hard stop at ds - x0.

  • If the Mechanical orientation parameter is set to Negative, then the lower hard stop is at -ds + x0 and the upper hard stop at x0.

The block is a composite component built from Simscape Foundation blocks. For more information on how the Double-Acting Actuator (IL-G) block works, see the reference pages of the constituent blocks:

Ports

Output

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Physical signal output port for the piston position data.

Conserving

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Hydraulic (isothermal liquid) conserving port representing the inlet to the hydraulic chamber of the actuator.

Gas conserving port representing the inlet to the gas chamber of the actuator.

Mechanical translational conserving port representing the actuator piston. The piston is capable of translational motion relative to the casing.

Mechanical translational conserving port representing the actuator casing. The casing serves as a mechanical reference for the motion of the piston.

Thermal conserving port representing the surface of the gas chamber through which heat exchange with the surroundings can occur. The thermal processes at this port determine the temperature in the gas chamber and therefore at port B.

Parameters

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Hydraulic Side

Orientation of the actuator piston relative to the direction of fluid flow. A positive orientation causes the piston to move in the positive direction relative to the actuator casing in response to a positive flow rate through port A. The mechanical orientation affects the placement of the piston hard stops. See the block description for more information on the hard stop placement.

Area normal to the direction of flow in the body of the hydraulic chamber. The block uses this area to calculate the hydraulic force due to the fluid pressure in the hydraulic chamber. This parameter must be greater than zero.

Total distance of travel available to the piston, from one hard stop to the other. The hard stops limit the piston motion so that the piston is confined to stroke of the piston. See the block description for more information on the locations of the hard stops.

Distance of the piston from the hard stop closest to the hydraulic inlet at the start of simulation. Use this parameter to change the starting position of the piston. This parameter affects the placement of the piston hard stops. For more information on the hard stop placement, see the block description.

Option to model the effects of dynamic compressibility inside the hydraulic chamber. The hydraulic fluid is treated as compressible if this parameter is set to On and as incompressible if it is set to Off. The block ignores the dependence of the hydraulic fluid density on pressure and temperature if Off is selected.

Hydraulic fluid volume remaining in the hydraulic chamber when the piston is pressed against the hard stop closest to the hydraulic inlet. The dead volume enables the block to capture the internal states of the hydraulic fluid volume—its pressure and temperature—when this volume is at a minimum. This parameter must be greater than zero.

Dependencies

This parameter is enabled when the Compressibility parameter is set to On.

Ratio of the specific heat of the gas entrained in the hydraulic fluid at constant pressure to that at constant volume. The block uses this parameter in the calculations of density for the hydraulic fluid.

Dependencies

This parameter is enabled when the Compressibility parameter is set to On.

Pressure inside the hydraulic chamber at simulation time zero relative to absolute zero. This parameter helps set the initial states of the hydraulic fluid volume.

Dependencies

This parameter is enabled when the Compressibility parameter is set to On.

Gas Side

Area normal to the direction of flow in the body of the gas chamber. The block uses this area to calculate the pneumatic force due to the fluid pressure in the gas chamber. This parameter must be greater than zero.

Area normal to the direction of flow at the entrance to the gas chamber. The cross-sectional area at the entrance can differ from that in the body of the chamber. Set the two cross-sectional areas to different values to model the effects of a sudden area change at the inlet. This parameter must be greater than zero.

Gas volume remaining in the gas chamber when the piston is pressed against the hard stop closest to the gas inlet. The dead volume enables the block to capture the internal states of the gas volume—its pressure and temperature—when this volume is at a minimum. This parameter must be greater than zero.

Pressure inside the gas chamber at simulation time zero relative to absolute zero. This parameter helps set the initial state of the gas volume.

Option to set the environment pressure of the gas chamber to the typical value of one earth atmosphere or to a custom value. Selecting Specified pressure exposes an additional parameter, Environment pressure, that you use to specify a custom pressure.

Pressure outside the gas chamber relative to absolute zero. This pressure acts against the pressure inside the gas chamber. A pressure of zero corresponds to a perfect vacuum.

Dependencies

This parameter is enabled when the Environment pressure specification is set to Specified pressure.

Hard Stop

Spring coefficient for use in the spring-damper model of the piston hard-stops. The spring force is assumed to be the same at both hard stops. Increase the coefficient value to model harder contact.

Damping coefficient for use in the spring-damper model of the piston hard stops. The damping force is assumed to be the same at both hard stops. Increase the coefficient value to reduce piston bounce on contact.

Modeling approach for hard stops. Options include:

  • Stiffness and damping applied smoothly through transition region — 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.

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.

Introduced in R2016b

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